HR: 0800h
AN: T41B-0585    [Abstracts]
TI: Co-seismic Static Stress Drops for Earthquake Ruptures Nucleated on Faults After Progressive Strain Localization
AU: * Griffith, W A
EM: wagrif@stanford.edu
AF: Stanford University, Department of Geological and Environmental Sciences Rm 118, Building 320, Stanford, CA 94305, United States
AU: Nielsen, S
EM: nielsen@ingv.it
AF: Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata,605, Roma, LAZ 00143, Italy
AU: Di Toro, G
EM: giulio.ditoro@unipd.it
AF: Università di Padov, Dipartimento di Geoscienze Via Giotto 1, Padova, VEN 35137, Italy
AU: Pollard, D D
EM: dpollard@stanford.edu
AF: Stanford University, Department of Geological and Environmental Sciences Rm 118, Building 320, Stanford, CA 94305, United States
AU: Pennacchioni, G
EM: giorgio.pennacchioni@unipd.it
AF: Università di Padov, Dipartimento di Geoscienze Via Giotto 1, Padova, VEN 35137, Italy
AB: We estimate the coseismic static stress drop on small exhumed strike-slip faults in the Mt. Abbot quadrangle of the central Sierra Nevada (California). The sub-vertical strike-slip faults cut ~85 Ma granodiorite, were exhumed from 7-10 km depth, and were chosen because they are exposed along their entire lengths, ranging from 8 to 13 m. Net slip is estimated using offset aplite dikes and shallowly plunging slickenlines on the fault surfaces. The faults show a record of progressive strain localization: slip initially nucleated on joints and accumulated from ductile shearing (quartz-bearing mylonites) to brittle slipping (epidote-bearing cataclasites). Thin (< 1 mm) pseudotachylytes associated with the cataclasites have been identified along some faults, suggesting that brittle slip may have been seismic. The brittle contribution to slip may be distinguished from the ductile shearing because epidote-filled, rhombohedral dilational jogs opened at bends and step-overs during brittle slip, are distributed periodically along the length of the faults. We argue that brittle slip occurred along the measured fault lengths in single slip events based on several pieces of evidence. 1) Epidote crystals are randomly oriented and undeformed within dilational jogs, indicating they did not grow during aseismic slip and were not broken after initial opening and precipitation. 2) Opening-mode splay cracks are concentrated near fault tips rather than the fault center, suggesting that the reactivated faults ruptured all at once rather than in smaller slip patches. 3) The fact that the opening lengths of the dilational jogs vary systematically along the fault traces suggests that brittle reactivation occurred in a single slip event along the entire fault rather than in multiple slip events. This unique combination of factors distinguishes this study from previous attempts to estimate stress drop from exhumed faults because we can constrain the coseismic rupture length and slip. The static stress drop is calculated for a circular fault using the length of the mapped faults and their slip distributions as well as the shear modulus of the host granodiorite measured in the laboratory. Calculations yield stress drops on the order of 100-200 MPa, one to two orders of magnitude larger than typical seismological estimates. The studied seismic ruptures occurred along small, deep-seated faults (10 km depth), and, given the fault mineral filling (quartz-bearing mylonites) these were "strong" faults. Our estimates are consistent with static stress drops estimated by Nadeau and Johnson (1998) for small repeated earthquakes.
DE: 7209 Earthquake dynamics (1242)
DE: 7215 Earthquake source observations (1240)
DE: 8010 Fractures and faults
DE: 8030 Microstructures
DE: 8118 Dynamics and mechanics of faulting (8004)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting