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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 EE 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. MCNALLYABSTRACT

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 d1 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 sb 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 e1 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 dr 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 el 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 ds 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 din 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 aa 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 sa 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 nt 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 tt 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 es 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 ana 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 fa 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 sa 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 sp 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 ea 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 et 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 .

INTRODUCTIONMany 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 ofsmaller earthquakes abutting to each other. One of the best examples is seen for thesubduction zone off the coast of Ecuador-Colombia. A great earthquake occurred in1906 along the coast of Ecuador-Colombia Ms = 8.7, estimated Mw = 8.8) (FigureI). Kelleher (1972) estimated the rupture length to be about 500 km on the basis ofthe macroseismic data. Abe (1979) estimated the tsunami magnitude Mt to be 8.7which 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 largeearthquakes 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, theevidence is strong that this segment of the Ecuador-Colombia subduction zonebehaved 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 olderevents, and the details of the rupture mode are unknown.

The purpose of the present paper is to investigate the nature of this type ofvariable rupture behavior by studying the Ecuador-Colombia sequence for whichinstrumental data are available. 1241

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242 HIROO KANAMORI AND KAREN C. McNALLYT 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 5R 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 et 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 f2 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 tt 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] N82E ~7.96 c m / y r . . / 1979'b.~...1/19 daysRupture . .z~. ~ I day(C) Lengtha n d / ~ _ - ~ / ~ /Direction / / i,~

, , / tKelleher (1972) ~ , ,9oe z . f COLO M al A

, 9 2 - t , J l '( b OuitoECUADOR

0 300 kmq ~ 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 thedirection c) are determined from azimu thal variation o f group arrival times of Rayleigh waves Kana moriand G iven, 1981). Th e m echanism diagram b) is from Kan am ori and Given 1981). Th e lower focalhemisphere is shown. Hatched areas show compressional quadrants. Th e arrow shown in a) indicatesthe convergence vector between the N azca and Sou th A merican plates at th e epicenter of the 1979 eventafter 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 nd 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 hs 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 sb 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 es 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 eN 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 dJ 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 243a 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 resultsin the uncertainty of the estimate of the rupture zone. The directivity methoddeveloped by Ben-Men ahem 1961) is often used for the determina tion of therupture length, but this event is not large enough to bring the directivity spectralholes in the period range with high signal-to-noise ratio. We, therefore, used theazimuthal variation of group arrival times of Rayleigh waves to determine therupture length.

Figure 3 compares band-passed synthetic seismograms computed for the sevenIDA stations used with the band-passed observed records. The band-pass filter iscenter ed at about 270 sec, and the synthetics are c omputed for a point source placedat 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 nsFIG. 2. Focal mechanism stereographic projection of the lower focal sphere) of the 1979 Colombiaearthquake 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 thosepropaga ted in the NE azimuth R3 at HAL and ESK, R4 at TWO). The waves whichpropaga ted in NW and SE azimu ths do not show significant delays CMO andSUR). This pa tte rn of group delays clearly indicates rupt ure propagation in the NEdirection. Also, the observed trains are delayed by 58 sec on the average with respec tto the synthetics comp uted for a point source. For a unilateral fault with the r uptur elength L, the phase delay at a station in the azimuth t? from the ru ptu re direction isgiven 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 yx 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 nb 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)Ua 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 ed 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 ht 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 MatchedC M O ^ i R 2 ~ [R 5

V1164H A L J ~ R 2 } ~ 3

[70E S K ~ R 2 R 5

] 2 A R 5

J , 6 0

W V~ R4

S U RoR 4

II2B D F / ~ R2 ~ R3

0 i O OOsecFIG. 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 byan arrow, and ~g is the delay time of the ob served trace with respect to the synthetics. A~g is the relativedelay time of the first Rayleigh wave train on the observed trace with respect to the second Kanamoriand Given, 1981).

