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Bulletinof the SeismologicalSocietyof America Vol. 72 No. 4 pp. 1241-1253 August 1982

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

E C U A D O R - C O L O M B I A C O A S T

BY HIROO KANAMORI AND KAREN C. MCNALLY

ABSTRACT

T h r e e l a rg e e a r t h q u a k e s o c c u r re d w i th i n t h e r u p tu r e z o n e o f t h e 1 9 0 6 C o l o m -

b i a - E c u a d o r e a r t h q u a k e M w = 8 . 8 ) : i n 1 9 4 2 M s = 7 . 9 ) ; 1 9 5 8 M s - - 7 . 8 ) ; a n d

1 9 7 9 M s = 7 . 7 ). W e c o m p a r e d t h e s iz e a n d m e c h a n i s m o f t h e s e e a r th q u a k e s

b y u s in g lo n g - p e r i o d s u r fa c e w a v e s , t s u n a m i d a ta , a n d m a c r o s e i s m i c d a t a . T h e

1 9 7 9 e v e n t i s a t h r u s t e v e n t w i t h a s e i s m i c m o m e n t o f 2 . 9 x 1 0 2 8 d y n e -c m , a n d

r e p r e s e n t s s u b d u c t i o n o f t h e N a z c a p l a te b e n e a t h S o u t h A m e r ic a . T h e r u p tu r e

l e n g t h a n d d ir e c t i o n a re 2 3 0 k m a n d N 4 0 ° E , r e s p e c t i v e l y . E x a m i n a t i o n o f o l d

s e i s m o g r a m s i n d ic a t e s t h a t t h e 1 9 0 6 e v e n t i s a l s o a t h ru s t e v e n t w h i c h r u p tu r e d

in t h e n o r t h e a s t d i r e c t io n . T h e s e i s m i c m o m e n t e s t i m a t e d f ro m t h e t s u n a m i d a t a

a n d t h e s iz e o f t h e r u p t u r e z o n e i s 2 x 1 02 9 d y n e - c m . T h e 1 9 4 2 a n d 1 9 5 8 e v e n t s

a r e m u c h s m a l l e r a b o u t ~ to ~-~oo f th e 1 9 7 9 e v e n t i n t h e s e i s m i c m o m e n t ) t h a n

t h e 1 9 7 9 e v e n t . W e c o n c l u d e t h a t t h e s u m o f t h e s e i s m i c m o m e n t s o f t h e 1 9 4 2 ,

1 9 5 8 , a n d 1 9 7 9 e v e n t s i s o n l y ~ o f th a t o f t h e 1 9 0 6 e v e n t d e s p i t e t h e f a c t th a t

t h e s e q u e n c e o f t h e 1 9 4 2 , 1 9 5 8 , a n d 1 9 7 9 e v e n t s r u p tu r e d a p p r o x i m a t e ly t h e

s a m e s e g m e n t a s t h e 1 9 0 6 e v e n t . T h is d if f e re n c e c o u l d b e e x p l a in e d b y an

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

a s p e r i ti e s w i th w e a k z o n e s i n b e t w e e n . T h e w e a k z o n e n o r m a l ly b e h a v e s

a s e i s m i c a l l y , b u t s l ip s a b r u p t l y o n l y w h e n i t i s d r i v e n b y f a i lu r e o f t h e a s p e r i t ie s .

A s m a l l e a r t h q u a k e r e p r e s e n t s f a il u r e o f o n e a s p e r i ty , a n d t h e r u p t u r e z o n e i s

p i n n e d a t b o t h e n d s b y a d j a c e n t a s p e r i t i e s s o t h a t t h e e f f e c t i v e w i d t h a n d t h e

a m o u n t o f s l ip a r e r e l a t i v e ly s m a l l. A g r e a t e a r t h q u a k e r e p r e s e n t s f a i l u re o f m o r e

t h a n o n e a s p e r i t y , a n d c o n s e q u e n t l y i n v o l v e s m u c h l a r g e r w i d t h a n d s li p .

INTRODUCTION

Many recent studies indicate that a long segment of a subduction zone sometimes

ruptures in a single great earthquake, but at other times it breaks in a series of

smaller earthquakes abutting to each other. One of the best examples is seen for the

subduction zone off the coast of Ecuador-Colombia. A great earthquake occurred in

1906 along the coast of Ecuador-Colombia

Ms

= 8.7, estimated Mw = 8.8) (Figure

I). Kelleher (1972) estimated the rupture length to be about 500 km on the basis of

the macroseismic data. Abe (1979) estimated the tsunami magnitude Mt to be 8.7

which is consistent with Kelleher s estimate of the size of the rupture zone. Approx-

imately this same segment ruptured again during the last 37 yr in three large

earthquakes which occurred in 1942

Ms

= 7.9), 1958

Ms

= 7.8), and 1979

Ms =

7.7). Although there is some uncertainty in the interpretation of the old events, the

evidence is strong that this segment of the Ecuador-Colombia subduction zone

behaved differently from sequence to sequence.

