swenson 1996

PAGEOPH, Vol. 146, No. 1 (1996) 0033-4553/96/010067 3551.50 + 0.20/0 �9 1996 Birkh/iuser Verlag, Basel

Historical 1942 Ecuador and 1942 Peru Subduction Earthquakes, and Earthquake Cycles along Colombia-Ecuador

and Peru Subduction Segments

JENNIFER L. SWENSON I a n d SUSAN L. BECK I

Abstract 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 magnitude 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 of 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.

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 understanding 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 of 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 heterogeneity.

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 (KANAMORI and 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.

Vol. 146, 1996 Historical 1942 Ecuador and Peru Earthquakes 69

2 8 0 " 2 9 0 " 3 0 0 ~

0"

-10"

_20 ~

O~

_10 ~

_20 ~

2 8 0 " 2 9 0 ~ 3 0 0 "

Figure 1 Map of the South American subduction zone showing the large Ecuador and Peru underthrusting

earthquakes which have occurred during this century.

The central and southern Peru trench has also failed in a series of large subduction zone earthquakes this century. From north to south these are the 17 October 1966 (Mw = 8.0), 24 May 1940 (M w = 7.8), 3 October 1974 (M w = 8.0)

and 24 August 1942 ( M w ~ 8.1) earthquakes. The fault areas as defined by the aftershocks do not overlap along strike (DEWEY and SPENCE, 1979). With the

exception of the 1942 event, each large event occurred north of the Nazca Ridge

intersection with the Peru trench. None of the events this century have ruptured this portion of the coastline where the crest of the Nazca Ridge intersects the Peru trench (BECK and NISHENKO, 1990). This suggests that there may be a 100-kin long gap between the 1942 and 1974 rupture zones which has not failed in several hundred years.

Tab

le

1

Stat

ions

use

d in

the

19

42 E

cuad

or e

vent

stu

dy;

Lon

g-pe

riod

ins

trum

ent

resp

onse

s 14

Ma

y 19

42 E

cuad

or E

arth

quak

e

Sta

tion

(C

OD

E)

A ~

q5 ~

(9;

com

p.

inst

. ph

ase

fm

T~

Te

dam

p

V,,,

(co

rrec

ted)

Ott

awa

(OT

T)

46.2

5.

6 18

8.0

Z

SH

P

c 1.

0 75

.0

0.0

160

Deb

ilt

(DB

N)

82.2

38

.0

258.

7 E

G

P

c 25

.0

25.0

0.

0 31

0 (1

28)

Kew

(K

EW

) 85

.1

38.3

26

2.6

E

G

P c

24.0

8.

0 0.

0 30

0 (1

25.5

) U

ccle

(U

CC

) 87

,8

39.0

24

8,7

E

G

P c

24,5

24

.5

0.0

1335

(42

8)

Pas

aden

a (P

AS

) 49

.0

318,

9 12

7.5

Z

B

P d

1,0

90.0

0.

0 20

00

Tuc

son

(TU

C)

43.0

32

2.5

134.

1 S

WA

P

d 10

.0

10.0

0.

02

450

( -

164.

4)

A i

s th

e ep

icen

tral

dis

tanc

e, ~

b th

e az

imut

h to

the

sta

tion

, q5

h th

e ba

ck a

zhnu

th f

rom

the

sta

tion

, T

s th

e pe

riod

of

the

seis

mom

eter

, T

g th

e pe

riod

of

the

ga

lvan

omet

er a

nd

V m

the

ins

trum

ent

mag

nifi

cati

on.

Inst

rum

ents

are

abb

revi

ated

as

foll

ows:

SH

is

a S

pind

ler-

Hoy

er,

G i

s a

Gal

itzi

n, W

A i

s a

long

-per

iod

Woo

d-A

nder

son

and

B i

s a

Ben

ioff

. Z

, ve

rtic

al c

ompo

nent

; E

, ea

st-w

est;

N,

nort

h-so

uth.

Tab

le 2

St

atio

ns u

sed

in t

he

1942

Per

u ev

ent

stud

y: L

ong-

peri

od i

nstr

umen

t re

spon

ses

24 A

ugus

t 19

42 P

eru

Ear

thqu

ake

8

Sta

tion

C

OD

E)

A ~

q5 ~

qS~,

com

p.

inst

. ph

ase

fm

2r~

Te

dam

p

V,,,

(co

rrec

ted)

==

Wes

tin

(WE

S)

54.2

4.

1 18

5.4

N

B

P c

1.0

60.0

0.

0 50

0 (3

22.4

) O

ttaw

a (O

TT

) 60

.3

0.2

180.

3 Z

S

H

P c

12.0

14

.0

0.1

160

Deb

ilt

(DB

N)

90.1

35

.6

, 24

5.9

E

G

Pdif

f C

25.0

25

~0

0.0

310

(112

.4)

Z

G

Pdm

~ c

12.0

12

.0

0.0

740

Kew

(K

EW

) 92

.7

37.4

24

9.7

E

G

Pdif

r C

24.8

23

.9

0.0

300

(63.

87)

Z

G

Pdif

f C

14

.4

I4.2

0.

0 30

0 U

ccle

(U

CC

) 95

.3

38.0

24

8.7

Z

G

Paif

f c

10.1

5 10

.10

0.0

1000

P

asad

ena

(PA

S)

63,9

32

1.4

133.

4 Z

B

P

c 1.

0 11

5.0

0.8

2000

T

ucso

n (T

UC

) 58

.2

325.

0 13

9.2

S W

A

P c

10.0

10

.0

0.02

45

0 (

- 19

0.0)

A i

s th

e ep

icen

tral

dis

tanc

e, ~

b th

e az

imut

h to

the

sta

tion

, q5

h th

e ba

ck

azim

uth

from

the

sta

tion

, T

s th

e pe

riod

of

the

sei

smom

eter

, Tg

the

per

iod

of t

he

galv

anom

eter

and

V

,,, th

e in

stru

men

t m

agni

fica

tion

. In

stru

men

ts a

re a

bbre

viat

ed a

s fo

llow

s: S

H i

s a

Spi

ndle

r-H

oyer

, G

is

a G

alit

zin,

WA

is

a lo

ng-p

erio

d W

ood-

And

erso

n, a

nd B

is

a B

enio

ff.

