1. Department of Earth and Space Sciences, University of California, Los Angeles, California 90095, USA
2. Department of Earth Sciences, University of California, Santa Cruz, California 95060, USA
3. Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California, San Diego, California 92093, USA

Correspondence to: Jean E. Elkhoury¹ Correspondence and requests for materials should be addressed to J.E.E. (Email: elkhoury@ess.ucla.edu).

Abstract

Earthquakes have been observed to affect hydrological systems in a variety of ways—water well levels can change dramatically, streams can become fuller and spring discharges can increase at the time of earthquakes^{1, 2, 3, 4, 5, 6, 7}. Distant earthquakes may even increase the permeability in faults⁸. Most of these hydrological observations can be explained by some form of permeability increase^{1, 5}. Here we use the response of water well levels to solid Earth tides to measure permeability over a 20-year period. At the time of each of seven earthquakes in Southern California, we observe transient changes of up to 24° in the phase of the water level response to the dilatational volumetric strain of the semidiurnal tidal components of wells at the Piñon Flat Observatory in Southern California. After the earthquakes, the phase gradually returns to the background value at a rate of less than 0.1° per day. We use a model of axisymmetric flow driven by an imposed head oscillation through a single, laterally extensive, confined, homogeneous and isotropic aquifer to relate the phase response to aquifer properties⁹. We interpret the changes in phase response as due to changes in permeability. At the time of the earthquakes, the permeability at the site increases by a factor as high as three. The permeability increase depends roughly linearly on the amplitude of seismic-wave peak ground velocity in the range of 0.21–2.1 cm s^-1. Such permeability increases are of interest to hydrologists and oil reservoir engineers as they affect fluid flow and might determine long-term evolution of hydrological and oil-bearing systems. They may also be interesting to seismologists, as the resulting pore pressure changes can affect earthquakes by changing normal stresses on faults¹⁰.

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