HR: 08:30h
AN: G51D-03 INVITED     [Abstracts]
TI: Evolution of Fault Strength Over the Seismic Cycle
AU: * Marone, C
EM: cjm@geosc.psu.edu
AF: Penn. State University, 536 Deike Bld., University Park, PA 16802 United States
AB: A fundamental premise of the seismic cycle is that faults restrengthen (heal) between earthquakes; thus, understanding fault healing is a key step in understanding the seismic cycle and reconciling geologic and geodetic estimates of fault slip rates. In this talk, I summarize laboratory measurements of frictional healing and discuss seismic estimates of the rate of fault healing in the context of the seismic cycle. Frictional healing, as indicated by increasing frictional strength as a function of time and slip during quasi-stationary contact, is considered the most likely mechanism of fault re-strengthening. Laboratory friction data show good agreement, in some cases, with seismic observations of fault healing. Such studies show that frictional healing varies with loading rate and shear stress level, and that small changes in temperature and fluid-state can produce significant changes in frictional re-strengthening. Notably, the change in frictional yield strength with time is negative in some cases, indicating a time dependent weakening process. However, detailed data from laboratory experiments with carefully controlled boundary conditions are available for only a small range of conditions, and primarily for materials that simulate fault gouge. Thus, much work remains for laboratory investigations of the physicochemical mechanisms of fault healing. Seismically-based estimates of the rate of fault healing have generally found that seismic stress drop varies as roughly the log of time between events. The early data sets showed that seismic moment increased systematically as a function of earthquake recurrence time, which is consistent with increasing frictional yield strength with time. However, recent, high-resolution observations of repeating earthquakes indicate that fault healing may be more complex. These data show that healing rate varies systematically with depth and distance from the mainshock. Notably, seismic moment decreases with increasing recurrence interval in some cases, indicating a time dependent weakening process and/or a slip-rate dependent strengthening effect that fades quickly in the postseismic period. Laboratory-based friction models and field observations show that postseismic deformation may occur within the region of coseismic rupture and that slip rates decay logarithmically with elapsed time following a mainshock. Thus, a difficulty in comparing geodetic and geologic strain rates may be non-linearity of slip rate evolution over the seismic cycle and uncertainty associated with timing relative to the mainshock. The available laboratory data and seismic observations of repeating earthquakes indicate that simple models of the seismic cycle, based on simple concepts of earthquake recurrence, may be too simplistic to describe the behavior of major faults.
DE: 1242 Seismic cycle related deformations (6924, 7209, 7223, 7230)
DE: 5104 Fracture and flow
DE: 7209 Earthquake dynamics (1242)
DE: 8118 Dynamics and mechanics of faulting (8004)
SC: Geodesy [G]
MN: Fall Meeting 2005