1400443160_690__swenson_1996.pdf

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PAGEOPH, Vol. 146, No. 1 (1996) 0033-4553/96/010067 3551.50 + 0.20/0

9 1996 Birkh/iuser Verlag, Basel

H istor ica l 1942 Ecua dor and 194 2 Peru Subduct ion E arthquakes

a n d Ea r t h q u a k e Cy c l e s a l o n g Co l o mb i a - Ec u a d o r

and Peru Subduct ion Segments

JENNIFE R L. SWENSON I an d SUSAN L. BECK I

bstract Two large shallow earthquakes occurred in 1942 along the South American subduction

zone in close proximity to subducting oceanic ridges: The 14 May event occurred near the subducting

Carnegie ridge off the coast of Ecuador, and the 24 August event occurred off the coast of southwestern

Peru near the southern flank of the subducting Nazca ridge. Source parameters for these two historic

events have been determined using long-period P waveforms, P-wave first motions, intensities and local

tsunami data.

We have analyzed the P waves for these two earthquakes to constrain the focal mechanism, depth,

source complexity and seismic moment. Modeling of the P waveform for both events yields a range of

acceptable focal mechanisms and depths, all of which are consistent with underthrusting of the Nazca

plate beneath the South American plate. The source time function for the 1942 Ecuador event has one

simple pulse of moment release with a duration of 22 seconds, suggesting that most of the moment

release occurred near the epicenter. The seismic moment determined from the P waves is 6-

8 x 102 ~ corresponding to a moment magnitude of 7.8 7.9. The reported location of the

maximum intensities (IX) for this event is south of the main shock epicenter. The relocated aftershocks

are in an area that is approximately 200 km by 90 km (elongated parallel to the trench) with the majority

of aftershocks north of the epicenter. In contrast, the 1942 Peru event has a much longer duration and

higher degree of complexity than the Ecuador earthquake, suggesting a heterogeneous rupture. Seismic

moment is released in three distinct pulses over approximately 74 seconds; the largest moment release

occurs 32 seconds after rupture initiation. The seismic moment as determined from the P waves for the

1942 Peru event is 10-25 x 1020 N. m, corresponding to a moment magni tude of 7.9 8.2. Aftershock

locations reported by the ISS occur over a broad area surrounding the main shock. The reported

locations of the maximum intensities (IX) are concentrated south o f the epicenter, suggesting that at

least part of the rupture was to the south.

We have also examined great historic earthquakes along the Colombia-Ecuador and Peru segments

of the South American subduction zone. We find that the size and rupture length of the underthrusting

earthquakes vary between successive earthquake cycles. This suggests that the segmentation of the plate

boundary as defined by earthquakes this century is not constant.

Key words

Earthquake cycle, source parameters, seismic moment, fault heterogeneity, P wavefomls,

historical earthquakes, source time function, seismic gap.

1 Department of Geosciences, University of Arizona, Tucson, AZ 85721, U.S.A.

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68 Jennifer L. Swenson and Susan L. Beck PAGEOPH,

Introduction

Plate boundary segments fail repeatedly in large earthquakes. However, the

seismic moment and recurrence interval for these major plate boundary events often

vary dramatically between successive earthquakes along the same segment (ANDo,

1975; KANAMORI and MCNALLY, 1982; BECK and RUFF, 1987; BECK and RUFF,

1989; THATCHER, 1990). We do not yet understand the fault zone heterogeneity nor

the interaction between plate boundary segments that give rise to these variations.

Because the earthquake cycle (if it is cyclic) is a long-term process, study of

different earthquakes over a long time interval is essential for a thorough under-

standing of earthquake phenomena. Restricting our analysis to recent earthquakes

can lead to erroneous or misleading results for some regions; for this reason,

detailed analyses of historic earthquakes are critical. For nearly a century, a large

number o f seismic stations have been recording global seismicity. Collecting historic

seismograms for important earthquakes and analyzing them with modern tech-

niques can yield valuable information about the rupture mode and fault heterogene-

ity.

The plate boundary between the South American plate and the subducting

Nazca plate has been the site of large destructive earthquakes for many centuries.

The Colombia-Ecuador and Peru segments in particular have been the site of

several large to great earthquakes in the past. We have analyzed two of these large

historic subduction zone earthquakes along the Colombia-Ecuador and Peru

trenches in order to determine source parameters. This information will help in

defining plate boundary segments, refining estimates of seismic potential, and

understanding fault heterogeneity and the variations in rupture mode between

successive earthquake cycles.

The Colombia-Ecuador subduction zone is probably one of the best examples of

different modes of earthquake rupture

KANAMORI

and MCNALLY, 1982). A great

earthquake

Mw

= 8.8) occurred 1906 along the Colombia Ecuador coast (Fig. 1)

with an estimated rupture length of 500 km (KELLEHER, 1972; KANAMOR and

MCNALLY, 1982). This same segment subsequently ruptured in three smaller

underthrusting events from south to north in 1942

M w

= 7.9), 1958

M w

= 7.8)

and 1979

Mw

= 8.2)

KANAMORI

and McNALLY, 1982). The fault areas of these

three events, as defined by aftershocks, about each other but do not overlap along

strike (MENDOZA and DEWEY, 1984). Previous studies indicate that the 1958 and

1979 earthquakes are underthrusting events that each failed with one dominant

asperity KANAMORIand GIVEN, 1982; BECK and RUFF, 1984). In contrast, very

little is known about the 1942 earthquake. Given that the time since the 1942

Ecuador event is larger than the 36 years between 1906 and 1942, we have evaluated

the 1942 event. It is not clear if this segment of the plate boundary fails as large

"1906" type ruptures or in smaller segments that only rarely fail as one great

earthquake.

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Vol. 146, 1996 Historical 1942 Ecuador and Per u Earthquakes 71

I n t h i s s t u d y w e u t i l i z e d l o n g - p e r i o d , t e l e s e i s m i c , P w a v e f o r m s , P - w a v e f i r s t

m o t i o n s , i n t e n s i t y r e p o r t s , l o c a l t s u n a m i d a t a a n d r e l o c a t e d a f t e r s h o c k s t o i n v e s t i -

g a t e so u r c e p a r a m e t e r s f o r t h e 1 9 42 E c u a d o r a n d 1 94 2 P e r u e a r t h q u a k e s . W e h a v e

e s t i m a t e d t h e fo c a l m e c h a n i s m s , h y p o c e n t r a l d e p t h , a n d s e i sm i c m o m e n t . I n

a d d i t i o n , w e p l a c e d c o n s t r a i n t s o n t h e r u p t u r e l e n g t h f o r t h e t w o e v e n t s a n d

e x a m i n e d t h e h i s to r i c e a r t h q u a k e r e c o r d i n th e c o n t e x t o f t h e e a r t h q u a k e c y cl e

a l o n g t h e S o u t h A m e r i c a n s u b d u c t i o n z o n e .

D a t a a n d M e t h o d s

L o n g - p e r i o d s e i s m o g r a m s a n d i n s t r u m e n t r e s p o n se i n f o r m a t i o n w e r e o b t a i n e d

f r o m a s m a n y s t a t i o n s a s p o s s i b le o p e r a t i n g i n 1 942 . T h e d a t a c a m e f r o m a v a r i e t y

o f s e i s m o m e t e r s i n c lu d i n g B e n i of f, G a l i t z in , l o n g - p e r i o d W o o d - A n d e r s o n a n d S p i n -

d l e r - H o y e r i n s t r u m e n t s . I n s t r u m e n t r e s p o n s e c h a r a c t er i s ti c s a s w e ll a s g e n e ra l

i n f o r m a t i o n f o r e a c h s t a t i o n a r e l is te d in T a b l e s 1 a n d 2 . I n s t r u m e n t a l r e s p o n s e s

w e r e o b t a i n e d f r o m s t a t i o n b u l le t in s , M C C O M B a n d W E S T ( 1 93 1 ) , e A R L I E R a n d

V A N G I L S ( 1 9 5 3 ) , B A K E R a n d L A N G ST O N ( 1 9 8 7 ) a n d E S TA B R O O K

e t a l . 1994) .

W e

w e r e a b l e to d i g it iz e si x l o n g - p e r i o d P w a v e f o r m s f o r th e 19 42 E c u a d o r e v e n t f r o m

S E I S M I C S T T IO N D IS T R I B U T IO N

80 240 ~

60

30"

P A : ~

0 9

-30"

- 6 0 P E R U 8 2 4 4 2

M w 8.1

180

~

240"

30 0" 0 ~ 60" 120 ~ 180 ~

300

O

Figure 2

60 ~ 120 ~

M ap show ing the location of stations from wh ich data were used.

60 ~

30 ~

0 o

-30"

-60"

180"

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72 Jennifer L. Swenson and Susan L. Beck PAGEOPH

I f i / t

~ . A f ~ ~ ~ i A ~

0 1 2 0 s

1 . 0 D B N E

2 4 A u g u s t 1 9 4 2

0.5

0.5

. . . . . . . i i i i i I - . I i

20 40 60 80 100 120

Seconds

Figure 3

Example of the original seismogram and the digitized vers ion from the seismic stat ion at DeBilt

Netherlands for the 24 August 1942 Peru earthquake.

I 100 SECONDS [

Figure 4

Lower hemisphere first motion focal mechanism plot for the 14 May 1942 Ecuador earthquake. The

solid circles are compressional first motions and the open circles are dilatational first motions. The P

waves are plotted starting at the arrival of the earthquake

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V o l . 1 4 6 , 1 99 6 H i s t o r i ca l 1 94 2 E cu ad o r an d P e ru E a r t h q u ak es 7 3

1 2 S E C O N D S I

F i g u re 5

L o w er h em i s p h e re f i rs t m o t i o n fo ca l m ech an i s m p l o t fo r t h e 2 4 A u g u s t 1 9 4 2 P e ru e a r t h q u ak e . T h e s o l id

c i rc les a re compress ional f i r s t mot ions and the open c i rc les a re d i la ta t ional f i r s t mot ions . The P waves

a re p l o t t ed s t a r t i n g a t t h e a r r i v a l o f t h e ea r t h q u ak e .

s i x s t a t i o n s , a n d n i n e l o n g - p e r i o d P w a v e f o r m s f r o m s e v e n s t a t i o n s f o r t h e 1 9 4 2

P e r u e v e n t F i g . 2 ).

S i x ty t o o n e - h u n d r e d t w e n t y s e c o n d s o f P w a v e f o r m s f r o m e a c h u s a b l e l o n g -p e -

r i o d r e c o r d w a s d i g it i z e d F i g . 3 ). W e u s e d o n l y d a t a w i t h v e r i f ia b l e p o l a r i t y a n d a

c l e a r w a v e f o r m f o r o p t i m a l d i g i t i z i n g r e s u l t s . W h e n p o s s i b l e , t h e v e r t i c a l c o m p o -

n e n t w a s u s e d . O f t e n t h e v e r t i c a l c o m p o n e n t w a s c l i p p e d o r i l l e g i b l e d u e t o t h e l a r g e

s iz e o f t h e s e e v e n t s . I n t h is c a s e , th e b a c k a z i m u t h t o t h e s t a t i o n w a s u s e d t o

d e t e r m i n e w h i c h o f th e h o r i z o n t a l c o m p o n e n t s h a d r e c o r d e d t h e m a j o r i t y o f s e is m i c

e n e r g y re l e a s e d a n d s h o u l d r e p l a c e th e v e r ti c a l c o m p o n e n t i n o u r s t u d y . H o r i z o n t a l

r e c e i ve r f a c t o r s w e r e d e t e r m i n e d b y c o m p a r i n g h o r i z o n t a l a n d v e r t ic a l c o m p o n e n t

a m p l i t u d e s o r b y a p p l y i n g a t h e o r e t i c a l c o r r e c t i o n f o l l o w i n g t h e m e t h o d o f B UL LE N

1 9 63 ) . T h e p o l a r i t y o f P - w a v e f i r st m o t i o n s f r o m e a c h r e a d a b l e s e i s m o g r a m

w h e t h e r o r n o t w e u s e d t h e w a v e f o r m ) w e r e u s e d t o c o n s t r a i n t h e f o c a l m e c h a n i s m

F i g s . 4 a n d 5 ). T o i n c r e a s e o u r a z i m u t h a l c o v e r a g e , w e h a v e i n c l u d e d d i f f r a c t e d P

w a v e s . T h e d i f f r a c t e d w a v e f o r m s a r e a s m o o t h v e r s i o n o f t h e n o n d i f f r a c t e d w a v e -

f o r m , w i t h t h e a d v a n t a g e t h a t t h e i r u s e l e ad s t o a n u n d e r e s t i m a t e o f t h e s e is m i c

m o m e n t .

