Experimental investigation of remote seismic triggering by gas bubble growth in groundwater

Abstract:

Remotely triggered seismicity is the process by which an earthquake at one location initiates others after a time delay ranging from seconds to days, over distances up to thousands of kilometers. Candidate mechanisms have been proposed, but none specifically address the role of carbon dioxide (CO₂) gas bubble growth in groundwater as a driver of remote seismic triggering in active volcanic and geothermal regions, where shallow crustal CO₂ gas is abundant. In the present study, we hypothesize that a seismic wave from a distant source can initiate rapid gas bubble growth in CO₂-rich groundwater, resulting in a persistent increase in pore fluid pressure and a reduction of effective stress, which can trigger failure on a critically loaded geologic fault. Under conditions representative of a confined aquifer, a Berea sandstone core flooded with an aqueous CO₂ solution was subjected to a six-period burst of 0.05–0.3 Hz, 0.1–0.4 MPa confining stress oscillations. After the oscillations were terminated, the pore fluid pressure exceeded its initial value by 13–60 cm equivalent freshwater head, scaling with the amplitude and frequency – a surplus that is consistent with borehole water level changes [Roeloffs et al. (1995) USGS Open File Report, 95-42] observed in response to the June 28 1992 M_W 7.3 Landers, California earthquake Rayleigh wave in Parkfield and Long Valley caldera, California, where remotely triggered earthquakes occurred [Hill et al. (1993) Science, 260(5114); Hill et al. (1995) Journal of Geophysical Research, 100(B7)]. Our experimental results indicate that seismically initiated gas bubble growth in groundwater is a physically plausible mechanism for remote seismic triggering in active volcanic and geothermal regions, suggesting that the aqueous CO₂ saturation state in a confined aquifer may be used to assess susceptibility to remote seismic triggering.