HR: 1330h
AN: S42C-0181    [PDF]
TI: Stress Drop and Stiffness for the 1992 Landers, 1994 Northridge, and 1995 Kobe Earhquakes
AU: * Fletcher, J B
EM: jfletcher@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025 United States
AU: McGarr, A
EM: mcgarr@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025 United States
AB: The new method of computing seismic energy and apparent stress of McGarr and Fletcher (2002) provides a means for computing the static stress drop on each sub fault from slip models by \[ \Delta\sigma=2\tau_{a}(E_{nf}/E_{a}-1) \] where $\Delta\sigma$ is the stress drop, $\tau_{a}$ is the apparent stress, $E_{nf}$ is the seismic energy in the nearfield, and $E_{a}$ the seismic energy propagated to the farfield. From this we can determine a local stiffness using the definition from Walsh (1971) of\\ \[ K_{e}=\Delta\sigma/u \] where $K_{e}$ is the stiffness, and $u$ is slip. Using this method we determined stress drop and stiffness for slip models of 1992 Landers, 1994 Northridge, and 1995 Kobe earthquakes. Although the depth dependence of stress drop is weak for Kobe, it is clear for both the Northridge and Landers events. Maximum values range from about 40 MPa at a depth of 17 km for the Northridge event to 6 MPa at a depth of 14 km for the Kobe event. Stiffnesses are much more constant with depth and range from a high value for Northridge of about 16 MPa/m to a low value for Kobe of about 4 MPa/m. The depth dependence of the maximum value of stress drop at each interval in depth is similar in shape to estimates of crustal strength versus depth.\\ The slope of stress versus slip from dynamic modeling of the Kobe earthquake has units of stiffness and following peak stress have values that are similar to that obtained from our method (about 7 MPa/m for the dynamic modeling at deeper points on the fault). This implies that the value of slip corresponding to the distance from peak stress to the frictional sliding stress obtained from the dynamic modeling is related to the amount of slip on the subfaults and not to the critical slip distance in the Dieterich (1981) slip-weakening model of friction. Further, if the slip is larger than the critical slip distance then the area under the stress unloading curve inferred from dynamic modeling following peak stress is probably an overestimate of the true fracture energy. McGarr, A. and J.B. Fletcher (2002). Mapping apparent stress and energy radiation over fault zones of major earthquakes, Bull. Seism. Soc. Amer., 92, 1633-1646.\\ Walsh, J.B. (1971). Stiffness in faulting and in friction experiments, Jour. Geophys. Res., 76, 8597-8598.\\ Dieterich, J. H. (1981). Constitutive properties of faults with simulated gouge, in mechanical Behavior of Crustal Rocks, edited by N.L. Carter, M. Friedman, J.M.Logan, and D.W. Stearns, Geophys. Monor. Ser., 24, AGU, Washington, D.C., 103-120.
DE: 7209 Earthquake dynamics and mechanics
DE: 7215 Earthquake parameters
DE: 7260 Theory and modeling
SC: Seismology [S]
MN: 2003 Fall Meeting