Similar examples are found for southwest Japan along the Nankai trough (Ima-

mura, 1928; Ando, 1975; Seno, 1977) and along the Aleutian Islands (Sykeset al.,

1980). However, in these examples, no instrumental data are available for the older

events, and the details of the rupture mode are unknown.

The purpose of the present paper is to investigate the nature of this type of

variable rupture behavior by studying the Ecuador-Colombia sequence for which

instrumental data are available.

1241

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242 HIROO KANAMORI AND KAREN C. McNALLY

T H E 1 9 7 9 E V E N T

K a n a m o r i a n d G i v e n 1 98 1) m a d e a d e t a i le d a n a l y s i s o f t h i s e v e n t b y u s in g 1 5

R a y l e i g h w a v e s r e c o r d e d a t s e v e n I D A I n t e r n a t io n a l D e p l o y m e n t o f A c c e l e r o g ra p h )

s t a t io n s . S i n c e t h e d e t a i l s a r e g iv e n in K a n a m o r i a n d G i v e n , w e b r i e fl y s u m m a r i z e

t h e r e s u l t s i n t h e f o ll o w i ng .

K a n a m o r i a n d G i v e n 1 98 1) in v e r t e d t h e R a y l e i g h - w a v e s p e c t r a a t th e p e r io d o f

2 56 se c b y u s in g a m o m e n t t e n s o r s o u r ce p l a c e d a t a d e p t h o f 3 3 k m . T h e m o m e n t

t e n s o r t h u s o b t a i n e d w a s d e c o m p o s e d i n to t h e m a j o r a n d th e m i n o r d o u b l e c o up l e

G i l b e r t , 1 98 0) . T h e s e i s m i c m o m e n t o f t h e m i n o r d o u b l e c o u p l e i s 0 .2 p e r c e n t o f

C] N82°E ~

7.96 c m / y r . . / 1979

'b.~...1/19 days

Rupture . .z~. ~ I day

(C) Lengtha n d / ~ _ - ~ / ~ /Direction / / i,~

, , / t

Kelleher (1972) ~ , ,

9oe z . f COLO M al A

, 9 2 - t , J l '

( b Ouito

ECUADOR

0 300 km

q ~ i

Fro. 1. Ru ptu re zon es of the 1906, 1942, and 1958 Colom bia-Ecuador even ts Kelleher, 1972). After-

shock zones 1 day and 19 days after the 1979 event asterisk} are shown. The rupture length and the

direction c) are determined from azimu thal variation o f group arrival times of Rayleigh waves Kana mori

and G iven, 1981). Th e m echanism diagram b) is from Kan am ori and Given 1981). Th e lower focal

hemisphere is shown.

Hatched areas

show compressional quadrants. Th e arrow shown in a) indicates

the convergence vector between the N azca and Sou th A merican plates at th e epicenter of the 1979 event

after Minster and Jordan, 1978).

t h a t o f t h e m a j o r d o u b l e c o u p l e , a n d i s c o n s i d e r e d n e g l ig i bl e . A s s h o w n b y F i g u r e 2 ,

t h e f a u l t g e o m e t r y o f t h e m a j o r d o u b l e c o u p l e is c o n s i s t e n t w i t h t h e f i r s t - m o t i o n

d a t a o b t a i n e d f r o m t h e W W S S N l o n g -p e r io d s e i s m o g r a m s . T h e s t r i k e o f t h e lo w -

a n g l e p l a n e i s p a r a l l e l t o t h e t r e n c h a x is , a n d t h e m e c h a n i s m i s c o n s i s t e n t w i t h

s u b d u c t i o n o f t h e N a z c a p l a t e b e n e a t h S o u t h A m e r i ca . A s i m i la r m e c h a n i s m h a s

b e e n r e p o r t e d b y H e r d e t a l . 1981) .

I f t h e l o w - a n g l e p l a n e d i p p i n g t o w a r d s S E i s t a k e n t o b e t h e f a u l t p la n e , t h e n t h e

s l i p d i re c t i o n i s i n n e a r l y E W d i r e c t io n a n d i s c o n s i s t e n t w i t h t h e m o t i o n o f t h e

N a z c a p l a t e w i th r e s p e c t t o th e S o u t h A m e r i c a n p l a te d e t e r m i n e d b y M i n s t e r a n d

J o r d a n 1 97 8 ). A s e i s m i c m o m e n t o f 2 .9 x 1 02 s d y n e - c m Mw = 8 .2 ) i s o b t a i n e d .

