On April 22, 1991, a large earthquake (Mw=7.7) occurred along the Caribbean coast of Costa Rica and western Panama. The rupture area of the fault mapped from the aftershocks is 45¿85 km2. The distribution of aftershocks and the local geological record suggest that faulting occurred on a blind thrust sheet that shallows toward the northeast. Uplift of the Caribbean coast ranging from 1.5 m near Puerto Lim¿n and decreasing gradually toward the southeast was observed along the Caribbean. Northeast of Puerto Lim¿n no significant coastal uplift was observed. This observation agrees with the aftershock data suggesting the rupture did not extend to the northwest of this location. The Lim¿n earthquake also triggered aftershocks on secondary faults in the crust. These events are apparently associated with a family of imbricate thrust and strike-slip faults that lie in the eastern piedmont the Talamanca Cordillera. The historical seismicity indicates that the Caribbean coast has been the site of several historical earthquakes with magnitudes greater than 7.0. On April 26, 1916, another earthquake (Ms=6.9) took place in the same region.

Summing the scalar seismic moment release along the Caribbean coast, the average rate of slip is appproximately 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 seismogenic 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 arc. The tectonic regime in the area appears to be dominated by the collision of the buoyant Cocos ridge with the subduction zone. The absence of a Wadati-Benioff zone where the Cocos ridge collides with the trench suggests the slab does not subduct beneath the Osa Pennisula; this is supported by the Pliocene gap of volcanism present in Costa Rica. Thus the predicted relative motion 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 zone of active crustal shortening and underthrusting in the back arc. This type of tectonic deformation resembles more a collisional regime than a typical subduction zone environment. ¿ American Geophysical Union 1995