HR: 1340h
AN: T23A-0573    [Abstracts]
TI: Stick-Slip as a Mechanism for Earthquakes Revisited
AU: * McGarr, A
EM: mcgarr@usgs.gov
AF: U.S. Geological Survey, MS977, 345 Middlefield Rd., Menlo Park, CA 94025 United States
AU: Fletcher, J B
EM: jfletcher@usgs.gov
AF: U.S. Geological Survey, MS977, 345 Middlefield Rd., Menlo Park, CA 94025 United States
AB: 38 years ago Brace and Byerlee (Science, 1966) proposed that shallow earthquakes represent stick-slip sliding along old or newly formed faults in the earth and that observed stress drops represent release of a small fraction of the stress supported by the rock at the earthquake focus. We have investigated the generality of this proposal by relating the results of stick-slip friction experiments on large granite samples to counterpart observations for earthquakes. The stick-slip experiments, entailing the measurements of fault slip, frictional stress, and loading stress as functions of time, can be analyzed to determine the maximum slip rate, the apparent stress, the dynamic stress drop and the static stress drop, which, in turn, can be related to the corresponding earthquake parameters using a stress adjustment factor. That is, the laboratory loading stresses must be multiplied so as to approximate stresses measured in the seismogenic crust. For example, a stick-slip experiment run at a normal stress of 2.76 MPa was adjusted for the state of stress measured at a depth of 6.8 km at the KTB site, Germany, the deepest available in situ measurement. Applying the resulting stress adjustment factor of 41 to laboratory measurements of static stress drop, dynamic stress drop, apparent stress, and peak slip rate yielded estimates of 12.7 MPa (compared to a total shear stress of 65 MPa), 10.2 MPa, 3.3 MPa, and 3.1 m/s, respectively. These stress-adjusted parameters, independent of earthquake size, are all typical of those observed or inferred for major earthquakes. In particular, maximum slip rates within the fault zones of earthquakes appear to be several m/s independent of magnitude or moment. To relate the laboratory slip to the maximum slip within the fault zone of an earthquake, differences in stiffness as well as the loading stresses must be taken into account; the stiffness adjustment results in maximum slip scaling according to the cube root of seismic moment. Applying both the stress and stiffness adjustments to a laboratory slip of 93 microns yielded a maximum slip of 11 m for the M7.9 Denali, Alaska earthquake, in good agreement with peak values of about 12 m from slip models developed for this event. Thus, the physics of stick-slip friction as observed in biaxial laboratory tests at modest loading stresses appears to govern the rupture processes of crustal earthquakes in general, even those with large magnitudes. If so, then there is no need to invoke rupture processes unique to large earthquakes, such as thermal pressurization.
DE: 8159 Rheology--crust and lithosphere
DE: 8164 Stresses--crust and lithosphere
DE: 7209 Earthquake dynamics and mechanics
DE: 7215 Earthquake parameters
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting