Tectonic implications of the seismic ruptures

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Tectonic implications of the seismic ruptures associated with the 1983 and 1991 Costa Rica earthquakes Fumiko Tajima Institute for Geophysics, The University of Texas at Austin, 8701 North Mopac Boulevard, Austin, Texas 78759-8397 Masayuki Kikuchi Department of Physics, Yokohama City University, 22-2 Seto, Kanazawa-ku, Yokohama, 236, Japan

ABSTRACT Two large earthquakes occurred in Costa Rica in 1983 and 1991. The source areas are in the vicinity of the triple junction of the Cocos, Nazca, and Caribbean plates, where a large bathymetric feature, the Cocos Ridge, is colliding/subducting beneath the Caribbean plate. The 1983 event occurred in the subduction zone of the Cocos Ridge near the southeastern terminus of the Middle America trench just west of the Panama Fracture zone. The 1991 earthquake took place about 100 km north of the 1983 epicenter and is considered to be a back-thrusting event representing the overthrusting of the North Panama deformed belt over the Caribbean plate. In this region the subduction of the buoyant Cocos Ridge should play an important role in the occurrence of unique seismological and tectonic features, for example, a decrease in seismicity, a cessation of volcanic activity, and a shallowing of the trench. This chapter summarizes several different seismological studies for the rupture processes and associated seismicity patterns of the 1983 and 1991 Costa Rica earthquakes, attempting to constrain the tectonic structure in this region. The results provide a clearer picture of the mechanical interaction and geometry between the subducting ridge and overriding Caribbean plate than previous hypothetical models schematically illustrated for ridge-trench intersections. INTRODUCTION T w o large earthquakes ( M s = 7.3, depth = 3 0 k m on April 3, 1983; and M s = 7.5, depth = 12 k m on April 22, 1991) occurred in C o s t a R i c a near the triple junction of the C o c o s , Nazca, and C a r i b b e a n plates. In this area a large b a t h y m e t r i c feature, the C o c o s Ridge, is colliding/subducting beneath the C a r i b b e a n plate (Fig. 1). T h e 1983 earthquake took place in the subduction zone, w h i c h had been the site of t w o previous events with similar m a g n i t u d e s — M s 7.6 in 1904 and M s = 7.5 in 1 9 4 1 — a n d therefore had been identified as a seismic g a p prior to the recent event (Astiz and Kanamori, 1984). T h e 1991 e a r t h q u a k e took place a b o u t 100 k m north of the 1983 epicenter, with noticeable vertical d e f o r m a t i o n of the coast, and is c o n s i d e r e d to be a back-thrusting event (thrusting on the

side a w a y f r o m the subduction zone). T h i s event represents the thrusting of the North P a n a m a d e f o r m e d belt ( N P D B ) o v e r the C a r i b b e a n plate ( G o e s et al., 1993; P l a f k e r and W a r d , 1992; Suarez et al., unpublished m a n u s c r i p t ; T a j i m a and K i k u chi, 1991). T h e o c c u r r e n c e of large back-thrusting events are rare in general. T h e intersection of the C o c o s R i d g e and the southeastern terminus of the M i d d l e A m e r i c a trench ( M A T ) is j u s t west of the P a n a m a Fracture Z o n e (PFZ), w h e r e the t w o C o s t a R i c a e a r t h q u a k e s took place. T h e C o c o s R i d g e is a track of the G a l a p a g o s hot spot and is genetically related to the C a r n e g i e R i d g e of the N a z c a plate (Hey, 1977). N u m e r o u s p a p e r s h a v e reported the u n i q u e seismological and tectonic features observed in this area, f o r e x a m p l e , a d e c r e a s e in seismicity, a cessation of volcanic arc activity, and a shallowing of the

Tajima, F., and Kikuchi, M., 1995, Tectonic implications of the seismic ruptures associated with the 1983 and 1991 Costa Rica earthquakes, in Mann, P., ed.. Geologic and Tectonic Development of the Caribbean Plate Boundary in Southern Central America: Boulder, Colorado, Geological Society of America Special Paper 295.

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Figure 1. Tectonic map of the Cocos Ridge-Middle America Trench (MAT) intersection area, including the epicenters of the 1983 and 1991 Costa Rica earthquakes. Subduction zone shallowing (with equidepth lines) and cessation of volcanic activity are shown. The shaded area between the 1983 and 1991 epicenters illustrates the topographic uplift (modified from Kolarsky et al., this volume). The plate convergence rate 9.2 cm/yr determined by DeMets et al. (1990) is indicated. MAT: Middle America Trench; PFZ: Panama Fracture Zone; NPDB: North Panama deformed belt; BCFZ: Ballena-Celmira fault zone; ENFZ: East Nicoya fault zone; MFZ: Medial fault zone.

trench as c o m p a r e d to other parts of the M A T . T h e s e o b s e r v a tions imply disturbance of the plate c o n v e r g e n c e , although the C o c o s - C a r i b b e a n c o n v e r g e n c e rate is estimated to be about 9 2 m m / y e a r ( D e M e t s et al., 1990).

u n i q u e seismo-tectonic features at ridge-trench intersections w o r l d w i d e , f o c u s i n g on the spatial g a p s of arc volcanism, and presented several s c h e m a t i c m o d e l s f o r the interaction bet w e e n a subducting ridge and the overriding plate.

T h e uplifted area b e t w e e n the t w o e a r t h q u a k e s (see the shaded area in Fig. 1; also M i y a m u r a , 1975; Rivier, 1985) is in line with the C o c o s Ridge and is the t o p o g r a p h i c high of the C o s t a Rican isthmus. Similar t o p o g r a p h y is o b s e r v e d worldw i d e at (aseismic) ridge-trench intersections, revealing the b u o y a n t nature of these features (e.g., Vogt et al., 1976; Kelleher and M c C a n n , 1976; C h u n g and K a n a m o r i , 1978a, b; M c C a n n and Sykes, 1984; Eissler and K a n a m o r i , 1982; Astiz and K a n a m o r i , 1984; L e F e v r e and M c N a l l y , 1985; M c G e a r y et al., 1985; Kolarsky et al., this v o l u m e ) , w h i c h should also h a v e an important role in the stress transfer b e t w e e n the earthq u a k e source area. M c G e a r y et al. (1985) s u m m a r i z e d the

A conventional a p p r o a c h to constrain hypothetical tectonic m o d e l s of a s e i s m o g e n i c area is to use properties obtained f r o m seismological analyses, such as m o m e n t tensor fault m e c h a n i s m solutions, or seismicity patterns. A s m e n tioned above, back-thrusting events are rare, and the g e o m e t r y of the southern terminus of the M A T is still a m b i g u o u s , b a s e d on seismicity studies. At present several papers are available that e x a m i n e seismological properties associated with the t w o recent C o s t a Rica earthquakes. S o m e of the studies described in these p a p e r s use m o d e r n digital seismological techniques to analyze the seismic rupture of the 1991 C o s t a Rica earthq u a k e . T h e results o b t a i n e d are s o m e w h a t c o m p l e m e n t a r y to


