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On April 22, 1991, a large earthquake (M w = 7.7) occurred along the Caribbean coast of Costa Rica 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. Uplift of the Caribbean coast ranging from 1.5 m near Puerto Lim6n and decreasing gradually toward the southeast was observed along the Caribbean. Northwest of Puerto Lim6n 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 Lim6n 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 of 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 (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 approximately 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 Peninsula; 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 Paper number 94TC02546. 0278-7407/95/94TC-02546510.00 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. 20-40-ß ß ß ß 24, Apr 22, Apr ß ß ß ß ß ß e ß ß A B gm , 0 20 40 60 80 Km 60, 1993. Bowland, C.L., Seismic stratigraphy and structure of the western Colombian basin, Caribbean Sea, M.S. thesis, 248 pp., Univ. of Tex., 1984. Brune, J., Seismic moment, seismicity, and rate of slip along major fault zones, J. Geophys. Res., 68, 777-784, 1968. Camacho, E., E1 tsunami del 22 de abril de 1991 en Bocas del Toro, Panamfi, Rev. Geol. Am. Cent., Spec. Vol., 61-64, 1994. Camacho, E. and V. Viquez, Historical and instrumental seismicity of the Caribbean region of Panama, U.S. Geol. Surv. Prof Pap., in press, 1994. Chinn, D.S., and B.L. Isacks, Accurate source depths and focal mechanisms of shallow earthquakes in western South America, Tec...

Two catalogs of earthquakes in the Mexican part of the Middle America trench are analyzed to elucidate principal relations between structure of the subducting Cocos plate and seismicity. A catalog of historical events that have occurred during the last two centuries with large magnitudes (Ms > 6.0) is used to obtain cumulative seismic moment (Mo) and seismic moment release rate (trueM˙0) distributions along the Mexican subduction zone. The catalog of instrumentally observed earthquakes from 1963 to 1990 (International Seismological Center and U.S. Geological Survey) with 4.5 ≤ mb < 6.0 is applied to investigate background seismicity for the region. The strength of coupling between the Cocos and North American plates would be expected to grow gradually from the southeast to the northwest according to the variation of convergence rate ( V) and age (A ) of the Cocos plate. This correlates in general with a steady reduction in background seismicity and a slight average increase of trueM˙0 and seismic energy release rate (trueW˙). At the sites where the main fracture zones of the Cocos plate; Tehuantepec, O'Gorman, Orozco and Rivera, undergo the subduction the general correlation breaks down. The background seismicity increases at fracture zones while M0 and trueM˙0 decrease significantly. This feature is interpreted as a drop of the coupling at the areas where transform faults are being subducted. Seismic slip rates along the trench obtained from trueM˙0 are lower then the values of plate convergence rates but the average seismic slip is in agreement with the estimates from the V model (interaction between lithospheric plates at convergent zones through the viscous layer of subducted sediments). Variability of trueM˙0 and seismic slip rate in relation with tectonics should be taken into account when the seismic gap model is being used for the prediction of strong earthquakes. An examination of space‐time plots for the historical catalogs supposes a probable tendency of northwest migration of strong events with a rate of ≈10 km/yr.

The seismicity in the Isthmus of Tehuantepec, Mexico is anomalously active compared with other parts of the Middle American subduction zone. The results of a microearthquake study conducted in this region show that the slab changes dip from an almost subhorizontal geometry west of longitude 96° W, to a dip of about 45° to 50° to the east of this longitude. The change in dip occurs gradually in a broad flexure of the subducting slab that takes place over 150 km. This geometry of the slab is reminiscent of that in southern Peru; the change in dip, however, takes place more gently in Tehuantepec. In addition to the change in the dip of the subducted Cocos plate, a rapid increase is observed in the maximum depth extent of the subducted slab. In the region of subhorizontal subduction, focal depths are consistently shallower than about 80 km, whereas earthquakes as deep as 200 km are found where the subducted slab dips at a steeper angle. Both the change in the dip of the slab and of the maximum depth of intermediate‐depth earthquakes appear to correlate with the presence of the Tehuantepec Fracture Zone which separates two distinct provinces of the oceanic Cocos plate. The age and the crustal thickness of the Cocos plate increase to the southeast of the Tehuantepec Fracture Zone relative to its northwestern counterpart. In Tehuantepec, the dip and maximum depth of the subducted slab appears to be correlated with the age of the oceanic plate, as has been observed in other subduction zones of the world.

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