HR: 0800h
AN: S21B-0281    [Abstracts]
TI: Fracturing From Seismic Waves At Regional Distances As Inferred From Water Well Tidal Response.
AU: * El-khoury, J E
EM: elkhoury@ess.ucla.edu
AF: Department of Earth and Space Sciences, University of California, Los Angeles., 595 Charles Young Dr. East, Los Angeles, CA 90095 United States
AU: Brodsky, E E
EM: brodsky@ess.ucla.edu
AF: Department of Earth and Space Sciences, University of California, Los Angeles., 595 Charles Young Dr. East, Los Angeles, CA 90095 United States
AU: Agnew, D
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: Water level responses to solid earth tides provide a means for measuring aquifer properties and, by extension, fracturing. At the time of five earthquakes in Southern California, we observe transient changes up to 20$^o$ in the phase of the water level response to the dilational volumetric strain of the $M_2$ tidal component of wells at the Pi\~non Flat Observatory (PFO). Larger, closer earthquakes generate larger phase changes than the smaller ones. The duration of the phase change is between two and ten months after which the response returns to the background value. We use a model of cylindrical flow driven by an imposed head oscillation through a single, laterally extensive, confined, homogeneous and isotropic aquifer to interpret changes in phase response as changes in aquifer properties. We interpret the changes as due to changes in the transmissivity (rate of water transmission through a unit width of aquifer under a unit hydraulic gradient). The transmissivity measures the ease with which water flows through the aquifer and is proportional to permeability. The higher the transmissivity, the smaller the observed tidal phase lag. At the time of the earthquakes, the transmissivity at the Pi\~non Flat Observatory increases by a factor that can be as high as four. Two different simple permeability models are used to interpret changes in transmissivity as changes in the geometry of the aquifer. Both a network of tubes model and a set of plane parallel fractures model give similar results. The transmissivity changes imply increases between 5% and 70% in the diameter of the tubes and the thickness of the fractures. For instance, the 1999 Hector Mine earthquake produces a 30% increase in fracture size for the tube model and a 40% increase for the fracture model. The fracture model always gives higher values for the fracture size change than the tube model. The fraction of average crack size increment shows a direct dependence on the amplitude of seismic waves at PFO as calculated from empirical magnitude relationships.
DE: 5104 Fracture and flow
DE: 5114 Permeability and porosity
DE: 7200 SEISMOLOGY
DE: 7209 Earthquake dynamics and mechanics
SC: Seismology [S]
MN: 2004 AGU Fall Meeting