Stress distribution and subduction of aseismic ridges in the Middle America Subduction Zone

Abstract

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.