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On March 25, 1990 a large earthquake (Mw = 7.0, ML = 6.8) occurred at the entrance of the Nicoya Gulf, Costa Rica, at 1322:55.6 UTC, producing considerable damage in central Costa Rica and generating much interest about whether or not the Nicoya seismic gap (Nishenko, 1989) had broken. The local country‐wide seismographic network recorded 6 years of activity prior to this large earthquake, 16 hours of foreshocks, the mainshock, and its aftershocks. This network is operated jointly by the Costa Rica Volcanological and Seismological Observatory at the National University (OVSICORI‐UNA), and the Charles F. Richter Seismological Laboratory at the University of California, Santa Cruz (CFRSL‐UCSC). We obtained high resolution locations from this network and located the mainshock at 9°38.5′N, 84°55.6′W (depth is 20.0 km) and the largest foreshock (Mw = 6.0, March 25, 1990, at 1316:05.8 UTC) at 9°36.4′N, 84°57.1′W (depth is 22.4 km). We find that the aftershock zone abuts the southeast boundary of the Nicoya seismic gap, suggesting that the seismic gap did not rupture. Since the installation of the local network in April 1984 to March 24, 1990, nearly 1900 earthquakes with magnitudes from 1.7 to 4.8 (318 with magnitude 3.0 or larger) have been located at the entrance of the Nicoya Gulf, one of the most active regions in Costa Rica. The March 25 earthquake occurred at the northwest edge of this region, where a sequence of foreshocks began 16 hours prior to the mainshock. The spatial‐temporal distribution of aftershocks and directivity analysis of the mainshock rupture process using teleseismic records both indicate a southeast propagating rupture. The mainshock ruptured an asperity of approximately 600 km2 of area, with this area expanding to 4000 km2 after 7 days. We present evidence that suggests that the ruptured asperity is produced by the subduction of a seamount. Inversion of teleseismic broadband and long‐period P and SH waves yields a thrust faulting mechanism with the shallow plane striking 292°, dipping 26°, and with a rake of 88°, in agreement with the subduction of the Cocos plate under the Caribbean plate. Local first motions for the largest foreshock and the mainshock agree with this solution. We also present evidence suggesting that the March 25, 1990, earthquake triggered and reactivated several seismic swarms in central Costa Rica and temporally decreased the activity in the epicentral area of the July 3, 1983 (Ms = 6.2), Pérez Zeledón earthquake.

Seismicity on and near the boundaries of the Rivera plate is examined in order to determine the subduction regime of the Rivera plate and its influence on the earthquake behavior of the northernmost section of the mid‐America trench. In particular, we address the question of a Rivera‐Cocos boundary.There have been several large historic earthquakes in the coastal areas of the Mexican states Colima and Jalisco, but the last large event was in June 1932 (the 1932 Jalisco earthquake, Ms = 8.1). The present quiescence has lasted longer than the average recurrence interval for the Mexican subduction zone, and the Jalisco area is now categorized as a seismic gap. However, if subduction in this area is dominated by the motion of the Rivera plate, which is subducting at half the rate of the Cocos plate, the deficiency of earthquake activity may not be unusual. In an attempt to better delineate the Rivera‐Cocos boundary,intermediate‐sized earthquakes along and near the Rivera fracture zone and the mid‐America trench were relocated with the joint epicenter determination method. We found that in general, the catalog locations are fairly accurate, with the average change in epicenter upon relocation being 12 km. Seismicity along the Rivera fracture zone follows the physiographic trend of the fracture zone, but east of its intersection with the East Pacific Rise, seismicity is more diffuse. Two intermediate‐sized earthquakes in this diffuse area have focal mechanisms suggesting that they represent Rivera‐Cocos boundary motion. Reexamination of arrival time data for the 1932 Jalisco earthquake yields an epicenter at 19.57°N, 104.42°W, close to the boundary zone inferred from these two earthquakes. We conclude that the possibility that the 1932 Jalisco earthquake broke the northernmost section of the Cocos‐North American plate interface, as opposed to the Rivera‐North American plate interface, cannot be ruled out.