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each other, but n o n e of t h e m has s u c c e e d e d in presenting a c o m p r e h e n s i v e m o d e l by itself. In addition, they are still in review (or in press). T h e r e f o r e , in this chapter w e s u m m a r i z e the recent source rupture studies presented f o r the 1983 and 1991 C o s t a Rica earthquakes, including G o e s et al. (1993), and associated seismicity (Fan et al., 1993; Protti et al., this v o l u m e ; and Suarez et al., unpublished m a n u s c r i p t ) and then attempt to constrain the model of interaction b e t w e e n the descending C o c o s R i d g e and the overriding C a r i b b e a n plate.

a b o u t 5 0 k m north of the 1983 epicenter. This implies that the stress created b e y o n d the 1983 source area in the direction n o r m a l to the trench axis could have b e e n transmitted t o w a r d the 1991 source area through the topographic uplift. In this circ u m s t a n c e the 1988 event is a peculiar and interesting event in terms of location ( P D E [Preliminary D e t e r m i n a t i o n s of Epicenters] depth = 2 3 k m and C M T depth = 37 k m ) and fault m e c h a n i s m (oblique strike-slip) and m a y o f f e r clues to understanding the stress p r o p a g a t i o n f r o m the 1983 source area to the 1991 rupture zone.

SEISMOLOGICAL STUDIES OF THE COSTA RICA EARTHQUAKES

T h e a f t e r s h o c k distribution of the 1991 event w i d e n e d in the direction n o r t h w e s t of the main rupture z o n e and m e r g e d with the activity near the trench. Since the 1991 event, the area b e t w e e n the t w o rupture z o n e s has r e m a i n e d seismically quiescent. T h e stress propagation through the topographic uplift s e e m s to be aseismic. T h e strike-slip m e c h a n i s m s northwest of the 1983 rupture z o n e (Fig. 2e) and the 1991 rupture zone (Fig. 2 f ) indicate segmentation of the C a r i b b e a n plate b o u n d ary in that area.

Overview of seismicity Figure 2 s u m m a r i z e s seismicity patterns (for events with m a g n i t u d e s > 4.7) obtained f r o m the International S e i s m o logical C e n t e r (ISC) and National E a r t h q u a k e I n f o r m a t i o n C e n t e r ( N E I C ) catalogs f o r the vicinity of the t w o Costa Rica e a r t h q u a k e s during the period b e t w e e n January 1964 and S e p t e m b e r 1991; a, f o r the entire period; b, f r o m 1964 to 1978; c, f r o m 1979 to M a r c h 1983; d, f r o m April to S e p t e m b e r 1983; e, f r o m April 1983 to M a r c h 1991; f, f r o m April to S e p t e m b e r 1991. Figures 2 b and c s h o w the activity before the 1983 event, 2d the f i v e m o n t h s of a f t e r s h o c k activity following the 1983 event, 2e the period b e t w e e n the 1983 and 1991 events including the 1983 a f t e r s h o c k s , and 2f the a f t e r s h o c k activity of the 1991 event. For s o m e of the events ( M > 5.5), C e n t r o i d M o m e n t T e n s o r ( C M T ) solutions d e t e r m i n e d by the H a r v a r d g r o u p are s h o w n . T h e rupture zones of the 1983 and 1991 m a i n events that were inferred f r o m o u r study are indicated with contours. In the f u t u r e rupture z o n e of the 1983 event there w e r e t w o m o d e r a t e events in 1979: one a n o r m a l event with substantial a f t e r s h o c k s near the southeastern b o u n d a r y (July 1, M s = 6.5, depth = 12 k m ) and the other a thrust event near the northwestern b o u n d a r y (August 24, M s = 6.4, depth = 19 k m ) (Fig. 2c). T h e normal faulting event is consistent with the present-day uplift of the Burica Peninsula, w h i c h m a y b e h a v e as a d e t a c h e d block f r o m the O s a Peninsula. T h e a f t e r s h o c k area of the 1983 event e x p a n d e d mostly to the northwest, parallel to the trench axis during the f i v e m o n t h s after the event (Fig. 2d). A d a m e k et al. (1987) noted a f t e r s h o c k s to the southeast of the epicenter as well. This a f t e r s h o c k e x p a n s i o n pattern indicates that there are n o distinct b o u n d a r i e s that act as barriers to stress propagation b e y o n d the m a i n source rupture area along the trench axis. T h e influences of the d e t a c h e d blocks or the distribution of partial c o u p l i n g are reflected in the a f t e r s h o c k e x p a n s i o n pattern and in their fault m e c h a n i s m s , w h i c h show substantial variation (Fig. 2e). Figures 2d and 2e s h o w that the area b e t w e e n the 1983 rupture z o n e and the f u t u r e 1991 rupture zone w a s very quiet during the period b e t w e e n the t w o e a r t h q u a k e s except for one event that occurred on M a r c h 11, 1988, with an M s = 6 at

Subduction zone geometry M a n y researchers pointed out that the dip of the B e n i o f f zone along the M A T b e c o m e s shallow w h e r e the C o c o s R i d g e is subducting ( M o l n a r and Sykes, 1969; D e a n and Drake, 1976; Liaw, 1981; B u r b a c h et al., 1984). A steeply d i p p i n g B e n i o f f z o n e is well d e f i n e d u p to Nicaragua, and the transition of the B e n i o f f zone seismicity is clearly o b s e r v e d f r o m N i c a r a g u a to C o s t a R i c a ( D e w e y and A l g e r m i s s e n , 1974). T h e h y p o c e n t r a l depths listed in the I S C catalog indicate little or n o d e e p (h > 7 0 k m ) activity f r o m southeast of the N i c o y a P e n i n s u l a to the southeastern terminus of the M A T . L i a w (1981) used data f r o m a local n e t w o r k and s h o w e d that the B e n i o f f zone of northwest C o s t a Rica is less well defined than beneath N i c a r a g u a and b e c o m e s distinctly shallower, with little activity below 100 k m near the N i c o y a Peninsula. B u r b a c h et al. ( 1 9 8 4 ) incorporated this observation with teleseismic data and proposed that the c h a n g e of the slab dip is d u e to a tear fault, w h i c h separates the subducting lithosphere beneath the N i c o y a Peninsula. T h e y also noted that d e e p e r seismic activity (h > 7 0 k m ) ceases southeast of the N i c o y a Peninsula, and accordingly the dip angle of the slab is u n d e t e r m i n e d . O t h e r researchers correlated the slab s e g m e n t a tion with the cessation of the volcanic chain in central C o s t a Rica (Stoiber and Carr, 1973; Carr and Stoiber, 1977). Protti et al. (this v o l u m e ) reevaluated the seismicity using high-resolution location data of 9 , 5 1 4 events with c o m p u t e d horizontal and vertical errors smaller than 4 and 5 k m , respectively. T h e y i m a g e d the g e o m e t r y of the W a d a t i - B e n i o f f zone u n d e r southern central A m e r i c a and s u m m a r i z e d the transition of the slab g e o m e t r y f r o m a steep dip angle to a shallow o n e f o c u s i n g on three areas: Nicaragua, N i c o y a Peninsula, and the C o c o s Ridge collision zone. T h e y f o u n d that the W a d a t i B e n i o f f zone u n d e r the N i c a r a g u a - C o s t a Rica b o r d e r contorts


