New Mexico Geological Society Annual Spring Meeting — Abstracts

The subsurface temperature regime, along with a number of other parameters, appears to be an important factor influencing the base of the continental seismogenic layer, and quite possibly the depth interval of the seismogenic layer as well. Heat flow measurements are the best way to estimate the subsurface thermal regime. There are however few heat flow measurements within several km of epicenter location where well determined hypocenter depths are available. In this study two widely separated location in very different geological environments are considered. The locations are along the San Andreas Fault near Parkfield, California, and along the Coyote fault near Socorro, New Mexico. There are heat flow measurements in these locations and the epicenters and hypocenter depths are reasonably well approximated. The San Andreas Fault is a major strike-slip fault between the Pacific and North American plates. The Coyote fault along the eastern boundary of the Rio Grande rift is mapped as a normal fault; interestingly, first motion studies of recent seismic swarms suggest primarily strike-slip fault events. Within data uncertainties, the hypocenter depth of the main event at Arroyo del Coyote, near the Coyote fault, is the same as the hypocenter depth of the two magnitude 6 events near Parkfield since 1966. There is also much overlap in the seismogenic layer at the two locations. Heat flow values at the 1966 magnitude 6 event site just north of Parkfield, and the value near The Arroyo del Coyote swarm just northeast of Socorro, suggest the main events at both locations are occurring at temperatures of ?260 degrees Celsius to ?300 degrees Celsius. This temperature range is in the upper part of the temperature field for the semi-brittle zone of wet quartz; a region where crustal strength is predicted to decrease. Is this observation a coincidence or does it suggest that strike-slip events depend on temperature and strain rates that affect the rock rheology and crustal fault strength?