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THE RUPTURE ZONE OF THE ECUADOR-COLOMBIACOAST 1245times is in good agreement with the extent of the aftershock area 1 day after themain shock, but is slightly shorter than that 19 days after the main shock. Thus, inthis 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 DATATable 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 edthe ts un ami mag ni tu de M~ of the 1906 eve nt to be 8.7. Using Ab e s metho d, M, = 8.2is obtained for the 1979 event. Thus, the tsunami data clearly indicate that the 1906event is subs tanti ally larger t ha n the 1979 event. Th e ab sence of repor ts of far-fieldtsunamis for the 1942 and 1958 earthquakes suggests that they are even smallerthan the 1979 event.

TABLEMAXIMUM T SUNAMI HE IGHT H IN ME T E RS

Tide Sta tion 1906 1979Honolulu 0.2 0.04Hilo 3.6 0.40*Hakodate 0.18 0.09Ayukawa 0.22 0.13Kushimoto 0.29 0.10Hosojima 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 EarthquakesP a s a de n a 3 0 - 9 0 R 4 I I cm1979

[958 /195S]_ I8 1 2 i m in ~ - 5 ~ A M w : O 5FIG. 4. Comparison of R4 recorded by a Press-Ewing seismograph at Pasadena between the 1958 and1979 events.

THE 1942 AND 1958 EVENTSThe 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 eventshave approximately the same mechanism, we can estimate the seismic moment ofthe 1958 event f rom the ampli tude ratio of long-period Rayl eigh waves of the 1958event to the 1979 event. As shown in Figure 4, the amplitude ratio is about 1:5.6whic h wou ld give a seismic mo me nt of 5.2 1027 dyn e- cm Mw = 7.7) to the 1958event.

Kelleh er (1972) estimat ed t he aft ersh ock area of the 1958 event by relocatingman y of the aftershocks. T he aftershock area determined by Kelleher (1972) isshown in Figure 1 and listed in Table 2. The empirical relation between theaftershock area and the seismic moment (e.g., Kanamori, 1977) suggests a seismicmo 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 testi 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 Ycm estimated from the Rayleigh-wave amplitude but in any case, this event issignficantly smaller th an the 1979 event.

No long-period seismogram is available for the 1942 event. However, the size ofthe aftershock area determined by Kelleher (1972) suggests that this event is ofabout the same size as the 1958 earthquake. We used Kelleher's aftershock area toestimate the seismic moment which is listed in Table 2. Although this estimate isindirect and is subject to some uncertainty, it is reasonable to conclude that thisevent is also significantly smaller th an the 1979 event.

THE 1906 EVENTThe epicenter of this event was located at IN and 81.5W by Gutenberg and

Richte r (1959). Since the location of the epicenter is critical for the determinationof the rupture direction, we examined the original data used by Gutenberg andRichter which are now available in the form of microfiche (see Goodstein e t a l . ,1980).

T A B L E 2C 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 mS,. (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 lociof constant S - P distance for five stations: Miinich; Baltimore; Tacubaya; Victoria;and GSttingen. The loci for Miinich, Baltimore, and Victoria intersect each othernear (within 200 km) the Gutenberg-Richte r epicenter. The loci from GSttingen andTac 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 southwesternend of the ru pture zone. It is possible that Gutenberg and Richter (1959) determinedtheir epicenter by more or less the same reasoning.

Kelleher (1972) estima ted the ruptu re zone of the 1906 event on the basis ofmacroseismic data that include reports of diminution of water level in the harborsof Manta (59'S) and Buena vent ura (354'N), and a broken submarine cable foundnear 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 ci : ~ ] [

M ~ J n , c h ~ : b OG - 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 rear 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 erup 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 s4 L t

8 0 7 8 7 6 W

19 ~

f J I IF 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 eR 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 na 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 yb 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 Vs 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 ti 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 YAlthough 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 inFigure 6 strongly suggests that the rup ture zone did not extend much further beyondKelleher s end points.