S i n c e t h e s iz e o f t h e r u p t u r e z o n e is c r i t ic a l fo r t h e p r e s e n t d i s c u s s i o n, w e m a d e

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T H E R U P T U R E Z O N E O F T H E E C U A D O R C O L O M B I A C O A S T 243

a special effort to determine it by using the observed long-period Rayleigh waves.

Usually, the size of the after shock area expands as a function of time, which results

in the uncertainty of the estimate of the rupture zone. The directivity method

developed by Ben-Men ahem 1961) is often used for the determina tion of the

rupture length, but this event is not large enough to bring the directivity spectral

holes in the period range with high signal-to-noise ratio. We, therefore, used the

azimuthal variation of group arrival times of Rayleigh waves to determine the

rupture length.

Figure 3 compares band-passed synthetic seismograms computed for the seven

IDA stations used with the band-passed observed records. The band-pass filter is

center ed at about 270 sec, and the synthetics are c omputed for a point source placed

at the epicenter. It is seen that the waves which propagat ed in the SW azimuth R2

W

• C o m p r e s s i o n

= 7 4 °

= 2 6 8 °

° l

I 4 1

+

E

S = 2 0 °

: 1 2 1 o

J o D i l a t a t i o n

s

FIG. 2. Focal mechanism stereographic projection of the lower focal sphere) of the 1979 Colombia

earthquake determined by a moment tensor inversion Kanamori and Given, 1981). The P-wave first-

motion data are obtained from the WWSSN records. 3 is the dip angle, and ~ is the dip direction.

at HAL and ESK, R3 at TWO) were delayed by about 65 sec with respect to those

propaga ted in the NE azimuth R3 at HAL and ESK, R4 at TWO). The waves which

propaga ted in NW and SE azimu ths do not show significant delays CMO and

SUR). This pa tte rn of group delays clearly indicates rupt ure propagation in the NE

direction. Also, the observed trains are delayed by 58 sec on the average with respec t

to the synthetics comp uted for a point source. For a unilateral fault with the r uptur e

length L, the phase delay at a station in the azimuth t? from the ru ptu re direction is

given by Ben-Menah em, 1961)

L v )

= V 1 - ~ c o s ~

where V is the rupture velocity, C is the phase velocity, and ~ is the angular

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2 4 4 H I R O O K A N A M O R I A N D K A R E N C M C N A L L Y

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

x L v )

Zg dw V 1 - - - ~ c o s t ? (1)

w h e r e U i s t h e g r o u p v e l o c i t y . U s i n g ( 1), t h e r a n g e a n d a z i m u t h a l a v e r a g e o f rg c a n

b e w r i t t e n r e s p e c t i v e l y b y

Arg = 2 - (2)

U

a n d

L

~:g - V (3)

S i n c e U = 3 .6 k m / s e c a t T = 2 2 5 s e c, w e e s t im a t e L = 2 3 0 k m f r o m ( 2), a n d V =

2 km /s e c fr o m (3) us ing th e o bs e r v e d v a lue s o f hTg a nd Yg. T he be s t - f i t r uptur e

d i r e c t i o n is N 4 0 ° E . T h e r u p t u r e le n g t h a n d t h e r u p t u r e d i r ec t i o n ar e c o m p a r e d w i t h

t h e a f t e r s h o c k a r e a i n F i g u re 1 . T h e r u p t u r e l e n g t h d e t e r m i n e d f r o m t h e g r o u p d e l a y

C o l o m b i a E c u a d o r

. /

. /

/

I A v g sec Matched

C M O ^ i R 2 ~ [R 5

V

1164

H A L

J ~ R 2 } ~

3

[70

E S K ~ R 2 R 5

] 2 °

A R 5

J

, 6 0

W V

~ R4

S U R

o

R 4

II

2

B D F / ~

R2 ~ R3

0 i O OOsec

FIG. 3. T he b and-pas s 150 to 1500 sec)-filtered observed solid curve) and synthet ic dashed curve)

seismograms of the 1979 event. The observed and synthetic traces are m atched at t he point indicated by

an arrow, and ~g is the delay time of the ob served trace with respect to the synthetics. A~g is the relative

delay time of the first Rayleigh wave train on the observed trace with respect to the second Kanamori

and Given, 1981).

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THE RUPTURE ZONE OF THE ECUADOR-COLOMBIACOAST 1245

times is in good agreement with the extent of the aftershock area 1 day after the

main shock, but is slightly shorter than that 19 days after the main shock. Thus, in

this case, the l-day aftershock area which is often used to estimate the fault area,

appe ars to be a good appr oxim ati on of the size of the r uptu re zone.

