HR: 1340h
AN: S23B-0325    [Abstracts]
TI: Depth Distribution of the Stress Sensitivity of Seismic Wave Velocities in the Crust
AU: * Furumoto, M
EM: furumoto@hakusan.s.kanazawa-u.ac.jp
AF: Graduate School of Science and Technology, Kanazawa University, Kakuma, Kanazawa, 920-1192 Japan
AU: Hiramatsu, Y
EM: yoshizo@hakusan.s.kanazawa-u.ac.jp
AF: Graduate School of Science and Technology, Kanazawa University, Kakuma, Kanazawa, 920-1192 Japan
AB: Seismic wave velocities in rocks are known to be sensitive to applied stress from laboratory experiments and field observations. The stress sensitivity of the velocity is attributed to opening and closing of such small-scaled defects as pores and cracks in the rocks. Then monitoring temporal changes in seismic wave velocities may provides a method to infer the stress state in the crust. In addition to this, the sensitivity depends also on the degrees of fluid saturation and pressure in the defects. Then there is a possibility that we obtain information on such interesting features of the defects in the crust. However, our knowledge about the sensitivity in the crust is far from the comprehension. To discuss the depth distribution of the sensitivity in the crust, we here compile stress sensitivities obtained by various field experiments. Analyzing the apparent stress sensitivity as a function of the epicentral distance of seismic station or the base line length of experiments, we estimate the depth distribution of the sensitivity. We define here the sensitivity S as S = dlogv/ds, where v is the average velocity along the wave path and s is the stress. The base line lengths in the compiled experiments are in a range from 100m to about 400 km. The observed sensitivities vary from 1 to 0.001 (1/MPa) depending on the base line length. The highest sensitivity is observed in experiments whose base lines are of the order of 100m, which can be explained by high concentrations of the small defects very near the surface. The sensitivity rapidly decreases with the increase of the base line length; the sensitivity decreases sharply with depth. If we assume that the depth distribution is an exponential function of depth, the observed relation between the sensitivities and the base line lengths is well explained by a depth constant (a depth increment with which the sensitivity becomes 1/e ) of the order of 1 km.
DE: 8164 Stresses--crust and lithosphere
DE: 7203 Body wave propagation
DE: 7205 Continental crust (1242)
SC: Seismology [S]
MN: 2004 AGU Fall Meeting