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Figure 2. Epicenter plots of events (M > 4.7) that occurred in the region of the Cocos Ridge-Middle America trench intersection using the International Seismological Center and National Earthquake Information Center catalogs and some Centroid Moment Tensor solutions by the H a r v a r d g r o u p d u r i n g d i f f e r e n t p e r i o d s : a, b e t w e e n 1964 and 1991; b, f r o m 1964 to 1978; c, f r o m 1979 t o M a r c h 1983

(note that there was a moderate event with a normal fault mechanism [Ms = 6.4, July 1] near the southeastern boundary of the future source area and a thrust event near the northwestern boundary in 1979 [Adamek et al., 1987]); d, from April to September 1983; e, from April 1983 to March 1991 (note that the aftershock activity of the 1983 event expanded along the fault strike but not widthwise; there were also some strike slip mechanisms to the northwest of the source areas); f, from April to September 1991. The future 1991 earthquake source area and its vicinity were quiet before the event in 1991 (b, c, d and e). There was some expansion of the aftershock activity of the 1991 event toward the 1983 source area.


Tectonic implications of Costa Rica earthquakes ( f r o m steep to shallow dip angles, n o r t h w e s t to southeast), but it does not s h o w evidence of a brittle tear, as postulated by others (Stoiber and Carr, 1973; B u r b a c h et al., 1984; B u r b a c h and Frohlich, 1986). Based on age variations of the s u b d u c t e d C o c o s plate along the M i d d l e A m e r i c a T r e n c h , Protti et al. (this v o l u m e ) p r o p o s e a n e w m o d e l that correlates well with the c o m p l e x g e o m e t r y of the W a d a t i - B e n i o f f z o n e u n d e r Nicar a g u a and C o s t a Rica (the Q u e s a d a S h a r p Contortion, c h a n g e s in dip angle, length and m a x i m u m depth of the seismic slab) as well as with regional tectonic features on the overriding C a r i b b e a n plate.

April 3,1983, earthquake and its aftershocks Preliminary solutions f o r the source m e c h a n i s m of the 1983 event indicate that this event is a thrust event along the M A T ( D z i e w o n s k i et al., 1983). T h e b o d y w a v e f o r m s s h o w a c o m p l e x time history of m o m e n t release as c o m p a r e d with the w a v e f o r m s of c o m p a r a b l e events along the M A T in southern M e x i c o ( T a j i m a , 1984). If the t i m e history of m o m e n t release is c o m p l e x , it also implies that the source rupture m a y include source m e c h a n i s m c h a n g e s d u e to the c h a n g i n g stress condi-

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tion during the rupture process. A d a m e k et al. (1987) studied the rupture process of this main event using a o n e - d i m e n s i o n a l inversion m e t h o d ( K i k u c h i and K a n a m o r i , 1982) with longperiod b o d y w a v e f o r m s and a fixed focal m e c h a n i s m ( = 295°, 8 = 32°, A. = 90°). T h e source-time f u n c t i o n obtained s h o w s a relatively long lasting source process with s o m e c o m plexity (see the u p p e r b o x in Fig. 3a). F r o m a directivity analysis, A d a m e k et al. d e t e r m i n e d the locations of strong m o m e n t release that indicate the rupture propagation to the northwest along the fault strike (Fig. 3a). T h e a f t e r s h o c k activity is m o d e r a t e and sparse ( s h o w n with crosses and small solid circles in Figs. 3a and b). T h e rupture z o n e b a s e d o n the o n e - d a y a f t e r s h o c k distribution 7 0 x 110 k m 2 (illustrated with a contour in Fig. 3a) w a s s o m e w h a t overestimated, and the stress d r o p (A ~ 2 to 3 bars) and the seismic slip ( D ~ 58 c m ) calculated f o r the rupture area w e r e underestimated according to our analysis. T h e first m o t i o n focal m e c h a n i s m s d e t e r m i n e d f o r m a j o r a f t e r s h o c k s s h o w oblique strike slip faults n o r t h w e s t and southeast b e y o n d the m a i n r u p t u r e z o n e (Fig. 3b; m o d i f i e d f r o m A d a m e k et al., 1987). A d a m e k et al. (1987) also c o m p a r e d the w a v e f o r m s of the 1941 large earthquake ( M s = 7.5) that occurred near the present

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Figure 3. a. Illustration for the location of moment release inferred for the 1983 main event by Adamek et al. (1987). The crosses and small solid circles (relocated by Adamek et al. [1987]) show the epicenters of aftershocks. The source time function shown in the upper box indicates two subevents during the main rupture, b, Aftershock distribution and some focal mechanism solutions. Events (1) 04/07/83, (2) 05/09/83, (3) 07/03/83, (4) 09/23/83, (5) 10/12/83, and (6) 04/09/84 (modified from Adamek et al., 1987).

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34°83°F. Tajima and M. Kikuchi

source area to those of the 1983 event and suggested a similar source process. T h e P w a v e directivity and a f t e r s h o c k distribution of the 1983 event as well as the source process similarity of the two large events suggest that the m a j o r zone of coupling along this section of the M A T is located just west of the epicenter of the 1983 event. This is the locally highest-stressed zone of contact between the C o c o s ridge and the overriding Caribbean plate. T h e gradual expansion of the aftershocks southeast and trenchward of the epicenter (east O s a and Burica area) suggests that the coupling beneath the outer forearc region is relatively w e a k and heterogeneous. H o w e v e r , as m e n tioned before, this main rupture process could also include fault m e c h a n i s m c h a n g e s as a result of the c h a n g i n g stress condition during the process. If this is the case, the assumption of a constant fault m e c h a n i s m in the inversion m a y not be adequate. W e were interested in the spatial and temporal proximity of the 1983 and 1991 earthquakes and analyzed the source rupture processes of these events in detail, using an iterative inversion m e t h o d (Kikuchi and K a n a m o r i , 1991) with long-period body waves. T h e P and S H w a v e f o r m s used in the inversion w e r e obtained f r o m the Global S e i s m o g r a p h i c N e t w o r k ( G S N ) and W o r l d W i d e S t a n d a r d i z e d S e i s m o g r a p h i c N e t w o r k ( W W S S N ) long-period records. T h e inversion m e t h o d m o d e l s the entire source rupture process with a series of subevents and determines the moment-tensors [M^ ] of subevents as well as the spatial extent of subevents and the time-function of m o ment release. Each of the subevents is determined separately and can h a v e a different m e c h a n i s m f r o m others; therefore, fault m e c h a n i s m c h a n g e s during the rupture can be accounted for. All the source parameters are allocated at assigned grids on a fault plane, and, accordingly, the fault g e o m e t r y and source area are well determined. U n l i k e the preliminary solutions m e n t i o n e d a b o v e , w e m o d e l e d the entire rupture process with three subevents of v a r y i n g m e c h a n i s m s f o r the main event ( d e n o t e d with s i , s2, and s3 in Fig. 4). T h e p r i m a r y subevent s h o w s a thrust m e c h a n i s m (4» = 357°, 8 = 46°, and X = 103°) with a source depth of 3 2 k m ; it w a s succeeded by t w o c o m p a r a b l e subevents, one located at 4 5 k m southeast of the epicenter with a strike-slip m e c h a n i s m ( = 126°, 8 = 78°, and I = 173°) and a depth of 21 k m in 13 s and the other one at 6 0 k m n o r t h w e s t of the epicenter with a thrust m e c h a n i s m (()) = 298°, 8 = 40°, and A. = 98°) and a depth of 37 k m in 23 s (see the fault m e c h a n i s m s and locations of the subevents in Fig. 4). T h e rupture area d e f i n e d by the t w o thrust subevents is about 3 0 x 75 k m 2 , but with the strike-slip subevent it is about 6 0 x 110 k m 2 . T h e rupture pattern determined by our analysis is r e m a r k ably consistent with the result by A d a m e k et al. (1987), w h o used a one-dimensional inversion m e t h o d (Kikuchi and Kanamori, 1982) with a fixed-thrust fault m e c h a n i s m . A c c o r d i n g to their analysis, the rupture front propagated w e s t w a r d 6 3 k m f r o m the epicenter to where the m a j o r m o m e n t release w a s truncated. T h e aftershock distribution, which also e x p a n d e d southeastward, indicates a possibility of bilateral rupture propa-

