THE LIMON, COSTA-RICA EARTHQUAKE OF APRIL 22, 1991 - BACK-ARC THRUSTING AND COLLISIONAL TECTONICS IN A SUBDUCTION ENVIRONMENT

On April 22, 1991, a large earthquake (M(w) = 7.7) occurred along the Caribbean coast of Costa Pica and western Panama. The rupture area of the fault mapped from the aftershocks is 45x85 km(2). The distribution of aftershocks and the local geological record suggest that faulting occurred on a blind thrust sheet that shallows toward the northeast. U plift of the Caribbean coast ranging from 1.5 m near Puerto Limon and decreasing gradually toward the southeast was observed along the Carib bean. Northwest of Puerto Limon no significant coastal uplift was obse rved. This observation agrees with the aftershock data suggesting the rupture did not extend to the northwest of this location. The Limon ea rthquake also triggered aftershocks on secondary faults in the crust. These events are apparently associated with a family of imbricate thru st and strike-slip faults that lie in the eastern piedmont of the Tala manca Cordillera. The historical seismicity indicates that the Caribbe an coast has been the site of several historical earthquakes with magn itudes greater than 7.0. On April 26, 1916, another earthquake (M(s) = 6.9) took place in the same region. Summing the scalar seismic moment release along the Caribbean coast, the average rate of slip is approx imately 0.8 cm/yr, compared with a value of 0.4 to 0.8 cm/yr along the Pacific subduction zone, depending on the estimated width of the seis mogenic zone. Therefore a large fraction of the relative plate motion between the Cocos and Caribbean plates (9.8 cm/yr) appears to be taken up by crustal deformation in the back are. The tectonic regime in the area appears to be dominated by the collision of the buoyant Cocos ri dge with the subduction zone. The absence of a Wadati-Benioff zone whe re the Cocos ridge collides with the trench suggests the slab does not subduct beneath the Osa Peninsula; this is supported by the Pliocene gap of volcanism present in Costa Pica. Thus the predicted relative mo tion between the Cocos and Caribbean plates appears to be absorbed by a low rate of seismic moment release in the forearc and by a broad zon e of active crustal shortening and underthrusting in the back are. Thi s type of tectonic deformation resembles more a collisional regime tha n a typical subduction zone environment.