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Microseismicity and stresses in the Lesser Antilles dipping seismic zone
Earth and Planetary Science Letters, 62 (1983) 340-348 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
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Microseismicity and stresses in the Lesser Antilles dipping seismic zone
Nicole Girardin and Roland Gaulon Laboratoire de Sismologie, L.A. 195, C.N.R.S., Institut de Physique du Globe, 4, place Jussieu, 75230 Paris Cedex 05 (France)
Received September 2, 1982 Revised version accepted November 30, 1982
Microearthquake data from a telemetered array have been used to study the central Antilles arc system. The zone lying between lat. 14-17°N and long. 59.5-62°W has been investigated. During the period 1979-1980 no shock with a magnitude larger than 4.5 was recorded. Results are well correlated with previous conclusions derived from teleseismic records. The dip of the subducting plate is 60 ° everywhere in this area. No composite focal mechanism can be produced for the region north of Dominique. For the southern region, composite focal mechanisms reveal two zones: For depths between 45 and 80 km, the stress field seems mainly related to the interplate action. Beneath 80 kin, the slab dominantly neither in compression nor in tension, earthquakes are more likely related to local stress fields. A double-layered structure has not been found in the earthquake distribution in the subducted plate nor in the stress field deduced from focal mechanisms.
1. Introduction
The Lesser Antilles arc is an extreme case of the subduction of old lithosphere (90 Ma) having a very slow convergence rate (2 c m / y r ) . The spatial distribution of earthquakes beneath the trench/island arc system is one of the most direct indications of the kinematic process operating there. Dorel [1,2] and Stein et al. [3] have described the seismicity pattern of the Lesser Antilles arc based on teleseismic data, Dorel [2] also used data from the French network. The regional French telemetered network was upgraded in late 1978 to include digital recording. Eleven stations were set up on the islands of Antigua, Guadeloupe, Dominique and Martinique (Fig. 1). The data of this network are mainly used for regional studies. Two seismological networks have also been set up around two volcanoes: La
Contribution I.P.G. No. 619. 0012-821X/83/$03.00
© 1983 Elsevier Science Publishers B.V.
Soufri6re in Guadeloupe and La Montagne Pel6e in Martinique for volcanic surveys. The slow slip-rate estimates based on seismic moment and macroseismic data and the lack of large thrust earthquakes (no information is available on the mechanism of the great earthquake of 1843) have led to the conclusion that subduction is at least partially decoupled and aseismic. A detailed cross-section of the seismicity can contribute to our understanding of the behavior of the South American plate beneath the Caribbean plate. Large-magnitude shocks in this area recorded by the global teleseismic network are relatively scarce so that, for this purpose, it was very important to improve regional networks in the islands of the arc. In this paper, the regional seismicity covering two years (1979-1980), using the French regional network, has been considered to study the subduction zone between latitude 14-17°N and longitude 59.5-62°W. For this period, no shock with a magnitude larger than 4.5 was recorded. Because of a
341
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partial lack of teleseismic data, it would be interesting to know if information given by this data set (seismicity and composite fault mechanisms) can be related to subduction in this region.
2. Data Fig. 1 shows the location of the regional network stations. Signals from eleven vertical shortperiod seismometers are transmitted via FM radio to a central digital recording station (PAG for the northern stations and F D F for the southern ones). P A G and F D F are three-component stations. The geometry of the network along the arc shows an evident gap in azimuth for the determination of
western and easternmost epicenters. About 400 hypocenters have been computed from these records. For most of them, the magnitude is less than 3.5. Computation was done with the program HYPO71 [4] using a velocity structure determined by Dorel et el. [5] from long-range refraction profiles. It consists of a three-layer model with P velocities of, respectively, 3.5 k m / s , 6.0 k m / s , 7.0 k m / s and a mantle velocity of 8.0 k m / s . The thicknesses of the three layers are 3, 12 and 15 km. According to Dorel's [1] study, the P- to S-wave velocity ratio is taken to be 1.85. This value is too small for areas southwest of the network where the ratio is likely larger, roughly 1.95 [1]. The main difficulty was the determination of depth. Five initial depths were tried (0, 30, 50, 100 and 150 km) and then four trial were again computed around the depth where the error was the smallest. A recent calibration shot at sea, 120 km northeast of Dominique has shown that, for a surface focus, the error in epicentral determination is less than 5 km in this region [2]. It is well known that the high P-wave velocity of the descending slab introduces a bias in the epicenter location [6]. The program CHEAP, written by Tarantola and Landre [7], allows one to compute the probability density of hypocentral location and especially probability density. It is also possible to take into account an estimation of the errors in the arrival times predicted by the propagation tables and with the reference to the stations involved; 0.5 seconds is certainly the maximum error due to the part of the path within the slab. Several determinations were recomputed with CHEAP. These hypocenters agree well with those calculated with the program HYPO71. Hence depth accuracy is estimated to be + 10 km and geographic location is better than 10 km. Precision is less for western and easternmost shocks. However, even doubtful hypocenter locations were kept as evidence for the seimicity of the region.
