1400443077_548__mendoza_1984

8/17/2019 1400443077_548__mendoza_1984

http://slidepdf.com/reader/full/1400443077548mendoza1984 1/17

Bulletin of the SeismologicalSocietyof America, Vol. 74, No. 2, pp. 577-593,April 1984

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

E A R T H Q U A K E S O F 1 942 , 19 58, A N D 1979: I M P L I C A T I O N S F O R

B A R R I E R M O D E L S O F E A R T H Q U A K E R U P T U R E

Y

C A R LO S M E N D O Z A A ND J A M E S W . D E W E Y

ABSTRACT

We have re loca ted ea r thquakes a long the Co lomb ia -Ecuador coas t w i th t he

method o f Jo in t Hyp ocen te r De te rm ina tion i n o rde r t o exam ine the na tu re o f f au lt

he te rogen e i t i es t ha t con t ro l l ed no r thward p ropaga t ion o f p la te -boundary rup tu re

f rom the sou rce reg ion o f t he ea r thquake o f 14 May 1942 to t he sou rce reg ion

o f t he 19 January 1958 ea r thquake and even tua l l y t o t he sou rce reg ion o f t he

ear th qua ke o f 12 De cem ber 1979. The ent i re reg ion o f the 1942 1958 and 1979

shocks had p rev ious l y r up tu red in t he s ing le g rea t ea r thquake o f 31 January

1906 . The re loca ted hypoce n te rs l ie on the sa me p lane to w i th in t he app rox ima te l y

20 -km unce r ta in t y o f the foca l dep ths . A lso the ma in shocks nuc lea ted a t nea rl y

the same d i s tance f r om the Co lomb ia t r ench . These obse rva t ions sugges t t ha t

the he te roge ne i t i es be tw een the 1942 and 1958 rup tu res and be tw een the 1958

and 1979 rup tu res do no t co r respond to a ma jo r d i s to r t ion o f t he downg o ing s lab

but ra ther to e i ther minor d is tor t ions o f the s lab or to reg ions o f h igh f r ic t ion or

low ava i lab le s t ra in en ergy on a cont inuo us fau l t sur face. In part icu lar the

he te roge ne i t y be tw een the 1958 and 1979 rup tu re zones seem s to have been a

h igh -s t reng th ba r r ie r w i th d imens ions much sma l le r t han the d imens ions o f e i t he r

o f t he rup tu re zones . The 1942 and 1958 ea r thquakes had so u rce d imens ions no

la rge r t han the 1979 ma in shock bu t t hey had s t ronge r a f te r shock seque nces

than the 1979 ea rthquake Th is obse rva t ion su gges ts tha t t he a r res t o f ea r thquake

rupture in 1979 le f t the p la te-boundary segment tha t had ruptured in 1906 in a

s ta te o f lower s t ress than i t had been fo l lowing the 1942 and 1958 ear thquakes.

Long- te rm se ism ic i t y i n t he decades p reced ing the 1979 ea r thquake occu r red

mos t l y ou ts ide o r on the bou ndar ies o f t he rup tu re a rea de f ined by the d i s t ri bu ti on

of 1979 a f te rshocks. The in tense a f te rshock ac t iv i ty tha t fo l lowed the 1958 main

shock w i th in t ens o f k i l ome te rs o f t he even tua l 1979 hypoce n te r may co r respond

to a l ong -te rm p recu rso ry se i sm ic sw arm fo r t he 1979 ea r thquake .

INTRODUCTION

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

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

t h a t h a v e p r o d u c e d m a j o r e a r t h q u a k e s m o r e t h a n o n c e d u r in g t h e i n s t r u m e n t a l

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

M s =

8 .7 ) e a r t h q u a k e o f 3 1 J a n u a r y 1 9 0 6 K e l l e h e r, 1 9 7 2) , a n d b r o k e a g a in i n

s e q u e n c e f r o m s o u t h t o n o r t h H e r d e t a l . , 1 9 8 1 ; K a n a m o r i a n d M c N a l l y , 1 9 8 2 ) i n

t h e e a r t h q u a k e s o f 1 4 M a y 1 9 42

M s

= 7 . 9 ), 19 J a n u a r y 1 9 5 8

M s =

7 .8) , and 12

D e c e m b e r 1 9 7 9 M s = 7.7) .

T h e d i f fe r e n t t im e i n te r v a l r e q u i r e d f o r r u p t u r e o f t h e p l a t e - b o u n d a r y s e g m e n t

i n th e 1 9 4 2 -1 9 5 8 - 19 7 9 s e r ie s , a s c o m p a r e d t o t h e s i n g l e 1 9 06 b r e a k , h a s i m p o r t a n t

i m p l i c a t i o n s f o r t h e h y p o t h e s i s t h a t t h e c h a r a c t e r i s t ic s o f l a rg e e a r t h q u a k e r u p t u r e

a r e d e t e r m i n e d b y t h e d i s t r i b u t i o n s o f h e t e r o g e n e i t i e s o n th e f a u l t s u r f a c e e .g .,

H u s s e i n i

e t a l . ,

1 9 7 5 ; D a s a n d A k i , 1 9 7 7 ; M i k u m o a n d M i y a t a k e , 1 9 7 8 ; A k i , 1 9 7 9 ;

L a y a n d K a n a m o r i , 1 98 1; L a y

e t a l . ,

1 9 8 2 ). T h e r e c e n t h i s t o r y o f e a r t h q u a k e s i n th e

C o l o m b i a - E c u a d o r r e g i o n s u g g e st s t h a t w h e n t h e p r o p a g a t i o n o f r u p t u r e a lo n g a

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

577

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57 8 CARLOS MENDOZA AND JAMES W DEWEY

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

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

O u r p r i m a r y d a t a a r e h y p o c e n t e r s o f e a r th q u a k e s f o ll o w in g e a c h o f t h e l a rg e

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

t h e s e h y p o c e n t e r s w i t h r e s p e c t to e a c h o t h e r , w e h a v e r e l o c a t e d t h e s h o c k s u si n g

t h e m e t h o d o f J o i n t H y p o c e n t e r D e t e r m i n a t i o n J H D ) .