Revised estimates of seismic slip rates along the Middle America Trench are lower on the average than plate convergence rates but match them locally (for example, Oaxaca). Along the Cocos‐North American plate boundary this can be explained by nonuniformities in slip at points of aseismic ridge or fracture zone subduction. For at least 81 yr (and possibly several hundred years), no major (Ms ≥ 7.5) shallow earthquake is known to have occurred near the Orozco Fracture Zone and Tehuantepec Ridge areas. Compared with the average recurrence periods for large earthquakes (33 ± 8 yr since 1898 and 35 ± 24 yr between 1542 and 1979), this suggests that either a large (M ≥ 8.4) event may be anticipated at such locations, or that these are points of aseismic subduction. Large coastal terraces and evidence suggesting tectonic uplift are found onshore near the Orozco Fracture zone. The larger discrepancy between plate convergence and seismic slip rates along the Cocos‐Carribbean plate boundary is more likely due to decoupling and downbending of the subducted plate. We used the limited statistical evidence available to characterize both spatial and temporal deficiencies in recent seismic slip. The observations appear consistent with a possible forthcoming episode of more intense seismic activity. Based on a series of comparisons with carefully delineated aftershock zones, we conclude that the zones of anomalous seismic activity can be identified by a systematic, automated analysis of the worldwide earthquake catalog (mb ≥ 4).

A new model, incorporating shear deformation within a subducting slab, is proposed to explain slip partitioning for oblique plate motion at subduction zones. On the basis of investigation of 450 interplate earthquakes at 24 subduction zone segments we find that the degree of slip partitioning is laxgely correlated with the calculated slab pull force. Such correlation suggests that other than the upper plate deformation, the slab pull force plays an important role in controlling oblique subduction. Our model proposes that the force balance between the slab pull force, the interplate coupling resistance, and the viscous mantle drag (the latter two are passive forces to the former one) produces a lateral shear within the slab, which causes the slab to deform and change its motion direction gradually toward trench normal as it subduers. The amount of direction change, which would be observed as the slip partitioning during earthquakes, therefore is closely related to the major plate driving force at the subduction zone, which is the slab pull force in out model.

The regional distribution of stresses associated with the subduction of the Cocos plate is inferred from a synthesis of 190 earthquake focal mechanisms, body and surface wave analyses of large earthquakes, and seismicity distributions. Broad patterns of consistent behavior are found across the region, from the Rivera Plate boundary in the northwest to the Guatemala/El Salvador border in the southeast, and are used as a framework to evaluate evidence for variations in local stresses due to the subduction of two aseismic ridges, the Tehuantepec Ridge and the Orozco Fracture Zone. Information which bears on the seismic potential at locations of aseismic ridge subduction is particularly important in that no large (Ms ≥ 7.5) earthquakes have occurred historically. We identify three major zones with consistent patterns in focal mechanisms and hypocentral distributions of seismicity. The first, closest to the trench and reflecting the mechanical interaction of the converging plates, is a zone of shallow thrust earthquakes extending 100–150 km inland from the trench. The second is a zone of normal faulting, beginning at about 200 km inland from the trench, h ≥ 60 km, which extends continuously along the entire length of the descending plate throughout the region. The third distinct zone exhibits a relatively low level of activity and separates the zones of thrust and normal faulting at about 150–200 km inland from the trench. This zone extends from the Rivera plate boundary in the northwest to the Guatamala region in the southeast. At this point, the quiet region pinches out, and the thrust and normal faulting zones abut and overlap. Superimposed on this overall pattern, we find locally only minor changes in areas of aseismic ridge subduction, aside from the prominent seismic slip gaps. Furthermore, on October 25, 1981, the Playa Azul earthquake (Ms = 7.3) occurred in the midregion of the Orozco Fracture Zone. Body and surface wave analyses of this event show a simple source rupture and shallow thrust fault mechanism, as are found elsewhere in the region. The seismic moment is MO = 1.3 × 1020 N m; the calculated stress drop is 4.5 MPa, not extraordinarily high, as might be expected in these pervasive seismic gaps. An event in the Tehuantepec Ridge region, on January 24, 1983, Ms = 6.7, was a large normal faulting event, but further interpretation is ambiguous due to the proximity of other normal faulting. We conclude that while aseismic slip may be occurring in the areas of ridge subduction, the possibility of large thrust earthquakes cannot be ruled out, due to the overall similarities with adjacent regions.

Aftershocks of the 29 November 1978 Oaxaca, Mexico, earthquake (surface-wave magnitude Ms = 7.8) define a rupture area of about 6000 square kilometers along the boundary of the Cocos sea-plate subduction. This area had not ruptured in a large (Ms >/= 7), shallow earthquake since the years 1928 and 1931 and had been designated a seismic "gap." The region has also been seismically quiet for small to moderate (M >/= 4), shallow (depth