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Figure 4. Results obtained for the 1983 main event source process by our study. The entire rupture was modeled with three subevents of varying focal mechanisms (denoted with si, s2, and s3 in the order of occurrence). The source-time function is also shown to distinguish the three subevents. The rupture zone is indicated with a hatched area.

gation, although the inversion with a fixed m e c h a n i s m could not resolve this. W i t h the n e w approach of varying fault m e c h anism, the rupture propagation to the southeast was also retrieved. A f e w m a j o r a f t e r s h o c k s (5 < M < 6.1; see A d a m e k et al. [1987] f o r the m e c h a n i s m s ) are located around the second and third subevents identified in our study (Fig. 3b). T h e total duration of the rupture is a b o u t 4 0 s f r o m the source-time f u n c t i o n , and the estimated rupture velocity ranges b e t w e e n 2.8 and 3.5 km/s. T h e m o m e n t s estimated f o r the three subevents are 0 . 5 4 x 10 2 7 , 0.32 x 10 2 7 , and 0 . 3 3 x 10 2 7 d y n e - c m , respectively. T h e total m o m e n t release is 1.2 x 10 2 7 , which is similar to the m o m e n t 1.35 x 10 2 7 d y n e - c m estim a t e d f r o m long-period s u r f a c e w a v e s at 2 5 6 s ( T a j i m a , 1985) but b e t w e e n the estimate f r o m the C M T solution, 1.8 x 10 2 7 d y n e - c m ( D z i e w o n s k i et al., 1983) and that listed in the 1983 P D E catalog by the United States Geological Survey. T h e m e c h a n i s m obtained by s u m m i n g u p the m o m e n t tensors of the s u b e v e n t s is similar to the C M T solution, including the n o n d o u b l e couple c o m p o n e n t ( 1 4 % ) . A description of the entire rupture process is that it started with a thrust m e c h a n i s m p r i m a r y rupture, p r o p a g a t e d bilaterally, and triggered t w o subevents, one with a thrust m e c h a n i s m n o r t h w e s t of the epicenter and the other with a strike-slip m e c h a n i s m southeast of


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the e p i c e n t e r . I n t e r e s t i n g l y , t h e strike of t h e p r i m a r y t h r u s t

c o u p l e w i t h strike () = 107 ± 5°, d i p (8) = 2 1 ± 10°, r a k e (X) =

s u b e v e n t is p a r a l l e l to the strike of t h e P a n a m a f r a c t u r e z o n e ,

5 6 ± 11°, c e n t r o i d d e p t h (h) = 2 2 ± 8 k m , a n d d u r a t i o n of

a n d t h e m e c h a n i s m of t h e third s u b e v e n t is p a r a l l e l to t h e

s o u r c e t i m e f u n c t i o n (x) = 4 0 ± 6 s, w i t h a n 8 % n o n - d o u b l e

strike of t h e P a n a m a f r a c t u r e z o n e , a n d t h e m e c h a n i s m of t h e

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

third s u b e v e n t is b e t w e e n t h e C M T s o l u t i o n ( = 3 1 0 ° , 8 =

depth earthquakes do not strongly excite surface w a v e m o m e n t

2 5 ° , X = 110°) a n d t h e first m o t i o n s o l u t i o n ( = 2 9 5 ° , 8 = 3 2 ° ,

tensor terms M x z and M y z

X = 9 0 ° ) b y A d a m e k et al. ( 1 9 8 7 ) , e x c e p t w i t h a s t e e p e r

large u n c e r t a i n t i e s in d i p a n d m o m e n t e s t i m a t e s .

dip angle.

(dip-slip component) and have

F r o m the broad band body w a v e f o r m inversion analysis,

T h o u g h a c h a n g e of t h e s u b d u c t i o n a n g l e f r o m 2 3 ° in

G o e s et al. ( 1 9 9 3 ) i d e n t i f i e d six s u b e v e n t s in t w o g r o u p s , t h e

n o r t h e r n C o s t a R i c a to 3 0 ° in s o u t h e r n C o s t a R i c a is s u g -

first o n e at a p p r o x i m a t e l y 15 k m d o w n - d i p of t h e h y p o c e n t e r

gested, the subduction geometry (dip angle and depth extent)

a n d t h e s e c o n d in an a r e a 15 to 3 0 k m u p - d i p . R e s u l t s s u g g e s t

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

o b l i q u e t h r u s t m o t i o n (X = 5 0 to 8 0 ° ) o n a s h a l l o w - d i p p i n g ( 8

ity s t u d y d u e to t h e l a c k of an a d e q u a t e c r u s t a l v e l o c i t y m o d e l

= 15 to 20°), s o u t h e a s t - s t r i k i n g ( 0 = 9 0 t o 120°) f a u l t p l a n e .

(Protri et al., this v o l u m e ) . A c o m p a r i s o n of t h e p r o p e r t i e s in-

T h e r e s o l v a b l e r u p t u r e a r e a is n o l a r g e r than 6 0 x 8 0 k m 2 , a n d

f e r r e d f o r t h e s u b d u c t i o n z o n e b y o u r study a n d Protti et al.

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

(this v o l u m e ) is g i v e n in T a b l e 1.

4 5 x 4 5 k m 2 (see F i g . 5). T h e y n o t e d that t h e e x t e n t of t h e d e t e r m i n e d r u p t u r e is q u i t e s m a l l f o r a n e v e n t w i t h M w = 7 . 7 .

April 22,1991, main event

After examining source models for various combinations of p a r a m e t e r s i n c l u d i n g v a r i a b l e f a u l t m e c h a n i s m s , G o e s et al.

