Bulletin of the Seismological Society of America, Vol. 72, No. 5, pp , October 1982

Transcription

1 Bulletin of the Seismological Society of America, Vol. 72, No. 5, pp , October 1982 SHALLOW-FOCUS SEISMICITY, COMPOSITE FOCAL MECHANISM, AND TECTONICS OF THE VALLE CENTRAL OF COSTA RICA BY WALTER MONTERO P. AND JAMES W. DEWEY ABSTRACT A network of seismographs operating in the Valle Central of Costa Rica has recorded many small earthquakes near the cities of Cartago and San Jose. This seismicity is similar in many ways to the shallow-focus intraplate seismicity of Central America to the north. The earthquakes occur within tens of kilometers of Quaternary volcanic centers at shallow focal depths. The earthquakes occur predominantly on strike-slip faults, with the nodal plane that would correspond to a left-lateral fault striking approximately east-northeast and the nodal plane that would correspond to a right-lateral fault striking approximately north-northwest. The shocks have a tendency to occur in seismic swarms. The region of highest seismicity in our study was located southwest of Cartago, about 10 km from the meizoseismal zones of destructive earthquakes of 1841 and In detail, the recently recorded small earthquakes seem to have occurred on different faults or fault segments than the 1910 earthquakes. The tendency for shallow-focus intraplate earthquakes to occur within kilometers of earthquakes that occurred several decades earlier has been noted elsewhere in Central America. The occurrence of shocks on distinct faults within the overall region of high activity appears similar to the occurrence of earthquakes on different fault strands in Managua, Nicaragua. We discuss the Valle Central seismicity in light of hypotheses proposed for the shocks farther north in Central America, Our data can be interpreted in terms of the hypothesis that shallowfocus intraplate earthquakes in Central America concentrate on zones of strikeslip faults that pass through offsets of the volcanic chain. Our data can also be interpreted in terms of the hypothesis that the earthquakes occur as the response of minor faults to high regional stresses throughout the region surrounding the volcanic chain. Both hypotheses leave some characteristics of the seismicity unexplained, although these characteristics are not crucial evidence against the hypotheses. A third hypothesis, that the Valle Central source regions are different than intraplate source regions northward in Central America and are occurring in a developing transform plate boundary between the Caribbean and Nazca plates, is plausible on the basis of the regional plate tectonic environment, but it is not strongly supported by the local geology of the Valle Central or by the characteristics of seismicity. INTRODUCTION For much of the densely populated inland region of Central America, the most destructive historical earthquakes have been shallow-focus earthquakes occurring in the overriding plates above the Benioff Zone, which dips northeast from the Middle America trench (Figure 1). Most of these destructive inland Central American shocks occur within the Caribbean plate; the Guatemala earthquake of 4 February 1976 was an exception, having occurred on the boundary between the Carribean and North American plates (U.S. Geological Survey, 1976). Well-studied examples of damaging or destructive intraplate shallow-focus earthquakes were the Jucuapa, E1 Salvador, earthquakes of 6 and 7 May 1951 (Meyer-Abich, 1952), the San Salvador, E1 Salvador, earthquake of 3 May 1965 (Lomnitz and Schulz, 1966), the Managua, Nicaragua, earthquakes of 31 March 1931 (Sultan, 1931), 4 January 1968 (Brown, 1968; Algermissen et al., 1974), and 23 December 1972 (Brown et al., 1611

2 1612 WALTER MONTERO P. AND JAMES W. DEWEY 1973; Algermissen et al., 1974), and the Tilaran, Costa Rica, earthquake of 13 April 1973 (Plafker, 1973; Matumoto et al., 1976). The larger of these earthquakes had magnitudes of 6.0 to 6.5, and their zones of maximum intensity were not large, but in the case of the E1 Salvador earthquakes and the 1931 and 1972 Managua earthquakes, they occurred beneath cities and produced heavy damage. This is a study of the shallow-focus (focal depths less than 30 km) seismicity of the Valle Central of Costa Rica. The Valle Central is a west-northwest-trending depression between the Cordillera de Talamanca and the Central Volcanic Cordillera 92ow 76 W H- NORT. AMERICAN PLATE.20 N w /,~.~ '- " CARIBBEAN PLATE ~, NIC~RAGL J UOCOS PLA TE./ z j, Go c i. F f:.:~ IL i NAZCA 50N ~ Present plate boundaries Major active w>leano II ' Ill I l, I Diffuse plate boundary dominated by east.west comp... I Possible future strike-slip plate boundary Highlands--average elevation 2000 m. or nmre FIG. 1. Location of Valle Central (V.C.) with respect to tectonic plates of the Central American region. The rectangle encloses the region of Figure 3. Broad arrows give direction of plate motion relative to the Caribbean plate (Jordan, 1975). Plate boundaries in the region southeast of Costa Rica are from Pennington (1981). Major active volcanoes are from Macdonald (1972). The 85 fault zone (F.Z.) is the future boundary between the Cocos and Nazca plates postulated by Van Andel et al. (1971). The Future Caribbean-Nazca Boundary (F.C.N.B.) is simply a line drawn from the future triple junction postulated by Van Andel et al. parallel to the direction of relative motion between the Caribbean and Nazca plates. (Figure 2). Most of Costa Rica's population lives in the Valle Central. The most destructive historical shocks in the Valle Central of Costa Rica were the Cartago earthquakes of 2 September 1841 and 4 May 1910 (Gonzalez Viquez, 1910). These earthquakes had isoseismals indicating that they also were moderate magnitude, shallow focus, earthquakes similar to the intraplate shocks farther north in Central America. The regional plate tectonic environment of the VaUe Central, however, suggests that the Valle Central might be expected to be experiencing higher tectonic strains than regions farther north, and that there might exist larger faults that could

