Rock Damage Constraints on Ground Motion along the San Andreas Fault during the Last Five Million Years

Department of Chemical Engineering and Materials Science Seminar

Rock Damage Constraints on Ground Motion along the San Andreas Fault during the Last Five Million Years --- New Interpretations of Fault Gouge and Lack of Sandstone Damage in Terms of Earthquake Physics

Featuring James Brune, Ph.D.
Director, Nevada Seismological Center, University of Nevada
Visiting Professor, Public Health
Department of Environmental Health, Science and Policy
UC Irvine

Location:  Engineering Lecture Hall 110
Free and open to the public.
Light refreshments served following the event.

Abstract:
Probabilistic seismic hazard analysis (PSHA) predicts extreme ground motions from earthquakes based on statistical assumptions which are very questionable when extended to very low probability maximum ground motions (the order of 10 g acceleration and10 m/s velocity, with 10-6 to 10-8 annual probabilities). Some models of fault rupture predict extreme ground motions due to waves trapped along fault zones. 

The short historical database for earthquake observations is not sufficient to resolve the uncertainties.  This suggests that we look for geomorphic and geologic evidence constraining ground motions over long periods in the past.  Such evidence might include near-fault damage, precariously balanced rocks, geologic evidence for rock avalanches from formerly unstable cliffs, strain-shattered rock, and motion along ancient cracks.

Recent mapping has established the existence of a ~100 m wide layer of pulverized crystalline and sandstone sedimentary rocks parallel to the slipping zone of the San Andreas Fault.  Details of the damage, e.g., lack of any shear parallel to the San Andreas Fault, suggest previously unexpected rupture mechanisms. The fabric of the pulverized rocks is compatible with a tensile type, very low friction, failure mechanism.

Farther from the fault un-fractured Tertiary sandstones exist at a distance between 1 km and a few km at several locations.  These sandstones have been exposed to stresses from overburden when buried, tectonic stresses related to the faulting, and transient dynamic stresses from thousands of large (M~8) earthquakes.  If the tectonic stresses are large enough, relatively low amplitude transient stresses will fracture the rock (in shear, or, at shallow depths, in tension).  Thus the observation that the sandstones are unfractured places an upper bound on the combination of tectonic stresses and extreme transient dynamic stresses.  Measurements of tensile strength of unfractured sandstones at several sites near the fault between Cajon pass and Tejon pass yield values of less than 5 bars.  The rocks at these sites have been at depths between 1 km and 0 km during the history of the San Andreas Fault. This places a strong constraint on the sum of the tectonic stress and transient stress.  Preliminary calculations suggest that the low tectonic stress model (assuming a coefficient of friction of 0.2), places an upper limit on particle velocities of about 1 m/s, and the high tectonic stress model (mu=0.6) about 30 cm/s.

Underground nuclear explosion provide examples of the type of surface geologic effects expected at extreme ground motions.  There is no evidence that such effects have ever occurred along the San Andreas Fault. The type of data discussed provides the potential of providing constraints on extreme ground motions and constraining the types of mechanisms for rupture along the San Andreas Fault.