Probing subsurface fault-zone frictional properties is a key to improving our understanding of the mechanics of postseismic deformations following larger earthquake that will control the occurrences of subsequent aftershocks and triggered earthquakes. Geodetic measurements have revealed spatially and temporally varying postseismic deformations in the crust immediately after large earthquakes (e.g., Johanson et al., 2006; Langbein et al., 2006). The estimations in deformation field at depth are, however, limited by surface measurements of strain. We propose a methodology for observing subsurface frictional properties of the fault zone by exploiting cumulative seismic slips derived from repeating earthquake sequences.
We explore subsurface cumulative fault slips extracted from a set of 27 repeating earthquake sequences extending over a depth range of 10 km at the Parkfield segment of the San Andreas fault. These sequences are chosen because around 5 repeating earthquakes in individual sequences typically occurred during the first month of the postseismic period, which allows us to illuminate time evolutions of cumulative seismic slips. Additionally, we have been monitoring selected sequences since 1984. We are able to well constrain background levels of cumulative seismic slips that needed to be corrected in order to address time-dependent postseismic deformations. Parkfield repeating earthquakes have been detected from continuous HRSN borehole seismograms, and their locations were determined using a double-difference relocation approach (Nadeau et al., 2004). The seismic moments also were reliably evaluated through a spectral ratio method introduced by (Nadeau and Johnson, 1998). Those precise determinations in source characteristics of individual sequences are crucial to identifying spatiotemporal variations of cumulative seismic slips.
We have employed rate-strengthening friction (Perfettini and Avouac, 2005) and viscoelastic models (Montési, 2004) to characterize fault-zone rheological properties from seismological observations of postseismic deformation following the 2004 Parkfield earthquake. We first show that 24 of the 27 sequences can be well-explained by the rate-strengthening friction model (Figure 2.43a). We then evaluate frictional parameter a where a is a frictional coefficient and is effective normal stress, by combining the static Coulomb stress change based on the coseismic slip model of the mainshock (Kim and Dreger, 2008). Our results suggest a to be 0.01-0.5 MPa (Figure 2.43b), a value that is generally consistent with previous studies (Marone et al., 1991).
Our preliminary result suggests that cumulative seismic slips inferred from repeating earthquake sequences provide a means of observing the in-situ frictional parameter. Continuing work will explore spatial variation in frictional parameters and the slip distribution of individual repeating earthquake sequences with finite source modeling.
The present study was supported by the National Science Foundation EAR-0910322. The Parkfield High-Resolution Seismic Network is operated by University of California, Berkeley Seismological Laboratory with financial support from the U.S. Geological Survey through National Earthquake Hazards Reduction Program awards 07HQAG0014 and G10AC00093. We would like to thank A. Kim for providing us with the coseismic slip model.
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