3 SEISMICITY, FOCAL MECHANISM, AND TECTONICS IN COSTA RICA 1613 produce larger shallow-focus earthquakes (see next section). The principal conclusion of the present study will be that small shallow-focus earthquakes recorded by a seismographic network in the Valle Central have modes of occurrence and focal mechanisms similar to intraplate earthquakes to the north. Taking our data together with regional geologic maps, we do not see good evidence for throughgoing faults that could produce major shocks of the sort that we might fear from the regional plate tectonic environment of the Valle Central. We cannot rule out the occurrence of such an earthquake in some part of the Valle Central that was inactive during the period of our study, but we propose that the principal seismic hazard in the source regions we have found is associated with the occurrence of moderate-magnitude earthquakes such as have historically affected Cartago or Managua, Nicaragua, and San Salvador and Jucuapa, E1 Salvador. 86oW 83 W lion ~: %, % ~ -~4 CZ~zc c~,v 9ON Major active volcano Quaternary volcanic rocks --elevations above 2000 in, Tertiary rocks--elevations above 2000 m. Concentrations of shallow seisrnicity identified in this study * Epicenter of April 13,1973 Fro. 2. The rectangle encloses the region covered by Figure 3 and shows the Valle Central as the region of lower elevation trending east-southeast between the Central Volcanic Cordillera and the Cordillera Talamanca. REGIONAL PLATE TECTONIC FRAMEWORK OF THE VALLE CENTRAL The Valle Central is located at the southern end of the principal Central American volcanic chain and in the northern end of a zone of mountains that are the highest in Central America south of the region of the triple junction of the North American, Caribbean, and Cocos plates in Guatemala (Figure 1). The location of the Valle Central at the foot of the Central Volcanic Cordillera, several hundred kilometers from the present Nazca-Cocos-Caribbean triple junction (Figure 1) as defined by seismicity and focal mechanism solutions {Molnar and Sykes, 1969), suggests that the seismicity of the Valle Central should be similar to intraplate volcanic regions to the north in Central America, such as in E1 Salvador or Nicaragua.

4 1614 WALTER MONTERO P. AND JAMES W. DEWEY On the other hand, the southeast termination of the principal Central America volcanic chain at the Valle Central and the high elevation of the Cordillera de Talamanca south of the Valle Central (Figures 1 and 2) have been postulated (Van Andel et al., 1971; De Boer, 1979) to be the result of the resistance to subduction of the aseismic Cocos Ridge, which has collided with the Middle America trench south of the Valle Central. A scarcity of intermediate-depth earthquakes beneath southern Costa Rica (Vogt et al., 1976) and a complex pattern of focal mechanism solutions in the region of the Isthmus of Panama (Pennington, 1981) also support the view that the Cocos Ridge is impeding the normal subcluction process. Our concern is that the collision of the Cocos Ridge could result in substantial strains and faulting in the Valle Central, well inland from the coast, and that there might be a chance for larger earthquakes in the Valle Central than in the interior of the Caribbean plate to the north. The distribution of regional stresses associated with the resistance to subduction is presumably quite complicated. Van Andel et al. (1971) have suggested that the end result of the anomalously strong coupling between the Cocos Ridge and the Caribbean plate will be that the triple junction of the Nazca, Caribbean, and Cocos plates will have changed from its present location near 7 N, 83 W to a position near 9 N, 85 W, approximately offshore of the Valle Central. At the completion of the change in triple junction anticipated by Van Andel et al., perhaps the simplest form for a plate boundary between the Caribbean and Nazca plates would be an east-northeast-striking left-lateral transform fault passing through or very near the Valle Central (Figure 1). METHOD OF ANALYSIS Arrival-time and first-motion data used in this study come principally from a network of five seismographic stations located in and near the Valle Central and operated by the Universidad de Costa Rica. Additional arrival-time data were provided by a portable seismograph temporarily located at various places in the Valle Central. Arrival times and first motions reported in bulletins of the Arenal network (Matumoto et al., 1977), situated northwest of the Valle Central, were used for some of the larger Valle Central shocks. Hypocenters were located with the program HYPO71 (Lee and Lahr, 1975). The velocity model we used is based on the P-wave velocity structure determined by Matumoto et al. (1977) for northwestern Costa Rica; we have assumed a Poisson's ratio of 0.25 in order to estimate theoretical S-wave arrival times. We defined two qualities of hypocenter location. Quality A hypocenters are those for which the gaps in azimuth between stations were less than 270, for which P-wave arrival times from four or more stations and S-wave times from two or more stations were used in the location process, for which the estimated standard deviations of the traveltime observations were less than 0.3 sec, and for which standard errors of the epicenters and focal depth were less than 5 km. Quality B hypocenters are those for which one or more of the above criteria were not met, but which could still be located such that the standard errors of the epicenter and focal depths were less than 10 kin, and the estimated standard deviations of the travel-time observations were less than 0.5 sec. Shocks that could not meet the requirements for quality A or quality B were judged indeterminant. Local magnitudes were determined with amplitudes of S waves measured on a vertical seismograph at station SJS, using the local magnitude formula of Richter (1958, p. 340). This procedure was calibrated with data from 5 months of recording

