HR: 11:35h
AN: T42B-05    [Abstracts]
TI: Observed Permeability Increases Generated by Seismic Waves
AU: * Elkhoury, J E
EM: elkhoury@ess.ucla.edu
AF: Dept. Of Earth & Space Sciences. University of California, Los Angeles., 595 Charles E Young Dr. East, Los Angeles, CA 90094 United States
AU: Brodsky, E E
EM: brodsky@ess.ucla.edu
AF: Dept. Of Earth & Space Sciences. University of California, Los Angeles., 595 Charles E Young Dr. East, Los Angeles, CA 90094 United States
AU: Agnew, D C
EM: dagnew@ucsd.edu
AF: Institute of Geophysics and Planetary Physics. University of California, San Diego, 9500 Gilman Dr., La Jolla, CA 92093 United States
AB: Earthquakes affect hydrological systems in a variety of ways: Water well levels, stream flows and spring discharge change at the time of earthquakes. Most of these hydrological observations can be explained by some form of permeability increase. However, to the best of our knowledge, direct measurements of permeability changes have never before been made for multiple earthquakes at a single, natural site. To address this gap, we use the response of wells to solid Earth tidal strains as a probe of the in situ permeability. The pressure in confined aquifers continually oscillates in response to the solid Earth tide. The phase lag of the tidal response is primarily determined by the permeability. If the permeability is high, then the tidal oscillations in the well are nearly in phase with the imposed tidal strain and the phase lag is negligible. If the permeability is low, then the tidal oscillations observed in the well lag significantly behind the solid Earth tidal strain as it takes some time for water to flow into the well. In this study we consider nearly 20 years of data from two wells at the Piñon Flat Observatory (PFO) in Southern California. We observe transient changes up to 20° in the tidal phase response in the two wells at the time of regional earthquakes. The duration of the change is up to a year with a rate of recovery less then 0.1° per day. The phase change reflects an increase in permeability by a factor of as much as 3. Changes in permeability at the time of the earthquakes are observed to be proportional to the measured peak ground velocity (PGV) which in turn is proportional to the peak dynamic stresses in the waves. Our results cannot be easily explained by either fracture growth or new fracturing given the observed dynamic stresses. Perhaps unclogging of temporarily sealed fractures may be a suitable mechanism for our observations.
DE: 5114 Permeability and porosity
DE: 7299 General or miscellaneous
DE: 8123 Dynamics: seismotectonics
SC: Tectonophysics [T]
MN: Fall Meeting 2005