RELATIONSHIP BETWEEN SUBDUCTION AND SEISMICITY IN THE MEXICAN PART OF THE MIDDLE AMERICA TRENCH

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 (M(s) > 6.0) is used to obtain cumulative seismic moment (M(o)) and seismic moment release rate (M(o)) 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 less-than-or-equal-to m(b) < 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 M(o) and seismic energy release rate (W). 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 M(o) and M(o) 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 M(o) are lower then the values of plate convergence rates but the averages eismic 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 M(o) 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 almost-equal-to 10 km/yr.