COMPARISON OF TSUNAMI DATA

Table 1 compares the tsunami data for the 1906 and 1979 events. No tsunamis at

teleseismic distances are re port ed for the 1942 and 1958 events. Abe (1979) estimat ed

the ts un ami mag ni tu de M~ of the 1906 eve nt to be 8.7. Using Ab e s metho d, M, = 8.2

is obtained for the 1979 event. Thus, the tsunami data clearly indicate that the 1906

event is subs tanti ally larger t ha n the 1979 event. Th e ab sence of repor ts of far-field

tsunamis for the 1942 and 1958 earthquakes suggests that they are even smaller

than the 1979 event.

TABLE

MAXIMUM T SUNAMI HE IGHT H IN ME T E RS

Tide Sta tion 1906 1979

Honolulu 0.2 0.04

Hilo 3.6 0.40*

Hakodate 0.18 0.09

Ayukawa 0.22 0.13

Kushimoto 0.29 0.10

Hosojima 0.19 --

* This value is given by H. G. Loomis (written com-

munication, 1980).

Forty centimeters is reported as peak-to-peak ampli-

tude in the NEIS monthly listing of earthquakes.

Ecuador Colomba Earthquakes

P a s a de n a 3 0 - 9 0 R 4 I I cm

1979

[958

/195S]_ I

8 1 2 i m in ~ - 5 ~ A M w : O 5

FIG. 4. Comparison of R4 recorded by a Press-Ewing seismograph at Pasadena between the 1958 and

1979 events.

THE 1942 AND 1958 EVENTS

The 1958 event was recorded by a Press-Ewing seismograph (30 to 90 sec) at

Pas ade na w hich also reco rded the 1979 event. If we assume t hat these two events

have approximately the same mechanism, we can estimate the seismic moment of

the 1958 event f rom the ampli tude ratio of long-period Rayl eigh waves of the 1958

event to the 1979 event. As shown in Figure 4, the amplitude ratio is about 1:5.6

whic h wou ld give a seismic mo me nt of 5.2 × 1027 dyn e- cm Mw = 7.7) to the 1958

event.

Kelleh er (1972) estimat ed t he aft ersh ock area of the 1958 event by relocating

man y of the aftershocks. T he aftershock area determined by Kelleher (1972) is

shown in Figure 1 and listed in Table 2. The empirical relation between the

aftershock area and the seismic moment (e.g., Kanamori, 1977) suggests a seismic

mo me nt of 2.8 × 102~ dy ne -c m (Mw = 7.6) whic h agree s re aso nab ly well with t ha t

esti mate d fr om the Rayle igh- wave amplitude. We prefer the value 5.2 x 1027 dyne-

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1246 H I R O O K A N A M O R I A N D K A R E N C . M c N A L L Y

cm estimated from the Rayleigh-wave amplitude but in any case, this event is

signficantly smaller th an the 1979 event.

No long-period seismogram is available for the 1942 event. However, the size of

the aftershock area determined by Kelleher (1972) suggests that this event is of

about the same size as the 1958 earthquake. We used Kelleher's aftershock area to

estimate the seismic moment which is listed in Table 2. Although this estimate is

indirect and is subject to some uncertainty, it is reasonable to conclude that this

event is also significantly smaller th an the 1979 event.

THE 1906 EVENT

The epicenter of this event was located at I°N and 81.5°W by Gutenberg and

Richte r (1959). Since the location of the epicenter is critical for the determination

of the rupture direction, we examined the original data used by Gutenberg and

Richter which are now available in the form of microfiche (see Goodstein e t a l . ,

1980).

T A B L E 2

C O L O M B I A - E C U A D O R E A R T H Q U A K E S S U M M A R Y

Eve nt Rupture Area* M ~ M o M ~ M t D m

S,. (km- ) (1027 dyne -cm )

1906 (1.14 × 10~)~ '¶ 8.7:~ (200) (8.8) 8.7§ (5.20)

1942 7.1 × 103¶ 7.9~ (3.2) (7.6) - - (1.30)

1958 6.6 × 10~¶ 7.81[ 5. 2 7.7 - - (2.30)

1979 2.8 × 104** 7.7 ~t 2955 8.2 8.1§§ (2.70)

S = Sr/1 .75

i s u s e d f o r t h e m o m e n t c a l c u l a t i o n t h r o u g h t h e r e l a t i o n M o = 1 .2 3 × 10 22 S 3/2 d y n e - c m

(e .g . , Kanamor i , 1977) .

t T h e v a l u e s in t h e p a r e n t h e s e s a r e o b t a i n e d i n d i re c t ly .

G e l l e r a n d K a n a m o r i (1 97 7).

§ Abe (1979) .

¶ Ke l l ehe r (1972) .

Jl Roth6 (1969).

R e l a t i v e t o t h e 1 9 7 9 e v e n t .

* * T h e a f t e r s h o c k a r e a f o r t h e p e r i o d 1 2 t o 3 1 D e c e m b e r 1 97 9.

t t N a t i o n a l E a r t h q u a k e I n f o r m a t i o n S e rv i c e ( N E I S ) .

