Abstract

From 1962 to 1995, 572 events with moment magnitudes greater than 1.3 occurred in the Socorro seismic area. The activity seems to be related to uplift resulting from magma replenishment of a cogeographical magma body at 19 km depth. A distinct microearthquake swarm at Arroyo del Coyote where focal depths are well determined, a nearby reliable heat-flow measurement, and a largely granitic upper-crust, make the site ideal for considering upper crustal strength inferred from experimentally determined elastic moduli as a function of temperature and confining pressure at in situ conditions. The experimental data may be interpreted to imply that strength across the seismogenic layer (∼6–10 km) is much more dependent upon in situ temperatures than in situ stresses. Normalized Young's moduli and bulk moduli decrease at temperatures predicted for the seismogenic layer at Arroyo del Coyote. The elastic moduli versus depth profile can be explained by the thermoelastic response of microcracking. It is suggested that the effect of microcracking weakens the rock and therefore the asperity and surface strength along fault surfaces where seismic events likely occur. The seismogenic layer is also within a temperature regime that generally corresponds to the temperature regime for a semibrittle zone model of crustal weakening with possible plastic deformation. Both the elastic moduli data and the semibrittle zone temperature regime suggest the seismic layer at Arroyo del Coyote is a layer of crustal weakening with depth, different from a stronger crust with depth across a seismogenic layer, as often presented in frictional resistance models. Whereas temperatures at the base of the Arroyo del Coyote swarm are consistent with in situ plastic deformation and the lack of upper-crustal earthquakes at deeper depths, the experimental data for elastic moduli at these same temperatures indicate elastic deformation. The conflicting character of deformation between experimental and in situ observations illustrates the dependence of rheology on strain rate.

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