Z,

vert

ical

com

pone

nt;

E,

east

-wes

t; N

, no

rth-

sout

h.

C~

O


In this study we utilized long-period, teleseismic, P waveforms, P-wave first

motions, intensity reports, local tsunami data and relocated aftershocks to investi-

gate source parameters for the 1942 Ecuador and 1942 Peru earthquakes. We have

estimated the focal mechanisms, hypocentral depth, and seismic moment. In

addition, we placed constraints on the rupture length for the two events and

examined the historic earthquake record in the context of the earthquake cycle

along the South American subduction zone.

Data and Methods

Long-period seismograms and instrument response information were obtained

from as many stations as possible operating in 1942. The data came from a variety of seismometers including Benioff, Galitzin, long-period Wood-Anderson and Spin-

dler-Hoyer instruments. Instrument response characteristics as well as general

information for each station are listed in Tables 1 and 2. Instrumental responses

were obtained from station bulletins, MCCOMB and WEST (1931), eARLIER and

VAN GILS (1953), BAKER and LANGSTON (1987) and ESTABROOK et al. (1994). We

were able to digitize six long-period P waveforms for the 1942 Ecuador event from

SEISMIC STATION DISTRIBUTION

80" 240 ~

m

60" .....

30" P A : ~

0 �9

-30"

-60" PERU 8/24/42 Mw 8.1

180 ~ 240"

300" 0 ~ 60" 120 ~ 180 ~

300" O"

Figure 2

60 ~ 120 ~

Map showing the location of stations from which data were used.

60 ~

30 ~

0 o

-30"

-60"

180"


Ifi / t ~ . A f ~ , ~ ~ i A ~

I I 0 120 s

1.0 D B N E

24 A u g u s t 1942

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 version from the seismic station 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,


120 SECONDS I

Figure 5 Lower hemisphere first motion focal mechanism plot for the 24 August 1942 Peru 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.

six stations, and nine long-period P waveforms from seven stations for the 1942 Peru event (Fig. 2).

Sixty to one-hundred twenty seconds of P waveforms from each usable long-period record was digitized (Fig. 3). We used only data with verifiable polarity and a

clear waveform for optimal digitizing results. When possible, the vertical component was used. Often the vertical component was clipped or illegible due to the large size of these events. In this case, the back azimuth to the station was used to determine which of the horizontal components had recorded the majority of seismic energy released and should replace the vertical component in our study. Horizontal receiver factors were determined by comparing horizontal and vertical component

amplitudes or by applying a theoretical correction following the method of BULLEN (1963). The polarity of P-wave first motions from each readable seismogram (whether or not we used the waveform) were used to constrain the focal mechanism

(Figs. 4 and 5). To increase our azimuthal coverage, we have included diffracted P waves. The diffracted waveforms are a smooth version of the nondiffracted wave-

form, with the advantage that their use leads to an underestimate of the seismic moment.

We inverted P waves using a multistation omnilinear inversion ( R U F F , 1989) and a single station inversion ( R U F F and KANAMORI, 1983; BECK and RUFF,

1984). Both methods invert for the source time function and seismic moment using an assumed focal mechanism and depth. The omnilinear inversion simultaneously determines the source time function and trace scaling factors to minimize scatter in


the amplitudes, resulting in a better match between observed and synthetic seismograms (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/cm 3 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.

Table 3

Fault plane solutions for the I942 Ecuador earthquake

Number Strike (q5 ~ Dip (3 ~ Rake (2~

1 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 25 20 115 12 30 20 90 13 30 20 120 14 34 20 120

Fault plane solutions tested for the 1942 Ecuador event. Solution 13 is the same as Solution 1, the only difference being that the multistation results for Solution 13 do not include stations PAS or KEW. The strike of Solution I4 is the strike of the Ecuador coastline near the epicenter.

Vol. 146, 1996 Hi s to r i ca l 1942 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 75

80 SECONDS ]

OTT % J ~ AZ= 5.6o

DIST = 46.2 o

AZ = 38,0 c DIST = 82.2 o

~ ~ ~ KEW.E AZ = 38.3 o DIST = 85.1 o

UCC.E AZ = 39.0 o DIST = 87.8 o

PAS ~ ~ / ~ AZ= 318'9 ~

DIST = 49,0 o

TUC.N ~ @ ~ AZ= 322.5 ~

DIST = 43.0 o

F igu re 6

O b s e r v e d (sol id line) a n d syn the t i c ( d a s h e d line) l o n g - p e r i o d P waves , focal m e c h a n i s m , a n d source t ime

func t ion f r o m the 1942 E c u a d o r mu l t i s t a t i on invers ion us ing o u r p re fe r r ed fau l t p l ane so lu t ion o f str ike,

d ip a n d slip q5 = 30 ~ ~ = 20 ~ a n d 2 = I20 ~ respect ively. S ta t ions codes a re in cap i t a l letters; i n s t r u m e n t

reponses are listed in T a b l e 1.

The multistation inversion method of RUFF (1989) was applied to the initial 80 seconds of six long-period teleseismic P waves from the 1942 Ecuador event to investigate characteristics of the source time function and to constrain the focal mechanism and hypocentral depth. We specified the focal mechanism and depth,

Results of Ecuador, 1942 InverSion: Depth v~ Normaliged Error

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

0.8

~ 0.7

z

0.5

0.5 0 20 40 60

Depth in Kilometer~

F igu re 7

N o r m a l i z e d e r r o r versus d e p t h for the 1942 E c u a d o r mu l t i s t a t i on invers ion us ing o u r p re fe r r ed focal

m e c h a n i s m . E r r o r m i n i m a exist a t 14 kin a n d 40 km.


