SIMULTANEOUS INVERSION FOR EARTHQUAKE LOCATION AND VELOCITY STRUCTUREBENEATH CENTRAL COSTA-RICA

We have imaged the complex crustal and upper mantle structure beneath central Costa Rica using P-wave arrival times from locally recorded ea rthquakes. Thurber's (1983) iterative inversion method is used to simu ltaneously estimate velocities along a three-dimensional grid and hypo central parameters of local earthquakes. Our data consist of over 12,0 00 arrival times from more than 1300 earthquakes recorded by stations of a permanent seismographic network in Costa Rica. Our resulting velo city model correlates well with mapped geologic units at very shallow depth and with tectonic features at greater depth. We find low velocit ies (4.0 to 4.8 km/sec) in the shallow crust (above 10 km) near the ac tive volcanoes and associated with a NW-SE trending late Cretaceous to late Tertiary sedimentary basin southeast of Herradura peninsula. Hig h velocities (5.4 to 5.7 km/sec) in the shallow crust correlate with o utcrops of late Jurassic to early Tertiary ultramafic ophiolitic units and with basic Tertiary volcanic units. At depths between 20 and 30 k m, high velocities (6.8 to 7.2 km/sec) are associated with the subduct ing Cocos plate under Costa Rica and two prominent low-velocity bodies (6.3 to 6.5 km/sec) are present about 30 km trenchward of the volcani c are and along the projection of the aseismic Cocos Ridge as it subdu cts beneath Costa Rica. The thickened oceanic crust of the Cocos Ridge is most likely responsible for its low velocities. The deep low-veloc ity anomaly located trenchward of the axis of the volcanoes may indica te the presence of a low-density intrusive resulting from an earlier p hase of magmatism, possibly the late Miocene episode that produced the Talamanca intrusive complex.