LOCATION METHOD

T h e J H D p r o c e d u r e u s e d is m o d i f i e d f r o m D e w e y 1 97 1) a n d h a s b e e n d e s c r ib e d

b y D e w e y a n d S p e n c e 1 9 79 ). F o r e a c h t i m e p e r i o d a n d g e o g r a p h i c r e g i o n c o n s i d e r e d ,

w e u se t h e m e t h o d o f J H D o n a g r o u p o f a m a x i m u m o f 1 5 w e l l - re c o r d e d e a r t h q u a k e s

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

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

a d j u s t m e n t s a n d w e i g h t s a re t h e n u s e d i n a s i n g l e - ev e n t l o c a ti o n m e t h o d t o

d e t e r m i n e a ll h y p o c e n t e r s w i t h i n t h e t i m e p e r i o d a n d g e o g ra p h ic r e g io n b e in g

s tu d i ed . T h e p r e c i si o n o f e a c h r e d e t e r m i n e d h y p o c e n t e r r e la t iv e t o t h e c a l ib r a t io n

e v e n t is e s t i m a t e d b y c o m p u t i n g 9 0 p e r c e n t c o n f i d e n c e e r r o r e ll ip s e s f o r e a c h p a i r

o f h y p o c e n t r a l c o o r d i n a t e s e .g ., F l i n n , 1 9 65 ; E v e r n d e n , 1 9 69 ). F o r t h e p u r p o s e s o f

t h i s s t u d y , w e d e f i n e q u a l i t y A h y p o c e n t e r s t o b e t h o s e f o r w h i c h a l l t h r e e e r r o r

e l li p se s h a v e s e m i - a x e s s m a l l e r t h a n 2 0 k i n . Q u a l i t y B h y p o c e n t e r s a r e th o s e w h o s e

e p i c e n t e r el li p se s h a v e s e m i - a x e s s m a l le r t h a n 2 0 k m , b u t w h o s e d e p t h - l a t i tu d e o r

d e p t h - l o n g i t u d e e r r o r e l l i p s e s e m i - a x e s a r e g r e a t e r t h a n 2 0 k m . W e a s s i g n q u a l i t y

C t o t h o s e h y p o c e n t e r s w h o s e e p i c e n t e r e ll ip s e s e m i - a x e s a r e b e t w e e n 2 0 a n d 5 0

k m . E a r t h q u a k e s w i t h e p i c e n t e r e ll ip s e se m i - a x e s g r e a t e r t h a n 5 0 k m h a v e b e e n

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

q u a l i ty A h y p o c e n t e r s ; th e s e d e p t h s a r e i n c l u d e d i n t h e t a b u l a t e d l is t in g o f o u r

r e l o c a t e d s e i s m i c i t y d a t a T a b l e 1 ) . W e h a v e a ls o in c l u d e d in T a b l e 1 o u r d e p t h

e s t i m a t e s f o r t h e q u a l i t y B 1 9 42 a n d 1 94 8 m a i n s h o c k s .

O u r r e l o c a t e d e a r t h q u a k e e p i c e n t e r s a re s h o w n i n F i g u r e 1 . I n o r d e r t o m i n i m i z e

l o c a ti o n b i as e s a r i s in g f r o m t r a v e l - t im e v a r i a t i o n s o v e r t h e l e n g t h o f t h e s t u d y

r e g io n , a n d t o e n s u r e t h a t t h e g r o u ps o f e a r t h q u a k e s b e i n g l o c a te d h a v e s i m i la r

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

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

a l l s h o c k s o c c u r r i n g b e t w e e n 1 4 M a y 1 94 2 a n d 1 9 A p r i l 1 95 8. E a r t h q u a k e s n o r t h o f

l a t i t u d e I ° N a n d o c c u r r i n g a f t e r 1 9 A p r i l 19 58 i.e . a f t e r 3 m o n t h s f o l l o w i n g t h e

1 9 5 8 m a i n s h o c k ) c o n s t i t u t e a s e c o n d g r o u p . T h e t h i r d g r o u p i n c l u d e s s h o c k s f r o m

t h e f i r s t tw o g r o u p s a n d c o n s is t s o f t h e 1 95 8 e a r t h q u a k e s o f F e b r u a r y 1 a n d 2 a n d

t h e D e c e m b e r 1 9 79 m a i n s h o c k ; t h i s g r o u p w a s d e f i n e d s p e c if i c al ly t o f o c u s o n

s e i sm i c i ty in t h e i m m e d i a t e v i c in i t y o f t h e 1 97 9 m a i n s h o c k h y p o c e n t e r .