Furthermore, the aseismic Carnegie Ridge intersects the Colombia trench atabou t 0 latitude (Figure 6), and t he rupture propagation probably did not extendsouthwest past this intersection. Concerning the northeast end, the Colombia trenchbends sharply from the NE-SW trend to the N-S trend at about 4N, where thechain of active volcanoes along the coast is interrupted. Probably the 1906 event didnot 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 subjectto some uncertainty. However, the value of the corresponding M~ is in closeagreement with Abe s (1979) tsuna mi magnitude Mr suggesting tha t it is reasonablyaccurate.

G 6 t t i n g e n1 9 3 3 S a n r i k uA:8 .zo :333o . . :

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

t 9 0 6 C o l o m b i a - E c u a d o rZ ~ = 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 e1 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 cm 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 e1906 even t .

Concerning the nature of the 1906 event, a possibility remains th at it was a largenormal-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 whichoccurred well over 100 km inland (see Figure 6). In the case of the 1933 Sanr ikuearthquake, the intensity inland was mostly VI on the JMA (Japan MeteorologicalAgency) scale (equivalent to VI to VII on the Modified Mercalli scale) and no severedestruc tion was caused. Comparison of this observation for the Sanriku ear thquakewith the intensity distribution for the 1906 event shown in Figure 6 does suggestth at the 1906 event is not a trench normal-fault event.

Since this problem is crucial for estimating the seismic recurrence rate in thisregion, we examined some old seismograms to resolve this problem. Unfortunately,we could collect only a few seismograms for this event. However, a Wiechertseismogram recorded at GSttingen, Germany, provides key information. One of thecharacteristic features of the large normal-fault events is a very sharp onset of bodywaves (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 249and 1906 Colombia-Ecuador earthquake recorded by a Wiechert seismograph ver-tical component) at GSttingen. The characteristics of the seismograph are almostidentical for the two events. The record of the Sanriku earthquake shows a verysharp onset followed by a large P-wave train. On the o ther hand, the 1906 eventshows a very gradual onset typical of subduction-zone thrust events. Also, the firstmotion is up for the 1906 event while it is down for the 1933 event. In view of thegeometry of the trench and the location of the station, it is very unlikely that theGSttingen station is located near the node of the r adiation pattern, and the upwardmotion indicates a thrust event.

The N-S-component Weichert seismograph at GSttingen registered a clear G3wave 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 forthe long-period Love wave i.e., 4.3 to 4.4 km/sec). However, no clear long-periodarrival is found at the time corresponding to the G2 wave, as shown by Figure 8. Ifa poin t source is assumed, the amplitude of G2 should be abo ut 3 times larger thanG3. This observation suggests a northeastward rupture propagation which is con-sistent with the location of the epicenter at the southwestern end of the rupturezone.

C o l o m b i o - E c u o d o r , Jon. 21, 1906GSt 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

~ I0 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 tW 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 NEdirection. Comparison of the amplitude of the synthetic seismogram with theobserved gives a seismic mo ment of 8 102s dyne-cm, which is about of theestimate given in Table 2. A most likely cause for this discrepancy is the solidfriction between the stylus and the recording paper of the seismograph. Since thistype of seismograph was designed for recording seismic waves with a period of up to100 sec or so, the effect of friction becomes very serious for very long-period waveswith 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 SVarious 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. McNALLYThe average values of slip are estimated from the seismic momen t and the rupturearea, 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 theuncerta 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 therupture area and the tsunami data. This difference is clearly larger than theuncert 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 thesequence of the 1942, 1958, and 1979 events ruptured approximately the samesegment of the subduction zone as the 1906 event. If the entire segment of theColombia trench from the intersection with Carnegie Ridge to the sharp bend brokein 1906, the rup ture length could have been as large as 600 km. Even if this were thecase, the conclusion that the seismic moment release per unit rupture length islarger for the 1906 event than for the 1942, 1958, and 1979 events would remainunchanged . If this is the general characteristic of the rupture behavior, the am oun tand extent of the coseismic displacement in a great earthquake would be significantlylarger than those for a series of smaller earthquakes which occur along the samesegment abu tting to each other. Sykes and Qui ttmeyer 1981) suggest this kind ofbehavior on the basis of a ruptu re model in which the stress drop increases with therupture length.