5 k m

10 k r n

15 k m

2 0 k r n

2 5 k m

A ~ z

V

~ 1 J ~ | I U ' ~ 1 I F

3 0 k m

4 0 k m

5 0 k m

6 0 k m

7 0 k m

t~j tJ,,

I T

0 80 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. Note the increase in ringing of the latter part of the source time function at

depths below approximately 20 kin.

and determined the source time function and the corresponding synthetic seis-

mograms for each station. We have started with the focal mechanism of the nearby

1979 Colombia underthrust ing earthquake, strike, dip, and rake o f q5 = 30 ~ ~ = 20 ~

and 2 = 120 ~ respectively (fault plane solution 1, Table 3). This mechanism is

consistent with the P-wave first motions. We tested 14 different fault plane solutions by varying the shallow dipping plane (listed in Table 3) at each of 31

different depths spanning 1 to 70 km. Our initial focal mechanism provided us with

the best fit between data and synthetics (Fig. 6). However, as expected the data are not very sensitive to small variations in the strike, dip and rake o f the shallow

plane.


In order to determine the best depth we examined the normalized error versus

depth for different focal mechanisms. Most of the mechanisms indicated a fairly

shallow depth, generally <40 kin. Hypocentral depths for such large earthquakes determined from historic data such as the 1942 Ecuador and Peru events can be poorly resolved at best. Figure 7 shows the error versus depth using our preferred

fault plane solution of q~ = 30 ~ ~ = 20 ~ and 2 = 120 ~ There are two error minima,

one for a source depth of 14 kin, and one at 40 kin. We also investigated the

behavior of the source time function with increasing depth as shown in Figure 8.

We find that at depths greater than approximately 20 km, the source time function

begins to exhibit the periodic ringing indicative of a depth overestimation. CHRIS-

TENSEN and RUFF (1985) show that at the best depth, the seismic moment is

concentrated toward the beginning of the deconvolved source time function. In our

case, the shallow depths show the majority of moment release in the initial 24 seconds. Our analysis indicates that the 1942 Ecuador earthquake was a relatively shallow event with the best point source at a depth of approximately 14 km. A

distributed depth between the surface and 30 km gives very similar results.

The source time function for the Ecuador event has one simple pulse of moment

release with a duration of 24 seconds, suggesting that most of the moment release

occurred near the epicenter and on a small part of the total fault area as defined by the aftershocks (Fig. 6). This source time function is characteristic of a single

asperity rupture, although we are not able to spatially locate the moment release on

the fault plane with directivity. Assuming an average rupture velocity of 2.0 km/s,

and from the 24 second duration of the moment pulse we estimate that most of the

seismic moment was released within approximately 50 km of the epicenter. We further investigated characteristics of the source time function, determined

seismic moment release and assigned a moment magnitude for the Ecuador event by using a single station inversion. This procedure inverted the initial 120 seconds of

each seismogram for a source time function. We used stations OTT and UCC to determine the seismic moment because the P waves from these stations are not

nodal and hence, are not sensitive to small variations in the focal mechanism. In

addition, we have reliable instrument response information for these stations. Stations PAS, TUC and BKS are near nodal for this event, and were thus too

sensitive to small changes in the focal mechanism. Stations OTT and UCC indicate a seismic moment of 6 - 8 x 1020 N - m, corresponding to a moment magnitude of 7.8-7.9.

24 August 1942 Peru Earthquake

The 24 August 1942 Peru earthquake has long and complex P waveforms as shown in Figure 5. The stations and instrument information used in this study are

listed in Table 2. All P waves have compressional first-motion arrivals and plot near the center of the focal sphere; hence, it is difficult to constrain the orientation of the


Table 4

Fault plane solutions for the 1942 Peru earthquake

Number Strike (q~o) Dip (6 ~ Rake (fl ~

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

Fault plane solutions tested for the 1942 Peru event. The strike of Solution 11 is the strike of the Peru coastline near the epicenter.

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 30 - 35 k m

�9 KEW.E ~ AZ=35.6

D1ST = 90.1 o ~ KEW.Z ~ DBN.E

AZ = 37,4 DIST = 92.7 o

'~ '~ DBN.Z

UCC,E AZ = 38 .0 DIST = 95.3 ~

@ ~ PAS + ~ AZ = 321.4 DIST = 63.9 o

TUC.N AZ = 325.0

" A1ST = 58.2~

U DIST = 60,3 o

WES.N AZ=4A DIST = 54.2 ~

I I 0 100 s

Figure 9 Observed (solid line) and synthetic (dashed line) long-period P waves, source mechanism, and source time function from the t942 Peru multistation inversion with our preferred fault plane solution of strike, dip and slip q~ = 345 ~ 3 = 25 ~ and ~. = 95 ~ respectively. Stations codes are in capital letters; instrument

responses are listed in Table 2.


0.54

"~ 0.52 .-_.

Results of Peru, 1942 Inversion: Depth vs Normalized Error

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.

We applied the multistation inversion technique to 9 long-period P waveforms from the 1942 Peru event to constrain the focal mechanism, depth, and to investigate source time function characteristics. Due to the complexity and long duration of the P wave, 100 seconds of each record were used. The seismic records

have been divided into three azimuthal groups (three European stations, two from southwestern North America, and two from northeastern North America). Each group was inverted separately to investigate the possibility of directivity resulting from a finite fault rupture. Each group of seismograms was simultaneously inverted at 31 depths between 1 and 70 km for each of 11 different focal mechanisms (Table 4). The focal mechanism which provided the best fit between the data and synthetics for all three groups was fault plane solution number 9 (q5 = 345 ~ 6 = 25 ~ 2 = 95 ~ (Table 4 and Fig. 9). Although this fault plane solution is not well constrained, the data are consistent with an underthrusting mechanism representing the subduction of the Nazca plate beneath the South American plate.

We determined a hypocentral depth of approximately 30-35 km through a combined analysis of normalized error versus depth curves and examination of the behavior of the source time function at increasing depths (Figs. 10 and 11). The depth-error curve for our preferred fault plane solution (r = 345 ~ ~ = 25 ~ 2 = 95 ~ does not show a pronounced minimum (Fig. 10). We are seeing the effects of the trade-off between depth and source time function. We constrain the depth as we did in our analysis of the Ecuador event, using the method of CHRISTENSEN and RUFF (1985), where the best depth concentrates the seismic moment release toward the


OTT, WES.N DBN.E, DBN.Z PAS, TUC.N Depth KEW.E, KEW.Z

UCC.Z

+ + + 10kin

+ # . 30,.