5 SEISMICITY, FOCAL MECHANISM, AND TECTONICS IN COSTA RICA 1615 with Wood-Anderson seismographs at SJS, so that our magnitude can be considered virtually equivalent to the local magnitude of Richter. Shocks plotted in Figure 3a occurred in the period from July 1976 through June However, only the 10 shocks of magnitude greater than 3.0 were plotted out of 52 locatable shocks that occurred in a swarm centered at 9 45'N, 83 58'W, in the period 16 to 26 December A list of the 119 plotted shocks is available from the authors upon request. First motions from these shocks were used in the composite focal mechanisms (Figure 4). Readers accustomed to viewing results from dense microearthquake networks that record on common time bases are hereby warned that most of our data were recorded on a rather sparse network of low magnification seismographs with separate clocks whose times, in turn, had to be corrected by linear interpolation between radio time signals. The accuracies of our hypocenters, while very good by the standards of teleseismic location, are not comparable to the accuracies associated with hypocenters recorded by dense modern microearthquake networks. In particular, the "Map of Auxiliary RMS Values" included in the HYPO71 output (Lee and Lahr, 1975) suggests that the sharp northeast-striking lineations northeast of San Jose could be, in part, artifacts of the distribution of seismographic stations. The location errors of these events are likely to be largest in northeast-southwest directions, and it is likely that the strong appearance of the lineations is due to mislocation of some epicenters that occurred in smaller source regions. The sensitivity of the Universidad de Costa Rica network is highest in the region within the network and least in the northeast, southeast, and southwest corners of the region covered by Figure 3. Within and near the region enclosed by the seismographic stations (SJS, SDS, LCR, and VPS in Figure 3b) shocks of magnitude 2.5 and greater would in normal circumstances have been locatable to A or B quality. Different stations in the network suffered temporary failures, so that the abovecited normal circumstances are appropriate to approximately 80 per cent of the time period from July 1976 through June The pattern of station failures was not such as to cause shocks of magnitude 2.5 or greater to be overlooked more frequently in one part of the region within the network than in another part of the region. For regions well outside the network, corresponding to the northeast, southeast, and southwest corners of Figure 3, the sensitivity of the network is reduced to the point that we could have overlooked individual shocks of magnitude 2.5 even when the network was operating well. We base this conclusion on the fact that shocks of magnitude 2.5 from slightly greater distances along the Pacific coast south and west of the area covered by Figure 3 were sometimes not well recorded by the network. However, we would still have been able to locate enough shocks of magnitude between 2.5 and 2.9 to identify important seismic sources in the northeast, southeast, and southwest corners of the region shown in Figure 3, had such sources been active. Shallow-focus shocks of magnitude 3 and greater from throughout the region covered by Figure 3 were well recorded and locatable to quality A or B during the 80 per cent of the time that the network was operating effectively. RESULTS The most intense shallow-focus seismic activity for the period July 1976 to June 1979 was northeast of San Jose and south of San Jose and Cartago (Figure 3a). Many of the earthquakes from the seismic zones near San Jose and Cartago occurred in swarms. Fifteen of the 24 shocks north and east of San Jose in Figure 3a occurred

6 .?, 1616 WALTER MONTERO P. AND JAMES W. DEWEY o '10" I0 ~ N o O e / ":; /~z. ]r.,,.. i.... " t ' ':.." ', - o 0 84o30'W ; i: :.., ~ - o o 0 o L o 0,9 40 ' N (a) 84"00 ~ ' W RAMON vps i 0 ~o~ 1(710' _;.. v..~,.a 7s ALAJ~EL:A ",. ~'~ ";',C O 6~ o 9, O 82. -~ \v\ ~',,'- V.*=AZU.." [ HEREDIA " " ~ ''- "... '%% 0 o, o. ~ ~. o o '~'sjs...~'--~w:-:l:.--s~)s... ;. \. ~JOSE / /~"v,," ~'~ \. " ; " : % k ~1 ~ X x " ', \, : 63.. \ '\ /v -,. \. \.. \ -,, -\ ~ti~l~. \ '-.,, " :... '~.~,. ~\~"'C'~-~:TAGO X \ 61 SB '. ~. v,~... ".. \.. ~ x \ \ o "v \ " "" " ~"".":X-.:-" :) ~ \ 0 "'. '\.,~'~;"~"=~'~ AGUA CALIENTE- 1' s.'. 08~ 4~ o 0 9~ O st'''.. \ 62 /.../.. 610~.:..LC R,_o ~.~.j/,." 03B 4e ". 0 3'd~ 7~. ) ~,~..,' ' 84' 30' 84"00 ~ 83 40' b) Mag. Quality ~ I K~ A B sj$ SEISMOGRAPHIC STATION... ISOSEISMALS EARTHQ. APR{L 13~t9{O,"---"~" * FAULT ~=" ">6" ACTIVE VOLCANIC CENTER... ISOSEISMALS EARTHQ, MAY 4, , o FTc. 3. (a) Shallow focus (depths less than 30 km) seismicity of the Valle Central and vicinity, July 1976 to June Criteria for selection of earthquakes to be plotted, and definitions of location quality are given in text. Subregions designated by Roman numerals are those used for construction of subregionalized focal mechanisms. We have only plotted two proposed faults that are discussed in the text, and we note that the existence of these faults is a matter of controversy. The stippled region shows the approximate position of the transverse shear zone expected under hypothesis A of the text. Intensity VII isoseismals of 1910 earthquakes are from Montero and Miyamura (1982). (b) Identification of geologic and geographic features of Figure 3a, more complete isoseismals of 1910 earthquakes, and epicenters of shocks with focal depths of 30 km or deeper that occurred in July 1976 to June Depths are given next to the epicenters.

7 N18*W 85osW N N N44 W, /'- ~ ^ ~ N46*E 45sw /,~oo\~ ~ o R ~ N72 E - \ of. - (- N10 W 86 SW N lll N N34 W ~ 9o...---'-'--'~'-P~ / r,~ ~ N56 E / ~ \oo o o ~,90 84 NW N N55oW 80 SW N50 E R ~ ' ~ NW LI, N I T, t,al, o %o IV ' %/ FIG. 4. Composite focal mechanism solutions for all shocks of Figure 3a ("ALL") and for subregions marked in Figure 3a. Data are not sufficient to uniquely determine the type of focal mechanism characteristic of subregion V. First motions are plotted on the lower focal hemisphere. Open circles are dilatations; closed circles are compressions. P and T are, respectively, the pressure and tension axes implied by the solution. At the end of each nodal plane are plotted its strike (top) and dip (middle), and whether slippage on the plane would correspond principally to right-lateral strike-slip (RL) motion, leftlateral strike-slip motion (LL), oblique slip combining left-lateral strike-slip motion and vertical motion with south side downthrown (OB), or reverse faulting, north side downthrown (RV). 1617

8 1618 WALTER MONTERO P. AND JAMES W. DEWEY in 4 days: 29 March 1978; 10 April 1978; 17 March 1979; and 18 March The zone south of San Jose and Cartago was more continuously active through the period of our study, but the swarm in this zone in December 1977 was the most intense swarm recorded in our study. The earthquakes of Figure 3a occurred for the most part in the upper crust; 75 per cent had computed focal depths of less than 15 km. The precision of focal depth determination is not sufficiently high to define the depth distribution of crustal earthquakes more precisely. We have plotted epicenters for shocks with focal depths of 30 km or deeper from the period July 1976 to June 1978 in Figure 3b. These subcrustal shocks are deeper members of the Benioff Zone dipping inland from the Pacific Coast. Throughout the region covered by Figure 3, a and b, the Benioff Zone is deep enough that we are confident that almost all of the earthquakes plotted in Figure 3a actually occurred in the overriding plate above the Benioff Zone. A composite focal mechanism (Figure 4, "ALL") determined from P-wave first motions of all shallow-focus shocks shown in Figure 3a implies that faulting is occurring predominantly as right-lateral strike-slip faulting on north-northweststriking faults or as left-lateral strike-slip faulting on east-northeast-striking faults, or both. We have attempted to account for some of the inconsistent first motions in the general focal mechanism solution ("ALL") of Figure 4 by dividing the region of highest seismicity into subregions for which plots of P-wave first motions are more internally consistent, following the method of Mendiguren (1980). The geographical subdivisions are shown in Figure 3a and the corresponding focal mechanisms in Figure 4. The effect of thus subdividing the Valle Central is to identify two subregions, II and III, with nearly pure strike-slip faulting similar to that of the general focal mechanism. Another subregion, I, has a focal mechanism indicating a component of vertical displacement in addition to strike-slip motion that is similar to that of the general focal mechanism. First motions of subregions IV and V can be interpreted in terms of vertical motion on faults. It is possible, however, that the reduction of the number of inconsistent first motions in going from the general focal mechanism solution ("ALL") to the subregionalized solutions is not significant, but is due only to the vastly increased number of parameters available to fit the first motions when the shocks are grouped in subregions. In addition, grouping of data into small subregions increases the likelihood of bias in focal mechanism due to location errors. We, therefore, are not convinced that the subregionalized focal mechanisms are more reliable than the overall focal mechanism solution of Figure 4 ("ALL"). In particular, although studies in other regions of high heat flow and/or Quaternary volcanoes (e.g., Hill, 1977; Klein et al., 1977; Freidline et al., 1976; Langer et al., 1974) show that strike-slip focal mechanisms and dip-slip mechanisms may coexist in a small region, we must regard the evidence for vertical displacement in the Valle Central earthquakes as suggestive but not conclusive. Both the general focal mechanism and the most reliable subregion mechanisms (Figure 4--"ALL," "I," "II," and "IIr') indicate a preponderance of strike-slip faulting. RELATIONSHIP OF RECENT SMALL EARTHQUAKES TO EARLIER DESTRUCTIVE SHALLOW EARTHQUAKES IN THE VALLE CENTRAL The earthquake sequence of April and May 1910 constitutes the major seismic disaster in the history of Costa Rica. The shock of 13 April caused important damage to public buildings and the fall of some houses in the region of San Jose and

9 SEISMICITY, FOCAL MECHANISM, AND TECTONICS IN COSTA RICA 1619 Cartago, and the earthquake of 4 May causes severe destruction in Cartago and more than 300 deaths (Gonzalez Viquez, 1910). Montero and Miyamura (1982) have studied the earthquakes of 13 April and 4 May 1910 and prepared isoseismal maps for them. Isoseismals for both shocks corresponding to Modified Mercalli intensity greater than V are shown on Figure 3b. The 1910 shocks occurred very close to the region of most intense small earthquake activity recorded in the period July 1976 to June 1979, so that a seismic source zone with dimensions of several tens of kilometers could encompass both the epicenters of the 1910 shocks and the epicenters of the recent small earthquakes. However, the 1910 isoseismals are too sharply defined to permit the 1910 earthquakes to occur at precisely the same location as the dense concentration of recent shocks; the 1910 earthquakes seem to be reliably located 5 to 10 km north of the recent shocks, and the 4 May 1910 earthquake must have been located very near to Cartago. In view of the fact that our composite focal mechanism solutions (Figure 4) show predominantly strike-slip faulting on either east-northeast-striking faults, with leftlateral displacement, or north-northwest-striking faults, with right-lateral displacement, we think it most likely that the 1910 earthquakes also occurred on a fault or faults with one of these orientations and senses of displacement. A single east-northeast-striking strike-slip fault could be made to pass through the meizoseismal regions of both the 13 April and 4 May 1910 earthquakes. If we propose that the 4 May earthquake occurred on a north-northwest-striking strike-slip fault, a different fault seems to be required for the 13 April earthquake, because the 13 April earthquake's meizoseismal region was significantly to the west of the meizoseismal region of the 4 May 1910 earthquake (Figure 3). As an alternative to a strike-slip focal mechanism, we might postulate that the focal mechanisms of the 1910 earthquakes were similar to the focal mechanism determined for subregion IV (Figure 4), the subregion nearest the sources of the 1910 earthquakes (Figure 3a). That focal mechanism is consistent with reverse faulting, north side downthrown, on a west-northwest-striking fault. However, as noted previously, the focal mechanism of subregion IV may be a spurious result arising from location errors and/or the availability of too many free parameters with which to fit the observed first motions. Presently available geological evidence does not decisively tip the scales in favor of any of the several fault types consistent with the seismological data. Berrang~ (1977) proposed that an alignment of thermal springs and sulfide mineralization south of Cartago marks the trace of a fault, which he names the Agua Caliente- Orosi fault, that produced the 4 May 1910 earthquake. The Agua Caliente-Orosi fault is shown in Figure 3 approximately as Berrang~ (1977) mapped it; Berrang~ suggested that the fault continues northwest of his region of study toward Cartago. The Agua Caliente-Orosi fault, as proposed by Berrang~, has a strike similar to the right-lateral strike-slip nodal plane of the overall Valle Central focal mechanism (Figure 4, "ALL"). However, R. D. Krushensky (personal communication, 1981) on the basis of his mapping experience in and near Cartago (Krushensky, 1972; Krushensky et al., 1976), cautions us that the hot spring alignments in this region may be due to other geologic processes than major throughgoing faulting. Because of poor bedrock exposures (Krushensky, personal communication, 1981), in much of the region in which the Agua Caliente-Orosi fault is mapped, a search for more concrete evidence of an active throughgoing Agua Caliente-Orosi fault would prob-

10 1620 WALTER MONTERO P. AND JAMES W. DEWEY ably require that detailed geologic mapping be supplemented by a detailed microearthquake study centered on Cartago. The city of Cartago was heavily damaged in an earthquake on 2 September Like the shock of 4 May 1910, the 1841 earthquake produced heavy damage and fatalities in a region of limited extent that included the city itself. Deaths or heavy damages were also reported in 1841 from sites about 5 km north and west of the intensity VII isoseismal shown in Figure 3 for the 4 May 1910 earthquake (Gonzalez Viquez, 1910). This suggests that the epicenter of the 1841 shock may have differed by several kilometers from those of the 1910 shocks, or that the 1841 earthquake may have been slightly larger than either of the two strongest 1910 earthquakes. The region northeast of San Jose was shaken by a teleseismically recorded shock on 30 December 1952 (Miyamura, 1980). Damage was concentrated in the region between the volcanoes Barba and Irazu, near 10 03'N, 83 57'W, and near the zone of small earthquakes recorded in our study (Figure 3a). The fact that the 1952 earthquake was well-recorded teleseismically indicates that it had a magnitude of about 6.0. The teleseismically determined epicenter of the 1952 shock was tens of kilometers northeast of the zone of highest damage (Miyamara, 1980), but mislocations of tens of kilometers are common in Central America (e.g., Algermissen et al., 1974). We tentatively associate the 1952 shock with the seismic zone north-northeast of San Jose. The north slopes of the Valle Central, north of Heredia, Alajuela, and San Ramon (Figure 3), have experienced destructive earthquakes, such as the earthquakes of 30 December 1888 (Gonzalez Viquez, 1910) and 6 June 1912 (Miyamura, 1912). The epicenters of these shocks are difficult to determine precisely from intensity data, but these shocks do not seem to have been associated with source regions active during the period of our study., THE ORIGINS OF VALLE CENTRAL SEISMICITY Before evaluating different hypotheses on the origins of Valle Central shocks, we list what we think are the most important characteristics of shallow focus seismicity and geology in the Valle Central. (1) The Valle Central shocks occur near Quaternary volcanoes, but not always directly beneath the volcanic edifices. The most active region in our study area, the region south of San Jose and Cartago, occurs beneath Tertiary outcrops at a distance of several tens of kilometers from the volcano Irazu. The distance of this principal Valle Central source from Irazu is about the same as the distance of the Tilaran, Costa Rica, earthquake of 13 April 1973 (Plafker, 1973; Matumoto et al., 1976) from the nearest major Quaternary volcano, Arenal (Figure 2). Farther north, however, the source regions at San Salvador, E1 Salvador (Lomnitz and Schulz, 1966), Jucuapa, E1 Salvador (Meyer Abich, 1952), and Managua, Nicaragua (Brown et al., 1973; Algermissen et al., 1974) are actually within the Quaternary volcanic terrane. It is not clear, therefore, that the seismic region south of San Jose and Cartago is as closely associated with volcanic structures as the E1 Salvador and Nicaragua shocks appear to be. (2) The composite focal mechanism for the Valle Central as a whole (Figure 4, "ALL") and the mechanisms for subregions II and III (Figure 4) are similar to the few focal mechanisms that have been determined for shallow intraplate earthquakes northward in Central America. Focal mechanisms of the earthquakes to the north (e.g., Molnar and Sykes, 1969; Brown et al., 1973; Algermissen et al., 1974; Matumoto et al., 1976), like the Valle Central mechanism, have been indicative of strike-slip