l

M I > M 2 M 3 M 4 , D > D i , 1 : 2 , 3 , 4FIG. 9. An asperity model which explains the larger moment per unit rupture length and slip of agreat ear thqua ke event 1) than the smaller events events 2 to 4). Mi and D~ are the seismic mom ent andthe 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 therupture dimension or not cannot be resolved in this study. Here, we attempt toexplain the present results by using a simple asperity model such as the onedescribed by Lay and Kanamo ri 1981) see also Lay e t a l . 1982). In this model, thefault zone is held by a discrete distribution of asperities and the zone between the mis considered weak. The weak zone normally behaves aseismically, but slips abruptlyonly when it is driven by failure of the asperities Figure 9). A recent study by Ruffand Kan amor i 1980) demonstra tes tha t this type of asperity distribution is inferredfrom complexities of body waveforms of large earthquakes. In th e framework of thissimplified model, a small earthquake represents failure of one asperity, and therupture zone is pinned at both ends by adjacen t asperities. Therefore, the effectivewidth and the amount of slip are relatively small. On the other hand, a greatearth quake represen ts failure of more tha n one asperity, and consequently involvesmuch larger width and slip. Although whether this model is mechanically feasible ornot must await further studies, it appears to explain qualitatively many of theobserved features of great earthquakes e.g., complexity of the waveform).In the case of the Colombia-Ecuador sequence studied here, the entire zone maybe 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 1251ones in the southwestern end of the rupture zone. As Kelleher (1972) suggests, the1942 and the 1958 events seem to have rupt ured from the s outhwest to the north eastend of the individual ruptu re zone. The 1979 event rupt ured f rom the north east ernend of the 1958 rupture zone toward northeast. On the basis of the location of themain shock, Kelleher (1972) suggests that the 1906 event ruptured also in thenortheast direction. The asymmetric radiation pattern of G2 and G3 demonstratedby 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 ewere pauses of abou t 20 yr between the successive events. The n the question is whythe trigger pattern changed from sequence to sequence. Using the model by Layand Kan amori (1981), two ex treme cases can be considered.

1. Each asperity has its own characteristic repeat time. Th e asperities normallybehave more or less independently, but occasionally they synchronize. If thecharac ter ist ic repeat times are 36, 52, and 73 yr for the 1942, 1958, and 1979zones, 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 sotha t when one as perity breaks, it tends to trigger the adjacent one even if thelatter is not quite rea dy to fail by itself. Only when th e stress in the adjacentasperity is very much lower than its strength, triggering fails to occur. In thiscase, 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 largeearth quake is repor ted in 1882, but this event seems to be located near the nort hernend of the 1906 event. As far as this catalog indicates, it appears t hat the 1906-typeevent is relatively infrequent.

Wh et he r events similar to t he 1942, 1958, or 1979 events occurred during the t imeperiod covered by Ramirez s catalog is more difficult to determine because thehistorical data which are mainly based on intensity data on land are considered lesscomplete for the events off shore.If the actual situat ion is close to case (1), long-term predict ion of seismic activitycan 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 bemore difficult.

Detailed studies on the waveforms of the events in these rupture zones may beable to de termine t he mechani cal conditions there, which in turn may provide cluesto the behavior of this subductio n zone in the future.

The result that the amount of displacement at a given point along a subductionzone depends on the rupture length has an important bearing on risk analysis.Recently, some attempts have been made to predict the nature of strong groundmotions and tsunamis which would be excited by failure of certain seismic gaps. Inthese 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 theprevious 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 isdemonstrated for the Colombia-Ecuador earthquake sequence, these parameters

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1252 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 Yshould be scaled appropriately for the dimension of the gap which is expected tobreak.In the case of the Tokai, Japan, gap Ishibashi, 1981), the length of the predictedevent is abou t 100 km, while the previous two events in 1854 and 1707 seem to havea much larger about 500 km) rupture length. Although the situation in the Tokaiarea may be different from the Colombia-Ecuador case, some caution should beexercised in using the data on the old events for risk estimate of future events.

