HR: 15:25h
AN: S33C-08    [Abstracts]
TI: Modeling Physical Limits on Extreme Earthquake Ground Motion
AU: * Andrews, D J
EM: jandrews@usgs.gov
AF: U.S. Geological Survey, Mail Stop 977, 345 Middlefield Road, Menlo Park, CA 94303 United States
AB: When predicting ground motion at critical structures for very low probability of exceedance, Probabilistic Seismic Hazard Analysis can yield values much larger than any ground motion that has actually been observed in an earthquake. Are there physical limits that can constrain such predictions? Two physical principles can be applied: (1) maximum stress drop available at the source, and (2) strength of material through which waves propagate. Unfortunately, we do not know the state of stress on faults in the earth's crust. Shear stress ranging up to 100 MPa is allowed by laboratory values of friction. A stress drop of 100 MPa may occur in small patches, but such stress drop over a large area of a fault would produce ground motion that has never been observed. Whatever the stress, complete stress drop may be possible. Thermal pressurization of pore fluid from frictional heating can produce near-complete stress drop in large events in sufficiently impermeable material. The character of ground motion near the northern part of the rupture of the Chi-chi earthquake suggests that thermal pressurization may have occurred there. More understanding is needed of when near-complete stress drop may be expected. Strength of material provides a physical constraint on ground motion. Particle velocity propagated in an S wave is limited to shear strength divided by shear impedance, which effectively limits short-period motion, even though velocity can increase further as waves reverberate in a layer. Non-elastic response near the earth's surface constrains short-period motion. In addition, non-elastic response near a rupture front increases fracture energy and limits particle velocity at the source. To establish physical limits on earthquake ground motion, we need to use non-linear calculational methods.
DE: 7223 Seismic hazard assessment and prediction
DE: 7209 Earthquake dynamics and mechanics
DE: 7212 Earthquake ground motions and engineering
SC: Seismology [S]
MN: 2004 AGU Fall Meeting