11 SEISMICITY, FOCAL MECHANISM, AND TECTONICS IN COSTA RICA 1621 faulting on nearly vertical fault planes, with the nodal plane that would correspond to a left-lateral fault striking approximately northeast and the nodal plane that would correspond to a right-lateral fault striking approximately northwest. The orientations of the focal mechanisms are not identical, however; the left-lateral nodal plane of the overall Valle Central mechanism (Figure 4) has a more easterly strike than has yet been observed for volcanic-terrane earthquakes to the north through E1 Salvador. (3) Consider the hypothesis that the Central American volcanic chain is segmented by transverse strike-slip shear zones that strike at a high angle to the chain (Stoiber and Carr, 1973; Schmoll et al., 1975). One such structure is south of the volcanos Irazu and Turrialba, truncating the volcanic chain on the southeast. The location of the principal Valle Central source south of Cartago and San Jose is consistent with that seismic source occurring on the postulated transverse structure that truncates the volcanic chain, assuming that the transverse structure has an east-northeast strike parallel to the east-northeast plane in the Valle Central focal mechanism (Figure 3a). However, the seismic region north of San Jose does not lie on a projection of a major transverse offset of the volcanic chain. (4) Consider again the possibility that a strike-slip shear zone strikes east-northeast across the southern Valle Central, truncating the volcanic chain at the southeast. There is no suggestion in geologic maps of the Valle Central (Dondoli and Chavez, 1968; Krushensky, 1972; Krushensky et al., 1976; Berrang6, 1977) for the existence of a sharply defined strike-slip shear zone or major fault corresponding to the postulated transverse shear zone. The zone, if it exists at all, must be very diffuse. The maps of Dondoli and Chavez (1968) and Berrang6 (1977) do have east or east-northeast-striking faults with lengths of 10 to 20 km that might be postulated to be members of a diffuse shear zone. One of these faults, the Navarro fault, is plotted in Figure 3, because its location and strike are so similar to the locations and strikes of faults suggested by the seismicity data that we would like to emphasize the correspondence. However, these faults of Dondoli and Chavez (1968) and Berrang6 (1977), including the Navarro fault, are not mapped as strike-slip faults, and the faults are not mapped at all by Krushensky (1972), Krushensky et al. (1976), and Castillo and Krushensky (1977). (5) The principal Valle Central source south of Cartago and San Jose defined by the small earthquakes recorded in the period from July 1976 to June 1979 does not extend east-northeast along the trend of the hypothetical transverse shear zone considered in items (3) and (4) immediately above (Figure 3a). (6) The tendency for Valle Central earthquakes to occur in seismic swarms is similar to that noted for shallow source regions northward in Central America (e.g., Schulz, 1963; Lomnitz and Schulz, 1966; Carr and Stoiber, 1977). (7) The reoccurrence, after many decades, of destructive moderate-magnitude shallow earthquakes at Cartago is similar to the reoccurrence of such earthquakes at other locations in Central America, such as Jucuapa, E1 Salvador (Meyer-Abich, 1952), San Salvador, E1 Salvador (Lomnitz and Schulz, 1966), and Managua, Nicaragua (Brown et al., 1973; Algermissen et al., 1974). (8) The offset of the 1910 Valle Central earthquakes from the zone of most recent activity (see section "Relationship of Recent Small Earthquakes to Earlier Destructive Shallow Earthquakes in the Valle Central"), notwithstanding that the two sources are within about 10 km of each other, is similar to the mutual offset, by kilometers, of the sources of the Managua, Nicaragua earthquakes of 1931, 1968, and 1972 (Sultan, 1931; Brown et al., 1973; Algermissen et al., 1974). Schmoll et al.

12 1622 WALTER MONTERO P. AND JAMES W. DEWEY (1975) suggested, on the basis of a lineament study, that the faults that ruptured in the 1931, 1968, and 1972 Managua earthquakes are members of a zone of northeaststriking faults whose width exceeds 10 km. Very few of the 1972 Managua aftershocks occurred in the source regions of the 1931 and 1968 earthquakes (Langer et al., 1974; Ward et al., 1974). Both the Managua and Valle Central results therefore suggest that repetition of strong shocks in the same source region, noted in item 7 above, occurs as the result of slippage on distinct faults within that source region, and that a fault that produces a destructive shock may be quite inactive decades later. Let us now consider three hypotheses on the causes of the Valle Central earthquakes, in light of the above-mentioned characteristics. Hypothesis A. The Valle Central seismicity recorded in our study is similar to shallow-focus intraplate seismicity of the inland regions of Central America to the north. This type of seismicity is concentrated on intraplate strike-slip shear zones passing through transverse offsets of the Central American Volcanic Chain at high angles to the trend of the chain. Because they are basically second-order effects of plate motion, these shear zones are more diffuse and less clearly defined than plate boundary transform fault zones such as the San Andreas fault zone of California or the Motagua fault zone of Guatemala. Two fundamentally different versions of hypothesis A have been proposed previously (e.g., Stoiber and Carr, 1973; Dewey and Algermissen, 1974; Schmoll et al., 1975) but both versions seem to us to make the same predictions for the Valle Central. The strengths of hypothesis A are that it directly accounts for the frequent occurrence of destructive earthquakes near transverse offsets of the volcanic chain (items 3 and 7, above), and it directly accounts for the composite focal mechanisms of the shocks (item 2). A weakness of hypothesis A is that, without further embellishment, it does not predict the occurrence of earthquakes associated with strike-slip fault displacement away from projections of transverse offsets of the volcanic chain (item 3). The meager geologic evidence for the postulated transverse shear zones in the Valle Central (item 4) must also be counted as evidence against hypothesis A, notwithstanding that such transverse shear zones are not expected to be as dramatic as large plate boundary transform faults. Finally, the lack of a linear trend of epicenters corresponding to the postulated transverse shear zone (item 5) argues against hypothesis A; however, because of the rather short-time period covered by our study, the lack of a linear trend in our data is not conclusive evidence against the hypothesis. Hypothesis B. The Valle Central seismicity we recorded is similar to shallowfocus intraplate seismicity of the inland region of Central America to the north and is due to the response of many minor small faults to regional tensional stresses oriented east-southeast or regional compressional stresses oriented north-northeast. Shocks occur preferentially near the volcanic chain either because there are more favorably oriented faults near the volcanic chain or because inhomogeneities in the elastic properties of the earth's crust near the volcanoes tend to amplify regional elastic stress at favorable locations near the volcanic chain. Hypothesis B is similar to models considered for the region of Managua, Nicaragua, by Matumoto and Latham (1973), Dewey and Algermissen (1974), and Ward et al. (1974). Without further refinement, hypothesis B does not account for the reoccurrence of shocks at a particular region in the vicinity of the volcanic chain (item 7, mentioned previously) or for the apparent association of many Valle Central shocks with the southern termination of the volcanic chain (item 3). However, hypothesis B does account for the occurrence of shocks away from the southern