W e i n v e r t e d P w a v e s u s i n g a m u l t i s t a t i o n o m n i l i n e a r i n v e r s i o n

R U F F ,

1989)

a n d a s i n g l e s t a t i o n i n v e r s i o n R U F F a n d K A N A M O R I , 1983; BECK and R U F F ,

1 98 4). B o t h m e t h o d s i n v e r t f o r th e s o u r c e t i m e f u n c t i o n a n d s e is m i c m o m e n t u s i n g

a n a s s u m e d f o c a l m e c h a n i s m a n d d e p t h . T h e o m n i l i n e a r i n v e r s i o n s i m u l t a n e o u s l y

d e t e r m i n e s t h e s o u r c e t i m e f u n c t i o n a n d t r a c e s c a li n g f a c t o r s to m i n i m i z e s c a t t e r i n

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7 4 J e n n if e r L . S w e n s o n a n d S u s a n L . B e c k P A G E O P H ,

the amplitudes, resulting in a better match between observed and synthetic seis-

mograms (RUFF, 1989). We used these methods in a comprehensive grid search by

varying the focal mechanism and depth to determine the best source parameters for

each earthquake. We use the geometric spreading constants of LANGSTON and

HELMBERGER (1975) to calculate the Green s function. Mantle attenuation is

modeled with a t* (travel time/Q .. .. ge) of 1.0 seconds. We use a density and

P-wave velocity of 2.7 g/cm3 and 6.7 km/s, respectively, for a source structure. We

also include a water layer with a depth of 2 kin. Although the historic data set is

limited, very important first-order source parameters can be determined.

P Wave Analysis

14 May 1942 Ecuador Earthquake

The P waveforms for the 14 May 1942 earthquake are relatively simple as

shown in Figure 4. The European stations all have compressional P-wave first

motion arrivals. Stations PAS and TUC have dilatational arrivals, and station BKS

is nodal. These North American stations help to constrain the maximum dip of the

steep nodal plane. The compressional first motion arrivals from the European

stations suggest an underthrusting focal mechanism.

T a b l e 3

Fault plane solutions for the I94 Ecuador earthquake

N u m b e r S t ri k e q5~ D i p 3 ~ R a k e 2 ~

30 20 120

2 25 20 120

3 35 20 120

4 30 15 120

5 30 25 120

6 30 20 125

7 30 20 115

8 25 15 115

9 35 25 125

10 35 20 12

t l 2 5 2 0 1 1 5

12 30 20 90

13 30 20 120

14 34 20 120

F a u l t p l a n e s o l u t i o n s t e s te d f o r t h e 1 9 4 2 E c u a d o r e v e n t . S o l u t i o n 1 3

i s t h e s a m e a s S o l u t i o n 1 , t h e o n l y d i f fe r e n c e b e i n g t h a t t h e m u l t i s t a -

t io n r e s u l ts fo r S o l u t i o n 1 3 d o n o t i n c l u d e st a t io n s P A S o r K E W . T h e

s t r ik e o f S o l u t i o n I 4 is th e s t r i k e o f t h e E c u a d o r c o a s t l i n e n e a r t h e

e p i c e n t e r .

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V o l . 1 46 , 1 9 9 6 H i s t o r i c a l 1 9 4 2 E c u a d o r a n d P e r u E a r t h q u a k e s 7 5

80 SECONDS ]

O T T

J ~ AZ= 5.6o

DIST = 46.2o

AZ = 38,0c

DIST = 82.2o

~ ~ ~ K EW .E

AZ = 38.3o

DIST = 85.1o

UCC.E

AZ = 39.0o

DIST= 87.8o

PAS

~ ~ / ~ AZ= 318'9~

DIST= 49,0o

TUC.N

~ @ ~ AZ= 322.5~

DIST = 43.0o

F i g u r e 6

O b s e r v e d ( s o l i d l i ne ) a n d s y n t h e t i c ( d a s h e d l in e ) l o n g - p e r i o d P w a v e s , fo c a l m e c h a n i s m , a n d s o u r c e t i m e

f u n c t i o n f r o m t h e 1 9 4 2 E c u a d o r m u l t i s t a t i o n i n v e r s i o n u s i n g o u r p r e f e r r e d f a u l t p l a n e s o l u t i o n o f s t r i k e ,

d i p a n d s l i p q5 = 3 0 ~ ~ = 2 0 ~ a n d 2 = I 2 0 ~ r e s p e c t i v e l y . S t a t i o n s c o d e s a r e i n c a p i t a l l e t t e r s; i n s t r u m e n t

r e p o n s e s a r e l i s t e d i n T a b l e 1.

T h e m u l t i s t a t io n i n v e r s i o n m e t h o d o f R U F F 1 9 8 9 ) w a s a p p l i e d t o th e in i ti a l 80

s e c o n d s o f s ix l o n g - p e r i o d t e le s e is m i c P w a v e s f r o m t h e 1 94 2 E c u a d o r e v e n t t o

i n v e s ti g a t e c h a ra c t e r is t i c s o f t h e s o u r c e t i m e f u n c t i o n a n d t o c o n s t r a i n t h e f o c a l

m e c h a n i s m a n d h y p o c e n t r a l d e p t h . W e s p e c i fi e d t h e fo c a l m e c h a n i s m a n d d e p t h ,

Resul ts o f E cuador 1942 InverSion: D epth v~ Normal iged Error

0 .9 ~ ' ' ' I ' ' ' I ' ' ' I '

0.8

0.7

z

0.5

0.5

0 20 40 60

Depth in Kilometer~

F i g u r e 7

N o r m a l i z e d e r r o r v e r s u s d e p t h f o r t h e 1 9 4 2 E c u a d o r m u l t i s t a t i o n i n v e r s i o n u s i n g o u r p r e f e r r e d f o ca l

m e c h a n i s m . E r r o r m i n i m a e x is t a t 1 4 k i n a n d 4 0 k m .

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76 Jennifer L. Swenson and Susa n L. Beck PAGE OPH,

5 k m

1 k r n

1 5 k m

2 k r n

2 5 k m

z

~ 1 J ~ | I U

~ 1 I F

3 k m

4 k m

5 k m

6 k m

7 k m

t ~ j t J , ,

T

8 s

Figure 8

A representative sampling of the source time functions for the 14 May 1942 Ecuador event deconvolved

from the simultaneously inversion of stations OTT, DBN.E, KEW.E, UCC.E, PAS and TUC.S. at

depths from 1 70 kin. N ote the increase in ring ing of the latter part of the source tim e function at

depths below approximately 20 kin.

a n d d e t e r m i n e d t h e s o u r c e t i m e f u n c t i o n a n d t h e c o r r e s p o n d i n g s y n t h e t ic s eis -

m o g r a m s f o r e ac h s t at io n . W e h a v e s t a r te d w i t h t h e f o c a l m e c h a n i s m o f t h e n e a r b y

1 9 7 9 C o l o m b i a u n d e r t h r u s t i n g e a r t h q u a k e , s t ri k e , d i p , a n d r a k e o f q5 = 3 0 ~ ~ = 2 0 ~

a n d 2 = 12 0 ~ r e s p e c t iv e l y ( f a u l t p l a n e s o l u t i o n 1 , T a b l e 3 ). T h i s m e c h a n i s m is

c o n s i s t e n t w i t h t h e P - w a v e f i rs t m o t i o n s . W e t e s t e d 1 4 d i f f e re n t f a u l t p l a n e

s o l u t i o n s b y v a r y i n g t h e s h a l l o w d i p p i n g p l a n e ( l is t e d i n T a b l e 3 ) a t e a c h o f 31

d i ff e re n t d e p t h s s p a n n i n g 1 t o 7 0 k m . O u r i n i ti al f o c a l m e c h a n i s m p r o v i d e d u s w i th

t h e b e s t f it b e t w e e n d a t a a n d s y n t h e t i c s ( F i g . 6 ). H o w e v e r , a s e x p e c t e d t h e d a t a a r e

n o t v e r y s e n si t iv e t o s m a l l v a r i a t i o n s i n t h e s t ri k e , d i p a n d r a k e o f t h e s h a l l o w

p l a n e .

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Vol. 146, 1996 Historical 1942 Ecuador and Pe ru Earthquakes 77

I n o r d e r t o d e t e r m i n e t h e b e s t d e p t h w e e x a m i n e d t h e n o r m a l i z e d e r r o r v e r s u s

d e p t h f o r d if f er e n t f o c a l m e c h a n i s m s . M o s t o f th e m e c h a n i s m s i n d i c a te d a f ai r ly

s h a l lo w d e p t h , g e n e r a l ly < 4 0 k in . H y p o c e n t r a l d e p t h s f o r s u c h la r g e e a r t h q u a k e s

d e t e r m i n e d f r o m h i s t o ri c d a t a s u c h a s t h e 1 94 2 E c u a d o r a n d P e r u e v e n t s c a n b e

p o o r l y r e s o l v e d a t b e s t . F i g u r e 7 s h o w s t h e e r r o r v e r s u s d e p t h u s i n g o u r p r e f e r r e d

f a u l t p l a n e s o l u t i o n o f q~ = 3 0 ~ ~ = 2 0 ~ a n d 2 = 1 20 ~ T h e r e a r e t w o e r r o r m i n i m a ,

o n e f o r a s o u r c e d e p t h o f 1 4 k i n , a n d o n e a t 4 0 k i n. W e a l s o i n v e s t i g a t e d t h e

b e h a v i o r o f t h e s o u r c e t i m e f u n c t i o n w i t h i n c r e a s in g d e p t h a s s h o w n in F i g u r e 8 .

W e f in d t h a t a t d e p t h s g r e a t e r t h a n a p p r o x i m a t e l y 2 0 k m , t h e s o u r c e t i m e f u n c t i o n

b e g i n s t o e x h i b i t t h e p e r i o d i c r i n g i n g i n d i c a t i v e o f a d e p t h o v e r e s t i m a t i o n . C H R I S -

T EN S EN a n d R U F F 1 9 8 5 ) s h o w t h a t a t t h e b e s t d e p t h , t h e s e i s m i c m o m e n t is

c o n c e n t r a t e d t o w a r d t h e b e g i n n i n g o f t h e d e c o n v o l v e d s o u r c e ti m e f u n c ti o n . I n o u r

c a se , t h e s h a l lo w d e p t h s s h o w t h e m a j o r i t y o f m o m e n t r e l e as e in t h e i n it ia l 2 4

s e c o n d s . O u r a n a l y s i s i n d i c a t e s t h a t t h e 1 94 2 E c u a d o r e a r t h q u a k e w a s a r e l a t iv e l y

s h a l l o w e v e n t w i t h t h e b e s t p o i n t s o u r c e a t a d e p t h o f a p p r o x i m a t e l y 1 4 k m . A

d i s t r i b u t e d d e p t h b e t w e e n t h e s u r f a c e a n d 3 0 k m g i v e s v e r y s i m i l a r r e s u lt s .

T h e s o u r c e t im e f u n c t i o n f o r t h e E c u a d o r e v e n t h a s o n e s im p l e p u l s e o f m o m e n t

r e le a s e w i t h a d u r a t i o n o f 24 s e co n d s , s u g g e s t in g t h a t m o s t o f t h e m o m e n t r e l e as e

o c c u r r e d n e a r t h e e p i c e n te r a n d o n a s m a l l p a r t o f th e t o t a l f a u l t a r e a a s d ef in e d b y

t h e a f t e r s h o c k s F i g . 6 ) . T h i s s o u r c e t i m e f u n c t i o n i s c h a r a c t e r i s t i c o f a s in g le

a s p e r i t y r u p t u r e , a l t h o u g h w e a r e n o t a b l e t o s p a t i al l y l o c a t e t h e m o m e n t r e l e as e o n

t h e f a u l t p l a n e w i t h d i re c t iv i ty . A s s u m i n g a n a v e r a g e r u p t u r e v e l o c i ty o f 2. 0 k m / s ,

a n d f r o m t he 2 4 s e co n d d u r a t i o n o f th e m o m e n t p u ls e w e e s ti m a t e th a t m o s t o f t he

s e is m i c m o m e n t w a s r e l e as e d w i t h i n a p p r o x i m a t e l y 5 0 k m o f th e e p i c e n te r .

W e f u r t h e r i n v e s t ig a t e d c h a r a c t e ri s t ic s o f t h e s o u r c e t i m e f u n c t i o n , d e t e r m i n e d

s ei sm i c m o m e n t r el ea se a n d a s si g ne d a m o m e n t m a g n i t u d e f o r t h e E c u a d o r e v e n t b y

u s i n g a s i n g l e s t a t i o n i n v e r s i o n . T h i s p r o c e d u r e i n v e r t e d t h e i n i t i a l 1 2 0 s e c o n d s o f

e a c h s e i s m o g r a m f o r a so u r c e t im e f u n c t io n . W e u s e d s ta t io n s O T T a n d U C C t o

d e t e r m i n e t h e s ei s m i c m o m e n t b e c a u s e t h e P w a v e s f r o m t h e se s t a t i o n s a r e n o t

n o d a l a n d h e n c e , a r e n o t s e n s i t i v e t o s m a l l v a r i a t i o n s i n t h e f o c a l m e c h a n i s m . I n

a d d i t i o n , w e h a v e r e l i ab l e i n s t r u m e n t r e s p o n s e i n f o r m a t i o n f o r th e s e s t a ti o n s .

S t a t i o n s P A S , T U C a n d B K S a r e n e a r n o d a l f o r t h is e v en t , a n d w e r e t h u s t o o

s e n si ti v e t o s m a l l c h a n g e s i n th e f o c a l m e c h a n i s m . S t a t i o n s O T T a n d U C C i n d i c a te

a se is m i c m o m e n t o f 6 - 8 x 1 2 N - m , c o r r e s p o n d i n g t o a m o m e n t m a g n i t u d e o f

7 . 8 - 7 . 9 .

24 August 942 Peru Earthquake

T h e 2 4 A u g u s t 1942 P e r u e a r t h q u a k e h a s l on g a n d c o m p l e x P w a v e f o r m s a s

s h o w n i n F i g u r e 5 . T h e s t a t i o n s a n d i n s t r u m e n t i n f o r m a t i o n u s e d i n t h i s s t u d y a r e

l is t ed i n T a b l e 2 . A l l P w a v e s h a v e c o m p r e s s i o n a l f i r s t - m o t i o n a r r i v a l s a n d p l o t n e a r

t h e c e n t e r o f t h e f o c a l s p h e r e ; h e n c e , i t is d i ff i cu l t t o c o n s t r a i n t h e o r i e n t a t i o n o f t h e

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7 8 J e n n i fe r L . S w e n s o n a n d S u s a n L . B e c k P A G E O P H ,

T a b l e 4

Fault plane solutions or the 942 Peru earthquake

N u m b e r S t r i k e q ~o ) D i p 6 ~ R a k e f l ~

1 340 20 90

2 335 20 9O

3 345 20 90

4 340 15 90

5 340 25 90

6 340 20 85

7 340 20 95

8 335 15 85

9 345 25 95

10 335 20 85

11 315 20 90

F a u l t p l a n e s o l u t i o n s t e s t e d f o r t h e 1 9 4 2 P e r u e v e n t . T h e s t r i k e o f

S o l u t i o n 1 1 i s t h e s t r i k e o f t h e P e r u c o a s t l i n e n e a r t h e e p i c e n t e r .

fault and auxiliary planes We start with an underthrusting mechanism with the

strike parallel to the trench consistent with the subduction of the Nazca Plate

beneath South America

D e p t h 3 0 3 5 k m

9 KEW.E

~ AZ=35.6

D1ST= 90.1o

~ KEW.Z

~ DBN.E

AZ=37,4

DIST= 92.7o

'~ '~ D B N .Z

UCC,E

AZ= 8 0

DIST= 5 3 ~

@ ~ PA S + ~

AZ= 2 1 4

DIST=63.9o

TUC.N

AZ= 2 5 0

" A1ST=58.2~

DIST= 60,3o

WES.N

AZ=4A

DIST=

4 2 ~

I I

0 100 s

F i g u r e 9

O b s e r v e d s o l i d l in e ) a n d s y n t h e t i c d a s h e d l in e ) l o n g - p e r i o d P w a v e s , s o u r c e m e c h a n i s m , a n d s o u r c e

t i m e f u n c t i o n f r o m t h e t 9 4 2 P e r u m u l t i s t a t i o n i n v e r s i o n w i t h o u r p r e f e r r e d f a u l t p l a n e s o l u t i o n o f s t r ik e ,

d i p a n d s l ip q~ = 3 4 5 ~ 3 = 2 5 ~ a n d ~. = 9 5 ~ r e s p e c t i v e ly . S t a t i o n s c o d e s a r e i n c a p i t a l l e t te r s ; i n s t r u m e n t

r e s p o n s e s a r e l i s t e d i n T a b l e 2 .

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Vol. 146, 1996 Historical 1942 Ecuador and Peru Earthquakes 79

0.54

~ 0.52

.-_.

Results of Peru, 1942 Inversion: Depthvs NormalizedError

0.50 , ~ _

20 40 60

Depth in Kilometers

Figure l0

Example of the normalized error versus depth for the 1942 Peru multistation inversion using the

European stations (see Figure 9). Error minima exist at depths of 30 km and below 55 kin.

W e a p p l i e d t h e m u l t i s t a t i o n i n v e r s i o n t e c h n i q u e t o 9 l o n g - p e r i o d P w a v e f o r m s

f r o m t h e 1 94 2 P e r u e v e n t t o c o n s t r a i n t h e fo c a l m e c h a n i s m , d e p t h , a n d t o

i n v e s ti g a t e s o u r c e t im e f u n c t i o n c h a r a c t e r is t ic s . D u e t o t h e c o m p l e x i t y a n d l o n g

d u r a t i o n o f t he P w a v e , 1 00 s e c o n ds o f e a c h r e c o r d w e r e u se d . T h e s e i sm i c re c o r d s

h a v e b e e n d i v id e d i n t o th r e e a z i m u t h a l g r o u p s t h r e e E u r o p e a n s t a ti o n s , t w o f r o m

s o u t h w e s t e rn N o r t h A m e r i c a, a n d t w o f r o m n o r t h e a s t e r n N o r t h A m e r ic a ) . E a c h

g r o u p w a s i n v e r t e d s e p a r a t e l y t o i n v e s t i g a t e t h e p o s s i b i l it y o f d i r e c t i v i t y r e s u l t i n g

f r o m a f in i te f a u l t ru p t u r e . E a c h g r o u p o f s e is m o g r a m s w a s s i m u l t a n e o u s l y in v e r t e d

a t 31 d e p t h s b e t w e e n 1 a n d 7 0 k m f o r e a c h o f 11 d i f fe r e n t f o c a l m e c h a n i s m s T a b l e

4 ). T h e f o c a l m e c h a n i s m w h i c h p r o v i d e d t h e b e s t fi t b e t w e e n t h e d a t a a n d s y n t h e ti c s

f o r a l l t h r e e g r o u p s w a s f a u l t p l a n e s o l u t i o n n u m b e r 9 q5 = 3 4 5 ~ 6 = 2 5 ~ 2 = 9 5 ~

T a b l e 4 a n d F i g . 9) . A l t h o u g h t h i s f a u l t p l a n e s o l u t i o n i s n o t w e l l c o n s t r a i n e d , t h e

d a t a a r e c o n s i s t e n t w i t h a n u n d e r t h r u s t i n g m e c h a n i s m r e p r e s e n t i n g t h e s u b d u c t i o n

o f th e N a z c a p l a te b e n e a t h t h e S o u t h A m e r i c a n p l a te .

W e d e t e r m i n e d a h y p o c e n t r a l d e p t h o f a p p r o x i m a t e l y 3 0 - 3 5 k m t h r o u g h a

c o m b i n e d a n a l y s i s o f n o r m a l i z e d e r r o r v e r s u s d e p t h c u r v e s a n d e x a m i n a t i o n o f t h e

b e h a v i o r o f t h e s o u r c e t i m e f u n c t i o n a t i n c r e a s i n g d e p t h s F i g s . 1 0 a n d 1 1). T h e

d e p t h - e r r o r c u r v e f o r o u r p r e f e r r e d f a u l t p l a n e s o l u t i o n r = 3 4 5 ~ ~ = 2 5 ~ 2 = 9 5 ~

d o e s n o t s h o w a p r o n o u n c e d m i n i m u m F i g . 1 0). W e a r e s e ei n g t h e e f fe c ts o f t h e

t r a d e - o f f b e t w e e n d e p t h a n d s o u r c e t im e f u n c t i o n . W e c o n s t r a i n t h e d e p t h a s w e d i d

i n o u r a n a l ys i s o f t h e E c u a d o r e v e n t , us i n g t h e m e t h o d o f C H RIS TE NS EN a n d R U F F

1 9 8 5) , w h e r e t h e b e s t d e p t h c o n c e n t r a t e s t h e s e i sm i c m o m e n t r e le a s e t o w a r d t h e

8/17/2019 1400443160_690__swenson_1996.pdf


8 0 J e n n i f e r L . S w e n s o n a n d S u s a n L . B e c k P A G E O P H ,

O T T , W E S . N D B N . E , D B N . Z P A S , T U C . N epth

K E W . E , K E W . Z

U C C . Z

+ + + 10kin

+ . 30,.

9 I

0 100 s

F i g u r e 1 1

R e p r e s e n t a t i v e s a m p l i n g o f t h e s o u r c e t i m e f u n c t i o n s c a l c u l a t e d a t 3 1 d i f f e re n t d e p t h s b e t w e e n 1 a n d

7 0 k m f o r t h e t h r e e a z i m u t h a l g r o u p s o f 1 9 42 P e r u r e c o rd s . N o t e t h e t r a d e - o f f b e t w e e n s o u r c e t i m e

f u n c t i o n a n d d e p t h .

b e g i n n in g o f t h e d e c o n v o l v e d s o u r c e t im e f u n c t i o n . A t d e p t h s l e ss t h a n 3 0 - 3 5 k m ,

t h e f it b e t w e e n d a t a a n d s y n t h e t i c s i s s a t i s f a c t o r y a n d t h e s e i s m i c m o m e n t r e l e a s e is

c o n c e n t r a t e d t o w a r d t h e b e g i n n i n g . A t d e p t h s g r e a t e r t h a n 3 0 - 3 5 k i n , w e b e g i n t o

s ee t h e e f fe c ts o f t h e t r a d e - o f f b e t w e e n s o u r c e t i m e f u n c t i o n a n d d e p t h : t h e s o u r c e

t i m e f u n c t i o n d e m o n s t r a t e s t h e p e r i o d i c r in g i n g c h a r a c t e r is t i c o f a d e p t h o v e r e s ti -

m a t i o n , a n d m o m e n t b e c o m e s d i s tr i b u t e d e v e n ly t h r o u g h o u t t h e 10 0 s e c o n d s o f

i n v e r t e d r e c o r d F i g . 1 1) .

W e d e t e r m i n e d t h e s ei sm i c m o m e n t a n d a s s ig n e d a m o m e n t m a g n i t u d e t o th e

1 9 42 P e r u e v e n t b y u s i n g a s i ng l e s t a t i o n i n v e r s i o n o f 1 20 s e c o n d s o f t h e P w a v e s

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Vol. 146, 1996 Historical 1942 Ecuador and Pe ru Earthquakes 81

f r o m s e v e ra l a z i m u t h a l y d i s t r ib u t e d s t a ti o n s . T h i s p r o c e d u r e f it s e a c h s e i s m o g r a m

u s i n g a d is t r i b u t e d d e p t h b e t w e e n t h e s u r f a c e a n d 4 0 k m . U s i n g o u r p r e f e r r e d f o c a l

m e c h a n i s m o f ~b = 3 4 5 ~ 6 = 2 5 ~ 2 = 9 5 ~ o u r b e s t r e s u l ts c a m e f r o m s t a t i o n s W E S ,

K E W , P A S a n d T U C , t h e s t a t i o n s w i t h t h e m o s t r e l i a b l e i n s t r u m e n t r e s p o n s e s . T h e

d e c o n v o l v e d l o n g - p e r i o d n o n d i f f r a c t e d P w a v e s g a v e u s a s e is m i c m o m e n t o f

1 0 - 2 5 x 1 0 2 ~ c o r re s p o n d in g t o a m o m e n t m a g n i t u d e

Mw

= 8 . 0 - 8 . 2 . F o r

c o m p a r i s o n , B R U N E a n d E N G E N ( 1 9 69 ) u s e d L o v e w a v e s a n d R a y l e i g h w a v e s to

d e t e r m i n e a m a n t l e w a v e m a g n i t u d e M M o f 7.9 f o r th e 1 94 2 P e r u e v e n t. K A N A M O R I

( 19 7 7 ) d e t e r m i n e d a m o m e n t m a g n i t u d e o f 8 .2 u s i n g a se is m i c m o m e n t o f

2 7 x 1 02 0 N - m o b t a i n e d f r o m t h e a f t e r s h o c k a r e a . O I CA L ( 1 99 2 ) d e t e r m i n e s m a n t l e

w a v e m a g n i t u d e s o f 8.1 5 ( L o v e w a v e s) a n d 8 .3 8 ( R a y l e i g h w a v e s ) f r o m U p p s a l a

r e c o r d s a n d a n M M o f 7 . 5 5 ( R a y l e i g h w a v e s ) a t P a s a d e n a .

T h e c o m p l e x n a t u r e o f t h e 1 942 P e r u s o u r c e ti m e f u n c t i o n s u g g e s t s a m o r e

h e t e r o g e n e o u s r u p t u r e t h a n s e en f o r th e s o u r c e t i m e f u n c t i o n o f t h e r e la t iv e l y

s i m p l e 1 9 42 E c u a d o r e v e n t. T h e t h r e e d i ff e re n t a z i m u t h s o f d a t a s h o w v a r i a t i o n s i n

t h e s o u r c e t im e f u n c t i o n s . P w a v e s f r o m s t a t io n s t o t h e n o r t h e a s t i n d i ca t e a s o u r c e

t i m e f u n c t i o n w i t h t w o m a i n p u l s e s o f m o m e n t r e l ea s e , a s m a l l i ni ti al p u l se f o l l o w e d

b y a m u c h l a r g e r p u l se . P w a v e s f r o m s t a t io n s t o t h e n o r t h s h o w a s o u r c e t i m e

f u n c t i o n w i t h 3 p u l se s o f m o m e n t r e le a s e. D a t a f r o m t h e n o r t h w e s t a z i m u t h a l s o

s h o w t h r e e d i s t i n c t p u l s e s o f m o m e n t r e l e as e . A l t h o u g h t h e r e a r e d if f e re n c e s , w e

h e s i t a t e t o i n t e r p r e t t h e s e d i f f e r e n c e s a s s o u r c e e f f e c t s . O u r s t a t i o n d i s t r i b u t i o n a n d

q u a l i t y o f d a t a d o n o t a l l o w u s t o re s o l v e d ir e c t i v i t y a n d h e n c e s p a t i a l l y l o c a t e t h e

m o m e n t r e l ea s e. T h e t h r e e d i st in c t p u l se s o f m o m e n t r e le a s e w i t h a t o t a l d u r a t i o n

o f a p p r o x i m a t e l y 7 4 se c o n d s c a n b e i n t e r p r e t e d a s c o r r e s p o n d i n g t o t h r e e a s p e ri ti e s

o n t h e u n d e r t h r u s t i n g f a u l t p l a n e . T h e l a r g e s t o f t h e s e t h r e e p u l s e s o c c u r s 3 2

s e c o n d s a f te r r u p t u r e i n it i at io n . G i v e n t h e d u r a t i o n o f t h e m o m e n t p u l se a n d

a s s u m i n g a n a v e r a g e r u p t u r e v e l o c i ty o f 2 .0 k i n/ s , w e a s s e rt t h a t m o s t o f th e s e i sm i c

m o m e n t w a s r e le a s e d w i th i n a p p r o x i m a t e l y 15 0 k m o f t h e e p ic e n t e r.

