MECHANICS OF BACK ARE DEFORMATION IN COSTA-RICA - EVIDENCE FROM AN AFTERSHOCK STUDY OF THE APRIL 22, 1991, VALLE DE LA ESTRELLA, COSTA-RICA, EARTHQUAKE (M(W)=7.7)

The April 22, 1991, M(W)=7.7 Valle de la Estrella, Costa Pica, earthqu ake represents back are thrusting of the Caribbean plate beneath the P anama block along the North Panama Thrust Belt. Large back are thrusti ng events are quite rare, occurring in only two other locations along the Sunda Are and Japan Sea. To better understand the mechanics of bac k are thrusting, we constrain the faulting geometry associated with th e 1991 Costa Rica earthquake using aftershock locations and focal mech anisms obtained from a three-component portable digital network deploy ed in and around the aftershock area following the mainshock. The spat ial distribution of aftershocks reveals a complicated faulting geometr y in the rupture area. Focal mechanisms determined from inversion of P wave and tangentially and radially polarized S wave (SH and SV, respe ctively) amplitudes recorded by this temporary network confirm fault c omplexity and indicate active thrust, normal and strike-slip faults in the back are of Costa Pica. Most of the thrust events are confined to the southern portion of the aftershock zone in the vicinity of the ma inshock. Their distribution suggests the existence of a near-horizonta l basal fault plane at a depth of about 15 km, with many imbricate fau lts having steeper dips extending from the basal plane toward the surf ace. Events with strike-slip mechanisms locate northwest of the thrust events and define a SW-NE trending, left-lateral strike-slip fault zo ne that represents the NW termination of the mainshock rupture and pos sibly the maximum NW extension of the Panama block. The superposition of the aftershock locations on a geologic map of the region shows that aftershocks are restricted to occur in the older, more competent rock units (volcanic and volcaniclastic rocks interbedded with carbonates) of the back are sedimentary basin. Shallow events (depth<5 km) occur only where these oldest units are exposed at the surface. This suggest s that (1) exposure of the lower units results from repeated earthquak e slip on the shallow crustal faults imaged by the aftershocks and (2) as much as 7 km of basin fill material, overlying the lower units, no rtheast of the mainshock, does not deform seismically but, instead, fo lds and possibly faults aseismically.