New Mexico Geological Society Annual Spring Meeting — Abstracts

The Socorro seismogenic zone is an upper crustal layer containing earthquakes above the intensely studied Socorro magma body within the Rio Grande rift. The depth of the seismogenic zone varies; however, microearthquake studies show that the thickness is typically between 2 km and 3 km. Near the Arroyo del Coyote swarm site, a reliable heatflow measurement allows reasonable subsurface temperature estimates. It has been suggested that the base of the seismic zone occurs at temperatures of ~300°C where quartz becomes ductile in response to tectonic stress. These temperature estimates, along with an approximation of the confining pressure with depth, allow depth equivalent estimates of normalized elastic moduli from experimental data which can be used to estimate rock strength across the entire seismic depth interval. Although the depth of a seismogenic zone is likely to depend on a number of parameters, data for several granitic rocks from two studies are combined to approximate normalized Young's moduli (E/Eo) at the top and bottom of the seismogenic zone (7 and 9 km depth) as ~0.8 and ~0.65. The normalized bulk moduli (K/K₀) at the top and bottom of the zone are ~0.45 and ~0.33. Reduction of the elastic moduli, and therefore rock strength, begins at depths below 3 km to 5 kIn because of temperature dependence. The reduction of elastic moduli predicted across the seismic zone at Arroyo del Coyote suggests a weakened crustal layer that is qualitatively consistent with a semi-brittle zone of reduced strength predicted from wet quartz rheologies. Experimentally derived elastic moduli values may be used to predict upper crustal strength-depth profiles for regions of different tectonic and thermal regimes. At Arroyo del Coyote increasing differential stress with depth may intersect a weakened semi-brittle zone to create the seismogenic depth interval.