HR: 15:25h
AN: G22D-08    [PDF]
TI: The Evolution of the Seismic-Aseismic Transition During the Earthquake Cycle: Constraints from the Time-Dependent Depth Distribution of Aftershocks
AU: * Rolandone, F
EM: frede@seismo.berkeley.edu
AF: UC Berkeley Seismological Laboratory, 215 McCone Hall, Berkeley, CA 94720 United States
AU: B\"urgmann, R
EM: burgmann@seismo.berkeley.edu
AF: UC Berkeley Seismological Laboratory, 215 McCone Hall, Berkeley, CA 94720 United States
AU: Nadeau, R
EM: nadeau@seismo.berkeley.edu
AF: UC Berkeley Seismological Laboratory, 215 McCone Hall, Berkeley, CA 94720 United States
AU: Freed, A
EM: freed@purdue.edu
AF: Earth and Atmospheric Science, Purdue University, 550 Stadium Mall Drive, West Lafayette, IN 47907 United States
AB: We have demonstrated that in the aftermath of large earthquakes, the depth extent of aftershocks shows an immediate deepening from pre-earthquake levels, followed by a time-dependent postseismic shallowing. We use these seismic data to constrain the variation of the depth of the seismic-aseismic transition with time throughout the earthquake cycle. Most studies of the seismic-aseismic transition have focussed on the effect of temperature and/or lithology on the transition either from brittle faulting to viscous flow or from unstable to stable sliding. They have shown that the maximum depth of seismic activity is well correlated with the spatial variations of these two parameters. However, little has been done to examine how the maximum depth of seismogenic faulting varies locally, at the scale of a fault segment, during the course of the earthquake cycle. Geologic and laboratory observations indicate that the depth of the seismic-aseismic transition should vary with strain rate and thus change with time throughout the earthquake cycle. We quantify the time-dependent variations in the depth of seismicity on various strike-slip faults in California before and after large earthquakes. We specifically investigate (1) the deepening of the aftershocks relative to the background seismicity, (2) the time constant of the postseismic shallowing of the deepest earthquakes, and (3) the correlation of the time-dependent pattern with the coseismic slip distribution and the expected stress increase. Together with geodetic measurements, these seismological observations form the basis for developing more sophisticated models for the mechanical evolution of strike-slip shear zones during the earthquake cycle. We develop non-linear viscoelastic models, for which the brittle-ductile transition is not fixed, but varies with assumed temperature and calculated stress gradients. We use them to place constraints on strain rate at depth, on time-dependent rheology, and on the partitioning of deformation between brittle faulting and distributed viscous flow associated with the earthquake cycle.
DE: 7209 Earthquake dynamics and mechanics
DE: 7230 Seismicity and seismotectonics
DE: 8123 Dynamics, seismotectonics
DE: 8159 Rheology--crust and lithosphere
SC: Geodesy [G]
MN: 2003 Fall Meeting