L o c a t i o n s o f th e g r o up s w i t h r e s p e c t to e a c h o t h e r a r e c o n s t r a i n e d b y a l lo w i n g at

l e a s t o n e w e l l - r ec o r d e d e a r t h q u a k e t o b e c o m m o n t o t w o a d j a c e n t g r o u p s a n d b y

r e q u ir in g t h a t t h e h y p o c e n t e r s o f t h e c o m m o n e a r t h q u a k e s b e th e s a m e i n t h e J H D

c o m p u t a t i o n s f o r t h e t w o g r o u p s . T h e a b s o l u t e p o s i t i o n s o f a ll th r e e g r o u p s a r e

d e t e r m i n e d b y fi x i n g o n e e a r t h q u a k e 0 0:1 0:2 6 U T C , 1 0 A u g u s t 19 76 , m b = 5 .5 ) to

a h y p o c e n t e r d e t e r m i n e d w i t h t h e s i n g le - e v e n t l o c a ti o n m e t h o d . W e c h o s e t h e 1 0

A u g u s t 1 97 6 e a r t h q u a k e a s a c a li b r a t i o n e v e n t b e c a u s e t h e s h o c k w a s w i d e l y

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

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

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

a m o u n t o f m i s l o c a ti o n p r e s e n t i n t h e e p i c e n t e r o f t h e c a l ib r a t io n e v e n t . H o w e v e r ,

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

579

T A B L E 1

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

MAY 1942 T O F E B R U A R Y 1980)

Day

Epicenter Magnitudet

Time

h:m:s) Latitude Longitude Depth* km)

°N) °W)

mb sx

Other

Quality

1942

14 May

15 May

16 May

17 May

7 June

16 June

1 July

2 July

3 July

4 July

5 July

7 July

8 July

10 July

12 July

29 July

9 Nov.

12 Nov.

14 Nov.

19 Nov.

15 Dec.

16 Dec.

02:13:27.6 0.01 80.39

02:54:23.9 -0 .1 7 80.57

08:38:52.5 0.14 80.82

15:46:49.6 0.40 80.50

10:50:45.1 0.65 80.12

11:51:28.4 0.69 80.10

18:12:13.1 0.89 80.00

05:28:45.6 -0 .2 7 81.14

18:58:07.9 -0 .1 7 81.02

19:31:28.8 -0 .1 1 80.98

15:14:24.6 0.56 80.45

10:48:14.2 0.78 80.76

07:43:00.0 0.89 80.65

21:05:19.6 0.75 80.74

21:33:47.0 0.88 80.58

07:52:06.2 0.77 80.66

23:46:28.3 0.51 80.93

00:40:41.4 1.18 80.46

01:53:13.2 0.78 80.58

04:59:36.7 0.90 80.53

06:08:39.8 0.82 80.52

10:29:56.0 0.96 80.55

14:11:19.2 1.20 80.31

12:37:49.9 0.94 80.35

22:30:59.9 0.78 80.48

04:50:01.9 0.85 80.42

05:05:23.5 -0 .0 1 80.07

21:18:53.4 0.44 80.73

21:55:35.4 0.12 80.82

15:26:17.3 0.17 81.01

18:00:06.5 -0 .1 5 81.30

22:30:11.8 0.04 81.14

06:19:05.0 0.43 80.80

17:58:52.4 0.02 81.24

20:56:58.4 -0 .0 6 81.30

08:51:56.9 -0 .1 9 81.72

09:09:01.3 -0 .6 7 81.82

14:39:57.9 0.29 81.07

16:33:00.1 -0 .4 9 81.65

09:09:02.6 0.02 81.28

02:44:46.9 0.17 81.02

19.7 6.0

5.7

5.4

5.9

20.4 5.6 5.9

23.9 5.4 6.2

5.4

5.7

5.5

5.6

25.1 5.4 6.4

5.6

16.9 5.7 6.0

13.7 6.0 6.3

5.2

5.2

5.4

5.3

20.8 5.6 6.2

5.5

6.3 5.6 6.2

7.0 5.6 6.1

5.3

5.7 6.1

5.8 6.3

5.4

29.2 5.8 6.6

7.9

s

B

C

C

C

A

A

C

C

C

C

A

C

A

A

C

C

C

C

A

C

A

A

C

C

B

C

A

C

C

C

C

C

C

C

C

C

C

C

C

C

C

1943

16 Mar.

15 Apr.

28 Apr.

22 July

24 Oct.

09:47:31.3 0.36 81.16

09:51:51.3 0.39 80.63

18:47:21.0 0.89 80.63

14:33:32.3 -1 .4 4 82.88

17:23:47.5 -0. 78 81.67

02:09:28.5 -0 .1 4 81.25

00:27:50.9 1.03 79.85

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58

C A R LO S M E N D O Z A A N D J A M E S W D E W E Y

TABLE

1 Continued

Time

Day

h:m:s)

Epicenter

Latitude Longitude

( N) (°W)

Depth* Ikm)