3. Seismicity Fig. 2 shows the seismicity map for the 1979-1980 time interval where foci are divided
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into three groups according to depth ranging from 0 to 30 kin, ~0 to 100 km and beyond 100 km. As already suggested by Stein et al. [3], there is no evidence of a plate boundary at 15°N between the North America (NOAM) and the South America (SOAM) plates which should show as a line of shallow seismicity extending into the Atlantic Ocean. The seismic gap located by Dorel [2] northeast of Guadeloupe is in the northern region of the zone investigated in this paper. Thus, the low level of seismicity observed in this area may be related to this gap or to the poor accuracy of the locations in this region.
The aseismic front noted by Yoshii [8] for northeastern Japan is not evident in the Lesser Antilles seismicity map but this may be due to the uncertainties of the location in the eastern part of the investigated region. Five cross-sections give the distribution of seismicity with depth. The first is along the arc and the four others are approximately perpendicular to the arc. No event has been discarded except when the longitude is larger than 62°W and smaller than 60°W. Along the arc (Fig. 3), two shallow clusters correspond to the Soufri6re in Guadeloupe and to a linear area from Caravelle to Le Lamentin in the central part of Martinique. The largest concentration of earthquakes in the investigated region (59.5-62°W, 14-17°N) lies south of 15.5°N, whatever the depth. The largest depth is 200 km for any latitude. Most of the seismicity is observed at depths between 25 and 35 km. This fact has already been noted by Stein et al. [3], but was considered to be an artefact of the default depth fixed at 33 km by I.S.C. and U.S.G.S. Depth locations determined with our regional network should not present this defect. The spatial distribution of the seismic activity gives a lower limit of about 35 km for the thickness of the seismically active layer. This high rate of seismic activity suggests that this is a region of high stress along the interface between the upper and lower plates. Fig. 4 shows the four cross-sections perpendicular to the central part of the arc (the width of
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to the arc (A 1. Bl to A4 - 84 shown in Fig. 2). The sections are 100 km
each section being 100 km). Dip for the four cross-sections are roughly 60’ but are shallower above 40 km. This distribution, as determined by the local network, is remarkably similar to that obtained by teleseismic data for the 1950-1978 time interval [2]. Hypocenters are located in a 25km band of the subducted plate, but, if we consider that orthogonal projection of the hypocenters, 50 km from each side of the vertical section, increases the apparent thickness of the
seismic layer which may be significantly thinner. Important points in this section are: (1) the 60” dip of the subducting plate; (2) the high level of seismicity located at a depth of 30 km; and (3) the seismicity being concentrated in a 2%km band of the subducting plate below 30 km. Small-magnitude earthquakes are representative of the regional subduction pattern and focal mechanisms may add information about the process of subduction.
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4. Composite fault mechanisms In this section, we have used deep events from 1978 to 198 1. During this period, no shock was large enough to be well recorded by teleseismic networks to obtain a reliable source mechanism. Composite mechanisms do not give convincing results if the focal mechanism orientation changes from place to place. In the case of a downgoing slab, it is important to identify areas where the recorded events have the same source mechanism. Following the method proposed by Mendiguren [9], regions with approximately identical mechanisms were isolated. Subregions were determined where P polarities are the same for different earthquakes at a given recording station. Subregions are thus obtained for each station and this is an indication that all events in these zones have the same source mechanism. But, obviously, this is neither a necessary condition (the case of a station close to a nodal plane) nor a sufficient one. It is, therefore, expected that a coherent composite mechanism may be obtained for each subregion. For events located from 14.3” to 15.6”N and 60” to 62’W, two large subregions with different depths are suggested by the results given by the stations FDF, CRM and PAG (Fig. 5): one from 45 to 80 km where first motions are compressions for the three stations and another one below 80 km where first motions are dilatations for FDF and CRM DISTANCE 1
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and compressions for PAG. Above 45 km, nc subregion could be isolated. Below 45 km, foci art situated within the downgoing plate. First motion5 are shown on an equal area projection of the lowel hemisphere. The number of inconsistent observations is very low even if the uncertainties of epicenter locations and the take-off angle are taken into account (Fig. 6a, b). For depths lying between 45 and 80 km, 12 events have been used. One plane is fairly well constrained (azimuth 316”, dip 30”); the other one has two extreme possibilities as shown in Fig. 6a. Both solutions show thrusting mechanism. Whatever the chosen solution is, this earthquake has a thrusting mechanism. Below 80 km, 24 events have been used. A possible extension mechanism with an important strike-slip component is defined by one plane (azimuth 80”, dip 66”); azimuths of the extreme positions of the second plane are 178” and 328” and dips are respectively 70” and 54” (Fig. 6b). The resolution of the transition between the two regions is given by the precision of the depth determination (10 km). At the latitude of Guadeloupe, such subregions cannot be isolated, perhaps because the number of data is too small, but it appears that the tectonics of this region is quite different from that of the southern region. An event occurred January 30, 1982, in this region, located northeast of Guadeloupe. Its magnitude
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,I Fig. 6. Focal compositemechanismsstudied in this paper (equal area, lower hemisphere). The dotted plane is the Benioffplane. Open circles represent dilatations and full circles, compressions.(a) Focal depth between 45 and 80 km. The dashed lines are the projections of the extremepositions of the less constrained plane. (b) Focal depth larger than 80 km. The dashed lines are the projections of the extreme positions of the less constrained plane.
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given by the U.S.G.S. is 6.0. Arrival times to the stations of the local network and synthetic waves (P, pP and sP) computed for several stations of the LP teleseismic network gave both a depth of 55 km. The mechanism obtained with P-wave polarities from local and teleseismic stations (Fig. 8) (P axis: azimuth 62 °, dip 35°; T axis: azimuth 275 °, dip 51 °) is a compressional one and disagrees with our composite one for the southern area, for this depth range. This may be related to the asymmetry of the subduction process suggested by Bouysse [10] between the north and south of the Lesser Antilles:
5. D i s c u s s i o n
Descriptions of subduction zone seismicity are mainly based on teleseismic determinations and are often incomplete and inaccurate. The small number of large events in the Caribbean region since the introduction of the World-Wide Seismic Station Network (W.W.S.S.N.) made the seismicity of this region poorly documented. Therefore
local networks may provide a wealth of data necessary to study the fine details of seismicity in this active subduction zone. Location precision is now within 10 km both in epicenters and focal depths. The vertical distribution of seismicity is rather uniform along the arc for the investigated region. N o segmentation is revealed by this distribution. However, focal mechanisms show a difference between the northern and the southern parts. Historic seismicity also appears to support this distinction: a great earthquake shook the northern area between north of Dominique and St. Kitts in 1843 with a magnitude, estimated after its isoseists, greater than 8 and no similar event is known in the southern area. In this latter region the maximum reported magnitude, about 7, is similar to that predicted by Ruff and Kanamori [11] for an old and slow slab. It is yet difficult to understand why such a large earthquake ruptured the northern Lesser Antilles. The dip of the slab is not significantly shallower here than in the southern zone. Perhaps it may be related to recent interaction of the northern segment with anomalous aseismic ridges (Barracuda and Tiburon Ridges) as
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suggested by Stein et al. [3] or due to oblique convergence. This problem remains unresolved and it is essential to make progress in this field for a correct estimation of seismic risk in this region. Seismic cross-sections do not show any twolayered structures of the descending slab. Such double Benioff zones have been identified for several subduction regions [12-16]. But this observation is not a general feature of Benioff zones [ 17,18]. The two layers of double seismic zones are separated by 30 or 40 km. Therefore the accuracy of the hypocenters as estimated in this study is able to reveal this separation. Seismic activity in the two layers varies greatly and it is often reported that the number of events of the lower plate is smaller, but magnitude would be greater. Therefore it is reasonable to think that no indication of a true double seismic zone can be seen in our cross-sections. This conclusion is corroborated by the seismicity previously mapped by Dorel [1] and Stein et al. [3]. Teleseismic locations are too much inaccurate in comparison to results of local networks to be used in detailed seismicity studies. Moreover they are more biased by the high P-wave velocity of the cold downgoing slab. However, where control on hypocentral depth is available, the results based on teleseismic data show no substantial bias or distortion of the spatial distribution [19]. Seismicity in the Virgin Islands region, at the north of the investigated zone provides a detailed picture at the corner boundary of the Caribbean plate. There is no evidence of a two-layered slab in the cross-sections presented by Frankel et al. [20]. Another view of a possible double seismic zone is the examination of the stress field as determined from focal mechanisms of intermediate depth earthquakes. It is clear that the coherence of the polarity of first motions of P waves excludes two different focal solutions in this range of depth. First motions readings are only consistent with a compressive slab in the depth range 45-80 km, without mixture. In conclusion, mechanisms in the depth range 45-80 km clearly show reverse fault events. The dip angle of the less constrained nodal plane is quite similar to that of the Benioff plane as mapped in the cross-sections and uncertainty is mainly
about the azimuth of that plane. In this case, it is reasonable to choose it as the fault plane. Moreover, the seismic layer appears to be thin as we have said before. Therefore these events could be due to friction at the interface of the two plates, although it may seem rather paradoxal that interplate earthquakes occur at such a depth. Abnormal low mantle P-wave velocity beneath the islands arc could influence depth determination and the dip of the Benioff plane would be shallower. In this case, focal mechanism solution would indicate that rupture would only affect downgoing plate. Moreover, the uncertainty of take-off angles may be rather larger for local stations and modifies significantly nodal planes. Therefore we tried to examine other data. For this order of depth large events are very unusual in this area and no shock having a magnitude larger than 6 has occurred in this region during the last 25 years. The two largest shocks have a magnitude smaller than 5.7 (August 20, 1964, and September 7, 1974). Some thrust mechanisms can be found in Dorel [1] and Stein et al. [3] but as Stein et al. have noted, at least two solutions could be reasonably be accepted for most of them. Moreover, for this range of magnitude (smaller than 5.7) first-motion readings of teleseismic records are often doubtful. So we will only examine the two largest events. The earthquake of August 20, 1964, has been studied by several authors [1,21,22]. With a depth of 72 km, the first motion observed at F D F was a compression and the mechanism was a normal fault (P axis: azimuth 69 ° , dip 53°; T axis: azimuth 279 ° , dip 33°). According to our classification as a function of depth this event should be located in the upper zone of 45-80 km, but it completely disagrees with the composite focal solution of that zone. The plot of the first-motion distribution is rather similar to that of the lower part. Teleseismic determination of depth for intermediate earthquakes was probably not accurate better than 30 kin. There is also an important azimuthal gap of teleseismic data for the east of this region for events with magnitude less than 6 (only few European stations report arrival times for this event). For the earthquake of September 7, 1974, Fig. 7 shows the plot of the polarities read on W.W.S.S.N. records. Nodal planes are unconstrained but first motions are
347 N
N
Fig. 7. W.W.S.S.N. first motions for the September 7, 1974, event. Open circles represent dilatations and full circles, compressions.