A special session to d i s c u s s t h e 1991 C o s t a R i c a e a r t h -

( 1 9 9 3 ) p r e f e r r e d a fixed f a u l t m e c h a n i s m s o u r c e m o d e l , w h i c h

q u a k e w a s h e l d at t h e 1991 A m e r i c a n G e o p h y s i c a l U n i o n fall

w a s d e t e r m i n e d c o n s i s t e n t l y w i t h t h e b o d y w a v e f o r m a n d sur-

m e e t i n g . P a p e r s p r e s e n t e d at t h e s e s s i o n p o i n t e d o u t that the

f a c e w a v e f o r m m o d e l i n g . T h i s m o d e l s u g g e s t s that t h e e v e n t

s o u r c e c o n s i s t s of m u l t i p l e s u b e v e n t s ( G o e s a n d

o c c u r r e d on a s h a l l o w s o u t h w e s t - d i p p i n g r u p t u r e p l a n e , that

Schwartz,

1991; M o n t e r a et al., 1991; T a j i m a a n d K i k u c h i , 1991), a n d the

t h e s o u r c e p r o c e s s is r e l a t i v e l y s i m p l e , a n d that m o s t e n e r g y is

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

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

s t e e p e r d i p a n g l e ( V e l a s c o et al., 1991; G i i e n d e l et al., 1991).

m o d e l h a s a f o c a l m e c h a n i s m w i t h strike 102 ± 10°, d i p 17 ±

P l a f k e r a n d W a r d ( 1 9 9 2 ) c o m b i n e d g e o d e t i c data a n d f o c a l

14°, a n d r a k e 6 3 ± 17°; a s e i s m i c m o m e n t of 3 . 8 ± 1.5 x 1 0 2 0

m e c h a n i s m s o l u t i o n s a n d s u g g e s t e d that the m a i n r u p t u r e of t h e

N m (1.2 to 2 . 0 x 1 0 2 0 N m f r o m b o d y w a v e f o r m s ) ; a n d a total

1991 e v e n t d i p s l a n d w a r d b e n e a t h C o s t a R i c a a n d n o r t h e r n

r u p t u r e d u r a t i o n of 4 0 ± 6 s. T h e y a l s o p o i n t e d o u t that

P a n a m a at a n a n g l e of a b o u t 30°, striking b e t w e e n 105° a n d

whether

120° a n d is a p p r o x i m a t e l y 4 0 k m w i d e a n d 8 0 k m l o n g .

b o u n d a r y is n o t clear.

the N P D B

is a f u l l - f l e d g e d o r d e v e l o p i n g

plate

W e s t u d i e d t h e s o u r c e r u p t u r e p r o c e s s of the 1991 m a i n

R e c e n t l y G o e s et al. ( 1 9 9 3 ) s t u d i e d t h e 1991 e a r t h q u a k e using teleseismic broad-band body w a v e f o r m s and long-period

e v e n t a l o n g w i t h the 1 9 8 3 e a r t h q u a k e , as d e s c r i b e d

above.

( 1 5 7 to 2 8 8 s) s u r f a c e ( L o v e a n d R a y l e i g h ) w a v e s o b t a i n e d

T h e y u s e d t h e iterative i n v e r s i o n m e t h o d of K i k u c h i

f r o m the Global Seismographic N e t w o r k (GSN), International

K a n a m o r i (1991) with broad-band b o d y w a v e f o r m s f r o m the

D e p l o y m e n t of A c c e l e r o m e t e r s ( I D A ) , a n d G E O S C O P E . T h e i r

GSN

p r e f e r r e d solution f r o m the s u r f a c e w a v e s h a s a m a j o r d o u b l e

long-period records. T h e broad- and intermediate-band wave-

and

GEOSCOPE

broad-band/intermediate-band

TABLE 1. SUBDUCTION ZONE FOR THE 1983 EARTHQUAKE* Authors Tajima and Kikuchi

Protti et al.

Objectives

Main source rupture process.

Subduction geometry.

Method and data

Teleseismic body waveform inversion for the main rupture.

Seismicity patterns using local and regional data.

Geometry of the subduction coupling.

Dip angle between 40° and 46°; the depth extent from 22 to 37 km (inferred from the mechanism of the main rupture).

30° (?) with a depth extent to as much as 70 km.

"From Protti et at, this volume, and Tajima and Kikuchi, present study.

and and


F. Tajima and M. Kikuchi a, SOURCE TIME FUNCTION

^.MECHANISM

strike=90°-120°

total moment

dip=15°-20° rake=50°-80° 102°/17°/63°

20 40 time (sec.)

Í DISTRIBUTION OF MOMENT RELEASE along strike

90 km

Figure 5. Results for the preferred model of the 1991 main source rupture obtained using teleseismic body waveforms by Goes et al. (1993). a, Source time function and six subevents identified, b, Source mechanism and its allowable range for the preferred model, c, Interpretation of the rupture propagation. The hatched regions depict the areas where most of the moment was released.

f o r m s of the 1991 event s h o w a s o m e w h a t c o m p l e x time history of m o m e n t release, w h e r e a s such a feature is not o b v i o u s f r o m the long-period w a v e f o r m s . T h i s rupture process w a s m o d e l e d as three thrust s u b e v e n t s f o l l o w e d by a m i n o r strikeslip subevent. T h e p r i m a r y m o m e n t release has been carried out by the thrust fault m e c h a n i s m in a time span of about 2 0 s (Fig. 6a). T h e subevent locations and onset times suggest that the rupture started at the h y p o c e n t e r (depth = 12 k m ) and the m a j o r m o m e n t release o c c u r r e d in the up-dip direction. T h e dip angles of the first and s e c o n d subevents are 36° and 39°, steeper than the C M T solution ( 8 = 25°). T h e dip angle of the third subevent located at a depth of 17 k m is 16°, less steep than the C M T solution. T h e varying dip angles of the subevents i m p l y a rupture z o n e that flattens with depth and m a y reflect the local tectonic structure. In c o m p a r i s o n , the study by G o e s et al. (1993) preferred a rather simple source m o d e l (e.g., with n o m e c h a n i s m c h a n g e ) in w h i c h a range of the dip angle is 17 ± 14°. H o w e v e r , the m e c h a n i s m obtained f r o m the s u m m a t i o n of the m o m e n t tensors of the f o u r subevents is a thrust m e c h a n i s m ( = 123°, 8 = 32°, and X = 89°) c o m p a r a b l e to those d e t e r m i n e d by C M T or G o e s et al. (1993). Figure 6 b illustrates the b e n d i n g rupture z o n e with a steeper dip angle (36 to 39°) at a shallow depth (12 to 7 k m ) and a smaller dip angle ( - 1 6 ° ) at a depth of 17 k m , inferred f r o m the b o d y w a v e f o r m inversion.