13 SEISMICITY, FOCAL MECHANISM, AND TECTONICS IN COSTA RICA 1623 termination of the volcanic chain (item 3). Hypothesis B accounts for gross similarities, but not the differences in detail, of the distribution of earthquakes with respect to volcanoes (item 1) and the focal mechanism (item 2) of Valle Central shocks compared with intrap ate earthquakes farther north in Central America. A major advantage of hypothesis B is that it does not require special pleading to account for the sparse geologic evidence of recent strike-slip faulting (item 4) or the absence of a strong regional linear trend of epicenters (item 5). Hypothesis C. The Valle Central seismicity we recorded is due to a different cause than the intraplate seismicity farther north in Central America. The Valle Central shocks that we recorded occurred on an east-northeast-striking left-lateral transform fault zone between northern Costa Rica, attached to the Caribbean plate, and southern Costa Rica, which is strongly coupled to the Cocos Ridge and which will ultimately become attached to the Nazca plate. Hypothesis C is a refinement of the hypothesis of Van Andel et al. (1971), constructed so as to be most consistent with the location and focal mechanisms of the Valle Central earthquakes. It is well to emphasize that Van Andel et al. {1971) do not insist, as hypothesis C does, that the future boundary between the Nazca and Caribbean plates pass through the Valle Central. Hypothesis C has the virtue of explaining several slight differences between the seismicity and focal mechanism of the Valle Central and the seismicity and focal mechanisms farther north. Under hypothesis C, the greater distance of the principal seismic source region of the Valle Central from the volcanic chain, compared with the distance of destructive Nicaragua and E1 Salvador shocks from the volcanic chain (item 1), might reflect the difference between a region of high shear stress across the entire width of Central America in Costa Rica, and a region of intraplate stress concentrations within kilometers of volcanic structures, farther north. The nodal plane of the composite Valle Central focal mechanism, whose strike differs slightly from focal mechanisms of earthquakes farther north (item 2), has the same strike and sense of displacement that would be anticipated for a transform fault between the Caribbean and Nazca plates (compare the east-northeast-striking plane of Figure 4, "ALL," with "F.C.N.B.", Figure 1). In general, however, the similarities between the seismicity and focal mechanisms of the Valle Central and the seismicity and focal mechanisms of regions farther north listed under items 1, 2, 6, 7, and 8 are more conspicuous than the slight differences noted in items 1 and 2. In addition, although hypothesis C has the same advantage as hypothesis A in explaining the occurrence of the principal Valle Central source near the southeastern terminus of the Central America volcanic chain, hypothesis C would seem to be even more difficult than hypothesis A to reconcile with the lack of geologic evidence of major recent eastnortheast-striking left-lateral faults in the Valle Central (item 4) or the absence of a strong regional linear trend of epicenters (item 5). Because the relative motion across the developing plate boundary of hypothesis C should be higher than the relative motion across the intraplate transverse shear zones of hypothesis A, the geological evidence for an elongated left-lateral shear zone should be more pronounced under hypothesis C than under hypothesis A. CONCLUSIONS Overall, the similarities between the small earthquakes recorded in our study and the shallow-focus seismicity of intraplate regions of Central America to the north seem to us to support hypotheses (such as hypotheses A and B of the preceding section) that would consider the Valle Central seismicity we recorded to be the same

14 1624 WALTER MONTERO P. AND JAMES W. DEWEY type of seismicity as that in the intraplate regions to the north. On this basis, we would look to the shallow-focus source regions in the north for analogs to the shallow focus source regions in the Valle Central near San Jose and Cartago. We suggest that the principal seismic hazard from these Valle C~ntral source regions is due to moderate-magnitude (magnitude 5.0 to 6.5) shallow-focus earthquakes that produce intense shaking in a rather small area, such as the Cartago earthquakes of 1841 and 1910 or the earthquakes of Managua, Nicaragua, and San Salvador, and Jucuapa, E1 Salvador. Validity of the hypothesis (called hypothesis A in the preceding section) that intraplate Central American earthquakes concentrate on transverse strike-slip fault zones passing through offsets of the volcanic chain would obviously be a major help in pinpointing regions of unusually high seismic risk in the Valle Central and in Central America generally. For the present, however, the hypothesis should probably not be used as a basis for seismic zoning in the Valle Central until some of the apparent difficulties discussed in the previous section have been resolved. In evaluating the seismic risk of the Valle Central, we would probably want to give weight to the observational results (items 3 and 7) that seem most to support hypothesis A but not use hypothesis A to extrapolate much beyond the observational results. For example, hypothesis A would predict a strong tendency for large shocks to be concentrated on east-northeast-striking faults, whereas hypothesis B, that earthquakes occur on favorably oriented minor faults in response to a regional stress field, would predict an approximately equal tendency for shocks to occur on northnorthwest- and east-northeast-striking faults, provided that faults of both orientation were present in equal numbers. One should probably follow hypothesis B and assign equal seismic potential to otherwise similar faults of either orientation until such time as more accurate hypocenters conclusively demonstrate a tendency for shocks to occur preferentially on faults of one or the other of the two orientations. We have considered only one (hypothesis C) of many effects on the seismicity of Costa Rica that might result from the change in plate boundaries postulated by Van Andel et al. (1971). Our judgment against hypothesis C, in favor of the view that the earthquake sources we have identified are similar to intraplate sources in Central America to the north, does not address the possibility that the tectonic processes discussed by Van Andel et al. (1971) might result in large earthquakes outside of the Valle Central or even within the Valle Central in some region that was quiescent during the period of our study. For example, the shallow Benioff Zone to the south and west of the Valle Central has produced a rather high proportion of shocks with focal mechanisms different than the pure thrust-fault mechanisms common in the Benioff Zone farther north in Central America (Dean and Drake, 1978); these different mechanisms may be an effect of the processes associated with the postulated change in plate boundaries. At the western end of the region covered by Figure 3, near San Ramon, there was an M = 7.0 earthquake on 4 March The damage caused by this earthquake was intense enough near San Ramon (Miyamura, 1980) that the shock might be postulated to have occurred at a shallow depth in the Caribbean plate rather than deeper in the Benioff Zone. This earthquake would be our likeliest candidate, among shocks in the historical record, for a large shallow earthquake that would be different both from Benioff Zone earthquakes and from the shallow-focus intraplate earthquakes northward in Central America. ACKNOWLEDGMENTS We thank M. J. Carr, E. Kuijpers, D. H. Harlow, R. D. Krushensky, and W. J. Spence for critical reading of the manuscript and suggestions for its improvement; we gratefully acknowledge R. D.