Fig. 8. Focal mechanism for the January 30, 1982 event. P axis: azimuth 62 °, dip 35°; T axis: azimuth 275 °, dip 51 °. Open circles represent dilatations and full circles, compressions.
consistent with our compressional mechanism. It seems, hence, that our result is compatible with previous local studies and that, in the range 45-80 km, stress field would be controlled by compression. For the second region below 80 km, it also seems important to discuss previous results [3] and those suggested by composite solution (Fig. 6b). The latter, whatever second plane is chosen, has a tension axis that is horizontal and nearly north-south. Its azimuth varies from 307-127 ° to 206 ° and may be nearly perpendicular to the direction of the plate convergence in this region. Two extreme solutions can be given for this mechanism: (1) lateral extension of the downgoing plate with a rather strong right-lateral component if the east-west nodal plane is chosen as the fault plane and with, roughly, in-plate P axis; or (2) nearly pure strike-slip and a fault plane striking east-west. In the last 25 years only two shocks with magnitude larger than 6 have been recorded for these depths (January 8, 1959, and March 19, 1953) and Stein et al. [3] have shown plots of first-motion readings for these two events. The first one (01-08-
59) presents a strike-slip mechanism with an eastwest P axis and a north-south T axis. For the second event (03-19-53) it is impossible to draw carefully nodal planes but polarities are compatible with our readings. Reliability of these solutions is questionable because readings are taken from bulletins. However, U.S. stations show clearly that polarities of the 1953 event are systematically different from those that would be seen for a focal mechanism like the composite solution. These different focal solutions suggest that, below 80 kin, the stress field in the downgoing plate is dominantly neither compressive nor in tension. Local, undetermined factors seem to overcome other driving forces for most of the cases.
6. Conclusion
The low level of seismicity of the Caribbean arc explains that the stress field in this region is poorly documented. Therefore the newly installed network in central Lesser Antilles may greatly improve the knowledge of the subduction process.
348 Two years of records were studied in this paper to map this Benioff zone and to examine focal m e c h a n i s m s . T h e d i p o f t h e s l a b is c o n s t a n t e v e r y w h e r e a l o n g t h e arc. T h e s e i s m o g e n i c l a y e r i n t h e d o w n g o i n g p l a t e is t h i n a n d m o s t o f t h e e a r t h q u a k e s s e e m to o c c u r n e a r t h e i n t e r p l a t e b o u n d a r y . First-motion readings show that, for the region north of Dominique, t h e r e exists n o s p a t i a l c o h e r e n c e f r o m o n e e v e n t to a n o t h e r p r e v e n t i n g the determination of composite mechanisms. Further studies are required to obtain individual focal solution in this region where the largest historic e v e n t s o c c u r r e d . N o d o u b l e s e i s m i c z o n e is s e e n i n c r o s s - s e c t i o n s n o r i n t h e f o c a l m e c h a n i s m det e r m i n a t i o n s . B e t w e e n 45 a n d 80 k m , a n d f o r t h i s class of low magnitude, only thrust events are found, indicating that, at these depths, there may e x i s t s o m e f r i c t i o n a n d c o u p l i n g . B e n e a t h 80 k m , t h e r e is n o e v i d e n c e o f a s l a b d o m i n a n t l y i n t e n s i o n as it m i g h t b e s u g g e s t e d f o r a c o l d o l d s l a b . I n f a c t , it s e e m s t h a t o b s e r v e d e v e n t s w o u l d b e m o r e o r less r e l a t e d t o l o c a l s t r e s s fields.
