Patterns of local seismicity preceding the petatlan earthquake of 14 March 1979

Abstract

Local seismic activity (M > 2.3) during the $412$-day period preceding the Petatlan earthquake of 14 March 1979 (M_s = 7.6) was monitored by a seismic network deployed by the Hawaii Institute of Geophysics. These data allow us to revise the location of that event, and to study its pattern of foreshocks. The foreshock hypocenters are concentrated above the ocean/continent plate boundary and landward of the hypocenter of the main shock. The spatial distribution of foreshocks suggests that the foreshocks occurred along existing zones of weakness within the continental plate. During the $112$-day period preceding the Petatlan event, foreshocks were concentrated within 100 km of the epicenter of that earthquake. The pattern of precursory seismic activity we observed during that period is similar to that observed during the same time period prior to the Oaxaca earthquake of 29 November 1978. However, the 17-hr period of seismic quiescence (for events with M > 2.8) preceding the Oaxaca earthquake is not found in our data where a magnitude 4 foreshock occurs within 28 min of and 2 km from the Petatlan event. This suggests that the spatially larger set of foreshocks may have greater diagnostic value than does the sequence of precursory events within 30 km of the main shock.