Assessment of Spatiotemporal Stress Associated with the November 29, 1978 M=7.8, Foreshocks in Oaxaca, Mexico

To study the spatiotemporal patterns of local earthquakes mb < 4.0, a local seismic network of six smoked paper seismographs was installed on the Oaxaca Coast three weeks before November 29, 1978, M=7.8, Earthquake, as part of a Research Project between the Institute of Geophysics of the UNAM and the California Institute of Technology. This study aimed to obtain relevant information on the Oaxaca Seismic Gap proposed by Kelleher et al. (1973). 339 earthquakes were located in this period, with a coda magnitude range of 1.0 to 4.4, most of the seismicity is located between the coastline and the Chacalapa Fault. The depths of the precursors are mainly between 10 and 35 km, basically in the continental crust. We observe an NS alignment of seismicity at 96.6° (east of the earthquake epicenter). Two clusters of seismicity are observed, one 25 km southwest of the epicenter and another 45 km west of the epicenter. Frames of quiescence period centered at the main event epicenter in profiles NS and EW are observed.  Using composite focal mechanisms, an analysis of the variation of previous stresses in the epicentral area of the earthquake is presented.