HR: 09:15h
AN: S31D-06    [Abstracts]
TI: Evaluating Thermoelastic Strain as an Earthquake Trigger
AU: * Ebel, J E
EM: ebel@bc.edu
AF: Weston Observatory Boston College, 381 Concord Rd., Weston, MA 02493, United States
AU: Ben-Zion, Y
EM: benzion@usc.edu
AF: University of Southern California, Department of Earth Sciences, Los Angeles, CA 90089, United States
AB: Spatio-temporal variations of temperature at the Earth's surface cause strains and stresses below the surface in two ways. The diffusion of heat into the ground leads directly to thermal deformation that may have a significant amplitude several meters below the Earth's surface. More importantly, variations of surface temperatures produce surface tractions that propagate stress-strain variations many kilometers into the crust. Under the latter mechanism, seasonal temperature variations with spatial wavelengths of 10-30 km can produce thermoelastic strains with amplitudes of ~10^-7-10^-8over the seismogenic depth range of 1-10 km in the crust. Since the threshold for triggering seismicity is expected to decrease with increasing period of excitation, these strain levels and associated stresses (~7x10²-7x10³ Pa using a nominal rigidity of 70 GPa) may trigger seismicity. We suggest that thermoelastic strains may explain the growing evidence for seasonal variations in the earthquake activity rates at a number of places in the western U.S. Seasonal variations of seismicity with increased activity in the summer and autumn were observed in California for earthquakes less than M1.5 during the several years following the 1992 Landers earthquake, for earthquakes with M>=2.0 in California and Nevada from 1990-2005, and for earthquakes with M>=4.0 in California and Nevada from 1932- 2005. A similar seasonality with increased seismicity during the autumn and winter has been documented at some of the Cascades volcanoes and in the Himalaya. Others have explained observed seasonal seismicity patterns by annual air pressure changes, rainfall effects or snow loads; however, some such explanations require unrealistic values of material properties. We argue that an annual thermoelastic effect appears to be the best explanation for the observed seasonal seismicity patterns in the dry, warm California climate.
DE: 7223 Earthquake interaction, forecasting, and prediction (1217, 1242)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
SC: Seismology [S]
MN: 2007 Fall Meeting