~ :$ K a n a m o r i a n d G i v e n ( 19 81 ).

§ § D e t e r m i n e d f r o m t s u n a m i h e i g h t a t H i lo a n d J a p a n e s e s t a t io n s .

For the data in 1906, it is probably best to use the

S - P

times. Figure 5 shows loci

of constant S - P distance for five stations: Miinich; Baltimore; Tacubaya; Victoria;

and GSttingen. The loci for Miinich, Baltimore, and Victoria intersect each other

near (within 200 km) the Gutenberg-Richte r epicenter. The loci from GSttingen and

Tac uba ya overshoot it by several hundr ed kilometers. Since the data are incomplete,

the result is inconclusive. Nevertheless, the three closely located intersections

(between Miinich and Baltimore, Baltimore and Victoria, and Miinich and Victoria)

are very close to the Gutenberg-Richter epicenter which is near the southwestern

end of the ru pture zone. It is possible that Gutenberg and Richter (1959) determined

their epicenter by more or less the same reasoning.

Kelleher (1972) estima ted the ruptu re zone of the 1906 event on the basis of

macroseismic data that include reports of diminution of water level in the harbors

of Manta (59'S) and Buena vent ura (3°54'N), and a broken submarine cable found

near Buenaventura (see Figure 6).

Rudolph and Szirtes (1911) made a detailed account of the macroseismic effects

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

247

o 2 o o k ~ S - P L o c

i : ~ ] [

M ~ J n , c h ~ : b O

G - R e

?

f l ngen

FIG. 5. S P

l o c i l o c i o f a

~oint corresponding

t o a c o n s t a n t

S P

t im e ) f o r th e 1 9 06 C o l o m b i a - E c u a d o r

ear thquake . Th e as te r isk ind ica tes the ep icen te r de te rmine d by Gutenbe rg and R i c h t e r 1 9 5 9 ). T h e

rup ture zone o f t h e 1 9 0 6 event es timated by Ke l le her 1972 ) is shown.

8 4 ° 8 2 °

8 ° I i

6 °

4 °

2 °

N

s

4 ° L t

8 0 ° 7 8 ° 7 6 ° W

19 ~

f J

I I

F I G 6

I n t e n s i t y d i s t ri b u t io n d e s c r i b e d b y R u d o l p h a n d S z i r t e s 1 9 1 1) . F o r th e e x p l a n a t i o n o f z o n e s ,

I t o I V , s e e t h e t e x t

o f t h i s e a r t h q u a k e . T h e y e s t i m a t e d t h e i n t e n s i t i e s i n l a n d to b e f r o m V t o X o n t h e

R o s s i - F o r e l s c a l e , a n d d i v i d e d t h e a f f e c t e d a r e a i n t o f o u r z o n e s I , I I, I II , a n d I V ) i n

a d e c r e a s i n g o r d e r o f t h e s t r e n g t h o f s h a k i n g a s s h o w n i n F i g u r e 6 . I n z o n e I , m a n y

b u i l d in g s w e r e c o m p l e t e l y d e s t r o y e d , a n d m a n y l i v e s w e r e l o s t . I n z o n e I I, d e s t r u c -

t i o n s w e r e l i m i t e d t o m a s o n r i e s i n c l u d i n g c h u r c h e s a n d p u b l i c b u i ld i n g s. Z o n e I V

s u f f e r e d o n l y v e r y m i n o r o r n o d a m a g e . V e r y l o n g - p e r i o d g r o u n d s h a k i n g s w e r e f e l t

i n z o n e I V .

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1248 H I R O O K A N A M O R I A N D K A R E N C . M c N A L L Y

Although Kellehe r (1972) considers that the evidence from which the end points

of the ruptu re zone are determined is marginal, the intensit y distribution shown in

Figure 6 strongly suggests that the rup ture zone did not extend much further beyond

Kelleher s end points.

Furthermore, the aseismic Carnegie Ridge intersects the Colombia trench at

abou t 0 ° latitude (Figure 6), and t he rupture propagation probably did not extend

southwest past this intersection. Concerning the northeast end, the Colombia trench

bends sharply from the NE-SW trend to the N-S trend at about 4°N, where the

chain of active volcanoes along the coast is interrupted. Probably the 1906 event did

not rupture past this sharp bend.

The seismic mom ent of this event is estimated to be 2 x 10e~ dyne-cm (Kanamori ,

1977) from Kelleher s (1972) estimate of the rupture zone, and is inevitably subject

to some uncertainty. However, the value of the corresponding M~ is in close

agreement with Abe s (1979) tsuna mi magnitude Mr suggesting tha t it is reasonably

accurate.