�9 I 0 100 s

Figure 11 Representative sampling of the source time functions calculated at 31 different depths between 1 and 70 km for the three azimuthal groups of 1942 Peru records. Note the trade-off between source time

function and depth.

beginning of the deconvolved source time function. At depths less than 30-35 km, the fit between data and synthetics is satisfactory and the seismic moment release is concentrated toward the beginning. At depths greater than 30-35 kin, we begin to see the effects of the trade-off between source time function and depth: the source time function demonstrates the periodic ringing characteristic of a depth overestimation, and moment becomes distributed evenly throughout the 100 seconds of inverted record (Fig. 11).

We determined the seismic moment and assigned a moment magnitude to the 1942 Peru event by using a single station inversion of 120 seconds of the P waves


from several azimuthal!y distributed stations. This procedure fits each seismogram using a distributed depth between the surface and 40 km. Using our preferred focal

mechanism of ~b = 345 ~ 6 = 25 ~ 2 = 95 ~ our best results came from stations WES, KEW, PAS and TUC, the stations with the most reliable instrument responses. The

deconvolved long-period nondiffracted P waves gave us a seismic moment of 10-25 x 102~ corresponding to a moment magnitude Mw = 8.0-8.2. For

comparison, BRUNE and ENGEN (1969) used Love waves and Rayleigh waves to determine a mantle wave magnitude MM of 7.9 for the 1942 Peru event. KANAMORI

(1977) determined a moment magnitude of 8.2 using a seismic moment of 27 x 1020 N - m obtained from the aftershock area. OICAL (1992) determines mantle

wave magnitudes of 8.15 (Love waves) and 8.38 (Rayleigh waves) from Uppsala

records and an MM of 7.55 (Rayleigh waves) at Pasadena. The complex nature of the 1942 Peru source time function suggests a more

heterogeneous rupture than seen for the source time function of the relatively simple 1942 Ecuador event. The three different azimuths of data show variations in

the source time functions. P waves from stations to the northeast indicate a source

time function with two main pulses of moment release, a small initial pulse followed

by a much larger pulse. P waves from stations to the north show a source time

function with 3 pulses of moment release. Data from the northwest azimuth also

show three distinct pulses of moment release. Although there are differences, we

hesitate to interpret these differences as source effects. Our station distribution and quality of data do not allow us to resolve directivity and hence spatially locate the moment release. The three distinct pulses of moment release with a total duration

of approximately 74 seconds can be interpreted as corresponding to three asperities on the underthrusting fault plane. The largest of these three pulses occurs 32

seconds after rupture initiation. Given the duration of the moment pulse and

assuming an average rupture velocity of 2.0 kin/s, we assert that most of the seismic

moment was released within approximately 150 km of the epicenter.

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

The qualitative comparison of events recorded on similar instruments during the same time period is often a useful tool when analyzing historic earthquake data.

Figure 12 shows a comparison of the P waves from several stations, each having recorded the 1942 Ecuador event, the 1942 Peru event as well as the nearby 1940 Peru event. P waves from the 1942 Ecuador and 1940 Peru events are found to be

relatively simple and of shorter duration than the complex P waves from the 1942 Peru event. We attribute the complexity of 1942 Peru P waves to source effects. BECK

and Ruvv (1989) found that the 1940 Peru earthquake was an underthrusting event with a simple source time function with a duration of 25 sec. Estimates of the seismic moment for the 1940 event range from 2 - 8 x 102o N m, overlapping with the seismic

moment determined for the 1942 Ecuador earthquake (BECK and RUFF, 1989).


Station OTT

Station PAS

Station TUC.N

14 May 1942 Ecuador 24 August 1942 Peru

Az 5.60 Dist 46.2 ~ Dist 60.3 ~

Az 318.9 ~ Az 321.4 ~ Dist 49.0 ~ Dist 63.9 ~

Az 322.5~ Az 325.0 ~ I Dist 43.0 Dist 58.2 ~

120s

14 May 1940 Peru

Az 1.8 ~ Dist 56,40

Az 321.4 ~

t ll Dist 58.90

Az 324.90 Dist 55.3 ~

Figure 12 Comparison of P waves recorded at stations OTT, PAS, and TUC for the 1942 Ecuador, 1942 Peru and

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

The 1942 Peru event has a complicated source time function with a total

durat ion o f 74 s e c o n d s - - o v e r three times the durat ion of the source time functions

for bo th the 1942 Ecuador and 1940 Peru events. The source time functions o f the

1942 Ecuador and 1940 Peru events are very similar, bo th in terms of main pulse durat ion ( ~ 25 sec.) and shape, suggesting similar rupture processes. The 1942 Peru

ear thquake is the largest o f the three events.

Discussion of Historic Earthquakes and Subduction Zone Segmentation

The Colombia-Ecuador and Peru subduction zones have ruptured in a number

of large to great earthquakes during this century. In addition, a 400-year long

written record exists that describes earthquakes that have occurred along both of

these South American zones. The record for the pre-20th century events includes

eyewitness reports, tsunami runup heights and intensity maps. Al though these data are difficult to interpret, we can gain some insight to previous earthquakes. We will

briefly discuss the historic ear thquakes for each zone.

Vol. 146, 1996 83 Historical 1942 Ecuador and Peru Earthquakes

Table 5

Colombia-Ecuador earthquakes

Intensity Tsumani Tsunami Date Type Latitude Longitude (maximum) Source (local; m) (Japan; cm)

15/05/1628 SA 0 . 2 2 ~ 78.5~ VIII A none 06/20/1968 SA 1.80~ 78.8~ X AA none 22/01/1766 SA 0 . 4 4 ~ 77.97~ 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 SA 1.3~ 76.9~ VIII AA none 16/08/1868 SA 0 . 3 1 ~ 78.18~ X AA none 31/01/1906 I 1.0~ 80.0~ IX A large 14/05/1942 I 0.75~ 81.5~ IX A none 01/04/1953 SA 0 . 1 4 ~ 80.66~ ? none 19/01/1958 I 1.14~ 79.59~ IX A none 12/12/t979 I 1.62~ 79.42~ IX PDE tsunami

36 none

none 10 25

Interplate (I) and intraplate events within the South American plate (SA) along the Colombia-Ecuador subduction zone. Sources for maximum intensities: AA (ASKEW and ALGERMJSSEN, 1985); A (ABE, 1981); PDE (Preliminary Determination of Epicenters). Tsunami heights from HATORI (1968). Loca- tions of 20th century events from MENDOZA and DEWEY (1984).

