SEISMIC TRIGGERING BY RECTIFIED DIFFUSION IN GEOTHERMAL SYSTEMS

Widespread seismicity was triggered by the June 28, 1992, Landers Cali fornia, earthquake at a rate which was maximum immediately after passa ge of the exciting seismic waves. Rectified diffusion of vapor from hy drothermal liquids and magma into bubbles oscillating in an earthquake can increase the local pore pressure to seismically significant level s within the duration of the earthquake. In a hydrothermal system mode led as a two-component H2O-CO2 fluid in porous rock the pressure initi ally increases linearly with time. The rate of pressure buildup depend s sensitively on the mean bubble radius and is large for small bubbles . The diffusion-induced pressure is relaxed by percolation and resorpt ion of vapor into the liquid solution. The induced seismicity itself a lso relieves stress. Values of parameters used in the present calculat ions give results consistent with observations of triggered seismicity at Long Valley caldera after the Landers earthquake. For one represen tative condition, at 250 degrees C and 5.6 km depth, oscillating strai n acting on 10-mu m-diameter bubbles increases pore pressure at the ra te 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 pe rcolation through soil of 0.2-mdarcy permeability in 53.6 hours. Obser vations of earthquake swarms at other locations suggest that self-indu ced buildup of pore pressure by rectified diffusion can provide a posi tive feedback mechanism for amplifying seismicity.