1991

aftershocks

T h e 1991 C o s t a Rica e a r t h q u a k e p r o d u c e d a n u m b e r of a f t e r s h o c k s . F a n et al. ( 1 9 9 3 ) studied w a v e f o r m data recorded by a n e t w o r k of three P r o g r a m f o r Array Seismic Studies of the Continental L i t h o s p h e r e ( P A S S C A L ) - t y p e , portable instruments that w a s d e p l o y e d to m o n i t o r the a f t e r s h o c k activity in southern C o s t a Rica t w o to six w e e k s after the m a i n shock. T h e y d e t e r m i n e d source p a r a m e t e r s of 15 relatively small a f tershocks with m a g n i t u d e s b e t w e e n 3.2 and 4.4, using a regional m o m e n t tensor inversion m e t h o d (Fan and W a l l a c e , 1991). In this analysis they a d m i t t e d that the f o c a l depths are not well constrained o w i n g to the uncertainty of the local velocity structure m o d e l , but the relative source depths can be compared. T h e focal m e c h a n i s m s obtained s h o w a variety of faulting behavior: T h r u s t faulting, strike-slip faulting, and n o r m a l faulting are all present. F r o m the distribution of the focal m e c h a n i s m s , F a n et al. ( 1 9 9 3 ) noted a clear spatial segmentation of focal m e c h a n i s m types. M o s t a f t e r s h o c k s near or southeast of the m a i n s h o c k w e r e thrusting events, with focal m e c h a n i s m s similar to the m a i n shock. On the other hand, a cluster of a f t e r s h o c k s n o r t h w e s t of the m a i n s h o c k s h o w e d d o m i n a n t l y left-lateral, strike-slip m o t i o n on a northeasterly striking nodal plane. Figure 7 s h o w s a block d i a g r a m m o d i f i e d f r o m Isacks et al. (1969) to illustrate the nature of faulting f o r the m a i n s h o c k and a f t e r s h o c k s of the 1991 earthquake. Suarez et al. ( u n p u b l i s h e d m a n u s c r i p t ) studied a f t e r s h o c k locations using data recorded by local n e t w o r k instruments, inc l u d i n g n i n e portable s e i s m o g r a p h s installed in the a f t e r s h o c k zone. Several h u n d r e d a f t e r s h o c k s of a duration m a g n i t u d e ( L e e et al., 1972) r a n g i n g f r o m 2.4 to 4 . 0 w e r e recorded d u r i n g the field c a m p a i g n f r o m 2 9 April through 4 M a y , 1991. T h e epicenters of the a f t e r s h o c k s d e t e r m i n e d f r o m the local netw o r k data are located in the back arc of C o s t a Rica, to the northeast of the T a l a m a n c a Cordillera (see Fig. 1). W h e n they c o m p a r e d the teleseismically d e t e r m i n e d epicenters to those b a s e d o n local arrival times, the teleseismic locations appear to be displaced 15 to 2 5 k m to the northwest. T h e a f t e r s h o c k z o n e d e f i n e s an ellipse of a p p r o x i m a t e l y 4 5 x 85 k m 2 . T h e long axis of the inferred rupture z o n e is parallel to the orientation of the C a r i b b e a n coast (see Fig. 8). T h e cross-section proj e c t i o n s s h o w that the a v e r a g e focal depth of the a f t e r s h o c k s increases f r o m a b o u t 15 k m near the coastline to an average of 2 0 to 3 0 k m beneath the continent. Suarez et al. (unpublished m a n u s c r i p t ) pointed out that the cross sections s h o w an almost horizontal distribution of seismicity; the distribution of the aftershocks along cross sections B B ' and C C ' (Fig. 8) suggest that nucleation of the source rupture b e g a n at a depth of about 2 0 k m . T h e a f t e r s h o c k distribution agrees with the h y p o c e n t r a l location (depth = 2 4 k m ) of the m a i n event d e t e r m i n e d with the local data. S u a r e z et al. (unpublished m a n u s c r i p t ) concluded that the actual rupture probably t o o k place on a thrust sheet that is subhorizontal beneath the continent and steepens


Tectonic implications of Costa Rica earthquakes 84

83

335

82

Figure 6. Results obtained for the 1991 main event by our study. The notations are the same as in Figure 4. a, The entire rupture was modeled with subevents SI, S2, S3, and S4. The primary moment release has been carried out with three thrust subevents that occurred in the first 20 sec. b, A schematic illustration of the bending rupture zone inferred from the inversion result.

t o w a r d the east, reaching the s u r f a c e o f f s h o r e at a h i g h angle (see Fig. 9). T h i s rupture pattern, w h i c h suggests a b e n d i n g rupture zone, is similar to our result. T h e seismological properties obtained f o r the 1991 C o s t a Rica e a r t h q u a k e by the present study, G o e s et al. (1993), Sudrez et al. ( u n p u b l i s h e d manuscript), and F a n et al. ( 1 9 9 3 ) are s u m m a r i z e d in T a b l e 2.

DISCUSSION Implications from seismological

approaches

A l t h o u g h there are s o m e disagreements of results or interpretations a m o n g the seismological studies above, w e can extract s o m e c o m m o n or c o m p l e m e n t a r y factors to describe the seismotectonic process in the Costa Rica region and the role of the subducting buoyant ridge. T h e seismicity patterns in Figure 2 illustrate the stress accumulation and release associated with the t w o large Costa Rica earthquakes and other m a j o r events in this region during the period between 1964 and 1991. T h e 1983 source area had s o m e precursory events including a n o r m a l faulting event ( M s = 6.5) with its aftershocks near the southeastern b o u n d a r y and a thrust event ( M s = 6.4) near the northwestern boundary. T h e distribution of the subevents suggests

that the rupture propagated bilaterally with its primary rupture in the C o c o s R i d g e subduction zone and a reactivated subevent in the loose fault zone southeast of the epicenter. T h e source duration time of about 4 0 s indicates a relatively slow rupture process. A s w a s suggested by B o n a f e d e et al. (1983), the e f f e c t of viscoelastic coupling near the rupture zone m a y p r o d u c e less-brittle failure, resulting in a slow rupture process as well as a depletion of h i g h - f r e q u e n c y seismic waves. This depletion has been observed f o r the source spectra of short period records f r o m the 1983 event ( H o u s t o n and Kanamori, 1986). T h e a f t e r s h o c k area e x p a n s i o n pattern supports the idea of bilateral rupture p r o p a g a t i o n of the 1983 m a i n event. A f e w m a j o r a f t e r s h o c k s (5 < M < 6.1; see A d a m e k et al. [1987] f o r the m e c h a n i s m s ) are located around the second and third s u b e v e n t s identified in the inversion of the present study. T h e a f t e r s h o c k e x p a n s i o n pattern of the 1983 event indicates that the fault c o u p l i n g n o r t h w e s t and southeast of the m a i n rupture z o n e is w e a k ( A d a m e k et al., 1987). T h e o b s e r v a t i o n s northwest of the rupture z o n e c o r r e s p o n d to the d i f f u s e , transcurrent d e f o r m a t i o n z o n e northwest of the 1991 m a i n rupture area pointed out by our study, G o e s et al. (1993), F a n et al. (1993), and Suarez et al. (unpublished manuscript). T h e f u t u r e 1991 source area had n o teleseismically observable activity (for events with m a g n i t u d e s > 4.7) b e f o r e the


336

34°83°F. Tajima and M. Kikuchi Suarez et al. (unpublished manuscript) also noted that the epicenter of the main event reported by the N E I C lies about 10 k m to the northeast of the location determined using local data. The focal depth of 24 km determined f r o m local data agrees better with the aftershock distribution (see Fig. 8) than the focal depth of about 10 k m determined teleseismically by G o e s et al. (1993) (note that the centroid depth determined with a long-period surface w a v e inversion by G o e s et al. [1993] is 22 ± 8 km). T h e aftershock distribution along cross sections C C ' and D D ' near the epicenter suggests a relatively flat fault at a depth of approximately 20 k m (Fig. 8). A clear spatial segmentation was observed f r o m the focal mechanism types of the 1991 aftershocks (Fan et al., 1993). A similar spatial segmentation occurred after the 1983 earthquake (Adamek et al., 1987). The two earthquakes with the aftershocks taken together define a diffuse deformation zone in central Costa Rica, characterized by left-lateral strike-slip motion (Goes et al., 1993; Fan et al., 1993). The diffuse, transcurrent deformation zone coincides with the termination of the volcanic chain in central Costa Rica and a change of the WadatiBenioff zone geometry along the southwestern coast of Costa Rica (Fan et al., 1993).

