HR: 13:40h
AN: S33C-01 INVITED     [Abstracts]
TI: What controls slip heterogeneity- prestress, fracture energy, or sliding friction?
AU: * Aagaard, B
EM: baagaard@usgs.gov
AF: U.S. Geological Survey, USGS MS977 345 Middlefield Rd, Menlo Park, CA 94025 United States
AU: Heaton, T
EM: heatont@caltech.edu
AF: California Institute of Technology, MS252-21 Caltech, Pasadena, CA 91125 United States
AB: We are exploring the physics of earthquake ruptures to understand what controls slip and stress heterogeneity. We consider many variations in the fault friction model and seek a set of parameters that allows the system to evolve into a stable heterogeneous state. In other words, earthquake ruptures with heterogeneous slip are able to occur across a wide range of length scales. We test friction models with and without shear restrengthening (friction rises as slip rate drops) and with and without spatial variations in the fracture energy and dynamic sliding stress. Results from 3-D finite-element dynamic rupture simulations of earthquakes with magnitudes between 5.0 and 7.7 on a planar strike-slip fault show that: (1) {\em heterogeneous prestress} with {\em no shear restrengthening}, {\em uniform fracture energy} and {\em uniform dynamic sliding stresses} rapidly evolves into a nearly homogeneous stress state with earthquakes that only rupture the entire fault and produce distributions of {\em slip} that are much {\em too smooth}; (2) {\em heterogeneous prestress} with {\em shear restrengthening}, {\em uniform fracture energy} and {\em uniform dynamic sliding stresses} slowly evolves into a homogeneous stress state with earthquakes that rupture larger and larger portions of the fault and produce distributions of {\em slip} that are {\em too smooth}; (3) {\em heterogeneous prestress} with {\em no shear restrengthening}, {\em heterogeneous fracture energy} and {\em heterogeneous dynamic sliding stresses} evolves into a slightly heterogeneous stress state with too many large earthquakes that produce {\em reasonable} distributions of {\em slip}; (4) {\em heterogeneous prestress} with {\em shear restrengthening}, {\em heterogeneous fracture energy} and {\em heterogeneous dynamic sliding stresses} evolves into a heterogeneous stress state with a wide variety of earthquake sizes that produce {\em realistic} distributions of {\em slip}. As the stress state on the planar fault tends toward a more homogeneous distribution, the slip distributions become smoother and fail to exhibit the degree of heterogeneity seen in kinematic source inversions. The set of parameters that appear to produce the most realistic conditions (set 4) yields faults that are in a critical state and can fail across a wide range of length scales with slip distributions that have more appropriate levels of heterogeneity. This set of parameters might also be a proxy for nonplanar fault geometry; we suspect that nonplanar geometry coupled with relatively homogeneous friction parameters would yield similarly realistic behavior.
DE: 7200 SEISMOLOGY
DE: 7209 Earthquake dynamics and mechanics
SC: Seismology [S]
MN: 2004 AGU Fall Meeting