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Time-dependent depth distribution of aftershocks: 
Implications for fault mechanics and crustal rheology

Time-dependent depth distribution of the seismicity following Landers earthquake for the Johnson Valley Fault.
The red curve shows the statistics for the average of the 5% of the deepest earthquakes.









Project Summary   The maximum depth of seismogenic faulting can be interpreted either as the transition from brittle faulting to plastic deformation  or as the transition from unstable to stable sliding. The maximum depth at which crustal earthquakes occur depends on four main factors: rock composition, temperature, strain rate, and fluid pressure. We focus on how the maximum depth of seismogenic faulting varies locally with time during the earthquake cycle in order to constrain the strain-rate dependent depth of the brittle-ductile transition. We investigate the time-dependent depth distribution of aftershocks following moderate to large earthquakes on strike-slip faults in southern California. We use the catalogs of Richards-Dinger and Shearer (2000) and Hauksson (2000) and we apply the double difference method of Waldhauser and Ellsworth (2000) to further improve event locations. We initially focus on the time-dependent depth distribution of aftershocks in the Superstition Hills area and in the Mojave desert. The different catalogs and our own relocation results show large variations in the hypocentral depths which seem mainly related to the choice of the velocity model for the upper crust. However, a persistent feature of the depth distribution of aftershocks is that in the immediate postseismic period the aftershocks are deeper than the background seismicity and the deepest aftershocks become shallower with time. We use numerical finite-element models to relate these observations to the mechanical evolution of strike-slip faults and to infer fault slip and the rheology near the base of seismogenic faults. We compare the seismological observations and calculations of postseismic stress in a power law rheology below a strike-slip rupture as a function of time. Our objective is to relate the time-dependent depth distribution of seismicity during the earthquake cycle to the evolution of the brittle-ductile transition and thus place constraints on strain rate at depth and on the partitioning  of deformation between brittle faulting and distributed deformation.
Tools We use the 69-year catalog of the Southern California Seismic Network (SCSN) to identify target events.
We use the catalog of relocated 1975-1998 earthquakes using the source specific station term method of Richards-Dinger and Shearer (2000), and the catalog of relocated events using a three-dimensional velocity model based on a joint-hypocenter-velocity (JHV) from 1981 to 2000 of Hauksson (2000) to do preliminary studies of aftershocks sequences.
We apply the double difference method of Waldhauser and Ellsworth (2000) to relocate earthquakes in the areas of interest using the HypoDD relocation code.
Geographic Location We investigate the time-dependent depth distribution of aftershocks following moderate to large earthquakes on strike-slip faults in southern California. Southern California offers a dense network of active strike-slip faults
and high seismic activity. It is a very well instrumented area.
We are currently analysing the seismicity around the 1987 Superstition Hills earthquake and around the 1992 Landers earthquake.
Group Members Involved Frederique Rolandone <email> <web site>
Bob Nadeau, Andy Freed, Roland Bürgmann
Project Duration In Progress 


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