Comparison of the 1942 Ecuador, 1942 Peru and 1940 Peru Earthquakes

T h e q u a l i ta t i v e c o m p a r i s o n o f e v e n t s r e c o r d e d o n s i m i l a r i n s t r u m e n t s d u r i n g t h e

s a m e t i m e p e r i o d i s o f t e n a u s e f u l t o o l w h e n a n a l y z i n g h i s t o r i c e a r t h q u a k e d a t a .

F i g u r e 1 2 s h o w s a c o m p a r i s o n o f t h e P w a v e s f r o m s e v e ra l s t a ti o n s , e a c h h a v i n g

r e c o r d e d t h e 1 9 42 E c u a d o r e v e n t , t h e 1 9 42 P e r u e v e n t a s w e l l a s t h e n e a r b y 1 9 40 P e r u

e v e n t . P w a v e s f r o m t h e 1 94 2 E c u a d o r a n d 1 94 0 P e r u e v e n t s a r e f o u n d t o b e

r e l at i v e ly si m p l e a n d o f s h o r t e r d u r a t i o n t h a n t h e c o m p l e x P w a v e s f r o m t h e 1 9 42

P e r u e v e n t . W e a t t r i b u t e t h e c o m p l e x i t y o f 19 4 2 P e r u P w a v e s t o s o u r c e e ff e ct s. B E C K

a n d

Ruvv 1989)

f o u n d t h a t t he 1 9 40 P e r u e a r t h q u a k e w a s a n u n d e r t h r u s t i n g e v e n t

w i t h a s i m p l e s o u r c e t i m e f u n c t i o n w i t h a d u r a t i o n o f 2 5 se c . E s t i m a t e s o f t h e s e i s m i c

m o m e n t f o r th e 1 94 0 e v e n t r a n g e f r o m 2 - 8 x 102o N m , o v e r l a p p i n g w i t h t h e s e is m i c

m o m e n t d e t e r m i n e d f o r t h e 1 94 2 E c u a d o r e a r t h q u a k e ( B EC K a n d R U F F , 1 98 9).

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82 Jennifer L. Swenson and Sus an L. Beck PAGE OPH,

S t a t io n O T T

S t a t io n P A S

S t a t io n T U C N

1 4 M a y 1 9 42 E c u a d o r 2 4 A u g u s t 1 9 4 2 P e r u

Az 5.60

Dis t 46 .2 ~ Dis t 60 .3 ~

A z 3 1 8 . 9 ~ A z 3 2 1 . 4 ~

Dis t 49 .0 ~ Dis t 63 .9 ~

A z 3 2 2 .5 ~ A z 3 2 5 . 0 ~

I D is t 43 .0 Dis t 58 .2 ~

12 s

1 4 M a y 1 9 4 0 P e r u

Az 1.8 ~

Dis t 56 ,40

A z 3 2 1 . 4 ~

l Dis t 58.90

Az 324.90

Dis t 55 .3 ~

Figure 12

Comparison o f P waves recorded at stations O TT, P AS , and T UC for the 1942 Ecuador, 1942 Peru and

1940 Peru events. Seismograms are plotted at their relative amplitudes for comparison.

T h e 1 9 42 P e r u e v e n t h a s a c o m p l i c a t e d s o u r c e ti m e f u n c t i o n w i t h a to t a l

d u r a t i o n o f 74 s e c o n d s - - o v e r t h r e e t im e s th e d u r a t i o n o f t h e s o ur c e t im e f u n c t i o n s

f o r b o t h t h e 1 9 42 E c u a d o r a n d 1 9 40 P e r u e v e n ts . T h e s o u r c e t i m e f u n c t i o n s o f t h e

1 9 42 E c u a d o r a n d 1 94 0 P e r u e v e n t s a r e v e r y s im i la r, b o t h i n t e r m s o f m a i n p u l s e

d u r a t i o n ~ 2 5 s ec .) a n d s h a p e , s u g g e s t i n g s i m i la r r u p t u r e p r o c e s s e s . T h e 1 9 4 2 P e r u

e a r t h q u a k e i s t h e l a r g e s t o f t h e t h r e e e v e n t s.

iscussion of Historic Earthquakes and Subduction Zone Segmentation

T h e C o l o m b i a - E c u a d o r a n d P e r u s u b d u c t i o n z o n e s h a v e r u p t u r e d in a n u m b e r

o f l a rg e t o g r e a t e a r t h q u a k e s d u r i n g t h i s c e n tu r y . I n a d d i t i o n , a 4 0 0 - y e a r l o n g

w r i t te n r e c o r d e x is ts t h a t d e sc r ib e s e a r t h q u a k e s t h a t h a v e o c c u r r e d a l o n g b o t h o f

t h e s e S o u t h A m e r i c a n z o n e s . T h e r e c o r d f o r t h e p r e - 2 0 t h c e n t u r y e v e n t s i n c l u d e s

e y e w i tn e s s r e p o r ts , t s u n a m i r u n u p h e i g h t s a n d i n t en s i ty m a p s . A l t h o u g h t h e s e d a t a

a r e d if f ic u lt t o i n t e r p r e t, w e c a n g a i n s o m e i n s i g h t t o p r e v i o u s e a r t h q u a k e s . W e w i ll

b r ie f ly d is c u s s t h e h i s t o r i c e a r t h q u a k e s f o r e a c h z o n e .

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Vol. 146, 1996 83

istorical 1 942 Ecuador and Peru E arthquakes

Table 5

Colombia-Ecuador earthquakes

Intensity Tsum ani Tsunam i

Date Type Lat itude Longitude maximum) S o u rc e loc a l ;m) Japan; cm)

15/05/1628 SA 0 . 2 2 ~ 78 .5~ VI I I A none

06/20/1968 SA 1.80 ~ 78 .8 ~ X AA none

22/01/1766 SA 0 . 4 4 ~ 77 .9 7~ tX AA none

02/04/1797 SA 1.67 ~ 78 .64 ~ XI AA none

31/12/1827 SA 2.5~ 76 .5 ~ IX AA none

20/01/1834 S A 1 . 3 ~ 7 6 . 9 ~ V I I I A A n o ne

16/08/1868 SA 0 . 3 1 ~ 78 .1 8~ X AA none

31/01/1906

I 1 .0 ~ 80 .0~ IX A la rge

14/05/1942 I 0.7 5~ 81 .5~ IX A none

01/04/1953 SA 0 . 1 4 ~ 80 .6 6~ ? none

19/01/1958 I 1.1 4~ 79 .59 ~ IX A none

12/12/t979 I 1.6 2~ 79 .42~ IX PDE tsunami

36

none

none

10 25

Interplate I) and intraplate events within the South American plate SA) along the Colomb ia-Ecuador

subduction zo ne . Sources for maximum intensities: AA ASKEW and ALGERMJSSEN, 1985); A ABE,

1981); PD E Preliminary Determ ination o f Epicenters). Tsunam i heights from HA TORI 1968). Loca-

tions of 2 0th century events from MENDOZAand DEWEY 1984).

Ear thquakes A long the Colombia-Ecuador Subduc t ion Zone

T h e b o u n d a r y b e tw e e n t h e N a c z a a n d S o u t h A m e r i c a n p l a te s a l o n g t h e c o a s t o f

C o l o m b i a - E c u a d o r h a s p r o d u c e d s ev e ra l m a j o r e a r th q u a k e s d u r i n g th is c e n t u r y

T a b l e 5 , F i g . 1) M E N D O Z A a n d D E W E Y , 1 98 4) . A n ~ 5 0 0 - k m l o n g s e g m e n t o f t h e

e n t i r e p l a t e b o u n d a r y b r o k e d u r i n g t h e

M w

= 8 .8 1 9 0 6 e v e n t K A N A M O R I a n d

M C N A L L Y , 1 9 8 2 ) . S u b s e q u e n t e v e n t s i n 1 9 4 2 M w = 7 . 9) , 1 9 5 8 M w = 7 . 7 ) a n d

1 9 79 M w = 8 .2 ) a p p e a r t o h a v e r e r u p t u r e d t h e sa m e p o r t i o n o f t h e i n te r p l at e

b o u n d a r y f r o m s o u t h t o n o r t h . M E N D O Z A a n d D E W E Y 1 9 8 4 ) r e lo c a te d a n d

i n v e s ti g a t ed t h e d i s t r ib u t i o n o f a f t e r s h o c k s o f t h e 1 9 4 2, 1 9 5 8 a n d t 9 7 9 E c u a d o r

e v e n ts . T h e y n o t e d t h a t t h e a f t e r s h o c k z o n e s a b u t w i t h o u t o v e r l a p p i n g , a n d

i n t e r p r e t e d t h i s a s a n i n d i c a t i o n t h a t t h e t h r e e e v e n t s i n v o l v e d p r o g r e s s i v e s t r a i n

r e le a s e a l o n g a s in g l e f a u l t s y s te m . I n d e e d , t h e d i s t r i b u t i o n o f h y p o c e n t e r s o f t h e

1 9 4 2, 1 9 5 8 a n d 1 9 7 9 s e q u e n c e s a r e c o n s i s t e n t w i t h t h e s e s h o c k s o c c u r r i n g a l o n g t h e

s a m e i n t e r p l a t e b o u n d a r y . T h e e v e n t s i n 1 9 0 6 , 1 9 58 a n d 1 9 7 9 w i ll b e b ri e fl y

d e s c r i b e d b e l o w , f o l l o w e d b y a d i s c u s s i o n o f t h e 1 9 4 2 e v e n t a n d a r e g i o n a l

d i sc u s si o n o f t h e C o l o m b i a - E c u a d o r s u b d u c t i o n z o n e.

31 January 1906

T h e 1 9 06 l a rg e u n d e r t h r u s t i n g e a r t h q u a k e

M w

= 8 .8 ) K A N A M O R I a n d M C -

N A L L Y , 1 9 82 ) g e n e r a t e d a fa r -f ie ld t s u n a m i r e c o r d e d i n A y u k a w a , J a p a n o f 36 c m

H A T O R I , 1 9 6 8 ) , a s w e l l a s a l a r g e l o c a l t s u n a m i . T o d i s c r e d i t t h e p o s s i b i l i t y t h a t

8/17/2019 1400443160_690__swenson_1996.pdf


a)

6o

82~ 80 ~ 78 ~ 76 ~

84 Jennifer L. Swenson and Susan L. Beck PAG EOP H,

b) I I 1 ,3 t

6~

] NAZCA ] [ ~ [ ]

I - - - t ~

I I I

82~ 80 ~ 78 ~ 76 ~

c) I I ,3 I 6~ d) [ ] e 6~

t

l .

AZCA I I | I= NAZCA I .4

PLATE l ~ I ~176 PLAT E] 1 ~ .

---t I?-6.

-- 4 . . . . + _ t - ~ - - ~ - 4 4 ~

I I , ( 6 1

_ _ _ / _ _ _ L _ 2ON _ 2 o

ly9 o . . . o~

(

~-- - -~ ~ ~ 4 -- - - --~ - - - - - - ~ - 2~ 2~

r / f AMERICAN / ~ #/ r AMERICAN

| t t / ] PLATE[ l / I / I PLATE

82 ~ 80 ~ 78 ~ 76 ~ 82 ~ 80 ~ 78 ~ 76 ~

Figure 13

Distribut ion of intensities reported during the a) 1906 Ecuad or event; b) 1942 Ecua dor event; c) 1958

Ecuado r event; and d) 1979 Ecuador event. The intensity patterns are crescent shaped. Note th at the

highest intensities for the 1979 earthquake occur north of the epicenter and correspond to the largest

moment release as determined from the body waves.

t h i s e v e n t w a s a l a r g e n o r m a l - f a u l t e v e n t n e a r t h e t r e n c h , K E L L E H E R 1 9 7 2 ) ci t es

s e v e re d e s t r u c t i o n w h i c h o c c u r r e d w e l l o v e r 1 0 0 k m i n l a n d . I n a d d i t i o n , t h e 1 9 06

e v e n t s h o w s a v e r y g r a d u a l o n s e t o f b o d y w a v e s , t y p i c a l o f s u b d u c t i o n z o n e t h r u s t

e v e n t s K E L L E H E R , 1 9 72 ). F i n a l l y , P - w a v e f i rs t m o t i o n s f r o m s e v e r a l s t a t i o n s f o r

t h e 1 9 06 e v e n t a r e c o m p r e s s i o n a l . K A N A M O R I a n d M C N A L L Y 1 9 8 2 ) a r g u e t h a t th e

8/17/2019 1400443160_690__swenson_1996.pdf



82 ~ 80 ~ 78 ~

979 958 942

0 see 80

I

Figure 14

Aftershocks white dots), rupture zones dashed lines) and maximum moment release stippled areas) of

the 1906, 1942, 1958 and 1979 Colombia-Ecuador subduction zone earthquakes. Aftershocks of the 1942

event and aftershock areas a re from MENDOZA and DEWEY 1984).

