SCEC Award Number 12214 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Better rheological models of deep coseismic slip, aseismic slip and shear localization
Investigator(s)
Name Organization
Terry Tullis Brown University
Other Participants
SCEC Priorities 3c, 3d, 3e SCEC Groups FARM, WGCEP, EFP
Report Due Date 03/15/2013 Date Report Submitted N/A
Project Abstract
 Uncertainty in seismic hazard arises from poor observational and theoretical constraints on the depth distribution of slip in large magnitude earthquakes. We investigate the implications of laboratory-identified frictional and flow behavior on coseismic down-dip and surface slip in strike-slip whole cycle faulting models. Our focus is to develop rigorous constitutive implementations of the laboratory observations most relevant to deep slip and to use these in models of the earthquake cycle. Using these models we examine scaling of surface slip with along-strike length for strike-slip faulting and aseismic to seismic partitioning at the base of the seismogenic zone. For constant stress drop elastic dislocation models, slip increases linearly with the smallest rupture dimension. So for large earthquakes that rupture the entire seismogenic depth, H, slip is expected to be independent of magnitude. H is defined by the depth extent of small earthquakes. In contrast, actual earthquake data show the average surface slip in large earthquakes increases with rupture length, L, well beyond the seismogenic crust depth. Based solely on a quasidynamic model having only velocity weakening and velocity strengthening friction, we show that for a rupture with an along-strike half-length of 90 km, rupture propagates 3.2 km into the creeping region, a 32% increase of coseismic depth over that which would be inferred from background seismicity. For a strike-slip fault with along strike half-lengths between 10 and 500 km, our quasidynamic model shows systematic increases in surface slip with fault length, similar to observations.
Intellectual Merit Following SCEC’s 2012 priority to characterize earthquake rupture, fault loading, degree of localization, and constitutive behavior at the base and below the seismogenic zone, our primary focus is to develop rigorous constitutive implementations of the laboratory observations most relevant to deep slip and to use these along with field observations from exhumed deep shear zones in models of the earthquake cycle. Using these models we examine scaling of surface slip with along strike length for strike slip faulting and aseismic to seismic partitioning at the base of the seismogenic zone. This is the first time laboratory-based constitutive laws have been used to allow the depth to the seismic-aseismic transition to result from the slip rates occurring in the model rather than being an a priori fixed input.
Broader Impacts Uncertainty in seismic hazard arises from poor observational and theoretical constraints on the depth distribution of slip in large magnitude earthquakes. The Working Group on California Earthquake Probabilities (WGCEP) has identified improvements in constraints on the depth distribution of slip in large magnitude earthquakes as a priority. Earthquake hazard is dominated by intermediate and large events; event slip determines the frequency of dominant events and the intensity of the most hazardous shaking. Thus, whether slip saturates with moment (area or length) is a key issue. Our work suggests that large earthquakes can penetrate significantly deeper than the depth of the seismogenic zone as defined by the lower limit of microsesismicity.
Exemplary Figure Figure 2. Simulated earthquake slip using the proposed constitutive relations for friction, equations (1). Left Panel: Same as Figure 1 left panel. Horizontal dashed reference line is the depth of the BDT, the transition depth for slip at the plate rate of 0.001 μm/s.. Center: Slip contoured at different times over a single simulated earthquake cycle using a brittle ductile transition fixed at 10km depth. Right: Slip over a single simulated earthquake cycle using a brittle ductile transition whose depth varies with slip speed. Horizontal dashed reference line is the coseismic extent of slip in the earthquake.
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