Main Shock Figure 7. A model proposed by Fan et al. (1993) showing the faulting characteristics associated with the 1991 main event and its aftershocks.

main event. The region between the 1983 and 1991 source areas was quiet as well. This implies that the coupling of the 1991 fault zone is strong; the stress produced by the 1983 event normal to the strike was not released seismically but possibly transmitted through the buoyant zone to the impending 1991 source area where the stress accumulation was high. The presence of the T a l a m a n c a range in Costa Rica and its geologic structure, together with the occurrence of the 1991 back-thrusting event, reflects an increase in the compressive stress transmitted to the upper plate by the subduction of the Cocos Ridge (Kolarsky et al., this volume). T h e concentration of m a j o r m o m e n t release in a relatively small rupture zone for the 1991 source process as analyzed by our study and G o e s et al. (1993) supports this hypothesis. T h e varying dip angles of the thrust subevents at different depths (39° at 7 km and 16° at 17 k m ) suggest a slightly bending rupture zone for the 1991 back-thrusting event (e.g., steeper angle at a shallower depth and flattening at a deeper depth). This rupture pattern supports the model proposed by Suarez et al. (unpublished manuscript) based on the aftershock locations (see the illustration in Fig. 9). T h e strong ground motion data recorded at 68 k m southwest of the epicenter suggest a multiple and complex rupture, and at least five subevents can be identified; the fifth subevent is the largest of the sequence, occurring at 21.2 s after the first subevent.

T h e rupture patterns and seismicity associated with the two large Costa Rica earthquakes provide us a clearer picture of geometry for the interaction between the subducting ridge and overriding Caribbean plate than the schematically illustrated models at ridge-trench intersections (McGeary et al., 1985). T h e models by M c G e a r y et al. (1985) as well as others (Kelleher and M c C a n n , 1976; C h u n g and Kanamori, 1978a, b) attempted to explain the m e c h a n i s m s of resisting forces to subduction, which affect both the overriding and underlying lithosphere and eventually d e f o r m the margin of the overriding plate. But the consequent results are described rather schematically, reflecting the simplified models, and d o not provide quantitative constraints for the mechanical interaction between the forearc and backarc source areas. T h e mechanical interaction between a subducting buoyant ridge and the overriding plate m a y be investigated further by assessing the associated crustal deformation for coseismic and postseismic displacement and interseismic strain accumulation, using the source parameters of large earthquakes. T h e gravitational stress due to the height of the buoyant ridge should also be accounted for. W h e n a buoyant ridge is thrust beneath an "island arc," the island arc rides up over the plate (backthrusting), which replaces the asthenospheric wedge. The topographic uplift illustrated in Figure 1 is evidence of this tectonic situation. Plate convergence

and seismic slip

ratio

Earthquake source characteristics and occurrence cycles at various convergent plate boundaries have been studied by many researchers. A s s u m i n g that the source m e c h a n i s m s of the historic events were similar to those of the 1983 event, the


Tectonic implications of Costa Rica earthquakes AFTERSHOCK

DISTRIBUTION

CAR18BEAN c o

SW

f

T

NE

y — V* 1991

''

m

•

MAIN

*

. tSlDRO

PANAMA

(slightly modified from Suarez et al., unpublished) 30 40

Figure 8. Aftershock distributions adopted and modified from Suarez et al. (unpublished manuscript). Left, Epicentral locations of aftershocks recorded by the temporarily installed network (solid circles) and those reported by NEIC (open circles) for the first seven days. The star indicates the main shock. (Right, Cross sections of the aftershocks shown on the left. The dashed lines on BB' and C C ' show the average dip of the fault used to model the coastal uplift. The solid lines represent the geometry of the fault plane inferred from the aftershocks.

'0 20

-D km

D' ,

l 70

l 60

I

I

I

l

1

50

40

30

20

10

km

r e c u r r e n c e t i m e of a l a r g e e v e n t in this s u b d u c t i o n z o n e w a s e s t i m a t e d to b e a p p r o x i m a t e l y 4 0 y e a r s ( A s t i z a n d K a n a m o r i , 1984). If t h e p l a t e c o n v e r g e n c e rate V , a v e r a g e slip of a c h a r -

P l a f k e r a n d W a r d ( 1 9 9 2 ) e s t i m a t e d that if t h e f u l l 9 2

acteristic l a r g e e a r t h q u a k e D , a n d t h e r a t i o of s e i s m i c slip to

m m / y ( N 2 9 ° ) ( D e M e t s et al., 1990) of C o c o s - C a r i b b e a n c o n -

total p l a t e m o t i o n t] a r e k n o w n , t h e r e c u r r e n c e t i m e x c a n b e

v e r g e n c e is p a r t i t i o n e d

calculated by

b a c k a r c ( C a r i b b e a n side) c o m p o n e n t s a n d b a c k a r c c o n v e r g e n c e

between

f o r e a r c ( P a c i f i c side)

and

falls b e t w e e n 2 a n d 11 m m / y at t h e C o c o s R i d g e s u b d u c t i o n x = D/T|V

(1)

z o n e , then t h e r e c u r r e n c e t i m e of a s i m i l a r b a c k - t h r u s t i n g e v e n t

A l t h o u g h V and D are k n o w n or c a n b e d e t e r m i n e d r e a s o n a b l y

is e s t i m a t e d to b e f r o m 2 0 0 to 1,100 y e a r s . G i v e n t h e e f f e c t s of

well, a large u n c e r t a i n t y is i n v o l v e d in r | ( K a n a m o r i a n d Astiz,

crustal d e f o r m a t i o n a s s o c i a t e d w i t h t h e b a c k - t h r u s t i n g e v e n t in

1985). W h e n t h e s e i s m i c slip ratio is not certain b e c a u s e of the

1991, t h e r e c u r r e n c e interval, e s t i m a t e d at a p p r o x i m a t e l y 4 0

lack of reliable historical data or t h e variable r u p t u r e length of

y e a r s in t h e s u b d u c t i o n z o n e ( 1 9 8 3 s o u r c e area), m a y b e dis-

large individual e a r t h q u a k e s f r o m t i m e to time, an e s t i m a t e of

t u r b e d as w e l l b e c a u s e of t h e C o c o s R i d g e i n v o l v e m e n t .

r e c u r r e n c e interval is not certain, either. T h e s o u r c e r u p t u r e p r o c e s s of t h e 1983 e v e n t ( A d a m e k et al., 1987; p r e s e n t study) i m p l i e s a small s e i s m i c slip ratio in t h e C o c o s R i d g e s u b d u c t i o n z o n e and also d i s t u r b a n c e in t h e r e c u r r e n c e interval d u e to the interaction of t h e t w o s o u r c e areas (see also M a n n et al., 1990).

