BEGINNINGS OF EARTHQUAKES IN THE MEXICAN SUBDUCTION ZONE ON STRONG-MOTION ACCELEROGRAMS

The Guerrero, Mexico, accelerograph network has recorded, at short dis tances, earthquakes with magnitudes from 3.0 to 8.1. In this article, the initial 3.5 sec of the P waves, on the vertical component, are com pared. In addition to unfiltered accelerograms, several causal bandpas s filters are applied. We do not see significant differences, in eithe r original or filtered records, between the initial. (e.g., 0.5 sec) s eismograms of moderate and large earthquakes. In the original and 2- t o S-Hz pass-band, the main differences among events with magnitudes ov er about 4.5 is in the duration of shaking. The initial ground motions of all the large events have very small amplitudes, which gradually g et larger. At lower frequencies, seismograms of the largest events sho w a complex series of multiple pulses, assumed related to failure of n umerous asperities. In our lowest frequency band (0.2 to 0.5 Hz), most of the large events that we examined begin with rupture of asperities that are not necessarily larger, and are sometimes smaller, than the asperities that fail during smaller events. The observations are consi stent with a model of faulting in which the largest asperities are loc ated at random on the eventual fault plane, and fail when the rupture front, propagating from the hypocenter, reaches them. This allows the largest asperities to occasionally be at the hypocenter and to fail im mediately, but for a large fault, it is more likely that they will be located elsewhere and fail later.