Magni tude t

mb Ms Other

Quality

1945

2 Feb. 21:37:51.6 -0 .4 5 81.05

23:27:57.7 -0 .2 5 80,85

12 Apr. 00:21:14.8 2.76 79.90

1950

C

C

C

28 May 05:06:35.5 -0 .3 9 81.19

B

1952

18 Jan. 22:53:27.4 3.84 77,78 C

1953

1 Apr. 10:49:51.9 -0 .1 4 80,66

11:21:27.3 -0 .0 4 80.38

B

C

1954

3 Apr . 07:59:54.8 2.70 79.47

1956

16 Jan . 23:37:43.5 -0 .6 8 80.40

9 Mar . 17:31:19.3 0.54 80.01

31 Mar. 08:19:24.3 2.07 78.41

21.7

7.3PAS

A

C

C

1958

19 Jan.

1 Feb.

2 Feb.

21 Feb.

3 Apr.

14 Apr.

15 Apr.

1961

29 Sept.

14:07:26.5 1.14 79.59 29.8

14:43:31.4 1.37 79.61 32.7

16:10:20.2 1.55 79.39 36.8

18:02:45.2 1.58 79.25 33.7

20:45:51.6 1.61 79.27 26.8

02:35:06.3 1.59 79.37 30.5

08:16:36.0 1.59 79.00

08:49:21.4 1.59 79.34 30.1

13:47:15.7 1.71 79.97 23.9

08:25:52.2 1.47 79.56 33.1

21:32:36.0 1.02 79.62 26.9

22:48:39.6 1.14 79.68 24.4

01:30:50.7 0.92 79.66 27.9

22:38:06.1 1.73 79.41

6.2 7.8 Ms

5.9 6.7 PAS

6.0 6.4 6.9 PAS

5.5 6.5

5.6 6.3 6.7 PAS

5.2 6.0

6.1

5.4 6.0

5.4

5.8

6.2 6.6 6.8 PA S

5.6 6.1 6.6 PAS

5.8

B

A

A

A

A

A

C

A

A

A

A

A

A

1964

26 July

18 Oct.

13:55:39.3 2.55 78.47 38.4 5.3

02:49:14.9 2.79 78.32 22.1 4.8

A

A

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S E I S M I C I T Y N D

THREE COLOMBIA-ECUADOR EARTHQUAKES

TABLE

1 Continued

581

Day

Epicenter Magnitude ~

Time

(h:m:s) Latitude Longitude Depth* (kin)

°N) °W)