G 6 t t i n g e n

1 9 3 3 S a n r i k u

A:8 .zo :333o . . :

~ - D . L ~ ~ : ~ ~ . ~

t 9 0 6 C o l o m b i a - E c u a d o r

Z ~ = 9 0 . 1 ° , , ~ = 3 9 ° ~ , ~ - - r:

- ~ 5 - - - ~ ~ ~ E : = : : t ~

• . _ .: W i e c h e r f . ~ _ ] -o . -- 4 .3 s e c , ~ = 1 5 , V o o = 1 7 0 -

F IG . 7 . C o m p a r i s o n o f t h e P w a v e f o r m o f t h e 1 9 06 C o l o m b i a - E c u a d o r e a r t h q u a k e w i t h t h a t o f t h e

1 93 3 S a n r i k u e a r t h q u a k e r e c o r d e d b y a v e r t i c a l- c o m p o n e n t W i e c h e r t s ei s m o g r a p h a t G S t t i ng e n G e r -

m a n y . T o i s t h e n a t u r a l p e r i o d o f t h e p e n d u l u m e i s t h e d a m p i n g c o n s t a n t a n d V 0 i s t h e s t a t i c

m a g n i f ic a t io n . N o t e t h e s h a r p o n s e t a n d t h e l a rg e a m p l i t u d e o f t h e 1 9 33 e a r t h q u a k e c o m p a r e d w i t h t h e

1906 even t .

Concerning the nature of the 1906 event, a possibility remains th at it was a large

normal-fault event near the trench axis such as the 1933 Sanriku (Kanamori, 1971)

and the 1977 Indonesian earthquake (Stewart, 1978; Given and Kanamori, 1980).

Kelleher (1972) dismissed this possibility on the basis of severe destruction which

occurred well over 100 km inland (see Figure 6). In the case of the 1933 Sanr iku

earthquake, the intensity inland was mostly VI on the JMA (Japan Meteorological

Agency) scale (equivalent to VI to VII on the Modified Mercalli scale) and no severe

destruc tion was caused. Comparison of this observation for the Sanriku ear thquake

with the intensity distribution for the 1906 event shown in Figure 6 does suggest

th at the 1906 event is not a trench normal-fault event.

Since this problem is crucial for estimating the seismic recurrence rate in this

region, we examined some old seismograms to resolve this problem. Unfortunately,

we could collect only a few seismograms for this event. However, a Wiechert

seismogram recorded at GSttingen, Germany, provides key information. One of the

characteristic features of the large normal-fault events is a very sharp onset of body

waves (see Kanamori, 1971). Figure 7 compares the waveform of the 1933 Sanriku

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T H E R U P T U R E Z O N E O F T H E E C U A D O R - C O L O M B I A C O A ST

249

and 1906 Colombia-Ecuador earthquake recorded by a Wiechert seismograph ver-

tical component) at GSttingen. The characteristics of the seismograph are almost

identical for the two events. The record of the Sanriku earthquake shows a very

sharp onset followed by a large P-wave train. On the o ther hand, the 1906 event

shows a very gradual onset typical of subduction-zone thrust events. Also, the first

motion is up for the 1906 event while it is down for the 1933 event. In view of the

geometry of the trench and the location of the station, it is very unlikely that the

GSttingen station is located near the node of the r adiation pattern, and the upward

motion indicates a thrust event.

The N-S-component Weichert seismograph at GSttingen registered a clear G3

wave as shown by Figure 8. Although the amplitude is very small peak-to-peak =

1 mm), it is clearly above the noise level and the group velocity is appropriate for

the long-period Love wave i.e., 4.3 to 4.4 km/sec). However, no clear long-period

arrival is found at the time corresponding to the G2 wave, as shown by Figure 8. If

a poin t source is assumed, the amplitude of G2 should be abo ut 3 times larger than

G3. This observation suggests a northeastward rupture propagation which is con-

sistent with the location of the epicenter at the southwestern end of the rupture

zone.

C o l o m b i o - E c u o d o r ,

Jon. 21, 1906

GSt t ingen Z~=90.1 , ~ ,=38 .6 °, ~e= 271 .8 ° )

W i echer t N -S T o=14 sec , V = I50 , ~= 13

- - ' - -: : ~ IL l : ; : - k m /s e 'C ._. :. : .

4 : 5 - ) . . ~ . ~ ' - 4 : 3 - 4 . Z ~ _ ., _ ~, .-

. ' : . . , , , ~ . : : , , , . . . . ~ , . - ' - : , . 1

~ I

0 5 min.

F IG . 8 . G 2 a n d G 3 w a v e s f r o m t h e 1 9 06 C o l o m b i a - E c u a d o r e a r t h q u a k e r e c o r d e d b y a N - S - c o m p o n e n t

W i e c h e r t s e i s m o g r a p h a t G S t t i n g e n G e r m a n y .