1906 ea r thquake was a th rus t - fau l t even t , r a the r than a g rea t no rma l - fau l t e a r th -

quake nea r the t r ench , on the ba s is o f the h igh in tens i ti e s in l and f rom the coas t,

wh ich have no t been obse rved wi th g rea t no rma l - fau l t e a r thquakes e l s ewhe re .

The S minus P t imes f ro m f ive s t a t ions we re used by KANAM ORI and M c -

NALLY (1982) to loca te the ep icen te r o f the even t . The loc i f rom th ree pa i r s o f

s t a t ions nea r ly in t e r s ec t a t t he p rev ious ly de te rmined Gutenbe rg -R ich te r ep icen te r

which i s nea r the sou thwes te rn end o f the rup tu re zone. KELLEHER (1972) ba sed an

es t ima te o f the rup tu re zone o f the 1906 even t (500 k in ) on mac ro se i smic da ta tha t

inc lude repor t s o f a d imin i shed wa te r l eve l in the ha rb ors o f M an ta (3~ and

B u e n a v e n t u r a ( 3 ~ a n d a b r o k e n s u b m a r i n e c a b le f o u n d n e a r B u e n a v e n t u r a .

The s e ismic mom en t i s e s t ima ted to be 2 x 1022N 9m KANAMORI, 1977) f rom

KELLEHER'S (1972) e s t ima te o f the rup tu re zone . The co r re spo nd ing m om en t

mag n i tud e i s in close ag reem en t w i th ABE'S (1979) t sunam i m agn i tu de o f M , = 8 .7 .

D is t r ibu t ion o f int ensi t ie s s t rong ly sugges ts tha t the rup tu re zone d id no t ex tend

8/17/2019 1400443160_690__swenson_1996.pdf



any appreciable distance beyond the end points specified by KELLENER (Figs. 13

and 14). The aseismic Carnegie Ridge intersects the Colombia trench at approxi-

mately 0 ~ latitude; it is unfikely that rupture propagated south of 0 ~ or north of the

sharp bend in the Columbia trench at about 4~

19 Janu ary 1958

The 1958

M w

= 7.7 earthquake was also interpreted as an underthrusting event

that ruptured a segment of the subduction zone that had previously ruptured during

the 1906 earthquake along the interface between the Nazca and South American

plates (Fig. 1) KANAMORI and MCNALLY, 1982; BECK and RUFF, 1984). The

source time function, determined from the deconvolution of long-period P-wave

records, has two pulses with a total duration of 24-26 seconds. The first pulse

contains most of the moment release and is 12-14 seconds in duration (BECK and

RUFF, 1984). BECK and RUFF (1984) determined a seismic moment of

~2.8 x 102~ 9m, corresponding to a moment magnitude of 7.6, KANAMORI and

MCNALLY (1982) estimate the seismic moment o f the 1958 event from the

amplitude ratio of long-period Rayleigh waves of the 1958 event to the 1979 event,

assuming that these two events have approximately the same mechanism. This gives

a seismic moment of 5.2 x 1020 N 9m, corresponding to a moment magnitude of

7.7. Both estimates are significantly smaller than that determined for the adjacent

1979 event. MENDOZA and DEWEY (1984) relocated the main shock and after-

shocks for the 1958 event. The 1958 event presumably ruptured unilaterally to the

northeast abutting, but not significantly overlapping, the 1979 rupture area, as the

epicenter is at the southwestern edge of the aftershock area (Fig. 14). The source

time function from teleseismic long-period P waves and aftershocks give a similar

rupture length of 50 km (BECK and RUFF, 1984), indicating that the 1958 rupture

stopped in the epicentral region of the 1979 earthquake.

12 Dece mber 1979

Several previous studies using body- and surface-wave modeling have identified

the 1979

M w

= 8.2 event as an underthrusting earthquake (HERD

et al.

1981;

KANAMORI and G~VEN, 1981) (Fig. 1). The strike of the low-angle plane of the

preferred fault plane solution is parallel to the trench axis, and the mechanism is

consistent with subduction of the Nazca plate beneath South America. Source time

functions for the 1979 event, deconvolved from long-period P waves, are approxi-

mately 60 seconds in duration and show two distinct truncations that exhibit

azimuthal directivity (BECK and

RUFF,

1984). BECK and RUFF (1984) locate the

two truncations at 56 and 116 km northeast of the epicenter, suggesting unilateral

rupture in that direction (Fig. 14). KANAMORI and MCNAELY (1982) also report

directivity and a best fit rupture direction of N40~ A P-wave analysis (BECK and

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Vol. 146, 1996 Historical 1942 Ecuado r and Peru Earth quake s 87

RUFF, 1984) yields a rupture length of approximately 120 km. However, BECK and

RUFF (1984) note that surface wave directivity requires a fault length of 180 to

240km. KANAMORI and MCNALLY (1982) determine a rupture length from

Rayleigh wave group delay times of 230 km. The 1958 event was smaller than the

1979 earthquake in terms of moment, rupture length, and dominant asperity size

(BECK and

RUFF

1984).

The absence of reports of far-field tsunamis for the 1942 and 1958 earthquakes

suggests tha t they are even smaller than the 1979 event. Tsunami data (Table 5)

indicate that the 1906 event is substantially larger than the 1979 event, although the

1979 event did generate a far-field tsunami in Japan of 10-25 cm.

4 May 942 Ecuador Earthquake

The large underthrusting earthquake on 14 May 1942 occurred off the coast of

Ecuador (0.01~ 80.39~ near the subducting Carnegie Ridge (Fig. 1). The 1942

earthquake presumably ruptured a segment of the plate boundary that previously

ruptured in the great 1906 earthquake. It has been longer than the 36-year interval

between 1906 and 1942; based on this single 36-year estimate, NISHENKO (1991)

placed the probability o f a large event occurring between 1991 and 2001 at the 66

o l o m b i a E c u d a d o r S u b d u c t i o n Z o n e

1900

1800

1700

1600

1979 8.1

IX

1766

IX

I n t r a p l a t e

5~ 4 ~

1958 7.8

w IX 1942 7.9

A

IX

1906 8.8 1901

X 9 7.8

t s u n a m i

1868 7.7

x

I n t r a p l a t e 1797 8.3

XI

I n t r a p l a t e

1 6 9 8 7.7

1587 7.7 A 1627 X

X

'~ VIII

I n t r a p l a t e

I n t r a p l a t e 9

3 ~ 2 ~ I ~ 0 o 1~ 2 ~

Figure 15

Space-time plot of the Colombia-Ecuador subduction zone. Time is shown on the y axis, distance along

the trench (in degrees) on the x axis. Dots represent the location and relative size of the earthquake; lines

extending from the dots represent the estimated rupture length of underthrusting events. Sources for

max imum intensities: ASKES and ALGERMISSEN (1985), ABE (1981) and Preliminary Determi nati on of

Epicenters.

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88 Jennifer L. Swenson and Susan L. Beck PAG EOPH ,

l ev e l. T h e s e f a c t o r s i l lu s t ra t e t h e i m p o r t a n c e o f e v a l u a t i n g t h e 1 94 2 e a r t h q u a k e

s e g m e n t . W e u s e o u r d e t e r m i n e d s o u r c e c h a r a c t e r i s t i c s a s w e l l a s r e l o c a t e d a f t e r -

s h o c k s a n d i n t e n s i t y r e p o r t s t o e x a m i n e t h e 1 9 4 2 E c u a d o r e v e n t i n t h e c o n t e x t o f

r e g i o n a l h i s t o r i c s e i s m i c it y a n d t h e e a r t h q u a k e c y c le .

T h e r e p o r t e d l o c a t i o n o f th e m a x i m u m i n te n s it ie s ( I X ) f o r t h e 1 9 42 E c u a d o r

e v e n t a r e south o f th e m a in sh o ck ep icen te r (A SK E W an d A L G E R MIS SE N , 19 85 ; A B E,

1 98 1; P r e l i m i n a r y D e t e r m i n a t i o n o f E p i c e n te r s ) , s u g g e st in g t h a t t h e m a j o r i t y o f

s e i s m i c m o m e n t r e l e a s e o c c u r r e d t o t h e s o u t h ( A S K E W a n d ALGERMISSEN, 1985)

( F i g . 1 3). M a n y o f t h e r e l o c a t e d a f t e r s h o c k s o c c u r r i n g d u r i n g a t h r e e - m o n t h p e r i o d

f o l l o w i n g t h e e v e n t a r e l o c a t e d i n a n a r e a

north

o f t h e e p i c e n t e r

MENDOZA

a n d

D E W E Y , 1 9 8 4 ) ( F i g . 1 4 ) . T h e r e l a t i o n s h i p b e t w e e n a f t e r s h o c k s t o t h e n o r t h a n d

h i g h e s t i n t e n s i t i e s t o t h e s o u t h c a n b e e x p l a i n e d a s f o l l o w s : a f t e r s h o c k s c o m m o n l y

o c c u r i n a r e a s o f

low

s e i s m i c m o m e n t r e l e a s e r a t h e r t h a n

high

SCHWARTZ

t

al.

1 98 9) . I f w e a s s u m e t h a t t h e s i n g le a s p e r i t y w h i c h r u p t u r e d d u r i n g t h i s e v e n t li es to

t h e s o u t h o f t h e m a i n s h o c k e p i c e n t e r i n t h e a r e a m a r k e d b y h i g h m o m e n t r e le a s e

a n d i n t e n s i t i e s , t h e n w e w o u l d expect a f t e r s h o c k s t o o c c u r t o t h e n o r t h .

T h e a f t e r s h o c k z o n e o f t h e 1 94 2 E c u a d o r e v e n t ( F ig . 14 ) is a p p r o x i m a t e l y

2 0 0 k m b y 9 0 k m ( e l o n g a t e d p a r a ll e l t o t h e t r e n c h ) a n d e x t e n d s t o a d e p t h o f

< 3 0 k i n ( M EN D O ZA a n d D E W E Y , 1 98 4). T h e a f t e r s h o c k s li e o n a s o u t h e a s t e r l y

i i

M a x i m u m I

I n t e n s i t y I

I

4 -

1 9 5 8 ( I

1 9 0 6 (9 )

1 9 4 2 (9 ) I

8 2 ~

. ~ _ . 1 7 6 6 ( 9 )

7 Oi

j Colombia

, ~ 1 182,7(9)

zg A 4

9 i

o

q J . , ~ - ~ 1 8 3 4 8 )

_ . i 8 8 1 0 ) 1 .

y ~ - ~ g 8 7 - - / 1 - 6 2 7 ( 8 - 9 -' ' ~

E c ua d or I 1 1

1 1 6 9 8 1 0 ) f

i 1 7 9 7 ( 1 0 - 1 1 ) i J

- r - O - - ~ . . . . ~ - -

r

8 0 ~ 78 ~ 76 ~

Figure 16

o

2 ~

o

2~

Distribution of large Colombia-Ecuadorevents; maximum reported intensities are shown in parentheses.

Source s fo r m axim um intensities: ASKEW and ALGERM1SSEN(1985), ABE (1981) and Preliminary

Determination of Epicenters. Locations of 20th ce ntury even ts from MENDOZA and DEWEY 1984).

8/17/2019 1400443160_690__swenson_1996.pdf



dipping plane, consistent with our preferred mechanism. The length and width of

the aftershock zone approximates the fault plane, which permits us to relate the

seismic mome nt to average displacement along the fault by the fo rmula M 0 =

I~AD

where A is the fault area, and D is the displacement along the fault and we take the

shear modulus # as 3.3 x 10 l~ N/ m 2. Using an average value for the seismic

moment, we calculated the average displacement along the thrust fault to be 1.2

meters. However, if we assume most of the moment release as defined by the P

waves occurred over a small portion of the fault, then locally the displacement

would be much larger. The source time function would indicate that most of the

ruptur e occurre d within a small area o f the fault plane. If we assume most o f the

mom en t release occur red in one domin ant asperity with dimensions of 50 km by

50 km (an estimated rup ture area based on the du rati on of the main seismic

moment pulse and an average rupture velocity of 2.0 km/s), then the displacement

would be 8.5 meters. The accumulated tectonic displacement based on the 36 years

between 1906 and 1942 and the conve rgence rate of 8.37 cm/year gives approxi -

mately 3 meters of displacement. Al tho ugh there are large uncertainties with the

seismic moment and area for the dominant asperity, this suggests that all of the

accumulated tectonic displacement was released in 1942.