Resolution and limitation of seismological

analyses

W e reviewed the source ruptures and seismicity patterns a s s o c i a t e d w i t h t h e 1 9 8 3 a n d 1991 C o s t a R i c a e a r t h q u a k e s w i t h t h e g o a l of a p p l y i n g s e i s m o l o g i c a l a p p r o a c h e s t o c o n -



COCOS RIDGE —

TRENCH

OSA

PENINSULA

FORE ARC BASIN

TALAMANCA CORDILLERA

N O R T H PANAMA DEFORMED 8 E L T

PACIFIC COAST

CARIBBEAN COAST

SEDIMENTS'

CARIBBEAN P L A T E RUPTURE Z O N E ^

j K

BY T A J I M A Í i KIKUCHK

EARTHQUAKE 1 2 / 0 3 / 1 9 8 3 Ms" 7.3

LIMON EARTHQUAKE E C / 2 2 / 1 9 9 1 Ms • 7.5 (from S u a r e z et a l , unpublished manuscript)

50

100

Figure 9. The rupture zone model for the 1991 source area obtained by our study is illustrated with thick dashed lines on a schematic cross section adopted from Suarez et al. (unpublished manuscript). The surface geology from the Osa Peninsula on the Pacific side to Puerto Limon on the Caribbean side was taken from Rivier (1985).

TABLE 2. COMPARISON OF SEISMOLOGICAL STUDIES FOR THE 1991 EARTHQUAKE* Authors Tajima and Kikuchi

Goes et al.

Suárez et al.

Fan et al

Determined quantities

Main rupture process.

Main rupture process.

Locations of aftershocks.

Moment tensors of 15 aftershocks (3.2 to 4.4).

Used data and method

Inversion modeling using teleseismic body waveforms (BWV) (IRIS and GEOSCOPE broad band, and DWWSSN mid-period).

Inversion with teleseismic BWV and surface waves (SWV).

Location determination using travel times from locally recorded data by temporarily installed seismometers.

Inversion using locally recorded body waveforms by three PASSCAL instruments.

Main source duration time.

29 s

40 s (20st) from BWV. 40 ± 6 s from SWV.

N.A.

N.A.

Source complexity of main rupture

Four subevents (three thrust subevents).

Single event from SWV. Six subevents from BWV.

N.A.

N.A.

Mechanism: Strike = 4> Dip = 8 Rake = X

4» = 123" (91°-138°)5 6 = 32° (16°-39°)5 X = 89° (69°-96°)5

s= 107 ± 5°, 5S = 10°, = 56 ± 11° SWV. B = 90°-120°, 8B = 20°, XB = 50° - 80° BWV.

N.A.

(Varying mechanisms also with time.)

45 x 85 km 2 ; depth «20 km.

Depth = 4 to 16 km

Rupture area

40 X 60 km2, depth 17 km.

7 to

21 ± from 15° from

60 x 80 km 2 (45 x 45 km 2 t ); depth = 15 to 30 km from BWV. Centroid depth = 22 ± 8 km from SWV.

•From Fan et al., 1993; Goes et al., 1993; SuSrez et al., unpublished; and Tajima and Kikuchi, present study. tMost of the moment was released within this area and process time (Goes et al., 1993). § The ranges are shown only for the three thrust subevents.


Tectonic implications of Costa Rica earthquakes strain regional tectonics and structural geology. U s i n g highquality digital s e i s m o g r a m s and robust inversion m e t h o d s , w e n o w h a v e the m e a n s to d e t e r m i n e c o m p l e x e a r t h q u a k e rupture processes that p r o v i d e u s e f u l clues f o r the involved tectonic structure. N o n u n i q u e n e s s and instability of the solutions rem a i n a p r o b l e m , h o w e v e r , and these are c o m m o n l y e n c o u n tered in general. O n e of the c a u s e s f o r such p r o b l e m s is the lack of k n o w l e d g e of the near-source velocity structure that a f f e c t s the source depth determination and w a v e f o r m inversion. T h e only available crustal m o d e l in this region, that of M a t u m o t o et al. (1977), w a s not a d e q u a t e to m o d e l the observed b o d y w a v e f o r m s (present study; G o e s et al., 1993). A s described by Suarez et al. (unpublished manuscript), the locally d e t e r m i n e d hypocentral depth of the m a i n event is 2 4 k m , w h e r e a s the h y p o c e n t r a l d e p t h d e t e r m i n e d teleseismically b y G o e s et al. (1993) is about 10 k m . T h e uncertainty of the near-source velocity structure and hypocentral depth can c a u s e the inversion results to be unstable. W e and G o e s et al. (1993) tested teleseismic b o d y w a v e f o r m s f o r an a d e q u a t e near-source structure. F a n et al. ( 1 9 9 3 ) tested regional w a v e f o r m data of the a f t e r s h o c k s f o r the i n f l u e n c e of the structure and admitted that the depth determinations are not stable within several k i l o m e ters but that the source m e c h a n i s m p a r a m e t e r s are well constrained. T h i s situation leads the results to be less c o n v i n c i n g even t h o u g h the varying dip angle of the fault m e c h a n i s m during the rupture p r o v i d e s u s e f u l information f o r the rupture zone and is consistent with the h y p o c e n t r a l distribution of the a f t e r s h o c k s obtained f r o m local data (Suarez et al., u n p u b lished manuscript). U n d e r this c i r c u m s t a n c e a p r e f e r r e d m o d el, w h i c h is a sort of averaged solution with a f i x e d fault m e c h a n i s m , is given with the a l l o w a b l e ranges of the dip and rake angles f o r the s u b e v e n t s ( G o e s et al., 1993). Currently high-quality global digital waveform data are accessible f r o m worldwide networks shortly after a large event occurs. However, local information such as the near-source structure is not generally available immediately after an earthquake, even though it is crucial in the following seismological analysis. W e are still in the process of improving our practical ability to obtain a stable solution of waveform modeling or inversion for events that take place in tectonically complex regions.

ACKNOWLEDGMENTS T h e authors thank P. M a n n f o r the opportunity to contribute a chapter to this special issue. W e benefited f r o m discussions with him, f r o m his useful review c o m m e n t s f o r the manuscript, and f r o m his patience throughout the preparation of this manuscript. T h e authors also w o u l d like to thank J. D e w e y and L. Astiz f o r their constructive review c o m m e n t s and S. Schwartz, G. Suarez, and G. P l a f k e r f o r sending preprints. K. M c i n t o s h and P. N y f f e n e g g e r read and provided helpful c o m ments f o r the final manuscript. This study w a s partially supported by N S F grants E A R - 8 8 1 6 9 9 3 , and E A R - 9 0 1 1 8 4 5 .

339

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1994

Printed in U.S.A.


Geological Society of America Special Papers Tectonic implications of the seismic ruptures associated with the 1983 and 1991 Costa Rica earthquakes Fumiko Tajima and Masayuki Kikuchi Geological Society of America Special Papers 1995;295; 327-340 doi:10.1130/SPE295-p327

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© 1995 Geological Society of America