mb stl Other

Quality

1965

7 Mar. 06:33:19.5 3.33 79.11 25.8 5.0

4 Apr. 05:53:46.0 1.44 79.85 4.8

1966

10 Oct. 14:28:28.9 2.91 78.83 20.5 4.8

1971

6 Sept. 01:09:53.2 2.45 78.93 22.4 5.0

9 Sept. 03:57:52.0 2.30 79.06 28.5 4.8

1976

11 Apr . 19:13:45.2 1.54 80.47 5.0

10 Aug. 00:10:29.5 2.23 78.92 32.2 5.5

23 Aug. 15:10:51.9 1.99 80.04 4.9

5.6

Ms

1977

24 Mar. 07:27:19.4 3.18 78.49 20.8 5.2

1978

3 Apr. 03:06:46.6 3.39 78.98 13.4 4.8

4.1

Ms A

1979

12 Dec. 07:59:07.2 1.62 79.42 37.5 6.4

08:14:58.9 2.92 78.97 5.5

08:32:00.8 2.03 79.07 5.3

08:33:51.4 2.90 78.71 5.7

11:53:52.2 2.47 79.30 34.4 5.2

12:03:47.2 1.61 79.46 25.4 5.0

14:59:03.4 2.86 79.00 4.8

17:49:07.9 3.75 78.31 30.7 5.0

18:36:27.1 2.14 79.68 12.0 4.8

20:08:59.3 2.47 79.44 13.2 4.9

13 Dec. 01:46:35.8 2.59 79.14 19.0 4.8

02:01:35.9 3.16 79.48 7.3 4.8

04:46:46.9 2.11 79.18 22.5 5.0

05:37:48.2 2.70 79.41 5.9 5.0

13:16:52.7 3.02 79.44 8.5 4.9

21:34:49.8 2.64 79.26 4.8

22:01:11.5 2.30 79.32 4.8

22:40:30.5 2.01 79.60 11.7 5.0

14 Dec. 06:14:13.6 2.57 79.08 21.5 4.9

06:16:06.4 2.55 79.09 18.5 5.2

15 Dec . 15:23:31.9 3.09 79.34 8.9 4.9

20:01:52.4 2.98 78.93 24.3 5.0

20:26:23.6 2.78 79.41 18.2 5.0

20:37:19.1 2.83 79.42 4.8

5.8

5.3

5.2

5.1

5.2

7.7Ms A

C

C

C

A

A

C

A

A

4.6 Ms A

A

A

A

5.8

Ms A

A

B

B

A

A

A

A

A

4.6

Ms A

C

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58

CARLOS MENDOZA AND JAMES W. DEWEY

TABLE 1--Continued

Epicenter Magnitude+

Time

Day h:m:s) Lat itud e Lo ngit ud e Depth* kin) Quality

°N) °W) mb Ms~

Other

19 Dec. 22:49:41.7 2.59 79.47 19.5 5.0 A

21 Dec. 22:57:50.8 2.42 79.26 33.1 4.8 A

23:02:17.3 3.25 77.93 31.6 4.9 A

23 Dec. 08:47:20.6 3.55 78.41 12.2 5.0 A

24 Dec. 12:46:07.4 2.30 79.07 31.8 5.1 5.4 5.0

Ms A

25 Dec. 14:55:59.4 2.16 79.49 17.7 4.8 A

28 Dec. 15:07:02.4 3.71 78.33 25.9 4.8 A

29 Dec. 22:08:04.8 2.24 79.33 17.4 5.0 A

30 Dec. 16:12:23.7 3.06 79.53 13.9 4.8 A

31 Dec. 11:58:40.3 3.67 78.30 15.4 5.1 A

23:07:25.4 2.12 79.02 29.5 5.3 5.5 5.5 Ms A

1980

2 Jan. 19:04:03.8 2.85 79.72 13.8 4.9 A

7 Ja n. 00:33:38.8 3.00 78.74 21.3 5.0 4.6 Ms A

26 Ja n. 15:27:17.1 2.36 79.38 25.5 5.0 5.3 Ms A

13 Feb. 11:13:12.7 3.50 77.96 18.7 4.8 4.0 Ms A

*

F o c a l d e p t h s a r e g i v e n o n l y f o r m a i n s h o c k s a n d f o r q u a l i t y A e v e n t s . E r r o r el l i p s e s e m i - a x e s f o r

f o c al d e p t h v e r s u s e p i ce n t r a l c o o r d in a t e s o f t h e 1 9 4 2 a n d 1 9 58 m a i n s h o c k s w e r e l e s s t h a n 2 5 k m b u t

l ar g er t h a n t h e 2 0 - k m t h r e s h o l d u s e d t o d e f i n e q u a l i t y

A.

After 1963,

m b v a l u e s a r e t h o s e a s s i g n e d b y N E I S . W e u s e d T U O r e c or d s t o c a lc u l a t e m b a n d

MsH

f o r e a r t h q u a k e s o c c u r ri n g w i t h i n 3 m o n t h s o f t h e 1 9 4 2 a n d 1 9 5 8 m a i n s h o c k s . F o r t h e 1 9 7 9 s e q u e n c e ,

MSH

v a l u e s w e re c a lc u l a t e d f r om t h e W W S S N r e co r d s a t T U C . O t h e r v a lu e s a r e P a s a d e n a

PAS)

m a g n i t u d e s a n d N E I S s u r f a c e - w a v e m a g n i t u d e s

Ms).

because our interpretat ions are based on the relat ive epicenter locat ions , the

pr inc ipa l conc lus ions o f th is s tudy are not a f f ected by such b ias.

W e read arriva l t imes o f depth phases for many earthquakes to he lp determine

foca l depth . We interpreted these phases a s

pw

phase s , i .e ., P wave s ref lected at

the a ir-water interface M endiguren, 1971; H on g and Fuj ita , 1981; Forsyth, 1982).

For the w ater and source depths cons idered here , mis ident i f ica t ion

o f p

phases as

pw would resu l t in the com puted foca l depths be ing sys temat ica l ly too sha l low by

as mu ch as 13 km, wi th the d iscrepancy be ing grea tes t for hypocenters beneath the

deepes t part o f the near- trench region . M is ident i f ica t ion o f

s

phases a s

pw

w o u ld

resu l t in the computed foca l depths be ing as much as 10 km too deep , w i th the

discrepancy be ing grea tes t for hypocenters beneath the sha l low cont inenta l she l f.

Depth phases for earthquakes occurring af ter 1963 were read from short-period

records o f se lec ted s ta t ions o f the W orldwide Standard Se ismograph Ne twork

W W SS N) and f rom short -per iod signa ls t e lemetered f rom U.S . s ta t ions to Nat iona l

Earthqu ak e Inform ation Service NE IS) develocorders. De pth pha ses for earth-

quakes occurring prior to 1963 w ere read from short-period records o f the pre-

W W S S N s t a ti o n a t T u c s o n T U O ) .

M ost o f the depths tha t we could es t imate to a prec is ion o f bet ter than 20 km

were determined in part by depth-phase data . T he main shock o f 12 December 1979

is an except ion; we could not conf ident ly ident i fy depth phases in the complex P-

wave t ra in o f th is shock , b ut da ta f rom reg iona l s ta t ions perm it ted the determinat ion

of a foca l depth with a prec is ion o f about 19 km us ing on ly P-wave arr iva l - t ime

data . Becau se o f the lack o f unambiguous depth phases and arr ival t imes f rom

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SEISMICITY AND T HR EE COLOMBIA-ECUADOREARTHQUAKES 583

r e g i o n a l s t a t i o n s , t h e c o m p u t e d d e p t h s o f t h e 1 9 4 2 a n d 1 9 5 8 m a i n s h o c k s a r e o n l y

e s t i m a t e d t o b e p r e c i s e to w i t h i n 2 5 k m .

I n t h i s s t u d y , w e d e f i n e a f t e r s h o c k z o n e s o n t h e b a s i s o f e a r t h q u a k e o c c u r r e n c e

w i t h i n 3 m o n t h s o f t h e c o r r e s p o n d i n g m a i n s h o c k . F o r t h e p e r i o d 1 9 6 4 t o 1 9 8 0, w e

r e l o c a t e d e a r t h q u a k e s w i t h b o d y - w a v e m a g n i t u d e s m b o f 4 .8 o r g r e at e r. A m a g n i t u d e -

f r e q u e n c y a n a l y s i s o f t h e 1 9 7 9 a f t e r s h o c k s e q u e n c e i n d i c a t e d i n c o m p l e t e d e t e c t i o n

f o r e a r t h q u a k e s b e l o w 4 .8 m b N E I S ) . F o r t h e p e r i o d 1 9 4 2 t o 1 9 6 3 , w e r e l o c a t e d

s h o c k s f o r w h i c h a t l e a s t 1 0 P - o r S - w a v e a r r iv a l s w e r e a v a i l a b l e t o u s. T h e a r r iv a l -

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

82w 8 f 8d

9 g

~ 1 _ ; 3 + - d

.

f

' ~' J ', / ~/ < ~ A

' ' - i :; -. ., ,, v,

...- . -- . A . ?