We computed a synthetic seismogram assuming the same mechanism as the 1979

event and a rupture length of 500 km with a rup ture velocity of 2 km/sec in the NE

direction. Comparison of the amplitude of the synthetic seismogram with the

observed gives a seismic mo ment of 8 × 102s dyne-cm, which is about ½ of the

estimate given in Table 2. A most likely cause for this discrepancy is the solid

friction between the stylus and the recording paper of the seismograph. Since this

type of seismograph was designed for recording seismic waves with a period of up to

100 sec or so, the effect of friction becomes very serious for very long-period waves

with an extremely small amplitude. We, therefore, consider this value a lower bound.

D I S C U S S I O N A N D C O N C L U S I O N S

Various source paramete rs for the four Colombia-Ecuador earthquakes are sum-

marized in Table 2.

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1250 HIROO KANAMOR I AND KARE N C. McNALLY

The average values of slip are estimated from the seismic momen t and the rupture

area, and are less accurate than the seismic moment. Nevertheless, the factor of

abou t 2 difference between the 1906 and other events is probably larger than the

uncerta inty involved in this calculation.

The sum of the seismic moments of the 1942, 1958, and 1979 events is 3.7 × 102s

dyne-cm and is only ~ of the seismic moment of the 1906 event e stimated from the

rupture area and the tsunami data. This difference is clearly larger than the

uncert aint y in the moment calculations. Thus, we conclude tha t the sum of the 1942,

1958, and 1979 events is not equivalent to the 1906 event, despite the fact that the

sequence of the 1942, 1958, and 1979 events ruptured approximately the same

segment of the subduction zone as the 1906 event. If the entire segment of the

Colombia trench from the intersection with Carnegie Ridge to the sharp bend broke

in 1906, the rup ture length could have been as large as 600 km. Even if this were the

case, the conclusion that the seismic moment release per unit rupture length is

larger for the 1906 event than for the 1942, 1958, and 1979 events would remain

unchanged . If this is the general characteristic of the rupture behavior, the am oun t

and extent of the coseismic displacement in a great earthquake would be significantly

larger than those for a series of smaller earthquakes which occur along the same

segment abu tting to each other. Sykes and Qui ttmeyer 1981) suggest this kind of

behavior on the basis of a ruptu re model in which the stress drop increases with the

rupture length.

l

M I > M 2 M 3 M 4 , D > D i , 1 : 2 , 3 , 4

FIG. 9. An asperity model which explains the larger moment per unit rupture length and slip of a

great ear thqua ke event 1) than the smaller events events 2 to 4). Mi and D~ are the seismic mom ent and

the amount of slip of the ith event.

Since the accurate size and the geometry of the rupture zones could not be

determined very well, the question of whether the stress drop increases with the

rupture dimension or not cannot be resolved in this study. Here, we attempt to

explain the present results by using a simple asperity model such as the one

described by Lay and Kanamo ri 1981) see also Lay e t a l . 1982). In this model, the

fault zone is held by a discrete distribution of asperities and the zone between the m

is considered weak. The weak zone normally behaves aseismically, but slips abruptly

only when it is driven by failure of the asperities Figure 9). A recent study by Ruff

and Kan amor i 1980) demonstra tes tha t this type of asperity distribution is inferred

from complexities of body waveforms of large earthquakes. In th e framework of this

simplified model, a small earthquake represents failure of one asperity, and the

rupture zone is pinned at both ends by adjacen t asperities. Therefore, the effective

width and the amount of slip are relatively small. On the other hand, a great

earth quake represen ts failure of more tha n one asperity, and consequently involves

much larger width and slip. Although whether this model is mechanically feasible or

not must await further studies, it appears to explain qualitatively many of the

observed features of great earthquakes e.g., complexity of the waveform).

In the case of the Colombia-Ecuador sequence studied here, the entire zone may

be modeled by thr ee asperities, a large one in the northeast ern end, and two smaller

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THE RUP TURE ZONE OF THE EC UADOR COLOMBIA COAST 1251

ones in the southwestern end of the rupture zone. As Kelleher (1972) suggests, the

1942 and the 1958 events seem to have rupt ured from the s outhwest to the north east

end of the individual ruptu re zone. The 1979 event rupt ured f rom the north east ern

end of the 1958 rupture zone toward northeast. On the basis of the location of the

main shock, Kelleher (1972) suggests that the 1906 event ruptured also in the

northeast direction. The asymmetric radiation pattern of G2 and G3 demonstrated

by Figure 8 supports Kelleher s suggestion.

In terms of the asperity model, triggering occurred instantaneously in 1906,

resulting in a single great earthquake , while in the sequence f rom 1942 to 1979, ther e

were pauses of abou t 20 yr between the successive events. The n the question is why

the trigger pattern changed from sequence to sequence. Using the model by Lay

and Kan amori (1981), two ex treme cases can be considered.

