The Milpa Alta earthquake of January 21, 1995

Abstract

In the past year there has been a significant increase in the number of seismic stations operating in the Valley of Mexico. Also, a dense network of six stations is in place around the Popocatepetl volcano, in response to the recent increase in seismic and fumarolic activity. For this reason, the recent earthquake of January 21, 1995 (Me= 3.9), located near the town of Milpa Alta, at a depth of about 12 km, is the best recorded event in the Trans-Mexican Volcanic Belt. The analysis of this earthquake was facilitated by two broadband seismographs, one in Ciudad Universitaria (CU) and the other near Tlamacas, both of which recorded this event. To constrain the focal mechanism we developed an algorithm which uses firstmotion data, polarities of SV₂ and SH phase, and P_z/SH and SV₂/SH ratios measured on displacement broadband seismograms. The focal mechanism corresponds to a normal-faulting event with significant (50%) strike-slip component (azimuth 106°, dip 63°, rake -39°). This mechanism is nearly identical to the composite focal mechanism of the aftershocks of February 7, 1984 (Mc= 3.9) earthquake, which occurred in the nearby town of Juchitepec. The T-axes of the two events are oriented NS, in agreement with the stress orientation inferred from the alignment of the cinder cones in the region. This extensional stress regime in the region may be due to the elevation of the altiplano and/or due to seaward migration of the trench.