\

[ ]

; i

l k m

D D . . . . ~ , : ~ , . . . . . . . . . . .

[ ]

E c u a d o r

F IG . 1 . E p i c e n t e r s o f 1 9 42 t o 1 9 8 0 C o l o m b i a - E c u a d o r e a r t h q u a k e s r e l o c a t e d i n t h i s s t u d y . T h e a r r o w s

i n d i c a t e t h e m a i n s h o c k s o f e a c h o f t h e 1 9 4 2 , 19 5 8, a n d 1 9 7 9 e a r t h q u a k e s e q u e n c e s . T h e b o x i n t h e i n s e t

m a p d e n o t e s t h e 5 ° b y 5 ° a r e a o f s t u d y . P o s t - 1 9 5 8 e a r t h q u a k e s o c c u r r i n g s o u t h o f t h e d a s h e d l i n e

l a t i t u d e 1 ° N ) w e r e n o t r e l o c a t e d . L a r g e s y m b o l s d e n o t e e a r t h q u a k e s o f m b => 5 .5 , a n d f i l l e d s y m b o l s

c o r r e s p o n d to e p i c e n t e r s o f q u a l it y A o r B . B a t h y m e t r i c c o n t o u r s i n k i l o m e t e r s a r e f r o m L o n s d a l e a n d

K l i t g o r d 1 9 7 8 ) .

l og i ca l b u l l e t i n s e .g ., t h e E a r t h q u a k e D a t a R e p o r t s o f t h e U . S . G e o lo g i c al S u r v e y ,

t h e I n t e r n a t i o n a l S e is m o l o g ic a l S u m m a r y , a n d b u l l e ti n s o f t h e B u r e a u C e n t r a l

I n t e r n a t i o n a l d e S e i s m o l o g i e ) .

M A G N I T U D E D E T E R M I N A T IO N S

W e c o m p u t e d s h o r t - p e r i o d T ~ 1 s e c) m b a n d i n t e r m e d i a t e p e r i o d T - 4 t o 12

sec MS G u t e n b e r g , 1 9 45 ) m a g n i t u d e s f o r s h o c k s o f t h e 1 9 42 a n d 1 9 5 8 s e q u e n c e s

u s i n g re c o r d s f r o m t h e p r e - W W S S N T u c s o n s t a t io n , T U O . F o r la rg e s h o c k s o f t h e

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

5 8 7

graben Figure 4) system . Altern atively, the 1942 and 1979 near -trench activity may

be due to thrust faulting in the sedimentary wedge above the interface thrust zone,

such as that postu lated by Fukao 1979), rather tha n being aftershock activity on

or near the main shock thrust plane.

T H E B A R R I E R A T T H E B O U N D A R Y B E T W E E N T H E 1 9 5 8 A N D 1 9 7 9

E A R T H Q U A K E R U P T U R E S

The location of the 1979 main shock hypocenter at the boundary between the

1958 and 1979 aftershock zones suggests that the 1979 earthquake began with the

f

C , , ' i

)) X..,.' , . i /

/

7. J -t't, Ji '.

J j i

C~

: ,

it

~< ; I 0

8 7 9° 7 8 ° 7 7

i t 3 .' t.. ~ I

,:~,

l

COLOMBI

4-, \ +

- p

l O O k m

1

+

E C U A D O R

FIG. 4.

Epicenters of earthquakes occurring

w ,~n m •

mon'fns o~ ~ne'l ~ ,

~I~dbB,

and 1979 main shocks.

Half-f i l led symbols are shocks occurring within 3 days of each main shock (f i l led symbol) . Large symbols

represent earthquakes of

mb --> 5.5.

The dashed line traces the strike (N 31 ° E) of the 1979 main shock

fault plane (Kanamori and Given, 1981). The dotted lines are boundaries of the aftershock zones drawn

so as to best separate the aftershock zones from the regions of prior seismicity (Figure 7) . Bathymetric

contours in kilometers are from Lonsdale and Klitgord (1978).

failure of a barrier that had earlier been responsible for arresting the 1958 rupture.

Th e follow ing characteristics of the seism icity of the 1958-1 979 boundary region

help define the nature of the barrier.

1. The region was the site of intense aftershock activity following the 1958

earthquake Figure 5). This im plies tha t the northern terminus of the 1958

rupture was highly stressed following the main shock. Therefore, the 1958

rupture was probably arrested by a region of high strength and did not stop as

a result of encountering a weak portion of the fault that is incapable o f storing

elastic stress.

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588

2.

.

.

CARLOS MENDOZA AND JAMES W. DEWEY

T h e h y p o cen t e r s o f t h e n o r th e r n m o s t 1 95 8 a f t e r sh o ck s an d t h e 1 97 9 m a in

shock occur red wi th in a vo lume or a rea wi th d imens ions o f t ens o f k i lometer s

Figure 5 ) . Th is sugges t s tha t the bar r ie r had smal l d imens ions r e la t ive to the

dim ensio ns >100 km) of the 1958 and 1979 source regions.