1. Each asperity has its own characteristic repeat time. Th e asperities normally

behave more or less independently, but occasionally they synchronize. If the

charac ter ist ic repeat times are 36, 52, and 73 yr for the 1942, 1958, and 1979

zones, respectively, they may synchronize in about 210 yr. During the 210 yr,

the 1942, 1958, and 1979 zones would break a bou t 6, 4, and 3 times, respect ively .

In this case, the 1906 earthquake represents a relatively rare event.

2. The degree of mechanical coupling between the asperities is very strong so

tha t when one as perity breaks, it tends to trigger the adjacent one even if the

latter is not quite rea dy to fail by itself. Only when th e stress in the adjacent

asperity is very much lower than its strength, triggering fails to occur. In this

case, the 1906-type event is the norm for the subduction zone.

Historic record in Colombia is not complete enough to test these two hypothetic al

cases against data. In the catalog for the period 1575 to 1915 compiled by Ramirez

(1933), no event simi lar to the 1906 event is repo rted before 1900. A very large

earth quake is repor ted in 1882, but this event seems to be located near the nort hern

end of the 1906 event. As far as this catalog indicates, it appears t hat the 1906-type

event is relatively infrequent.

Wh et he r events similar to t he 1942, 1958, or 1979 events occurred during the t ime

period covered by Ramirez s catalog is more difficult to determine because the

historical data which are mainly based on intensity data on land are considered less

complete for the events off shore.

If the actual situat ion is close to case (1), long-term predict ion of seismic activity

can be ma de relati vely accurately, but if triggering controls the sequence, as in case

(2), prediction of recurrence time at a given point of the subduction zone would be

more difficult.

Detailed studies on the waveforms of the events in these rupture zones may be

able to de termine t he mechani cal conditions there, which in turn may provide clues

to the behavior of this subductio n zone in the future.

The result that the amount of displacement at a given point along a subduction

zone depends on the rupture length has an important bearing on risk analysis.

Recently, some attempts have been made to predict the nature of strong ground

motions and tsunamis which would be excited by failure of certain seismic gaps. In

these studies, various paramet ers such as the intensit y, the size of the affected area,

the observed tsunami height, and the magnitude of crustal deformation of the

previous events are used to constrain the models for the earthquake in the future.

However, if the behavior of the seismic gap varies from sequence to sequence as is

demonstrated for the Colombia-Ecuador earthquake sequence, these parameters

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T H E R U P T U R E Z O N E O F T H E E C U A D O R - C O L O M B I A C O A ST 253

R u d o l p h , E . an d S . S z i r t e s 1 9 11 ). D aN K o l u m b i an s k c h e E r d b e b en a m 3 1 J an u a r y 1 9 0 6 , G. Beitr. 11 ,

132-199, 207-275.

R u f f , L . an d H . K an a m o r i { 19 8 0) . V a r i a t i o n o f a s p e r i t y s i z e i n f e r r ed f ro m t h e b o d y w av e s o f g rea t

e a r t h q u a k e s , EOS 61, 1026-1027.

S e n o , T . 1 97 7) . T h e i n s t a n t a n e o u s r o t a t i o n v e c t o r o f t h e P h i l i p p in e S e a p l a t e r e l a t i v e t o t h e E u r a s i a n

p l a t e , Tectonophysics 42, 209-226.

S t ew ar t , G . S . 1 97 8 ). Im p l i c a t i o n s fo r p l a t e t e c t o n i c s o f t h e A u g u s t 1 9 , 1 9 7 7 , In d o n e s i an d ec o u p l i n g

n o r m a l - f a u l t e a r t h q u a k e a b s t ra c t ) ,

EOS

59, 326.

S y k es , L . R . , J . B . K i s s l i n g e r , L . H o u s e , J . N . D av i e s , an d K . H . J aco b { 19 8 0) . R u p t u re zo n es o f g rea t

ea r t h q u ak e s , A l a s k a -A l eu t i an a r c , 1 7 84 -1 9 80 , Science 210, 1343-1345.

S y k e s , L . R . a n d R . C . Q u i t t m e y e r 1 98 1) . R e p e a t t i m e s o f g r e a t e a r t h q u a k e s a l o n g s i m p l e p l a t e

b o u n d a r i e s , i n Earthquake Prediction D . W . S i m p s o n a n d P . G , R i c h a r d s , E d i t o r s , A m e r i c a n

G eo p h y s i ca l U n i o n , W as h i n g t o n , D .C . , 21 7-24 7.

SEISMOLOGICALLABORATORY

DIVISION OF GEOLOGICALAND PLANETARYSCIENCES

CALIFOR NIA INSTITU TE OF TECHNOLOGY

PASADENA, CALIFORNIA91125

CONTRIBUTION No . 3603

M a n u s c r i p t r e c e i v e d 6 N o v e m b e r 1 98 1

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