Mechanics of back arc deformation in Costa Rica: Evidence from an aftershock study of the April 22, 1991, Valle de la Estrella, Costa Rica, earthquake (Mw=7.7)

Abstract

The April 22, 1991, Mw=7.7 Valle de la Estrella, Costa Rica, earthquake represents back arc thrusting of the Caribbean plate beneath the Panama block along the North Panama Thrust Belt. Large back arc thrusting events are quite rare, occurring in only two other locations along the Sunda Arc and Japan Sea. To better understand the mechanics of back arc thrusting, we constrain the faulting geometry associated with the 1991 Costa Rica earthquake using aftershock locations and focal mechanisms obtained from a three‐component portable digital network deployed in and around the aftershock area following the mainshock. The spatial distribution of aftershocks reveals a complicated faulting geometry in the rupture area. Focal mechanisms determined from inversion of P wave and tangentially and radially polarized S wave (SH and SV, respectively) amplitudes recorded by this temporary network confirm fault complexity and indicate active thrust, normal and strike‐slip faults in the back arc of Costa Rica. Most of the thrust events are confined to the southern portion of the aftershock zone in the vicinity of the mainshock. Their distribution suggests the existence of a near‐horizontal basal fault plane at a depth of about 15 km, with many imbricate faults having steeper dips extending from the basal plane toward the surface. Events with strike‐slip mechanisms locate northwest of the thrust events and define a SW–NE trending, left‐lateral strike‐slip fault zone that represents the NW termination of the mainshock rupture and possibly 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 arc sedimentary basin. Shallow events (depth<5 km) occur only where these oldest units are exposed at the surface. This suggests that (1) exposure of the lower units results from repeated earthquake 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, northeast of the mainshock, does not deform seismically but, instead, folds and possibly faults aseismically.