We have demonstrated that aftershock distributions after several large earthquakes show an immediate deepening from pre-earthquake levels, followed by a time-dependent postseismic shallowing (Rolandone et al., 2002). We use these seismic data to constrain the depth variations with time of the seismic-aseismic transition throughout the earthquake cycle. Most studies of the seismic-aseismic transition have focussed on the effect of temperature and/or rock composition and 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, with time during the earthquake cycle.
The mechanical behavior of rocks in the crust is governed by frictional behavior and at greater depth by plastic flow. The coupling between the brittle and ductile layers and the depth extent and behavior of the transition zone between these two regimes is a fundamental question. Mechanical models of long-term deformation (Rolandone and Jaupart, 2002) suggest that the brittle-ductile transition is a wide zone where deformation is caused both by slip and ductile flow. Geologic observations (Sibson, 1986; Scholz, 1990; Trepmann and Stockhert, 2002) indicate that the depth of the seismic-aseismic transition varies with strain rate and therefore is also expected to change with time throughout the earthquake cycle. The maximum depth of seismogenic faulting is interpreted either as the transition from brittle faulting to plastic flow in the continental crust, or as the transition in the frictional sliding process from unstable to stable sliding. The seismic-aseismic transition therefore reflects a fault zone rheology transition or a more distributed transition from brittle to ductile deformation mechanisms.
We investigate the time-dependent depth distribution of aftershocks in the Mojave Desert. We apply the double difference method of Waldhauser and Ellsworth (2000) to the region of the M 7.3 1992 Landers earthquake to relocate earthquakes. Time-dependent depth patterns of seismicity have been identified in only few previous studies (Doser and Kanamori, 1986; Schaff et al., 2002) and never quantified. This was mainly due to the problem of the accuracy of the hypocenter locations. Accurately resolving depth is the most challenging part of earthquake location. With new relocation techniques, we can investigate the time-dependent depth distribution of seismicity to reveal more intricate details in the patterns of deformation which take place during an earthquake cycle.
In this study, we focus on quantifying the temporal pattern of the deepest aftershocks. We calculate the d, the depth above which 95 of the earthquakes occur, and we also calculate the d, the average of the 5 of the deepest earthquakes for a constant number of events. We compare our results with the same statistics for the Hauksson relocations (catalog from Hauksson (2000) with a vertical error cutoff of 1.5 km). 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. Figure 18.1 shows the time-dependent depth distribution of seismicity for the Johnson Valley fault that ruptured in the 1992 Landers earthquake. Our analysis reveals a strong time-dependence of the depth of the deepest aftershocks. In the immediate postseismic period, the aftershocks are deeper than the background seismicity, followed by a time-dependent shallowing. Figure 18.2 shows the same data but in the form of histograms and relate them to the deepening of the brittle-ductile transition after the mainshock. The temporal variations of the depth of the brittle-ductile transition reflect the strain-rate changes at the base of the seismogenic zone.
The analysis of seismic data to resolve the time-dependent depth distribution of the seismic-aseismic transition provides additional constraints on fault zone rheology, which are independent of geodetic data. 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.
This research is supported by the Southern California Earthquake Center and an IGPP/LLNL grant.
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