| Abstract | Aseismic creep refers to fault slip that does not produce seismic radiation. Both geological and geodetic observations document evidence of creep along many fault segments in California. Fault creep releases elastic strain and reduces the hazard from future earthquakes, making it an important part of seismic hazard estimation. Comprehensive measurements of fault creep can be used to infer the variations in creep rate and creep depth, but testing the physical models will require additional information. As discussed above, the candidate physical mechanisms include: (1) Low normal stress and mature fault gouge in the upper 3 km of the crust causes the frictional properties of the fault to undergo velocity strengthening [Marone and Scholz, 1988; Linker and Dieterich, 1992]. (2) The creep behavior depends on the rock type and pore pressure [e.g. Sieh and Williams, 1990]. This model predicts that creep will be correlated with rock type. (3) The creep behavior depends on the shallow stressing rate, which in turn depends on the deep locking depth D [Savage and Lisowski, 1993]. This model predicts that fault creep will be proportional to shallow stressing rate and creeping depth. We propose to focus on testing the third mechanism. Exploring the other factors using more comprehensive models and rheologies is beyond the scope of this investigation. We will begin with the model of Smith and Sandwell [2003; 2007] to calculate the shallow stress accumulation rate. |
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(a) Fault slip versus depth for the complete earthquake cycle, after Tse and Rice [1986]. At each depth, the sum of the co-, post- and inter-seismic slip must equal the geologic displacement. (b) Surface displacement across the fault for the co-, post- and inter-seismic parts of the cycle. The near-field surface displacement for both the co- and inter-seismic phases depends on the depth distribution of fault creep during the inter-seismic period. InSAR is the best tool for monitoring near-field co- and inter-seismic displacement over large areas. GPS is the best tool for monitoring far-field velocity and time-dependent, post-seismic displacement. (c) Schematic for interseismic strain accumulation. At shallow depths (0 - 4 km), a velocity strengthening zone occurs along well-developed faults resulting in stable sliding at shallow depth [Marone and Scholz, 1988]. Similarly, there is a velocity strengthening zone at greater depth (> ~12 km). (d) Fit of GPS velocity data across the Imperial fault [Lyons et al., 2002] reveals a locked zone between 2.9 and 10 km depth. We use the Savage and Lisowski [1993] model to fit the GPS velocity data across the fault. |
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