Acknowledgements W e w o u l d like to t h a n k t h e F r e n c h V o l c a n o O b s e r v a t o r y t e a m s f o r t h e i r h e l p i n field o p e r a t i o n a n d d a t a p r o c e s s i n g , in p a r t i c u l a r J.C. D e l m o n d a n d J.P. V i o d e . J. D o r e l a n d R. M a d a r i a g a a r e gratefully acknowledged for helpful discussions on the topics of this paper.
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Microseismicity and stresses in the Lesser Antilles dipping seismic zone
Nicole Girardin and Roland Gaulon Laboratoire de Sismologie, L.A. 195, C.N.R.S., Institut de Physique du Globe, 4, place Jussieu, 75230 Paris Cedex 05 (France)
Received September 2, 1982 Revised version accepted November 30, 1982
Microearthquake data from a telemetered array have been used to study the central Antilles arc system. The zone lying between lat. 14-17°N and long. 59.5-62°W has been investigated. During the period 1979-1980 no shock with a magnitude larger than 4.5 was recorded. Results are well correlated with previous conclusions derived from teleseismic records. The dip of the subducting plate is 60 ° everywhere in this area. No composite focal mechanism can be produced for the region north of Dominique. For the southern region, composite focal mechanisms reveal two zones: For depths between 45 and 80 km, the stress field seems mainly related to the interplate action. Beneath 80 kin, the slab dominantly neither in compression nor in tension, earthquakes are more likely related to local stress fields. A double-layered structure has not been found in the earthquake distribution in the subducted plate nor in the stress field deduced from focal mechanisms.
1. Introduction
The Lesser Antilles arc is an extreme case of the subduction of old lithosphere (90 Ma) having a very slow convergence rate (2 c m / y r ) . The spatial distribution of earthquakes beneath the trench/island arc system is one of the most direct indications of the kinematic process operating there. Dorel [1,2] and Stein et al. [3] have described the seismicity pattern of the Lesser Antilles arc based on teleseismic data, Dorel [2] also used data from the French network. The regional French telemetered network was upgraded in late 1978 to include digital recording. Eleven stations were set up on the islands of Antigua, Guadeloupe, Dominique and Martinique (Fig. 1). The data of this network are mainly used for regional studies. Two seismological networks have also been set up around two volcanoes: La
Contribution I.P.G. No. 619. 0012-821X/83/$03.00
© 1983 Elsevier Science Publishers B.V.
Soufri6re in Guadeloupe and La Montagne Pel6e in Martinique for volcanic surveys. The slow slip-rate estimates based on seismic moment and macroseismic data and the lack of large thrust earthquakes (no information is available on the mechanism of the great earthquake of 1843) have led to the conclusion that subduction is at least partially decoupled and aseismic. A detailed cross-section of the seismicity can contribute to our understanding of the behavior of the South American plate beneath the Caribbean plate. Large-magnitude shocks in this area recorded by the global teleseismic network are relatively scarce so that, for this purpose, it was very important to improve regional networks in the islands of the arc. In this paper, the regional seismicity covering two years (1979-1980), using the French regional network, has been considered to study the subduction zone between latitude 14-17°N and longitude 59.5-62°W. For this period, no shock with a magnitude larger than 4.5 was recorded. Because of a
341
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partial lack of teleseismic data, it would be interesting to know if information given by this data set (seismicity and composite fault mechanisms) can be related to subduction in this region.
2. Data Fig. 1 shows the location of the regional network stations. Signals from eleven vertical shortperiod seismometers are transmitted via FM radio to a central digital recording station (PAG for the northern stations and F D F for the southern ones). P A G and F D F are three-component stations. The geometry of the network along the arc shows an evident gap in azimuth for the determination of
western and easternmost epicenters. About 400 hypocenters have been computed from these records. For most of them, the magnitude is less than 3.5. Computation was done with the program HYPO71 [4] using a velocity structure determined by Dorel et el. [5] from long-range refraction profiles. It consists of a three-layer model with P velocities of, respectively, 3.5 k m / s , 6.0 k m / s , 7.0 k m / s and a mantle velocity of 8.0 k m / s . The thicknesses of the three layers are 3, 12 and 15 km. According to Dorel's [1] study, the P- to S-wave velocity ratio is taken to be 1.85. This value is too small for areas southwest of the network where the ratio is likely larger, roughly 1.95 [1]. The main difficulty was the determination of depth. Five initial depths were tried (0, 30, 50, 100 and 150 km) and then four trial were again computed around the depth where the error was the smallest. A recent calibration shot at sea, 120 km northeast of Dominique has shown that, for a surface focus, the error in epicentral determination is less than 5 km in this region [2]. It is well known that the high P-wave velocity of the descending slab introduces a bias in the epicenter location [6]. The program CHEAP, written by Tarantola and Landre [7], allows one to compute the probability density of hypocentral location and especially probability density. It is also possible to take into account an estimation of the errors in the arrival times predicted by the propagation tables and with the reference to the stations involved; 0.5 seconds is certainly the maximum error due to the part of the path within the slab. Several determinations were recomputed with CHEAP. These hypocenters agree well with those calculated with the program HYPO71. Hence depth accuracy is estimated to be + 10 km and geographic location is better than 10 km. Precision is less for western and easternmost shocks. However, even doubtful hypocenter locations were kept as evidence for the seimicity of the region.