Only one 1979 af te rsho ck w ith mb -> 4 .8 occu rred within 45 km of the 1979

main shock hypoc en ter F igure 4 ). Th is sugges t s tha t f a i lu re o f the bar r ie r a t

the 1979 main shock hypocen ter was essen t ia l ly comple te .

P-w ave f i r s t mo t ions o f the no r thern 1958 af te r shocks are cons is ten t wi th

s l ippage a long the same fau l t su r face tha t p roduced the 1979 main shock

Figure 3B) . Thi s suggests tha t the ba rr ier was ei ther a) a zone of high f r ict ion

on the sam e fau l t su r face tha t p rodu ced bo th the 1958 and 1979 shocks , o r b )

an o f f se t be tw een two fau l t p lanes o f s imi lar o r ien ta t ion . The f ac t tha t the

1958 af te r shock and 1979 main shock hypocen ter s a re no t s t r ic t ly cop lanar

1.6

1 4 °

v9 8°w v9

6 °

79 4 v9

z

,~o /' ..- ' 1958 AFTERSHOCKS

:gxc:o

~ \ ~ ~ thers

FIG. 5. Relocated epicenters of the 1979 main shock and nor ther n quality A aftershocks of the 1958

sequence. Epicenter ellipses denote th e est imat ed location error relative to the cal ibration shock at a 90

per cen t level of confidence. Bathym etri c contour of 1000 m is from Lonsdale and Klitgord 1978).

Figure 2C) may no t be s ign i f ican t in l igh t o f the e s t ima ted unc er ta in t ies o f

the foca l dep ths .

5 . Th e 1958 af te r shock ac t iv i ty in the v ic in i ty o f the 1979 main shock hypo cen te r

occur red in a per iod o f 20 h r tha t began near ly 2 weeks af te r the 1958 main

shock Tab le 1 ). The r eg ion o f the bar r ie r then bec ame qu iescen t , p roducing

only one o ther t e lese i smica l ly r ecorded shock 29 Sep tem ber 1961) p r io r to the

1979 ear thquake . The conc en t ra t ion o f ac t iv i ty in a nar row t ime in terva l

wi th in the tec ton ic s t r ess cyc le ma y ind ica te a f ami ly o f subfau l t s o f near ly

iden t ica l s t r eng ths wi th in the bar r ie r Kanam or i , 1981; Lay

et al.

1982).

W e em phas ize tha t the p reced ing observa t ions per ta in to the r eg ion o f the 1979

main shock hypocen ter , i . e . , where the 1979 rup ture began , no t to the r eg ion f rom

which mos t o f the se i smic mom ent o r ig ina ted . Bec k and Ruf f 1983) conclude from

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SEISMICITY AND THREE COLOMBIA-ECUADOR EARTHQUAKES

89

a study of long-period waveforms that the dominant asperity in the 1979 source

zone, in terms of its contribution to the moment release of the mainshock, lay in

the region of principal aftershock activity that is located from 50 to 120 km north-

nor theast of the main shock epicenter. The focal depths of our most reliably located

aftershock hypocenters suggest that the principal source region identified by Beck

and Ruff 1983) was substantially shallower than the main shock hypocenter; the

mean and median focal depths of quality A hypocenters from this source region are

both about 19 km. Considering the three different total-moment spectra computed

for the 1979 earthquake by Silver and Jordan 1983), these depths suggest that the

relatively frequency-independentmoment spectrum computed for an assumed depth

of 15 km may be preferred over the frequency-dependent spectra computed for

assumed depths of 29 km and 50 km.

2

N

c 10

v

L

w

z

[ ] 1942 A 1958 0 1 9 7 9

A ~

A

I ] A

~ A

.

5.0 5 5 0 6 0 6

Magni tude

MsH)

FIG. 6. Magnitude MsH) versus cumulative-frequency plot for earthquakes of the 1942 1958 and

1979 aftershock sequences calculated from seismograms of the Tucson stations TUO and TUC.

LEVEL OF AFTERSHOCK ACTIVITY AS AN INDICATION OF A CONTINUING

POTENTIAL FOR LARGE EARTHQUAKES IN A PLATE-BOUNDARY SEGMENT

The aftershock activity in the months following the 1958 earthquake was sub-

stantially stronger than the aftershock activity in the months following the 1979

earthquake Figure 6), notwithstanding tha t the 1958 earthquake had a seismic

moment about one-fifth tha t of the 1979 earthquake Kanamori and McNally,

1982). The 1942 earthquake also had substantially stronger aftershock activity than

the 1979 earthquake Figure 6). The moment of the 1942 earthquake was n o t

computed from long-period waves by Kanamori and McNally 1982), but the zones

of 1942 and 1979 aftershocks with mb ---- 5.5 have approximately the same lengths

Figure 4), suggesting similar source dimensions. We conclude that the 1979 earth-

quake had substantially less aftershock activity than the 1942 and 1958 earthquakes,

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590

C A R L O S M E N D O Z A A N D J A M E S W . D E W E Y

and tha t the d i f ferences are no t s imply a re f l ec t ion of the s ize of the main shocks .

Th e lower af t e rshock ac t iv i ty fo llowing the 1979 ear thq uake re la t ive to tha t fo llow-

ing the 1942 and 1958 ear thquakes may imply tha t the subduct ion zone in the

reg ion of the 1979 ear thqua ke w as unde r lower s tress o r exp er iencing lower s t ra in

ra tes McG arr , 1976) fo l lowing the main shock than were the reg ions sur round ing

the 1942 and 1958 earthquakes.