NAZCA

PLATE

I I 4

I

/

7 9 7

- - [ - -

o

? 1 9

6 ~

I

I I _=

AMERICAN

I PLATE I

o

~

2 ~

8 2 ~ 8 ~ 7 8 ~ 7 6 ~

Figure 17

Map showing the distribution of maximum intensities VIII and IX reported during the 1797 Ecuador

event. The bulls-eye pattern is indicative of an event occurring within the South American plate. This

intensity pattern is very different from the intensity pattern for the underthrusting events as shown in

Figure i3. Source for maximum intensities is ASKEW and ALGERMISSEN 1985).

8/17/2019 1400443160_690__swenson_1996.pdf



T h e C o l o m b i a - E c u a d o r s e g m e n t o f t h e N a z c a - S o u t h A m e r ic a n p la te b o u n d a r y

s h o w s t w o d i f f e r e n t r u p t u r e m o d e s b e t w e e n s u c c e s s i v e e a r t h q u a k e c y c l e s . W e n o w

a d d r e s s t h e q u e s t i o n o f w h e t h e r t h e 1 9 0 6 -t yp e r u p t u r e o f th e t h r e e a d j a c e n t b u t

s e p a r a te e a r t h q u a k e s i s a m o r e c o m m o n o c c u r re n c e a l o n g t h is p o r t i o n o f t h e

s u b d u c t i o n z o n e . F i g u r e 15 sh o w s a s p a c e ti m e p l o t o f t h e C o l o m b i a - E c u a d o r

s u b d u c t i o n z o n e s o u r c e s f o r m a x i m u m i n te n s it ie s : A SK E W a n d ALGERMISSEN, 1985;

A B E , 1 9 8 1 ; P r e l i m i n a r y D e t e r m i n a t i o n o f E p i c e n t e rs ) . P r i o r t o 1 90 6 t h e r e w e r e m a n y

l a rg e e v e n t s. T h o u g h s o m e i n t e r p l a t e e v e n t s t h a t b r e a k t h e l o w e r i n t e r p l a t e in t e r f a c e

a l s o m i g h t n o t p r o d u c e t s u n a m i s , w e i n t e r p r e t th e s e e v e n t s a s i n t r a p l a t e w i t h i n t h e

S o u t h A m e r i c a n p l a t e ) , b a s e d o n i n t e n s i t y r e p o r t s a n d t h e l a c k o f e i t h e r l o c a l o r

f a r- f ie l d t s u n a m i s r e p o r t s F i g . 1 6) s o u r c e s f o r m a x i m u m i n t e n s it ie s : A S K E W a n d

ALGERMISSEN,1 9 8 5 ; A B E , 1 9 81 ; P r e l i m i n a r y D e t e r m i n a t i o n o f E p i c e n t e rs . L o c a t i o n s

o f 2 0 t h c e n t u r y e v e n t s f r o m M E N D O Z A a n d D E W E Y , 1 98 4). I n t e n s i t y p a t t e r n s f o r

e a r t h q u a k e s w h i c h h a v e o c c u r r e d within t h e S o u t h A m e r i c a n p l a t e t y p i c al ly f o r m a

b u l ls - e y e a r o u n d t h e e p i c e n t e r a s s h o w n f o r th e 1 79 7 e v e n t in F i g u r e 17 s o u r c e s f o r

m a x i m u m i n te n s it ie s f r o m A S K E W a n d

ALGERMISSEN,

1985) . In con t r as t , i n t ens i t y

p a t t e r n s f o r e a r t h q u a k e s w h i c h o c c u r a t t h e i n t e r f a c e b e t w e e n t h e s u b d u c t i n g a n d

o v e r r i d i n g p l a te s f o r m a c r e sc e n t s h a p e a l o n g t h e c o a s t a s s h o w n f o r t h e 1 94 2 e v e n t

F i g . 1 3 ) . W e d o n o t f i n d a n y c a n d i d a t e s f o r a 1 94 2 o r 1 9 0 6 - ty p e r u p t u r e a l o n g t h i s

s e g m e n t . T h i s s u g g e s t s t h a t t h e p r e v i o u s s u b d u c t i o n z o n e u n d e r t h r u s t i n g e v e n t

o c c u r r e d a t l e a s t 2 0 0 y e a r s p r i o r t o 1 9 0 6 . H e n c e , w e c a n n o t d e t e r m i n e w h a t t y p e o f

e a r t h q u a k e s e q u e n ce is m o r e t y p i c a l o f th e C o l o m b i a - E c u a d o r s e g m e n t . T h e r e d o e s

n o t a p p e a r t o b e c h a r a c t e r is t i c e a r t h q u a k e s f o r t h i s s e g m e n t .

T h e b o u n d a r y b e t w e e n th e 1 9 5 8 a n d 1 97 9 a f t e r s h o c k z o n e s a n d t h e b o u n d a r y

b e t w e e n t h e 1 94 2 a n d 1 9 5 8 a f t e r s h o c k z o n e s l ie in l a n d o f t h e i n t e r s e c t i o n o f t h e in a c t i v e

M a l p e l o R i f t - Y a q u i n a g r a b e n s y s t e m . W i t h t h is i n m i n d , M E N D O ZA a n d D E W E Y 1 9 8 4 )

s u g g e s t t h a t t h e h e t e r o g e n e i ti e s s e p a r a t i n g t h e 1 94 2, 1 9 5 8 a n d 1 97 9 f a u l t r u p t u r e s m a y

b e t r a n s i e n t h e t e r o g e n e i t ie s i n th e s u b d u c t i n g N a z c a p l a t e r e s u l ti n g f r o m t h e p r e s e n c e

o f t h e e x t i n c t s p r e a d i n g c e n t e r - t r a n s f o r m f a u l t s y s t e m . T h e s e h e t e r o g e n e i t i e s w e r e

b y p a s s e d o r b r e a c h e d d u r i n g t h e 1 90 6 e a r t h q u a k e .

T h e p r e v i o u s l y m e n t i o n e d s e r ie s o f e v e n t s w h i c h o c c u r r e d i n t h e l a s t 1 00 y e a r s

d e m o n s t r a t e t h e t w o d i f fe r e n t m o d e s o f r u p t u r e t h a t h a v e o c c u r r e d a l o n g t h e s u b d u c t i o n

z o n e s e g m e n t o f f t h e c o a st o f C o l o m b i a - E c u a d o r . T h e 1 90 6 g r e a t e a r t h q u a k e

M w = 8 .8 ) r u p t u r e d a 5 00 k m s e g m e n t o f t h e p la t e b o u n d a r y M C N A L L Y a n d

KANAMORI 1 9 8 2 ) . T h i s s a m e t h r u s t f a u l t p l a t e b o u n d a r y s e g m e n t s u b s e q u e n t l y

r u p t u r e d i n t h r e e s m a l l e r e a r t h q u a k e s f r o m s o u t h to n o r t h i n 1 94 2 M w = 7 .8 ), 1 9 5 8

M w = 7 .7) , an d 1979 Mw = 8 .2 ) F igs . 13 an d 14) . Th e 1942 eve n t i s s imi l a r t o bu t

s l ig h t l y l a r g e r t h a n t h e a d j a c e n t 1 95 8 e v e n t , a l t h o u g h b o t h e a r t h q u a k e s i n i t i a t e d r u p t u r e

a t th e d o m i n a n t th r u s t p l a n e a s p e ri ty . I n c o n t r a st , t h e 1 9 7 9 e a r t h q u a k e h a d a m u c h

l o n g e r s o u r c e d u r a t i o n a n d i n it ia t e d r u p t u r e a p p r o x i m a t e l y 60 k m f r o m t h e d o m i n a n t

a s p e r i ty . T h e s u m m e d s e i sm i c m o m e n t f r o m t h e t h r e e p o s t - 19 06 e v e n ts is a p p r o x i m a t e l y

one - f i f t h a s la rge as t he e s t im a te fo r t he 1906 even t KANAMORIand MCNALLY, 1982) .

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H i s t o r i c E a r t h q u a k e s o f t h e P e r u a n d N o r t h e r n C h i l e S u b d u c t i o n Z o n e

The historic seismicity record along coastal Peru and northern Chile extends back

to personal accounts during the 16th century Fig. 1, Table 6; sources for maximum

intensities: ASKEW and

ALGERMISSEN, 1985; SILGADO,

1985. Local tsunami heights

are from DORBATH

et a l . ,

1990 and

LOCKRIDGE,

1985. Far-field tsunami heights are

from HATORI, 1968 and DORBATH

et al. ,

1990). The central Peru subduction zone

in particular has had a long history of destructive earthquakes DORBATH

et a l . ,

1989)

Fig. 1, Table 6). The majority of large events this century have occurred along the

plate boundary north of the Nazca Ridge-South American trench intersection. There

have not been any events this century that ruptured through the Nazca Ridge

intersection with the South American trench BECK and NISHENKO, 1990). We have

compared the large events that occurred this century with historic events in terms

of detailed descriptions often quite colorful) of damage and tsunami runup heights.

We will briefly describe the historic events that occurred along the Peru-Chile

subduction zone. This will be followed by a description of events that have occurred

during the 20th century and a regional discussion of the Peru-Chile subduction zone.

9 Ju l y 1586

This 16th century event appears larger than its 20th century reference events

those of 1966 and 1974). It caused a local tsunami runup of 24m near Callao

LOCKR~DGE, 1985), as well as considerable tsunami damage from a 2 m tsunami

runup in Japan HATORI, 1968). A maximum Modified Mercalli Intensity of XII

for the 1586 event was reported near the town of Lima, Peru.

2 4 N o v e m b e r 1 60 4

This great underthrusting tsunamigenic earthquake

M w

= 8.7-9.0) is one of

the largest to occur in Peru during the last 400 years DORBATH

et al . ,

1990). It

generated a local tsunami of 10-15 meters. Comparison with the 1868 event

discussed below) yields an approximate rupture length for this event of 400 km

NISHENKO,

1991). The exact southern boundary of the rupture extent is uncon-

strained N~SHENKO, 1991), but DORBATH

et aI.

1990) use macroseismic data to

estimate an area of extreme damage that extends from approximately 15.5~ to 18~

corresponding to a rupture zone similar in length to that cited by NISHENKO

1991), on the order o f 450 km long.

2 0 O c t o b e r 1 6 8 7

The 20 October 1687 event is the largest known earthquake to have occurred

between Lima and Pisco Fig. 18). This event caused strong shaking maximum

8/17/2019 1400443160_690__swenson_1996.pdf


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8/17/2019 1400443160_690__swenson_1996.pdf


Vol. 146, 1996 Historical 1942

Ecua do r a nd Peru Ea rthqua kes

93

intens i ty IX) and tsunami damage from Lima to Ica. Intens i ty IX reports reached

further south than the 1746 event (discussed below) , so i t has been postulated that

the 1687 event ruptured further south than tha t of 1746 (BEC K and NISHENKO,

1990) . T o as s i gn a s e i smi c moment and a moment magni tude to the 1687 event

BECK and NISHENKO (199 0) c ho os e as a reference even t the 1974 earthquake, and

compared their local and global t sunami ef fects .

From these t sunami re l a t i onshi ps (T abl e 5 ) and the known se i smi c moment o f

the 1974 re ference event , a s e ismi c m om ent o f 1 -2 x 1022 N . m wa s as si gned to the

1687 event , correspondi ng to a m om ent m agni tude o f 8 . 6 8 .7 . B E C K and

NISHENKO (1990) suggested that the rupture extent o f the 1687 event encom passed

and exceeded that of the 1974 event , perhaps extending south to Pisco or Ica (Figs

18, 19 and 20). DORBATH

e t a l . 1 9 9 0 )

est imate a rupture length for this event of

175 km . The significance o f the 1687 event l ies in the obse rva tion that the 1687

rupture zon e ma y very wel l extend throu gh the se i smic gap exis t ing along the

Nazca Ridge- trench intersect ion (Fig . 1) . This in turn would indicate that the

intersect ion of the Nazca Ridge and Peru trench i s

n o t

a permanent barrier (BEc~:

and NISHENKO, 1990).

28 October 746

Th e 28 October 174 6 underthrushing earthquake (Fig . 18) was the largest wel l

doc um ented ev ent to emerge from the wri t ten accou nts o f the preinstrument era.

T a b l e 6

C e n t r a l P e r u e a r t h q u a k e s

I n t e n s i t y T s u n a m i T s u n a m i

Date Latitude

Lo ng i tude ma x im um ) lo ca l ; m) J a pan; cm)

09/07/1586 12.2~ 77.8~ IX 24 2 m

24/11/1604 17.9~ 70.9~ IX 10-15

13/11/1655 12.3~ 77.6~ IX

n o n e

17/06/1678 11 ~ 77.6~ IX 5?

20/10/1687 12.5~ 77~ IX 5-10 1 m

28/10/1746 11.5~ 78~ X 24

30/03/1828 12~ 77~ VIII

n o n e

13/08/1868 16~ 71.5~ X 14 2 m

24/05/1940 11.2~ 77.2~ VIII 3

n o n e

24/08/1942 15.2~ 76.0~ IX 3

n o n e

17/10/1966 10.92~ 78.79~ VIII 2.6 <50

31/05/1970 9.36~ 78.87~ IX 0.5 5

03/10/1974 12 .3 9~ 77.66~ VIII 1.6 4 20

La rg e ea rthqua kes o f the Peru su bduct io n zo ne . So urce fo r ma x im um in tensi ties wa s AS KEW a nd

ALGERM1SSEN (1985) and SmGADO (1985).