Earthquakes Along the Colombia-Ecuador Subduction Zone

The boundary between the Nacza and South American plates along the coast o f

Co lombia -Ecuador has produced several major ear thquakes during this century

(Table 5, Fig. 1) (MENDOZA and DEWEY, 1984). An ~500 -km long segment o f the

entire plate boundary broke during the Mw = 8.8 1906 event (KANAMORI and

MCNALLY, 1982). Subsequent events in 1942 (Mw = 7.9), 1958 (Mw = 7.7) and

1979 (Mw = 8.2) appear to have reruptured the same port ion o f the interplate

boundary f rom south to north. MENDOZA and DEWEY (1984) relocated and

investigated the distribution o f aftershocks o f the 1942, 1958 and t979 Ecuador

events. They noted that the aftershock zones abut without overlapping, and

interpreted this as an indication that the three events involved progressive strain

release along a single fault system. Indeed, the distribution of hypocenters o f the

1942, 1958 and 1979 sequences are consistent with these shocks occurring along the

same interplate boundary . The events in 1906, 1958 and 1979 will be briefly

described below, followed by a discussion of the 1942 event and a regional

discussion o f the Co lombia -Ecuador subduction zone.

31 January 1906

The 1906 large underthrust ing ear thquake (M w = 8.8) (KANAMORI and MC- NALLY, 1982) generated a far-field tsunami recorded in Ayukawa, Japan of 36 cm

(HATORI, 1968), as well as a large local tsunami. To discredit the possibility that

a) 6o

82~ 8 0 ~ 78 ~ 76 ~


b) I I 1 , 3 t 16~ ] NAZCA ] [ ~ [ ]

I - - - t '~ I I

I I I I

82~ 80 ~ 78 ~ 76 ~

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

t l . NAZCA I I | I= NAZCA I .4 PLATE l ! ~ I ~176 PLATE] 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 I

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

Figure 13 Distribution of intensities reported during the (a) 1906 Ecuador event; (b) 1942 Ecuador event; (c) 1958 Ecuador event; and (d) 1979 Ecuador event. The intensity patterns are crescent shaped. Note that 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.

th i s e v e n t was a l a rge 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 , KELLEHER (1972) c i tes

severe d e s t r u c t i o n w h i c h o c c u r r e d wel l o v e r 100 k m i n l a n d . I n a d d i t i o n , t h e 1906

e v e n t s h o w s a ve ry g r a d u a l o n s e t o f b o d y waves , t yp i ca 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 (KELLEHER, 1972). F i n a l l y , P - w a v e first m o t i o n s f r o m seve ra l s t a t i o n s fo r

t h e 1906 e v e n t a r e c o m p r e s s i o n a l . KANAMORI a n d M C N A L L Y (1982) a r g u e t h a t t h e


82 ~ 80 ~ 78~

1979 1958 1942

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 are from MENDOZA and DEWEY (1984).

1906 earthquake was a thrust-fault event, rather than a great normal-fault earth-

quake near the trench, on the basis of the high intensities inland from the coast,

which have not been observed with great normal-fault earthquakes elsewhere. The S minus P times from five stations were used by KANAMORI and Mc-

NALLY (1982) to locate the epicenter of the event. The loci from three pairs of stations nearly intersect at the previously determined Gutenberg-Richter epicenter

which is near the southwestern end of the rupture zone. KELLEHER (1972) based an estimate of the rupture zone of the 1906 event (500 kin) on macroseismic data that include reports of a diminished water level in the harbors of Manta (3~ and Buenaventura (3~ and a broken submarine cable found near Buenaventura. The seismic moment is estimated to be 2 x 1022N �9 m (KANAMORI, 1977) from

KELLEHER'S (1972) estimate of the rupture zone. The corresponding moment magnitude is in close agreement with ABE'S (1979) tsunami magnitude of M, = 8.7.

Distribution of intensities strongly suggests that the rupture zone did not extend


any appreciable distance beyond the end points specified by KELLENER (Figs. 13 and 14). The aseismic Carnegie Ridge intersects the Colombia trench at approximately 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 January 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~ �9 m, corresponding to a moment magnitude of 7.6, KANAMORI and MCNALLY (1982) estimate the seismic moment of 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 �9 m, 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 aftershocks 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 December 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 approximately 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


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 that 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.

14 May 1942 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 of a large event occurring between 1991 and 2001 at the 66%

Colombia Ecudador Subduction Zone

1900

1800

1700

1600

1979 8.1 IX

1766 IX

Intraplate

5~ 4 ~

1958 7.8 w IX 1942 7.9

A IX v

1906 8.8 1901 X �9 7.8

tsunami 1868 7.7 x @

Intraplate 1797 8.3 XI

Intraplate

1698 7.7

1587 7.7 A 1627 X X ' ~ VIII Intraplate

Intraplate �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 maximum intensities: ASKES and ALGERMISSEN (1985), ABE (1981) and Preliminary Determination of

Epicenters.


level. These factors illustrate the importance of evaluating the 1942 earthquake segment. We use our determined source characteristics as well as relocated aftershocks and intensity reports to examine the 1942 Ecuador event in the context of

regional historic seismicity and the earthquake cycle. The reported location of the maximum intensities (IX) for the 1942 Ecuador

event are south of the main shock epicenter (ASKEW and ALGERMISSEN, 1985; ABE, 1981; Preliminary Determination of Epicenters), suggesting that the majority of seismic moment release occurred to the south (ASKEW and ALGERMISSEN, 1985) (Fig. 13). Many of the relocated aftershocks occurring during a three-month period following the event are located in an area north of the epicenter (MENDOZA and DEWEY, 1984) (Fig. 14). The relationship between aftershocks to the north and highest intensities to the south can be explained as follows: aftershocks commonly occur in areas of low seismic moment release rather than high (SCHWARTZ et al., 1989). If we assume that the single asperity which ruptured during this event lies to the south of the main shock epicenter in the area marked by high moment release and intensities, then we would expect aftershocks to occur to the north.

The aftershock zone of the 1942 Ecuador event (Fig. 14) is approximately 200 km by 90 km (elongated parallel to the trench) and extends to a depth of <30k in (MENDOZA and DEWEY, 1984). The aftershocks lie on a southeasterly

i i M a x i m u m I I

I n t e n s i t y I I 4 -

1958(I 1906(9)

1942(9) I

8 2 ~

. ~_ .1766(9 ) 7 O i

j Colombia , ~ 1 182,7(9)

zg --A- 4 - - - �9 i , o ,

q J . , ~ - ~ 1834(8)

_ . i8 8(10) 1. y ~ -~g87--/1-627(8-9- - ' ' ~ Ecuador I 1 1

1 1 6 9 8 ( 1 0 ) f i "1797(10-11) i J

- r - O - - ~ . . . . ~ - - + r 8 0 ~ 78 ~ 76 ~

Figure 16

0

o

2 ~

o

2~

Distribution of large Colombia-Ecuador events; maximum reported intensities are shown in parentheses. Sources for maximum intensities: ASKEW and ALGERM1SSEN (1985), ABE (1981) and Preliminary Determination of Epicenters. Locations of 20th century events from MENDOZA and DEWEY (1984).


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 moment to average displacement along the fault by the formula 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

rupture occurred within a small area of the fault plane. If we assume most of the

moment release occurred in one dominant asperity with dimensions of 50 km by

50 km (an estimated rupture area based on the duration 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 convergence rate of 8.37 cm/year gives approxi-

mately 3 meters of displacement. Although 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.

I I I NAZCA I I I PLATE I I I

I I I I. . . . . . . I . . . . 4

I ,,/

1 7 9 7

,--[--

1

o3 ? 1 9

6 ~

I

I I _=

AMERICAN I PLATE I

o

~

o

2 ~

8 2 ~ 8 0 ~ 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).


The Colombia-Ecuador segment of the Nazca-South American plate boundary shows two different rupture modes between successive earthquake cycles. We now address the question of whether the 1906-type rupture of the three adjacent but separate earthquakes is a more common occurrence along this portion of the subduction zone. Figure 15 shows a space time plot of the Colombia-Ecuador subduction zone (sources for maximum intensities: ASKEW and ALGERMISSEN, 1985; ABE, 1981; Preliminary Determination of Epicenters). Prior to 1906 there were many large events. Though some interplate events that break the lower interplate interface also might not produce tsunamis, we interpret these events as intraplate (within the South American plate), based on intensity reports and the lack of either local or far-field tsunamis reports (Fig. 16) (sources for maximum intensities: ASKEW and ALGERMISSEN, 1985; ABE, 1981; Preliminary Determination of Epicenters. Locations of 20th century events from MENDOZA and DEWEY, 1984). Intensity patterns for earthquakes which have occurred within the South American plate typically form a bulls-eye around the epicenter as shown for the 1797 event in Figure 17 (sources for maximum intensities from ASKEW and ALGERMISSEN, 1985). In contrast, intensity patterns for earthquakes which occur at the interface between the subducting and overriding plates form a crescent shape along the coast as shown for the 1942 event (Fig. 13). We do not find any candidates for a 1942 or 1906-type rupture along this segment. This suggests that the previous subduction zone underthrusting event occurred at least 200 years prior to 1906. Hence, we cannot determine what type of earthquake sequence is more typical of the Colombia-Ecuador segment. There does not appear to be characteristic earthquakes for this segment.

The boundary between the 1958 and 1979 aftershock zones and the boundary between the 1942 and 1958 aftershock zones lie inland of the intersection of the inactive Malpelo Rift-Yaquina graben system. With this in mind, MENDOZA and DEWEY (1984) suggest that the heterogeneities separating the 1942, 1958 and 1979 fault ruptures may be transient heterogeneities in the subducting Nazca plate resulting from the presence of the extinct spreading center-transform fault system. These heterogeneities were bypassed or breached during the 1906 earthquake.

The previously mentioned series of events which occurred in the last 100 years demonstrate the two different modes of rupture that have occurred along the subduction zone segment off the coast of Colombia-Ecuador. The 1906 great earthquake (Mw = 8.8) ruptured a 500 km segment of the plate boundary (MCNALLY and KANAMORI, 1982). This same thrust fault plate boundary segment subsequently ruptured in three smaller earthquakes from south to north in 1942 (Mw = 7.8), 1958 (Mw = 7.7), and 1979 (Mw = 8.2) (Figs. 13 and 14). The 1942 event is similar to but slightly larger than the adjacent 1958 event, although both earthquakes initiated rupture at the dominant thrust plane asperity. In contrast, the 1979 earthquake had a much longer source duration and initiated rupture approximately 60 km from the dominant asperity. The summed seismic moment from the three post- 1906 events is approximately one-fifth as large as the estimate for the 1906 event (KANAMORI and MCNALLY, 1982).


Historic Earthquakes of the Peru and Northern Chile Subduction Zone

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 al., 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 al., 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 July 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.

24 November 1604

This great underthrusting tsunamigenic earthquake (Mw = 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 of 450 km long.

20 October 1687

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

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intensity IX) and tsunami damage from Lima to Ica. Intensity IX reports reached further south than the 1746 event (discussed below), so it has been postulated that the 1687 event ruptured further south than that of 1746 (BECK and NISHENKO, 1990). To assign a seismic moment and a moment magnitude to the 1687 event BECK and NISHENKO (1990) choose as a reference event the 1974 earthquake, and compared their local and global tsunami effects.

From these tsunami relationships (Table 5) and the known seismic moment of the 1974 reference event, a seismic moment of 1-2 x 1022 N . m was assigned to the 1687 event, corresponding to a moment magnitude of 8.6 8.7. BECK and NISHENKO (1990) suggested that the rupture extent of the 1687 event encompassed and exceeded that of the 1974 event, perhaps extending south to Pisco or Ica (Figs 18, 19 and 20). DORBATH et al. (1990) estimate a rupture length for this event of 175 km. The significance of the 1687 event lies in the observation that the 1687 rupture zone may very well extend through the "seismic gap" existing along the Nazca Ridge-trench intersection (Fig. 1). This in turn would indicate that the intersection of the Nazca Ridge and Peru trench is not a permanent barrier (BEc~: and NISHENKO, 1990).