3. Seismicity Fig. 2 shows the seismicity map for the 1979-1980 time interval where foci are divided
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into three groups according to depth ranging from 0 to 30 kin, ~0 to 100 km and beyond 100 km. As already suggested by Stein et al. [3], there is no evidence of a plate boundary at 15°N between the North America (NOAM) and the South America (SOAM) plates which should show as a line of shallow seismicity extending into the Atlantic Ocean. The seismic gap located by Dorel [2] northeast of Guadeloupe is in the northern region of the zone investigated in this paper. Thus, the low level of seismicity observed in this area may be related to this gap or to the poor accuracy of the locations in this region.
The aseismic front noted by Yoshii [8] for northeastern Japan is not evident in the Lesser Antilles seismicity map but this may be due to the uncertainties of the location in the eastern part of the investigated region. Five cross-sections give the distribution of seismicity with depth. The first is along the arc and the four others are approximately perpendicular to the arc. No event has been discarded except when the longitude is larger than 62°W and smaller than 60°W. Along the arc (Fig. 3), two shallow clusters correspond to the Soufri6re in Guadeloupe and to a linear area from Caravelle to Le Lamentin in the central part of Martinique. The largest concentration of earthquakes in the investigated region (59.5-62°W, 14-17°N) lies south of 15.5°N, whatever the depth. The largest depth is 200 km for any latitude. Most of the seismicity is observed at depths between 25 and 35 km. This fact has already been noted by Stein et al. [3], but was considered to be an artefact of the default depth fixed at 33 km by I.S.C. and U.S.G.S. Depth locations determined with our regional network should not present this defect. The spatial distribution of the seismic activity gives a lower limit of about 35 km for the thickness of the seismically active layer. This high rate of seismic activity suggests that this is a region of high stress along the interface between the upper and lower plates. Fig. 4 shows the four cross-sections perpendicular to the central part of the arc (the width of
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to the arc (A 1. Bl to A4 - 84 shown in Fig. 2). The sections are 100 km
each section being 100 km). Dip for the four cross-sections are roughly 60’ but are shallower above 40 km. This distribution, as determined by the local network, is remarkably similar to that obtained by teleseismic data for the 1950-1978 time interval [2]. Hypocenters are located in a 25km band of the subducted plate, but, if we consider that orthogonal projection of the hypocenters, 50 km from each side of the vertical section, increases the apparent thickness of the
seismic layer which may be significantly thinner. Important points in this section are: (1) the 60” dip of the subducting plate; (2) the high level of seismicity located at a depth of 30 km; and (3) the seismicity being concentrated in a 2%km band of the subducting plate below 30 km. Small-magnitude earthquakes are representative of the regional subduction pattern and focal mechanisms may add information about the process of subduction.
344
4. Composite fault mechanisms In this section, we have used deep events from 1978 to 198 1. During this period, no shock was large enough to be well recorded by teleseismic networks to obtain a reliable source mechanism. Composite mechanisms do not give convincing results if the focal mechanism orientation changes from place to place. In the case of a downgoing slab, it is important to identify areas where the recorded events have the same source mechanism. Following the method proposed by Mendiguren [9], regions with approximately identical mechanisms were isolated. Subregions were determined where P polarities are the same for different earthquakes at a given recording station. Subregions are thus obtained for each station and this is an indication that all events in these zones have the same source mechanism. But, obviously, this is neither a necessary condition (the case of a station close to a nodal plane) nor a sufficient one. It is, therefore, expected that a coherent composite mechanism may be obtained for each subregion. For events located from 14.3” to 15.6”N and 60” to 62’W, two large subregions with different depths are suggested by the results given by the stations FDF, CRM and PAG (Fig. 5): one from 45 to 80 km where first motions are compressions for the three stations and another one below 80 km where first motions are dilatations for FDF and CRM DISTANCE 1
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and compressions for PAG. Above 45 km, nc subregion could be isolated. Below 45 km, foci art situated within the downgoing plate. First motion5 are shown on an equal area projection of the lowel hemisphere. The number of inconsistent observations is very low even if the uncertainties of epicenter locations and the take-off angle are taken into account (Fig. 6a, b). For depths lying between 45 and 80 km, 12 events have been used. One plane is fairly well constrained (azimuth 316”, dip 30”); the other one has two extreme possibilities as shown in Fig. 6a. Both solutions show thrusting mechanism. Whatever the chosen solution is, this earthquake has a thrusting mechanism. Below 80 km, 24 events have been used. A possible extension mechanism with an important strike-slip component is defined by one plane (azimuth 80”, dip 66”); azimuths of the extreme positions of the second plane are 178” and 328” and dips are respectively 70” and 54” (Fig. 6b). The resolution of the transition between the two regions is given by the precision of the depth determination (10 km). At the latitude of Guadeloupe, such subregions cannot be isolated, perhaps because the number of data is too small, but it appears that the tectonics of this region is quite different from that of the southern region. An event occurred January 30, 1982, in this region, located northeast of Guadeloupe. Its magnitude
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given by the U.S.G.S. is 6.0. Arrival times to the stations of the local network and synthetic waves (P, pP and sP) computed for several stations of the LP teleseismic network gave both a depth of 55 km. The mechanism obtained with P-wave polarities from local and teleseismic stations (Fig. 8) (P axis: azimuth 62 °, dip 35°; T axis: azimuth 275 °, dip 51 °) is a compressional one and disagrees with our composite one for the southern area, for this depth range. This may be related to the asymmetry of the subduction process suggested by Bouysse [10] between the north and south of the Lesser Antilles:
5. D i s c u s s i o n
Descriptions of subduction zone seismicity are mainly based on teleseismic determinations and are often incomplete and inaccurate. The small number of large events in the Caribbean region since the introduction of the World-Wide Seismic Station Network (W.W.S.S.N.) made the seismicity of this region poorly documented. Therefore
local networks may provide a wealth of data necessary to study the fine details of seismicity in this active subduction zone. Location precision is now within 10 km both in epicenters and focal depths. The vertical distribution of seismicity is rather uniform along the arc for the investigated region. N o segmentation is revealed by this distribution. However, focal mechanisms show a difference between the northern and the southern parts. Historic seismicity also appears to support this distinction: a great earthquake shook the northern area between north of Dominique and St. Kitts in 1843 with a magnitude, estimated after its isoseists, greater than 8 and no similar event is known in the southern area. In this latter region the maximum reported magnitude, about 7, is similar to that predicted by Ruff and Kanamori [11] for an old and slow slab. It is yet difficult to understand why such a large earthquake ruptured the northern Lesser Antilles. The dip of the slab is not significantly shallower here than in the southern zone. Perhaps it may be related to recent interaction of the northern segment with anomalous aseismic ridges (Barracuda and Tiburon Ridges) as
346
suggested by Stein et al. [3] or due to oblique convergence. This problem remains unresolved and it is essential to make progress in this field for a correct estimation of seismic risk in this region. Seismic cross-sections do not show any twolayered structures of the descending slab. Such double Benioff zones have been identified for several subduction regions [12-16]. But this observation is not a general feature of Benioff zones [ 17,18]. The two layers of double seismic zones are separated by 30 or 40 km. Therefore the accuracy of the hypocenters as estimated in this study is able to reveal this separation. Seismic activity in the two layers varies greatly and it is often reported that the number of events of the lower plate is smaller, but magnitude would be greater. Therefore it is reasonable to think that no indication of a true double seismic zone can be seen in our cross-sections. This conclusion is corroborated by the seismicity previously mapped by Dorel [1] and Stein et al. [3]. Teleseismic locations are too much inaccurate in comparison to results of local networks to be used in detailed seismicity studies. Moreover they are more biased by the high P-wave velocity of the cold downgoing slab. However, where control on hypocentral depth is available, the results based on teleseismic data show no substantial bias or distortion of the spatial distribution [19]. Seismicity in the Virgin Islands region, at the north of the investigated zone provides a detailed picture at the corner boundary of the Caribbean plate. There is no evidence of a two-layered slab in the cross-sections presented by Frankel et al. [20]. Another view of a possible double seismic zone is the examination of the stress field as determined from focal mechanisms of intermediate depth earthquakes. It is clear that the coherence of the polarity of first motions of P waves excludes two different focal solutions in this range of depth. First motions readings are only consistent with a compressive slab in the depth range 45-80 km, without mixture. In conclusion, mechanisms in the depth range 45-80 km clearly show reverse fault events. The dip angle of the less constrained nodal plane is quite similar to that of the Benioff plane as mapped in the cross-sections and uncertainty is mainly
about the azimuth of that plane. In this case, it is reasonable to choose it as the fault plane. Moreover, the seismic layer appears to be thin as we have said before. Therefore these events could be due to friction at the interface of the two plates, although it may seem rather paradoxal that interplate earthquakes occur at such a depth. Abnormal low mantle P-wave velocity beneath the islands arc could influence depth determination and the dip of the Benioff plane would be shallower. In this case, focal mechanism solution would indicate that rupture would only affect downgoing plate. Moreover, the uncertainty of take-off angles may be rather larger for local stations and modifies significantly nodal planes. Therefore we tried to examine other data. For this order of depth large events are very unusual in this area and no shock having a magnitude larger than 6 has occurred in this region during the last 25 years. The two largest shocks have a magnitude smaller than 5.7 (August 20, 1964, and September 7, 1974). Some thrust mechanisms can be found in Dorel [1] and Stein et al. [3] but as Stein et al. have noted, at least two solutions could be reasonably be accepted for most of them. Moreover, for this range of magnitude (smaller than 5.7) first-motion readings of teleseismic records are often doubtful. So we will only examine the two largest events. The earthquake of August 20, 1964, has been studied by several authors [1,21,22]. With a depth of 72 km, the first motion observed at F D F was a compression and the mechanism was a normal fault (P axis: azimuth 69 ° , dip 53°; T axis: azimuth 279 ° , dip 33°). According to our classification as a function of depth this event should be located in the upper zone of 45-80 km, but it completely disagrees with the composite focal solution of that zone. The plot of the first-motion distribution is rather similar to that of the lower part. Teleseismic determination of depth for intermediate earthquakes was probably not accurate better than 30 kin. There is also an important azimuthal gap of teleseismic data for the east of this region for events with magnitude less than 6 (only few European stations report arrival times for this event). For the earthquake of September 7, 1974, Fig. 7 shows the plot of the polarities read on W.W.S.S.N. records. Nodal planes are unconstrained but first motions are
347 N
N
Fig. 7. W.W.S.S.N. first motions for the September 7, 1974, event. Open circles represent dilatations and full circles, compressions.