Lo ca l t suna mi he ig ht s f ro m

DORBATH

et al

(1990)

a n d

LOCKRID6E (1985).

Far-f ield tsunami heights from

HAI OR~ (1968)

a nd DORBATH et al

(1990).

8/17/2019 1400443160_690__swenson_1996.pdf


94 Jennifer L. Swenson and Susan L. Beck PAGE OPH,

A n i n t e n s i t y m a g n i t u d e M 1 = 9 . 2 w a s a s s i g n e d t o t h i s e v e n t o n t h e b a s i s o f t h e a r e a

e n c o m p a s s e d b y a m a x i m u m i n te n s it y o f M M = - J F ig . 1 8) . T h e 1 7 4 6 ev e n t

g e n e r a t ed a m a x i m u m l o ca l ts u n a m i r u n u p o f 24 m n e a r C a l l a o, P e r u LOCKRIDGE,

1 98 5) . G i v e n t h e s iz e o f t h e 1 74 6 e v e n t , w e w o u l d e x p e c t a n o b s e r v a b l e f a r - f i e ld

t s u n a m i a l o n g t h e c o a s t o f J a p a n b u t t h e r e is n o m e n t i o n o f

one WATANABE,

1983).

B E C K an d N ISH E N K O 1990) c h o s e a s t h e i r r e f e r e n c e e v e n t i n t h is c a s e t h e 1 9 66 e v e n t

w h i c h c a u s e d a m a x i m u m l o c a l t s u n a m i r u n u p o f 2.1 m i n C a l l a o T a b l e 6). T a k i n g

t h e r a t i o b e t w e e n l o c a l t s u n a m i r u n u p s a n d t h e s e is m i c m o m e n t o f th e 19 66 e v e n t,

a s e is m i c m o m e n t o f 2 . 2 6 x 1 02 2 N - c m c a n b e a s s i g n e d t o t h e 1 7 46 e v e n t , c o r r e -

s p o n d i ng t o a m o m e n t m a g n i t u d e o f 8 . 8 - 9 .5 . B a s e d o n i n te n s it y a n d t s u n a m i d a t a

f r o m t h e 1 7 46 , 19 4 0, 1 9 66 a n d 1 9 74 e v e n t s , B EC K a n d N I S n E N K O 1 9 9 0 ) c o n c l u d e d

t h a t t h e 1 74 6 e v e n t r u p t u r e d t h e s a m e p o r t i o n o f th e c o a s t a s d i d b o t h t h e 1 94 0 a n d

1 9 66 e v e n t s , a n d a p o r t i o n o f t h e 1 9 74 s e g m e n t a s w e l l F i g . 2 0 ) . D O R BA T H

et a l .

1 9 9 0) d e l i n e a te a r u p t u r e z o n e f r o m a b o u t 1 0 ~ d o w n t o 13 ~ a p p r o x i m a t e l y

3 50 k m , a n d a s s ig n a m o m e n t m a g n i t u d e o f 8 .6 t o t hi s e v e nt . T h e s o u r c e s o f b o t h

t h e 1 94 0 a n d 1 9 66 e v e n t s w e r e s im p l e . W e c a n c o n c l u d e t h a t b e c a u s e t h e 1 74 6 e v e n t

w a s s o c o n s i d e r a b l y l a r g e r , i t w a s l i k e l y a m u l t i p l e a s p e r i t y r u p t u r e . I t is p o s s ib l e t h a t

t h e t h r e e a s p e r it i e s c o r r e s p o n d t o t h e l a t e r m a i n s h o c k l o c a t i o n s o f t h e 1 94 0 e v e n t ,

t h e 1 9 6 6 ev en t , an d p o ss ib ly a l so th e 1 9 7 4 ev e n t B E C K an d N IS H E NK O , 1 9 9 0 ).

1 4 A u g u s t 1 8 6 8

T h e u n d e r t h r u s t i n g e v e n t i n 1 86 8 g e n e r a t e d a t s u n a m i r u n u p o f ~ 2 m i n J a p a n

HATORI, 1 96 8) a n d l o c a l t s u n a m i o f 1 4 m DORBATH et a l . , 1 9 90 ) f r o m w h i c h A B E

1 9 7 9 ) c a l c u la t e d a t s u n a m i m a g n i t u d e o f M t = 9 . 0 . D O RB A TH

et a l .

1 9 9 0 )

d e t e r m i n e a r u p t u r e z o n e e x t e n t o f 15 .5 ~ t o 1 9~ a p p r o x i m a t e l y 4 0 0 k i n , f r o m

w h i c h t h e y c a l c u l a te a m o m e n t m a g n i t u d e o f 8.8 . D O R B AT tt

e t a l. 1990 )

a l s o m a k e

t h e o b s e r v a t i o n t h a t t h e s i m i la r i ty b e t w e e n t h e 1 6 0 4 a n d 18 68 e a r t h q u a k e s m a y b e

i n t e r p r e t e d a s t h e l a t t e r b e i n g a r e p e a t o f th e f o r m e r .

2 4 M a y 1 94 0

O n e o f t h e f i rs t l a r g e h is t o r i c e a r t h q u a k e s a l o n g t h e P e r u t r e n c h t o b e s t u d i e d

ex ten s iv e ly w as th e 1 9 40 ev e n t F ig . 1 ). G U T E N B ER G an d R IC H T E R 1 9 5 4 ) a s s ig n ed

t h e e v e n t a m a g n i t u d e o f 8.0 . T h e I S S p r e l im i n a r y i n l a n d l o c a t i o n , d e e p e r a s s u m e d

d e p t h ~ 6 0 k m ) a n d s m a l l n u m b e r o f a f t e r s h o c k s s u g g es t e d th a t t h e 19 40 e v e n t

o c c u r r e d w i t h i n t h e d o w n g o i n g N a z c a p l a t e B E C K a n d N I SH E N KO , 1 9 90 ). A

d e t a i l e d a n a l y s i s o f t h e 1 94 0 e a r t h q u a k e b y B EC K a n d R U F F 1 9 8 9 ) i n d i c a t e s i t is

a s h a l l o w u n d e r t h r u s t i n g e v e n t. H e n c e , t h e e n t i r e p l a t e b o u n d a r y b e t w e e n 10 ~ a n d

1 4~ h a s f a i l e d d u r i n g th i s c e n t u r y F i g . 2 0) B E C K a n d N I S n E N K O , 1 99 0) .

T h e 2 4 M a y 1 940 e v e n t h a s o n e d o m i n a n t p u l s e o f m o m e n t r e le a s e w i t h a t o t a l

d u r a t i o n o f 2 4 t o 3 0 se c o n d s . T h i s is h a l f o f t h e d u r a t i o n o f t he a d j a c e n t 1 96 6 a n d

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V o l . 1 46 , 1 9 96 H i s t o r i c a l 1 9 42 E c u a d o r a n d P e r u E a r t h q u a k e s 9 5

S P A C E T IM E P L O T F O R T H E P E R U S U B D U C T I O N Z O N E

1 9 0 0

1 8 0 0

1 7 0 0

1 6 0 0

,x~x~

....... --- 1974

1 9 6 6 O - - ~ I= 8

I= 8 1 9 4 0 T = 2m

T=3m I=8

T=2m

1746 9

1=10 1828

T= 24m I = 8

, d h

V ?

? A

9 I W

1878 1655 9 1687

I=9 I=9 I=9-11

9 T=Sm

I 586

I=9

T = 2 6 m

1942

M=8.2

?

A

w

1868

I = l 1

T=21m

1784

I=8-9

? 9

9 1687

9 1664 I=8

1650 I=8 9

I=8

A

' q F

1604

I = l l

T=16m

0 250 500 km

J

F i g u r e 1 9

S p a c e - t i m e p l o t o f th e P e r u s u b d u c t i o n z o n e . T i m e i s s h o w n o n t h e y a x is , d i s t a n c e a l o n g t h e t re n c h ( i n

k i l o m e t e r s ) o n t h e x a x i s . D o t s r e p r e s e n t t h e l o c a t i o n a n d r e l a ti v e si ze o f t h e u n d e r t h r u s t i n g e a r t h q u a k e ;

a n d l in e s e x t e n d i n g f r o m t h e d o t s r e p r e s e n t e s t i m a t e d r u p t u r e l e n g th . T h e 1 9 70 e v e n t is n o t s h o w n i n t h i s

f ig u r e b e c a u s e i t i s a n o r m a l f a u l t in g e v e n t r a t h e r t h a n a n u n d e r t h r u s t i n g e v e n t .

1 97 4 e v e n t s w h i c h o c c u r r e d n o r t h a n d s o u t h o f th e 1 94 0 e v e n t , r e s p e c t iv e l y . A

s e is m ic m o m e n t o f 2 - 8 x 1 0 2 ~ w a s d e t e r m i n e d f o r t hi s e v e n t ( BE C K a n d

RUFF, 1989) .

7 October 966

T h i s u n d e r t h r u s t i n g e a r t h q u a k e t r i g g e re d a s er ie s o f a f te r s h o c k s w h i c h s p a n a n

8 0 k m l o n g s e g m e n t o f th e P e r u t r e n c h ( F i g . 1). A l o c a l t s u n a m i r u n u p o f 2 .1

m e t e r s w a s r e p o r t e d i n C a l l a o ( L o c K R I D G E , 1 98 5) . M o s t o f th e 2 0 x 1 0 2o N . m o f

s e is m i c m o m e n t t h a t w a s g e n e r a t e d b y t h e 19 66 e v e n t w a s r e l e a s e d in o n e m a i n

p u l s e 5 0 s e c o n d s i n d u r a t i o n i n t e r p r e t e d a s a n a s p e r i t y n e a r t h e e p i c e n t e r ( B E C K

a n d

RUFF

1989).

3 October 974

T h e m o s t r e c e n t u n d e r t h r u s t i n g e v e n t i n c lu d e d i n th i s s tu d y o c c u r r e d i n th e

se i smic g ap id en t i f i ed b e tw een th e 1 9 4 0 an d 1 9 4 2 ev en t s (K E L L E H E R , 1 9 7 2 ) (F ig . 1 ) .

T h e a f t e r s h o c k a r e a is 2 40 k m b y 5 0 k m e l o n g a t e p a r a l l e l to t h e t r e n c h ( D EW E Y

a n d S P E N C E , 1 979 ). T h e m a j o r i t y o f m o m e n t r e le a s e f o r t h is M w = 8 . 0 e v e n t

8/17/2019 1400443160_690__swenson_1996.pdf


9 6 J e n n i f e r L . S w e n s o n a n d S u s a n L . B e c k P A G E O P H ,

~ _ ~b=530

966 ~ A =106~

A T U ~\\\~.~-.~s.,~.o._.,-.~

1940 ~ *=37 ~

K E W : = 91 ~

1 97 4 ~ t ~ 0 = 5 4 ~

A T U ~ A = 1 06~

cb =3 21 o

1942

P A S

I I

0 120 s

8 ~

10 ~

12 ~

14 ~

16 ~

8 0 o W

F i g u r e 2 0

18 o

78 ~ 76 ~ 74 ~

S u m m a r y o f t h e s o u r c e t i m e f u n c t i o n s a n d a s p e r i ty d i s t r ib u t i o n s o f t h e f o u r u n d e r t h r u s t i n g e a r t h q u a k e s

t h a t h a v e o c c u r r e d a l o n g t h e c e n t r a l P e r u s u b d u c t i o n z o n e d u r i n g t h e p r e s e n t c e n tu r y . H a c h u r e d r e g i o n s

i n d i c a t e t h e l o c a t i o n s o f d o m i n a n t a s p e r i t ie s , a n d s o l id c o n t o u r s r e p r e s e n t a f t e r s h o c k a r e a s a s d e t e r -

m i n e d b y B ~ C K a n d R U F F ( 1 9 8 9 ) ; s t a r s r e p r e s e n t r e s p e c t i v e e p i c e n t e r s . T h e a f t e r s h o c k a r e a s f o r t h e

1 9 6 6, 1 9 7 0 a n d 1 9 7 4 e a r t h q u a k e s a r e f r o m D E W E Y a n d S P E N CE (1 9 7 9 ) . T h e l o c a t i o n o f t h e 1 9 4 0 a n d

1 9 4 2 e a r t h q u a k e s a r e f r o m D E W E Y ( u n p u b l i s h e d ) ,

o c c u r r e d i n t h e n o r t h w e s t h a l f o f th e e l o n g a t e d a f t e r s h o c k z o n e . T h e e v e n t r u p t u r e d

b i la t er a ll y : 4 0 k m n o r t h w e s t a n d 6 0 k m s o u t h e a s t o f t h e e p ic e n te r . T h e m o s t

i m p o r t a n t f e a tu r e o f th i s e a r t h q u a k e is t h e w e l l -c o n s t r ai n e d e x t e n t o f i ts r u p t u r e

z o n e F i g . 1 9 ) . I t a b u t s t h e r u p t u r e a r e a o f t h e 1 94 0 e v e n t to t h e n o r t h , b u t l ea v e s

a g a p b e t w e e n i t s s o u t h e r n e x t e n t a n d t h e 1 94 2 e v e n t . T h i s s ig n if ie s t h e i m p o r t a n c e

o f c o n s t r a i n i n g t h e r u p t u r e l e n g t h o f t h e 1 94 2 e v e n t F i g . 2 0).