A C K N O W L E D G M E N T SW e t h a n k J o h n K e l l e h e r f o r p r o v i d i n g u s w i t h v a l u a b l e c o m m e n t s a n d i n f o r m a t io n o n t h e 1 90 6 e v e n ts .

J i m B r u n e s u g g e s t e d t h a t w e s h o u l d e x a m i n e t h e p o s s i b i l it y t h a t t h e 1 90 6 e v e n t i s a n o r m a l - f a u l t e v e n ts i m i l a r to t h e 1 93 3 S a n r i k u e a r t h q u a k e . W e t h a n k t h e d i r e c t o r s o f s e i s m o g r a p h i c s t a t i o n s a t G S t t i n g e n ,G e r m a n y , a n d U p p s a l a , S w e d e n , f o r k in d l y s e n d i n g u s t h e r e c o r d s o f t h e 1 90 6 C o l o m b i a e a r t h q u a k e .K a t s u y u k i A b e , T o k u t a r o H a t o r i, H a r o l d L o o m i s, a n d J a c k F a n c h e r p r o v i d e d u s w i t h v a l u a b le i nf o r-m a t i o n o n t h e t s u n a m i h e i g h t o f t h e 1 90 6 a n d 1 97 9 e v e n t s . G e o r g e P u r c a r u a n d I n e s C i f u e n t e s s e n t u su s e f u l i n f o r m a t i o n o n t h e 1 97 9 a n d 1 90 6 e v e n t s , r e s p e c t iv e l y . F u m i k o T a j i m a t r a n s l a t e d f o r u s s o m e k e ys e n t e n c e s i n R u d o l p h a n d S z i r t e s (1 91 1). T h i s r e s e a r c h w a s s u p p o r t e d b y t h e U . S . G e o l o g ic a l S u r v e yC o n t r a c t s 1 4- 08 -0 0 01 -1 92 6 5 a n d 1 4 -0 8- 00 01 -1 97 5 5 a n d a l s o b y t h e N a t i o n a l S c i e n c e F o u n d a t i o n G r a n tE A R 7 8 - 0 5 3 5 2 .

R E F E R E N E SA b e , K . { 19 79 ). S i ze o f g r e a t e a r t h q u a k e s o f 1 8 3 7- 1 97 9 i n f e r r e d f r o m t s u n a m i d a t a , J. Geophys. Res. 8 4 ,

1561-1568 .A n d o , M . ( 19 75 ). S o u r c e m e c h a n i s m s a n d t e c t o n i c s i g n i fi c a n ce o f h i s t o r i c a l e a r t h q u a k e s a l o n g t h e N a n k a iT r o u g h , J a p a n , Tectonophysics 27 , 119-140 .

B e n - M e n a h e m , A . ( 19 61 ). R a d i a t i o n o f s e i s m i c s u r f a c e w a v e s f r o m f i n i te m o v i n g s o u r c e s, Bull. Seism.Soc. Am. 5 1 , 4 0 1 - 4 3 5 .G e l l e r, R . J . a n d H . K a n a m o r i { 19 77 ). M a g n i t u d e o f g r e a t s h a l l o w e a r t h q u a k e s f r o m 1 90 4 t o 1 9 52 , Bull.Seism. Soc. Am. 67 , 587-598 .G i l b e r t , F . ( 19 80 ). A n i n t r o d u c t i o n t o l o w - f r e q u e n c y s e i s m o l o g y , i n Physics of the Earth s Interior, A. M.D z i e w o n s k i a n d E . B o s c h i , E d i t o r s , N o r t h - H o l l a n d P u b l i s h i n g C o ., A m s t e r d a m , 4 1 -8 1 .

