Seismic triggering by rectified diffusion in geothermal systems

B. Sturtevant, H. Kanamori, E.E. Brodsky
Graduate Aeronautical Laboratories, California Institute of Technology, Pasadena, CA 91125

Abstract:

Widespread seismicity was triggered by the 28 June 1992 Landers CA earthquake at a rate which was maximum immediately after passage of the exciting seismic waves. Rectified diffusion of vapor from hydrothermal liquids and magma into bubbles oscillating in an earthquake can increase the local pore pressure to seismically significant levels within the duration of the earthquake. In a hydrothermal system modeled as a two-component HO-CO fluid in porous rock the pressure initially increases linearly with time. The rate of pressure buildup depends sensitively on the mean bubble radius, and is large for small bubbles. The diffusion-induced pressure is relaxed by percolation and resorption of vapor into the liquid solution. The induced seismicity itself also relieves stress. Values of parameters used in the present calculations give results consistent with observations of triggered seismicity at Long Valley caldera after the Landers earthquake. For one representative condition, at 250 C and 5.6 km depth, oscillating strain acting on 10 m diameter bubbles increases pore pressure at the rate of 151 Pa/s resulting in a pressure increase of 12 kPa in the 80 s duration of the Landers earthquake. The elevated pressure induced by a single 26 m diameter cloud of bubbles in saturated rock relaxes by percolation through soil of 0.2 mDarcy permeability in 53.6 hr. Observations of earthquake swarms at other locations suggest that self-induced buildup of pore pressure by rectified diffusion can provide a positive feedback mechanism for amplifying seismicity.

AGU Index Terms: 7209 Earthquake dynamics and mechanics; 8424 Hydrothermal systems; 7280 Volcano seismology
Keywords/Free Terms: Hydrothermal systems, Volcano seismology

JGR-Solid Earth 96JB02654
Vol. 101 , No. B11 , p. 25,269