24 August 942 Peru Earthquake

T h e 2 4 A u g u s t 1 94 2 P e r u e a r t h q u a k e o c c u r r e d a t 1 5.2 1~ 7 5 . 2 5 ~ r e l o c a t e d

b y D E W E Y , p e r s o n a l c o m m u n i c a t i o n ) o n t h e s o u t h e r n f l a n k o f th e s e e m i n g l y

a s e is m i c N a z c a R i d g e F i g s . 1 a n d 2 0). A l t h o u g h n o f a r -f ie l d t s u n a m i w a s r e p o r t e d

in Jap an , t he even t gen era t ed a l oca l t su na m i o f 2 .0 m LOCKRIDOE, 1985) . Th i s

e v e n t is k e y t o o u r u n d e r s t a n d i n g t h e m o d e o f e a r t h q u a k e r u p t u r e a l o n g t h e

P e r u - C h i l e t r e n c h a n d w h e t h e r o r n o t t h a t m o d e v a r i e s t e m p o r a l l y . T h i s P e r u

e a r t h q u a k e h a s n o t b e e n t h e f o c u s o f a n y p r e v i o u s d e t a i l e d s t u d y . A s o u t l i n e d i n

p r e v i o u s s e c t i o n s , w e i n v e s t i g a t e d t h e s o u r c e t i m e f u n c t i o n , f a u l t p l a n e s o l u t i o n ,

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Vol. 146, 1996

280

Historical 1942 Ecuador and Peru Earthquakes

8 5 ~

97

-10

-15

-10

-1 5 ~

280 285

Figure 21

Map showing the 1942 Per u ma in shock as located by DEWEY (unpublished; large triangle), KELLEHER

(1972; larg e circle) and ISS (lar ge cross). Also show n are two aftershocks relocated by Dewey

(unpublished; sm all triangles), four aftershocks relocated by KELLEHER (1972; small circles) and three

months of aftershocks reported by ISS (small crosses).

hypo cen t ra l dep th , and se i smic m om en t re lea se o f the 1942 Pe ru ea r thquak e . We

exam ine th i s even t in the con tex t o f r eg iona l h i s to r i c se i smic i ty and the g loba l

tectonic cycle.

The rep or t ed loca t ions o f the m ax im um in tens it i es ( IX) (ASKEW and AL6ER-

MISSEN, 1985) (F ig . 18) for the 1942 Peru event l ie near and sou th of the mai n

shock ep icen ter . The h igh conce n t ra t ion o f in t ens i ty IX repor t s sou th o f the

ep icen te r sugges t s a t le a s t a po r t ion o f the rup tu re was to the sou th . H owe ve r ,

in t ens i ty repor t s a re h igh ly sub jec t ive and dependen t upon popu la t ion and bu i ld ing

s tyles , and m us t be c r it i ca l ly in t e rp re ted . In tens i ty VI , VI I and VI I I r epor t s a re a l so

foun d no r th o f the ep icen ter , so the rup tu re a ssoc ia t ed wi th the Pe ru even t m ay

8/17/2019 1400443160_690__swenson_1996.pdf



have been bilateral. A comparison of relative timing of source time function pulses

from the station groupings gives little evidence to support a conclusion of rupture

occurring in any specific direction. Aftershocks recorded for a period of three

months following the Peru earthquake and reported by the ISS (Fig. 21) occur over

a broad area surrounding the main shock. Aftershocks are located within a zone

approximately 330 km by 220 km, elongated parallel to the trench. A concentration

of aftershocks exists immediately adjacent to the epicenter, with a sparse collection

further south. These aftershock locations are subject to debate given the poor

station coverage in the southern hemisphere in 1942. Unfortunately, most of the

aftershocks are not large enough to be relocated. Although KELLEHER(1972) does

not determine an actual rupture length for the 1942 Peru event, he relocated several

aftershocks and used S minus P times from La Paz to determine that the rupture

zone was relatively small (Fig. 21). DEWEY and SPENCE (1979) also relocated

two of the largest aftershocks of the 1942 Peru event (Fig. 21).

The previously-mentioned estimate of 148 km of rupture is based on the

duration of the three main pulses of the source time function. Given the small

number of aftershocks large enough to be relocated, it is difficult to accurately

allocate the 148 km of rupture north and south of the epicenter. The distribution of

intensities and aftershocks would indicate that at least part of rupture and seismic

moment release during the 1942 event occurred south of the epicenter (Figs. 18 and

21). We consider two possibilities: (1) the 1942 Peru event ruptured in a purely

bilateral fashion 75 km north and 75 km south; and (2) the event ruptured 40 km

to the north and 110 km to the south. Case number one would suggest that had the

1942 event ruptured in a purely bilateral fashion, there would be a gap between the

1974 and 1942 rupture zones approximately 100 km long (Figs. 19 and 20). Case

number two, perhaps the more realistic of the two, increases the gap between the

1974 event and 1942 event to nearly 175 km. For the 1942 Peru event to have

ruptured through the seismic gap would require a 200-km northward rupture

which, given our source time function, is unlikely. Therefore we conclude that a

seismic gap of 100 km or more remains between the 1974 and 1942 rupture events.

Figures 19 and 20 show the estimated rupture lengths of great historic earth-

quakes based on intensities and tsunami data (Table 6). Although there are many

problems associated with estimating earthquake rupture lengths from intensity and

historic data, these data give us a first-order approximation of earthquake size. The

1942 Peru earthquake probably did not rupture through the segment where the

Nazca Ridge intersects the trench. I t seems possible that the 1868 and 1604

earthquakes ruptured all or part of the 1942 segment and possibly part of the

Nazca Ridge-Peru trench intersection. The 1687 event also appears to have

ruptured through the Nazca Ridge-trench intersection, suggesting that it was

possible for great historic earthquakes to rupture through the Nazca Ridge trench

intersection. Based on these estimated rupture zones there exists a 80-100 km long

portion of the trench which has not ruptured in at least 300 years. BECK and

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Vol. 146, 1996 Historical 1942 Ecua dor and Peru Earthquakes 99

NISHENKO 1990) p r o p o s e d t h a t r u p t u r e z o n e s w e r e s e g m e n t e d b y s t r u c t u r e s o n t h e

d o w n g o i n g p l a te only during this century O u r s t u d y s u p p o r t s t h e i d e a t h a t t h e m o d e

o f e a r t h q u a k e r u p t u r e a n d e a r t h q u a k e s iz e a l o n g t he c o a s t o f P e r u h a s c h a n g e d

b e t w e e n s u c c e s s i v e g r e a t e a r t h q u a k e c y c l e s .

Conclusions

L o n g - p e r i o d t e l e s e i s m i c P w a v e s h a v e b e e n a n a l y z e d t o d e t e r m i n e t h e s o u r c e

c h a r a c t e r is t i c s o f t h e 1 9 42 E c u a d o r a n d P e r u s u b d u c t i o n z o n e e a r t h q u a k e s . F o r t h e

1 94 2 E c u a d o r e a r t h q u a k e o u r p r e f e r r e d f o c a l m e c h a n i s m is q5 = 3 0 ~ 6 = 2 0 ~

2 = 1 20 ~ w i t h a d e p t h o f a p p r o x i m a t e l y 1 4 k in . W e d e t e r m i n e d a s e is m i c m o m e n t

o f 6 - 8 x 1 02o N - m , c o r r e s p o n d i n g t o a m o m e n t m a g n i t u d e o f M w = 7 . 8 - 7 . 9 f o r

t h e E c u a d o r e a r t h q u a k e . M o s t o f t h e s e is m i c m o m e n t w a s r e l e a se d in a s im p l e p ul s e

w i t h a d u r a t i o n o f 2 4 s e co n d s . A s s u m i n g a n a v e r a g e r u p t u r e v e l o c i t y o f 2 .0 k m / s ,

w e e s t i m a t e d a r e g i o n o f m a x i m u m m o m e n t r e l ea s e w i t h i n 4 8 k m o f th e e p i c e n te r .

W h i l e a f t e r s h o c k s a r e l o c a t e d n o r t h o f t h e e v e n t , m a x i m u m i n te n s it ie s I X ) a r e

c o n c e n t r a t e d t o t h e s o u t h , i n d ic a t i n g t h a t m o s t o f t h e r u p t u r e w a s l ik e l y s o u t h o f

t h e m a i n s h o c k e p i c e n t e r .

T h e s a m e i n v e r s i o n p r o c e d u r e a p p l i e d t o t h e 1 94 2 P e r u e v e n t g i ve s a b e s t fa u l t

p l a n e s o l u t i o n o f ~b = 3 4 5 ~ c5 = 2 5 ~ 2 = 9 5 ~ a n d a h y p o c e n t r a l d e p t h o f 3 0 - 3 5 k i n .

S e i sm i c m o m e n t w a s c o m p l e x w i t h t h r e e d i s t i n c t p ul se s o v e r 7 4 s e c o n ds , t h e l a r ge s t

p u l s e o c c u r r i n g 3 2 s e c o n d s a f t e r r u p t u r e i n i ti a t io n . T o t a l s e is m i c m o m e n t r e l e a s e a s

d e t e r m i n e d b y t h e n o n d i f f r a c t e d P w a v e s w a s 1 0 - 2 5 x 1 0 2 ~ m , c o r r e s p o n d i n g t o

a m o m e n t m a g n i tu d e o f M w = 7 . 9 - 8 . 2 . T h e m a x i m u m i n te n s it ie s I X ) f o r t h e P e r u

1 94 2 e v e n t a r e p r e d o m i n a n t l y t o t h e s o u t h o f t h e e p i c e n te r , s u g ge s t in g t h a t a t l e a s t

a p o r t i o n o f th e r u p t u r e w a s s o u t h o f t h e e p i c e n te r . I t is u n li k e l y t h a t t h e r u p t u r e

e x t e n t o f t h e 1 9 42 P e r u e v e n t a b u t s t h a t o f t h e 1 97 4 P e r u e v e n t ; t h u s a l a r g e p o r t i o n ,

1 00 k m o r m o r e , o f th e P e r u t r e n c h h a s n o t r u p t u r e d i n se v e r a l h u n d r e d y e a r s .

W e h a v e e x a m i n e d l a r g e a n d g r e a t h i s t o r i c e a r t h q u a k e s a l o n g t h e C o l o m b i a -

E c u a d o r a n d P e r u s e g m e n ts o f t h e S o u t h A m e r i c a n s u b d u c t i o n zo n e . R u p t u r e

p r o c e s s e s o f e a r t h q u a k e s o c c u r r i n g a l o n g t h e se s e g m e n t s a r e t h o u g h t t o b e c o n -

t r o l l e d i n p a r t b y t h e s u b d u c t i o n o f s e e m i n g ly a se i sm i c f e a t u r e s o n t h e N a z c a p l a t e

s u c h a s th e C a r n e g i e R i d g e o f f t h e c o a s t o f E c u a d o r a n d t h e N a z c a R i d g e o f f t h e

c o a s t o f P e r u . T h e s e f e a t u r e s a c t t o l a t e r a l ly s e g m e n t t h e t r e n c h . T w e n t i e t h c e n t u r y

e a r t h q u a k e s t e n d n o t t o r u p t u r e t h r o u g h t h e s e s u b d u c t i n g r i d g e s . H o w e v e r , a t l e a s t

o n e h is t o r i c e a r t h q u a k e 1 6 8 7 P e r u ) h a s r u p t u r e d a c r o s s t h e N a z c a R i d g e , le a d i n g

u s t o s p e c u l a t e t h a t t h e s e g m e n t d e f i n e d b y t h e i n t e r s e c t io n o f t h e N a z c a R i d g e w i t h

t h e t r e n c h i s c a p a b l e o f s e is m i c fa i l u r e a n d t h a t t h e s e g m e n t a t i o n o f th e p l a t e

b o u n d a r y a s d e fi n e d b y e a r t h q u a k e s t h is c e n t u r y i s n o t c o n s t a n t . T h e h i s t o ri c

e a r t h q u a k e r e c o r d s u gg e st s th e m o d e o f e a r t h q u a k e f a il u re a n d s e g m e n t a t i o n a l o n g

t h e S o u t h A m e r i c a n t r e n c h v a r i e s t h r o u g h t i m e .

8/17/2019 1400443160_690__swenson_1996.pdf


1 00 J en n i f e r L . S w en s o n an d S u s an L . B eck P A G E O P H ,

cknowledgments

W e thank the many s ta t i on operators that prov i ded us w i th copi es o f the

hi s tor i c s e i smograms . W e thank T erry W al l ace and Randy Ri chardson for the i r

thought fu l revi ews and Renata Dm ow ska and an anonym ous rev iewer for he l pfu l

comments . Special thanks to John Cass idy for providing records from the Canadian

S e is m i c N e t w o r k . T h e s tu d y w a s f u n d ed b y N S F g ra nt E A R - 9 0 1 7 3 5 8 . S A S O

contribut ion number 31.

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Submit ted July 23, 1994, revised/accepted May 8, 1995)

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