RELATIONSHIP BETWEEN SUBDUCTION AND SEISMICITY IN THE MEXICAN PART OFTHE 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 ma gnitudes (M(s) > 6.0) is used to obtain cumulative seismic moment (M(o )) and seismic moment release rate (M(o)) distributions along the Mexi can subduction zone. The catalog of instrumentally observed earthquake s from 1963 to 1990 (International Seismological Center and U.S. Geolo gical 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 coup ling between the Cocos and North American plates would be expected to grow gradually from the southeast to the northwest according to the va riation of convergence rate (V) and age (A) of the Cocos plate. This c orrelates 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; Te huantepec, O'Gorman, Orozco and Rivera, undergo the subduction the gen eral correlation breaks down. The background seismicity increases at f racture 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 obtai ned 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 t hrough 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 histor ical catalogs supposes a probable tendency of northwest migration of s trong events with a rate of almost-equal-to 10 km/yr.