THE SEISMIC SOURCE PARAMETERS OF THE 1991 COSTA-RICA AFTERSHOCK SEQUENCE - EVIDENCE FOR A TRANSCURRENT PLATE BOUNDARY

The April 22, 1991, Valle de la Estrella, Costa Rica earthquake (M(s) = 7.6) was a back-arc thrusting event associated with the underthrusti ng of the Caribbean plate beneath Central America. A network of three PASSCAL-type, portable instruments was deployed to monitor the aftersh ock activity in southern Costa Rica 2 to 6 weeks after the main shock. The waveforms recorded on three-component midperiod seismometers were used to recover source information for 15 small aftershocks (magnitud es between 3.2 and 4.4) with a linear moment tensor inversion method. We conducted several tests to investigate the effects of unknown struc ture and event mislocation on source parameter recovery. The longer-pe riod waveforms, in general, are less sensitive to the effects of the s tructural details so that the essential source information can be succ essfully extracted from the waveform data. The earlier pan of the seis mic waveforms has proven to be the most important carrier of the sourc e information. A gross crustal model can be used to describe the struc ture for the source study. The small changes in the waveform character resulting from the mislocation of the events, or inexact Green's func tions generated from the oversimplified crustal model, do not prohibit us from the recovery of the source orientation at local distances. In contrast, the determination of the focal depth is subject to uncertai nty because of the lack of detailed structural information. Our focal mechanisms are generally in good agreement with P wave first-motion fa ult plane solutions. determined from a local short-period network. The aftershocks show a clear spatial segmentation based on focal mechanis m type. Most aftershocks near or southeast of the main shock were thru sting events with focal mechanisms similar to the main shock. In contr ast, a cluster of aftershocks northwest of the main shock showed domin antly left-lateral, strike-slip motion on a northeasterly striking nod al plane. This suggests that a diffuse deformation zone exists in cent ral Costa Rica and is characterized by left-lateral strike-slip motion . This diffuse, transcurrent deformation zone coincides with several g eologic and geophysical features, and perhaps is a result of the slowe r subduction rate of the buoyant Cocos Ridge, than its adjacent segmen ts along the Middle America Trench (MAT). The diffuse transcurrent bou ndary may intersect with the North Panama Deformed Belt (NPDB) near Li mon, Costa Rica, and is very likely a plate boundary for the proposed Panama block.