28 October 1746

The 28 October 1746 underthrushing earthquake (Fig. 18) was the largest well documented event to emerge from the written accounts of the preinstrument era.

Table 6

Central Peru earthquakes

Intensity Tsunami Tsunami Date Latitude Longitude (maximum) (local; m) (Japan; 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 none 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 none 13/08/1868 16~ 71.5~ X 14 2 m 24/05/1940 11.2~ 77.2~ VIII 3 none 24/08/1942 15.2~ 76.0~ IX 3 none 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 12.39~ 77.66~ VIII 1.6 4 20

Large earthquakes of the Peru subduction zone. Source for maximum intensities was ASKEW and ALGERM1SSEN (1985) and SmGADO (1985). Local tsunami heights from DORBATH et al. (1990) and LOCKRID6E (1985). Far-field tsunami heights from HAI'OR~ (1968) and DORBATH et al. (1990).


An intensity magnitude M1 = 9.2 was assigned to this event on the basis of the area encompassed by a maximum intensity of M M =-J( (Fig. 18). The 1746 event generated a maximum local tsunami runup of 24 m near Callao, Peru (LOCKRIDGE, 1985). Given the size of the 1746 event, we would expect an observable far-field tsunami along the coast of Japan but there is no mention of one (WATANABE, 1983). BECK and NISHENKO (1990) chose as their reference event in this case the 1966 event

which caused a maximum local tsunami runup of 2.1 m in Callao (Table 6). Taking the ratio between local tsunami runups and the seismic moment of the 1966 event, a seismic moment of 2.26 x 1022 N - cm can be assigned to the 1746 event, corre-

sponding to a moment magnitude of 8.8-9.5. Based on intensity and tsunami data from the 1746, 1940, 1966 and 1974 events, BECK and NISnENKO (1990) concluded that the 1746 event ruptured the same portion of the coast as did both the 1940 and 1966 events, and a portion of the 1974 segment as well (Fig. 20). DORBATH et al.

(1990) delineate a rupture zone from about 10~ down to 13~ approximately 350 km, and assign a moment magnitude of 8.6 to this event. The sources of both

the 1940 and 1966 events were simple. We can conclude that because the 1746 event was so considerably larger, it was likely a multiple asperity rupture. It is possible that

the three asperities correspond to the later main shock locations of the 1940 event, the 1966 event, and possibly also the 1974 event (BECK and NISHENKO, 1990).

14 August 1868

The underthrusting event in 1868 generated a tsunami runup of ~ 2 m in Japan (HATORI, 1968) and local tsunami of 14 m (DORBATH et al., 1990) from which ABE (1979) calculated a tsunami magnitude of Mt =9.0. DORBATH et al. (1990) determine a rupture zone extent of 15.5~ to 19~ approximately 400 kin, from which they calculate a moment magnitude of 8.8. DORBATtt et al. (1990) also make the observation that the similarity between the 1604 and 1868 earthquakes may be

interpreted as the latter being a repeat of the former.

24 May 1940

One of the first large historic earthquakes along the Peru trench to be studied extensively was the 1940 event (Fig. 1). GUTENBERG and RICHTER (1954) assigned the event a magnitude of 8.0. The ISS preliminary inland location, deeper assumed depth ( ~ 60 km) and small number of aftershocks suggested that the 1940 event occurred within the downgoing Nazca plate (BECK and NISHENKO, 1990). A detailed analysis of the 1940 earthquake by BECK and RUFF (1989) indicates it is a shallow underthrusting event. Hence, the entire plate boundary between 10 ~ and 14~ has failed during this century (Fig. 20) (BECK and NISnENKO, 1990).

The 24 May 1940 event has one dominant pulse of moment release with a total duration of 24 to 30 seconds. This is half of the duration of the adjacent 1966 and

Vol. 146, 1996 His tor ica l 1942 Ecuador and Peru Ea r thquakes 95

SPACE-TIME PLOT FOR THE PERU SUBDUCTION ZONE

1 9 0 0

1800

1700

1600

,x~x~ O ....... --- 1974 1 9 6 6 O - - ~ I=8 I=8 1940 T=2m

T=3m I=8 T=2m

1746 �9 1=10 1828

T=24m I=8 ,dh V ?

? A �9 I W

1878 1655 �9 1687 I=9 I=9 I=9-11

�9 T=Sm

I586 I=9

T=26m

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

I J I Figure 19

Space-t ime plot of the Peru subduc t ion zone. Time is shown on the y axis, dis tance a long the t rench (in

ki lometers) on the x axis. Dots represent the loca t ion and relat ive size of the unde r th rus t ing ear thquake ;

and lines ex tend ing f rom the dots represent es t imated rupture length. The 1970 event is no t shown in this

figure because it is a no rma l faul t ing event ra ther than an under th rus t ing event.

1974 events which occurred north and south of the 1940 event, respectively. A seismic moment of 2 -8 x 102~ was determined for this event (BECK and

RUFF, 1989).

17 October 1966

This underthrusting earthquake triggered a series of aftershocks which span an 80km long segment of the Peru trench (Fig. 1). A local tsunami runup of 2.1 meters was reported in Callao (LocKRIDGE, 1985). Most of the 20 x 102o N . m of

seismic moment that was generated by the 1966 event was released in one main pulse 50 seconds in duration interpreted as an asperity near the epicenter (BECK and RUFF, 1989).

