SOME SIMILARITIES IN SUBSURFACE TEMPERATURES AND SEISMOGENIC DEPTHS ALONG THE SAN ANDREAS FAULT NEAR PARKFIELD AND ALONG THE COYOTE FAULT NEAR SOCORRO

Temperature significantly influences the base of the continental seismogenic layer and likely the depth interval of the seismogenic layer as well. The best way to estimate subsurface temperature regimes is to acquire representative heat flow measurements. There are however few heat flow data within several km of main event epicenters. Two areas are examined for possible similarities in subsurface temperatures and seismogenic layers. The two widely separated areas have greatly differing geology; they are along the San Andreas Fault near Parkfield, California, and along the Coyote fault near Socorro, New Mexico. At both of these locations first motions indicate mainly strike slip events (although the Coyote fault is mapped as a normal fault). Heat flow data at Middle Mountain (near Parkfield) and at Arroyo del Coyote (near Socorro) are used to estimate main event focal depth temperatures at both locales of ~ 270 ± 30 °C. This temperature is in the higher part of the semi-brittle temperature regime for wet quartz (200-300 °C). The seismogenic layer near Parkfield is twice the depth interval of the layer along the Coyote fault near Socorro; probably because larger main events near Parkfield influence the local stress field over a much greater volume, causing a greater depth interval for subsequent swam events. Therefore main event focal depths may represent a more explicit inter-dependence between regional tectonic stresses, subsurface temperatures, and crustal srength. Three main events recorded along the Coyote fault, and four main events near Parkfield between 1992 and 1994, all have hypocenters between ~ 6 – 10 km depth. Along the Coyote fault events are small enough that three distinctly separated swarm sites have different depths and likely different temperature regimes. Future heat flow measurements should indicate if the temperatures correlate with event depths along the Coyote fault.