15 SEISMICITY, FOCAL MECHANISM, AND TECTONICS IN COSTA RICA 1625 Krushensky, in addition, for extensive correspondence and discussion on the nature of faulting in the Cartago region. This investigation was partially funded by the Proyecto Multinacional de Ciencias de la Tierra of the Organization of American States and by the Vicerrectoria de Investigacion, Universidad de Costa Rica. REFERENCES Algermissen, S. T., J. W. Dewey, C. J. Langer, and W. H. Dillinger (1974). The Managua, Nicaragua, earthquake of December 23, 1972: location, focal mechanism, and intensity distribution, Bull. Seism. Soc. Am. 64, Berrang6, J. P. (1977). Reconnaissance geology of the Tapanti quadrangle Talamanca Cordillera, Institute of Geological Sciences, Overseas Division, London, Report no. 37, Brown, R. D. (1968). Managua, Nicaragua earthquake of January 4, 1968, Proj. Rept. Nicaragua Invest., U.S. Geol. Surv. Internal Rept., NI-1, 16 pp. Brown, R. D., Jr., P. L. Ward, and G. Plafker (1973). Geologic and seismologic aspects of the Managua, Nicaragua, earthquakes of December 23, 1972, U.S. Geol. Survey Profess. Paper 838, 34 pp. Carr, M. J. and R. E. Stoiber (1977). Geologic setting of some destructive earthquakes in Central America, Bull. Geol. Soc. Am. 88, Castillo, M. R. and R. D. Krushensky (1977). Geologic map and cross section of the Abra Quadrangle, Costa Rica 1:50,000, U.S. Geol. Surv. Misc. Invest. Series, Map Dean, B. W. and C. L. Drake (1978). Focal mechanism solutions and tectonics of the Middle America Arc, J. Geol. 86, DeBoer, J. (1979). The outer arc of the Costa Rica orogen (oceanic basement complexes of the Nicoya and Santa Elena Peninsulas), Tectonophysics 56, Dewey, J. W. and S. T. Algermissen (1974). Seismicity of the Middle America Arc-Trench system near Managua, Nicaragua, Bull. Seism. Soc. Am. 64, Dondoli, C. and R. Chaves (1968). Mapa adjunto al estudio geologico del Valle Central 1:150,000, Direccion de Geologia, Minas y Petroleo, Ministerio de Industria, y Comercio, San Jose, Costa Rica. Freidline, R. A., R. B. Smith, and D. D. Blackwell {1976). Seismicity and contemporary tectonics of the Helena, Montana area, Bull. Seism. Soc. Am. 66, Gonzalez Viquez, C. {1910). Temblores, terremotos, inundaciones y erupciones volcanicas en Costa Rica, , Tipografia de Avelino Alsina, San Jose, Costa Rica, 200 pp. Hill, D. P. (1977). A model for earthquake swarms, J. Geophys. Res. 82, Jordan, T. H. (1975). The present-day motions of the Caribbean Plate, J. Geophy. Res. 80, Klein, F. W., P. Einarsson, and M. Wyss (1977). The Reykjanes Peninsula~ Iceland, earthquake swarm of September 1972 and its tectonic significance, J. Geophys. Res. 82, Krushensky, R. D. (1972). Geology of the Istard Quadrangle, Costa Rica, U.S. Geol. Surv. Bull. 1358, 46 pp. Krushensky, R. D., E. Malavassi V., and R. Castillo M. (1976). Reconnaissance geologic map and cross sections of central Costa Rica 1:100,000, U.S. Geol. Surv. Misc. Invest. Series, Map Langer, C. J., M. G. Hopper, S. T. Algermissen, and J. W. Dewey (1974). Aftershocks of the Managua, Nicaragua earthquake of December 23, Bull. Seism. Soc. Am. 64, Lee, W. H. K. and J. G. Lahr (1975). HYPO 71 (revised): a computer program for determining hypocenter, magnitude, and first motion pattern of local earthquakes, U.S. Geol. Surv., Open-File Rept , 111 pp. Lomnitz, C. and R. Schulz (1966). The San Salvador earthquake of May 3, 1965, Bull. Seism. Soc. Am. 56, Macdonald, G. A. {1972). Volcanoes, Prentice-Hall, Englewood Cliffs, New Jersey, 510 pp. Matumoto, T. and G. Latham (1973). Aftershocks and intensity of the Managua, Nicaragua, earthquake of December 1972, Science 181, Matumoto, T., G. Latham, M. Ohtake, and J. Umana (1976). Seismic studies in northern Costa Rica (abstract), EOS 57, 290. Matumoto, T., M. Ohtake, G. Latham, and J. Umana (1977). Crustal structure in Southern Central America, Bull. Seism. Soc. Am. 67, Mendiguren, J. A. (1980). A procedure to resolve areas of different source mechanism using the method of composite nodal plane solution, Bull. Seism. Soc. Am. 70, Meyer-Abich, H. {1952). Das Erdbeben von Jucuapa in E1 Salvador (Zentralamerika) vom 6 and 7 Mai 1951, Neues Jahrb. Geol. Palaeontol., Abhandl. 95, Miyamura, S. (1980). Sismicidad de Costa Rica, Editorial Universidad de Costa Rica, 190 pp. Molnar, P. and L. R. Sykes {1969). Tectonics of the Caribbean and Middle America regions from focal mechanisms and seismicity, Bull. Geol. Soc. Am. 89, Montero, W. and S. Miyamura (1982). Distribucion de intensidades y estimacion de los parametros

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