SEARCH

SEARCH BY CITATION

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.