G i v e n , J . W . a n d H . K a n a m o r i ( 19 80 ). T h e d e p t h e x t e n t o f t h e 1 97 7 S u m b a w a , I n d o n e s i a e a r t h q u a k e( a b s t r a c t ) , EOS 61, 1044.G o o d s t e i n , J . R . , I -: I .- K a n a m o r i, a n d W . H . K . L e e ( 1 98 0) . S e i s m o l o g y m i c r o f i c h e p u b l i c a t i o n s f r o m t h eC a l t e c h A r c h i v e s ( a n n o u n c e m e n t s ) , Bull. Seism. Soc. Am. 70 , 657-658 .

G u t e n b e r g , B . a n d C . F . R i c h t e r ( 1 9 5 9 ) . Seismicity of the Earth, P r i n c e t o n U n i v e r s i t y P r e s s , P r i n c e t o n ,N e w J e r s e y , 3 1 0 p p .H e r d , D . G . , T . L . Y o u d , H . M e y e r , J . L . A r a n g o C . , W . J . P e r s o n , a n d C . M e n d o z a ( 19 81 ). T h e g r e a tT u m a c o , C o l o m b i a e a r t h q u a k e o f 12 D e c e m b e r 1 97 9, Science 2 1 1 i 4 4 1 - 4 4 5 .

I m a m u r a , A . ( 19 28 ). O n t h e s e i sm i c a c t i v i t y o f c e n t r a l J a p a n , Jap. J. Astron. Geophys. 6 , 119-137 .I s h i b a s h i , K . ( 1 9 81 ). S p e c i f i c a t i o n o f a s o o n - t o - o c c u r s e i s m i c f a u l t i n g i n t h e T o k a i d i s t r i c t, C e n t r a l J a p a n ,b a s e d o n s e i s m o l e c t o n i c 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 e o p h y s i c a l U n i o n , W a s h i n g t o n , D . C . , 2 9 7 - 3 3 2 .K a n a m o r i , H . (1 97 1). S e i s m o l o g i c a l e v i d e n c e fo r a l it h o s p h e r i c n o r m a l f a u l t i n g - - T h e S a n r i k u e a r t h q u a k eof 1933, Phys. Earth Planet. Interiors 4 , 289-300 .K a n a m o r i , H . ( 19 77 ). T h e e n e r g y r e l e a s e i n g r e a t e a r t h q u a k e s , J. Geophys. Res. 82 , 2981-2987 .K a n a m o r i , H . a n d J . W . G i v e n ( 19 81 ). U s e o f l o n g - p e r i o d s u rf a c e w a v e s f o r f a s t d e t e r m i n a t i o n o fe a r t h q u a k e s o u r c e p a r a m e t e r s , Phys. Earth Planet. Interiors 27 , 8 -31 .

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L a y , T . a n d H . K a n a m o r i { 19 81 ). A n a s p e r i t y m o d e l o f l a r g e e a r t h q u a k e s e q u e n c e s , i n EarthquakePrediction, 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 e o p h y s i c a l U n i o n , W a s h i n g t o n ,D.C . , 579-592 .L a y , T ., H . K a n a m o r i , a n d L . R u f f (1 98 2) . T h e a s p e r i t y m o d e l a n d t h e n a t u r e o f l a rg e s u b d u c t i o n z o n ee a r t h q u a k e s , Earthquake Prediction Research ( i n p re ss ) .M i n s t e r , J . B . a n d T . H . J o r d a n ( 19 78 ). P r e s e n t - d a y p l a t e m o t i o n s , J. Geophys. Res. 83 , 5331-5354 .

R a m i r e z , J . E . { 1 93 3). E a r t h q u a k e h s t o r y o f C o l o m b i a , Bull. Seism. Soc. Am. 23 , 13 -22 .R o t h ~ , J . P . ( 19 66 ). T h e s e i s m i c i t y o f t h e e a r t h 1 9 5 3 - 19 6 5 , U N E S C O , 3 3 6 p p .

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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 253R 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 gn 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 nG 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.

SEISMOLOGICALLABORATORYDIVISION OF GEOLOGICALAND PLANETARYSCIENCESCALIFOR NIA INSTITU TE OF TECHNOLOGYPASADENA, CALIFORNIA91125CONTRIBUTION 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