Distribution of (mb - Ms) of moderate earthquakes along the Mexican seismic zone

Abstract

We study the spatial distribution of mb-Ms for 55 earthquakes from 1978 to 1994 with focal depth from 0 to 80 km, mb from 4.5 to 5.5, which occurred along the Middle America trench, between 95° to 107° W. The mb-Ms values have a bimodal distribution, with peaks at values of 0.3 and 0.8 and minimum at mb-Ms=0.6. Events of the first group, mb-Ms <0.6, were distributed along the trench over the whole region, and events of the second group, mb-Ms ³ 0.6, were found mainly in the central part of the region, within the Guerrero block. These data show that the Oaxaca block consists of a relatively homogeneous lithosphere, where low- frequency events dominate, while the Guerrero block is more complex radiating both low- and high-frequency events.