Al though the 1942 to 1980 Colombia-Ec uador se i smic i ty da ta a re no t cons i s ten t

wi th the speci fi c c r i te r i a p roposed b y K ei l i s-Borok

et al.

1980) to identify shocks

wi th h igh af te rshock ac t iv i ty as long- term precursors to s t ronger ear thquakes , our

8 w

z >

f

o

8 t 8 ° 7 ° 7g 77 °

,~ .~ . . . . / t ~ ~ r /.

_~ ~ ,

_

.... 6 ¢ ? S

-,'.., i \ / / .U,U r1,: ~e~ / (,~ Y ' A .J

.' ',: i ~iv/ ~ .,,. , ~ d ~

: : ' ~ :~ .'

' . i

V , ~I /

~ .; .' .' :' :< ~, ,. ; .. ~: \ U +

........J'" ~ ,a ... ._.~ d O L O M B I A

~*/ oz . 4

[ ] /,' / ?g ] ..7/ .....

[] ~.l, ?O ?

O ~ . ~'J /.

i /il d v

l k m

4

ECU DOR

Fro. 7 . Epicente rs• f re • •ca tedC•• •mbia•Ecuad• rea r thquakes•ccurr inga t t imes• the r than3m•nths

a f te r the 1942, 1958, and 1979 ma in shocks . L arge symbols a re ea r th quak es of mb ~ 5 .5 . The dot ted l ines

out l ine the a f te rshoc k a reas de f ined in F igure 4 . Pos t -1958 ea r thqu akes south of l a t i tude 1 ° N were not

re l oc a te d . B a t hym e t r i c c on t ou rs i n k i l ome t e r s a re f rom Lonsda l e a nd Kl i t go rd (1978) .

resu l t s a re cons i s ten t wi th the genera l hypothes i s tha t abnormal ly l a rge numbers

of a f t e rshocks to a s t rong ear thq uak e m ay po in t to a h igh po ten t ia l fo r fu ture s t rong

ear thq uake s in the same tec ton ic p rov ince wi th in the same ac t ive per iod of a rup ture

cycle.

S E I SM I C IT Y P R I O R T O T H E 1 2 D E C E M B E R 1 9 7 9 E A R T H Q U A K E

Th e d i s t r ibu t ion of se i smic i ty p r io r to the 1979 ear thqu ake m ay be cons i s ten t

with previou sly recognize d pa t ter ns of prior seism ici ty e .g ., Mogi , 1969; Ke l leher

and Savino, 1975; Ohtake et al. 1977) in which m os t o f the rup tu re zone of the

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

59

Lay, T. and H. Kanamori 1981). An asperity model of large earthquake sequences, in E a r th q u a ke

Prediction

D. W. Simpson and P. G. Richards, Editors, American Geophysical Union, Washington,

D.C., 579-592.

Lay, T., H. Kanamori, and L. Ruff {1982). The asperity model and the nature of large subduction zone

earthquakes, Earth. Pred. Res. 1, 3-71.

Lonsdale, P. and K. D. Klitgord 1978). Structure and tectonic history of the eastern Panama Basin,

Bull. Geol. Soc. Am .

89, 981-999.

McGarr, A. 1976). Dependence of magnitude statistics on strain rate, Bull . Se ism Soc . Am . 66, 33-44.

Mendiguren, J. A. 1971). Focal mechanism of a shock in the middle of the Nazca plate,

J. Geophys. Res.

76, 3861-3879.

Mikumo, T. and T. Miyatake 1978). Dynamical rupture process on a three-dimensional fault with non-

uniform frictions and near-field seismic waves,

Geophys. J .

54, 417-438.

Mogi, K. 1969). Some features of recent seismic activity in and near Japan, 2, activity before and after

great earthquakes,

Bull . Ear thqua ke Res. Inst . Tokyo Univ.

47, 395-416.

Ohtake, M., T. Matumoto, and G. V. Latham 1977). Seismicity gap near Oaxaca, southern Mexico as a

probable precursor to a large earthquake,

Pu re Appl. Geophys.

115, 375-385.

Pennington, W. D. 1981). Subduction of the eastern Panama Basin and seismotectonics of northwestern

South America, J. Geophys. Res. 86, 10753-10770.

Poppe, B. B. 1979). Historical survey of U.S. seismograph stations,

U.S. G eol. Surv . P rofess. Pa per I096

389 pp.

Reyners, M. 1981). Long- and intermediate-term precursors to ear thquake--State of the art, in

Earthquake Prediction

D. W. Simpson and P. G. Richards, Editors, American Geophysical Union,

Washington, D.C., 333-347.

Silver, P. G. and T. H Jordan 1983). Total-moment spectra of fourteen large earthquakes,

J. Geophys.

Res. 88, 3273-3293.

U.S. Department of Commerce 1966). Principles underlying the interpretation of seismograms,

Spec.

Publ. No. 254 50 pp.

Wyss, M., R. E. Habermann, and A. C. Johnston 1978). Long-term precursory seismicity fluctuat ions,

in Methodology[or Iden ti fying Seism ic Gaps and Soon-to-B reak Gaps Conference VI, Natl. Eq. Haz.

Red. Prog., 869-894.

U.S. GEOLOGICALSURVEY

BOX 25046, MS 967

DENVER FEDERAL CENTER

DENVER, COLORADO80225

Manuscr ipt received 5 May 1983

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