HR: 10:45h
AN: S41G-02 INVITED     [PDF]
TI: Slip localization on narrow surfaces within exhumed fault zones: implications for earthquake rupture processes and seismic energy budgets
AU: * Shipton, Z K
EM: shiptonz@tcd.ie
AF: Department of Geology Trinity College, College Green, Dublin, 2 Ireland
AU: Evans, J P
EM: jpevans@cc.usu.edu
AF: Department of Geology Utah State University, 4505 Old Main, Logan, UT 84322 United States
AB: Observations of exhumed faults, with net offset ranging from cm to km, indicate that fault slip is often accommodated on slip surfaces only mm's to cm's thick embedded in a damage zone 10's to 100's m thick. The same deformation textures observed in the smallest faults are seen along faults of greater length and offset. We use rock mechanics and earthquake energy and moment relationships to show that slip zone thickness t is a function of seismological parameters (seismic efficiency $\eta$, defined as the proportion of the total driving stress that contributes to seismic radiation, $\tau_{a}$), fault gouge properties (grain surface area per unit mass of fault gouge S, density $\rho$), co-seismic slip, D and the fracture energy (crushing energy per unit area of rupture G), according to the relationship t = $\tau_{a}$ D /$\eta$ S $\rho$ G. By using reasonable values of these parameters this relationship predicts that the thickness should be of the order of millimeters, close to our field observations. Given that G scales with rupture size and that the other parameters vary over a narrow range, this equation shows that co-seismic slip should remain restricted to narrow zones across a large range of slip magnitudes (rupture areas). During a rupture, stored strain energy is partitioned into radiated seismic energy, E$_{a}$, work done against frictional resistance to slip, E$_{f}$, (resulting in heating of the fault zone) and the energy required to create a new rupture surface and/or re-rupture a healed surface, E$_{g}$. Given the relatively low seismic efficiency of earthquakes, much of the energy from an earthquake goes into propagating slip along the narrow slip zone. There are a number of complex 3-D processes that could potentially consume energy on faults, which are not typically represented in most fault zone energy balance models. Energy must be consumed by deformation of the damage zone, movement of fluid flow, heating of the focal region of the fault, and chemical alteration in the fault zone. Preliminary calculations indicate that alteration consumes a small proportion of the energy, leaving mechanical, thermal, and fluid motion processes as possible energy sinks.
DE: 5104 Fracture and flow
DE: 7209 Earthquake dynamics and mechanics
DE: 7215 Earthquake parameters
DE: 8010 Fractures and faults
DE: 8030 Microstructures
SC: Seismology [S]
MN: 2003 Fall Meeting