Persistent seismic activity in the epicentral region of the 1977 double earthquake, Sierra de Pie de Palo, SAN JUAN, Argentina

This work analyzes the modern seismic activity in the crust around the Sierra de Pie de Palo, where have occurred the largest shallow earthquake (M = 7.5) ever recorded in Argentina. Hypocentral parameters have been quantitatively characterized from seismicity recorded during 2008 and 2009 by two temporary seismological experiments, in the most active portion of the continental crust of the Andean back-arc of the southern central Andes. Total of 127 events have been relocated using a local velocity model. Epicenters from these 127 events are heterogeneously distributed beneath the Sierra de Pie de Palo, and occurring mainly at middle-to-lower crustal levels with depth between 10.9 and 39 km. Local magnitudes 0.6 ≤ ML ≤ 3.7 and moment magnitudes 1.5 ≤ MW ≤ 3.7 have been estimated. The focal mechanisms determined from P-wave first motion polarities are predominantly reverse and, in general, the azimuths follow a north-south trend. It has been possible to observe that the orientation of the average stress field σ1 is consistent with the direction of convergence of the Nazca plate in this sector of the Andean back-arc. The pattern of hypocenters determined in the present work is consistent with that observed for the aftershocks of the 1977 double earthquake, recorded by a portable network during 10 days in December 1977, and with the depth distribution of the seismicity recorded during 1988 and 1989 by the PANDA temporary network. At present, the hypocentral zones of the 1941 earthquake and 1977 double earthquake are still active with small-to-moderate seismicity.