HR: 16:45h
AN: S34A-04    [Abstracts]
TI: Thermal Pressurization Explains Enhanced Long-Period Motion in the Chi-chi Earthquake
AU: * Andrews, D J
EM: jandrews@usgs.gov
AF: U.S. Geological Survey, Mail Stop 977 345 Middlefield Road, Menlo Park, CA 94025 United States
AB: Ground motion recorded in the 1999 M 7.6 Chi-chi, Taiwan, earthquake differed between points near the southern and near the northern portions of the fault. Spectral response velocity in the south was roughly flat at periods from 1 s to 10 s, while spectral velocity in the north was about the same at 1 s, but increased with period to be more than double the southern response value at 10 s. The fault is in different lithologic units in the north and south, being within a low-permeability shale in the north. The spatial correlation strongly suggests that properties of the shale determined the enhanced long-period motion. Thermal pressurization of pore fluid due to frictional heating during fault slip reduces effective pressure and so reduces shear stress resisting slip. The process of thermal pressurization operates in a Coulomb solid, unlike elasto-hydrodynamic lubrication (Brodsky and Kanamori, 2001), which requires fluid fault gouge. Stress drop due to thermal pressurization can be nearly complete in calculated models, if diffusion of pressurized fluid away from the fault is sufficiently limited. For given heat input, fluid pressure rise is inversely proportional to the thickness of the pressurized zone. Complete stress drop is approached when slip is 4 to 8 times the pressurized thickness. Laboratory measurements of permeability in the Chinshui shale imply that fluid pressure will diffuse only 2.5 cm in 10 s. For a slip zone a few centimeters thick, complete stress drop will be achieved at slip on the order of a decimeter. This prediction is highly uncertain, because the stress concentration at the rupture front could greatly increase permeability in a damage zone. Shale is both less permeable and less subject to dilatant damage than other rocks. Dynamic simulations of the Chi-chi earthquake have been performed. Average initial stress is constrained by the geometry of the accretionary prism. Self-similar spatial fluctuations in initial stress produce ground motions that match the southern spectra. Thermal pressurization at shallow depths in the north produces complete stress drop there and matches long period amplitudes. In order to match shorter period amplitudes, however, it is necessary that fluid pressure rise over a period of several seconds. A fit is obtained by assuming that the pressurized fluid, although it may be generated by heating in a thin slip zone, is promptly spread through a damage zone 1.25 m thick. Then the rate of rise of fluid pressure is reduced, and complete stress drop is reached after a critical slip displacement of Dc = 8 m, a large fraction of the final slip.
DE: 5114 Permeability and porosity
DE: 7209 Earthquake dynamics (1242)
DE: 7212 Earthquake ground motions and engineering seismology
SC: Seismology [S]
MN: Fall Meeting 2005