3 October 1974

The most recent underthrusting event included in this study occurred in the seismic gap identified between the 1940 and 1942 events (KELLEHER, 1972) (Fig. 1). The aftershock area is 240 km by 50 km elongate parallel to the trench (DEWEY and SPENCE, 1979). The majority of moment release for this M w = 8.0 event


~ _ ~b =530 1966 ~ A =106 ~ ATU ~\\\~.~-.~s.,~.o._.,-.~

1940 ~ *=37 ~

KEW :' =91~

1974 ~ t ~ 0=54~ ATU ~ A =106 ~

cb =321 o

1942

PAS

I I 0 120 s

8~

10 ~

12 ~

14 ~

16 ~

80 o W

Figure 20

18 o 78 ~ 76 ~ 74 ~

Summary of the source time functions and asperity distributions of the four underthrust ing earthquakes that have occurred along the central Peru subduction zone during the present century. Hachured regions indicate the locations of dominant asperities, and solid contours represent aftershock areas as determined by B~CK and RUFF (1989); stars represent respective epicenters. The aftershock areas for the 1966, 1970 and 1974 earthquakes are from DEWEY and SPENCE (1979). The location of the 1940 and

1942 earthquakes are from DEWEY (unpublished),

occurred in the northwest half of the elongated aftershock zone. The event ruptured bilaterally: 40 km northwest and 60 km southeast of the epicenter. The most important feature of this earthquake is the well-constrained extent of its rupture zone (Fig. 19). It abuts the rupture area of the 1940 event to the north, but leaves a gap between its southern extent and the 1942 event. This signifies the importance of constraining the rupture length of the 1942 event (Fig. 20).

24 August 1942 Peru Earthquake

The 24 August 1942 Peru earthquake occurred at 15.21~ 75.25~ (relocated by DEWEY, personal communication) on the southern flank of the seemingly aseismic Nazca Ridge (Figs. 1 and 20). Although no far-field tsunami was reported in Japan, the event generated a local tsunami of 2.0 m (LOCKRIDOE, 1985). This event is key to our understanding the mode of earthquake rupture along the Peru-Chile trench and whether or not that mode varies temporally. This Peru earthquake has not been the focus of any previous detailed study. As outlined in previous sections, we investigated the source time function, fault plane solution,

Vol. 146, 1996

280"

Historical 1942 Ecuador and Peru Earthquakes

2 8 5 ~

97

-10"

-15"

-10"

-1 5 ~

280" 285"

Figure 21 Map showing the 1942 Peru main shock as located by DEWEY (unpublished; large triangle), KELLEHER (1972; large circle) and ISS (large cross). Also shown are two aftershocks relocated by Dewey (unpublished; small triangles), four aftershocks relocated by KELLEHER (1972; small circles) and three

months of aftershocks reported by ISS (small crosses).

hypocentral depth, and seismic moment release of the 1942 Peru earthquake. We examine this event in the context of regional historic seismicity and the global

tectonic cycle. The reported locations of the maximum intensities (IX) (ASKEW and AL6ER-

MISSEN, 1985) (Fig. 18) for the 1942 Peru event lie near and south of the main shock epicenter. The high concentration of intensity IX reports south of the epicenter suggests at least a portion of the rupture was to the south. However, intensity reports are highly subjective and dependent upon population and building

styles, and must be critically interpreted. Intensity VI, VII and VIII reports are also found north of the epicenter, so the rupture associated with the Peru event may


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 earthquakes 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. It 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


NISHENKO (1990) proposed that rupture zones were segmented by structures on the downgoing plate only during this century. Our study supports the idea that the mode

of earthquake rupture and earthquake size along the coast of Peru has changed between successive great earthquake cycles.

Conclusions

Long-period teleseismic P waves have been analyzed to determine the source characteristics of the 1942 Ecuador and Peru subduction zone earthquakes. For the

1942 Ecuador earthquake our preferred focal mechanism is q5 = 30 ~ 6 = 20 ~ 2 = 120 ~ with a depth of approximately 14 kin. We determined a seismic moment of 6 -8 x 102o N - m, corresponding to a moment magnitude of Mw = 7.8-7.9 for

the Ecuador earthquake. Most of the seismic moment was released in a simple pulse with a duration of 24 seconds. Assuming an average rupture velocity of 2.0 km/s, we estimated a region of maximum moment release within 48 km of the epicenter. While aftershocks are located north of the event, maximum intensities (IX) are concentrated to the south, indicating that most of the rupture was likely south of the main shock epicenter.

The same inversion procedure applied to the 1942 Peru event gives a best fault plane solution of ~b = 345 ~ c5 = 25 ~ 2 = 95 ~ and a hypocentral depth of 30-35 kin.

Seismic moment was complex with three distinct pulses over 74 seconds, the largest pulse occurring 32 seconds after rupture initiation. Total seismic moment release as determined by the nondiffracted P waves was 10-25 x 102~ m, corresponding to

a moment magnitude of Mw = 7.9-8.2. The maximum intensities (IX) for the Peru 1942 event are predominantly to the south of the epicenter, suggesting that at least a portion of the rupture was south of the epicenter. It is unlikely that the rupture extent of the 1942 Peru event abuts that of the 1974 Peru event; thus a large portion, 100 km or more, of the Peru trench has not ruptured in several hundred years.

We have examined large and great historic earthquakes along the Colombia- Ecuador and Peru segments of the South American subduction zone. Rupture processes of earthquakes occurring along these segments are thought to be con- trolled in part by the subduction of seemingly aseismic features on the Nazca plate such as the Carnegie Ridge off the coast of Ecuador and the Nazca Ridge off the coast of Peru. These features act to laterally segment the trench. Twentieth century earthquakes tend not to rupture through these subducting ridges. However, at least one historic earthquake (1687 Peru) has ruptured across the Nazca Ridge, leading us to speculate that the segment defined by the intersection of the Nazca Ridge with the trench is capable of seismic failure and that the segmentation of the plate boundary as defined by earthquakes this century is not constant. The historic earthquake record suggests the mode of earthquake failure and segmentation along the South American trench varies through time.


Acknowledgments

We thank the many station operators that provided us with copies of the historic seismograms. We thank Terry Wallace and Randy Richardson for their thoughtful reviews, and Renata Dmowska and an anonymous reviewer for helpful comments. Special thanks to John Cassidy for providing records from the Canadian Seismic Network. The study was funded by NSF grant EAR-9017358. SASO contribution number 31.

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Vol. 146, 1996 Historical 1942 Ecuador and Peru Earthquakes I01

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(Submitted July 23, 1994, revised/accepted May 8, 1995)

swenson 1996

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