Fig. 8. Focal mechanism for the January 30, 1982 event. P axis: azimuth 62 °, dip 35°; T axis: azimuth 275 °, dip 51 °. Open circles represent dilatations and full circles, compressions.
consistent with our compressional mechanism. It seems, hence, that our result is compatible with previous local studies and that, in the range 45-80 km, stress field would be controlled by compression. For the second region below 80 km, it also seems important to discuss previous results [3] and those suggested by composite solution (Fig. 6b). The latter, whatever second plane is chosen, has a tension axis that is horizontal and nearly north-south. Its azimuth varies from 307-127 ° to 206 ° and may be nearly perpendicular to the direction of the plate convergence in this region. Two extreme solutions can be given for this mechanism: (1) lateral extension of the downgoing plate with a rather strong right-lateral component if the east-west nodal plane is chosen as the fault plane and with, roughly, in-plate P axis; or (2) nearly pure strike-slip and a fault plane striking east-west. In the last 25 years only two shocks with magnitude larger than 6 have been recorded for these depths (January 8, 1959, and March 19, 1953) and Stein et al. [3] have shown plots of first-motion readings for these two events. The first one (01-08-
59) presents a strike-slip mechanism with an eastwest P axis and a north-south T axis. For the second event (03-19-53) it is impossible to draw carefully nodal planes but polarities are compatible with our readings. Reliability of these solutions is questionable because readings are taken from bulletins. However, U.S. stations show clearly that polarities of the 1953 event are systematically different from those that would be seen for a focal mechanism like the composite solution. These different focal solutions suggest that, below 80 kin, the stress field in the downgoing plate is dominantly neither compressive nor in tension. Local, undetermined factors seem to overcome other driving forces for most of the cases.
6. Conclusion
The low level of seismicity of the Caribbean arc explains that the stress field in this region is poorly documented. Therefore the newly installed network in central Lesser Antilles may greatly improve the knowledge of the subduction process.
348 Two years of records were studied in this paper to map this Benioff zone and to examine focal m e c h a n i s m s . T h e d i p o f t h e s l a b is c o n s t a n t e v e r y w h e r e a l o n g t h e arc. T h e s e i s m o g e n i c l a y e r i n t h e d o w n g o i n g p l a t e is t h i n a n d m o s t o f t h e e a r t h q u a k e s s e e m to o c c u r n e a r t h e i n t e r p l a t e b o u n d a r y . First-motion readings show that, for the region north of Dominique, t h e r e exists n o s p a t i a l c o h e r e n c e f r o m o n e e v e n t to a n o t h e r p r e v e n t i n g the determination of composite mechanisms. Further studies are required to obtain individual focal solution in this region where the largest historic e v e n t s o c c u r r e d . N o d o u b l e s e i s m i c z o n e is s e e n i n c r o s s - s e c t i o n s n o r i n t h e f o c a l m e c h a n i s m det e r m i n a t i o n s . B e t w e e n 45 a n d 80 k m , a n d f o r t h i s class of low magnitude, only thrust events are found, indicating that, at these depths, there may e x i s t s o m e f r i c t i o n a n d c o u p l i n g . B e n e a t h 80 k m , t h e r e is n o e v i d e n c e o f a s l a b d o m i n a n t l y i n t e n s i o n as it m i g h t b e s u g g e s t e d f o r a c o l d o l d s l a b . I n f a c t , it s e e m s t h a t o b s e r v e d e v e n t s w o u l d b e m o r e o r less r e l a t e d t o l o c a l s t r e s s fields.
Acknowledgements W e w o u l d like to t h a n k t h e F r e n c h V o l c a n o O b s e r v a t o r y t e a m s f o r t h e i r h e l p i n field o p e r a t i o n a n d d a t a p r o c e s s i n g , in p a r t i c u l a r J.C. D e l m o n d a n d J.P. V i o d e . J. D o r e l a n d R. M a d a r i a g a a r e gratefully acknowledged for helpful discussions on the topics of this paper.
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