HR: 09:15h
AN: S21A-06    [PDF]
TI: New Insights into Crustal Attenuation from Deep Borehole Studies
AU: * Prejean, S G
EM: sprejean@usgs.gov
AF: US Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025 United States
AU: Abercrombie, R E
EM: rea@bu.edu
AF: Boston University, Dept. of Earth Sciences; 685 Commonwealth Avenue, Boston, MA 02251 United States
AU: Ellsworth, W L
EM: ellswrth@usgs.gov
AF: US Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025 United States
AU: Imanishi, K
EM: hisao.itou@aist.go.jp
AF: National Institute of Advanced Industrial Science and Technology, AIST Tsukuba Central 7; Higashi 1-1-1, Tsukuba, 305-8567 Japan
AU: Ito, H
EM: hisao.itou@aist.go.jp
AF: National Institute of Advanced Industrial Science and Technology, AIST Tsukuba Central 7; Higashi 1-1-1, Tsukuba, 305-8567 Japan
AU: Stork, A
EM: anna@blacknest.gov.uk
AF: National Institute of Advanced Industrial Science and Technology, AIST Tsukuba Central 7; Higashi 1-1-1, Tsukuba, 305-8567 Japan
AB: Teleseismic and regional attenuation studies commonly find that S-waves are more attenuated than P-waves. Four recent studies that have estimated Q as part of the process of determining local earthquake source parameters using data recorded in deep boreholes (800 m to 2500 m), however, find the opposite result for ray paths that sample the seismogenic crust: P-waves are more strongly attenuated than S-waves. The difference in Qs/Qp between the borehole, regional, and teleseismic studies reflects the depth dependence of attenuation in the crust. In this presentation, we summarize attenuation measurements from the SAFOD Pilot Hole in Parkfield, California, the Long Valley Exploratory Well (LVEW) in eastern California; the Cajon Pass borehole in southern California; and the Ontake borehole in western Nagano, Japan, and we discuss the implications these measurements have for physical properties of the seismogenic crust. The seismometers in all four boreholes were installed well below the water table in competent basement rock and were used to observe nearby earthquakes with focal depths from 2 to 10 km. At all sites S-waves are less attenuated than P-waves (Qs/Qp = 1.2-2). The ratio Qs/Qp does not vary systematically with the overall degree of attenuation; at Cajon Pass Qp$\sim$900 and at SAFOD Qp$\sim$250, yet Qs/Qp = 1.2 in both areas. In the case of Ontake, the only site where frequency dependent attenuation was estimated, Qs/Qp also does not vary with frequency. Furthermore, at all sites, neither Qp nor Qs varies systematically with corner frequency, as it would were Q to have a strong frequency dependence. Because these four boreholes are located in widely varying tectonic and lithologic environments, Qs$>$Qp may be a common property of the Earth's crust in the 1-10 km depth range. The two boreholes in geothermally active provinces that we have studied have higher Qs/Qp ratios than the other sites (LVEW Qs/Qp$\sim$2 and western Nagano Qs/Qp $\sim$1.7 versus Cajon Pass and SAFOD Qs/Qp$\sim$1.2). Theoretical calculations and laboratory rock mechanics experiments suggest that Qs/Qp reaches a maximum of 2-2.5 when pore spaces are 70-90% saturated (see Winkler and Nur, {\it Geophysics}, 1995 V. 47, p.1 for summary). Although it is not clear if these rock-physics observations are applicable to the crust at seismogenic depths, they suggest that pore spaces are not fully saturated. Particularly high Qs/Qp in Long Valley Caldera and Ontake might reflect the presence of a steam phase trapped in pore spaces.
DE: 7200 SEISMOLOGY
DE: 7205 Continental crust (1242)
DE: 7215 Earthquake parameters
SC: Seismology [S]
MN: 2003 Fall Meeting