SCEC Award Number 12004 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Intact rock and regolith as fragile geological features and nonlinear attenuation
Name Organization
Norman Sleep Stanford University
Other Participants
SCEC Priorities 6e, 4e, 3d SCEC Groups Geology, GMP, EEII
Report Due Date 03/15/2013 Date Report Submitted N/A
Project Abstract
Strong seismic waves damage shallow (10s to 100s m) rock nonlinearly sapping energy from the wave. Strong Love waves impose essentially strain boundary conditions on the upper few 100 m. Dynamic stress imposed strain (particle velocity / phase velocity) time the stress modulus. Failure occurs when this stress exceeds frictional strength. Cracking during each event reduces the shear modulus, which self-organizes so that typical shaking barely causes failure and adds few new cracks. The shear modulus in fact increases linearly with depth in basins near Los Angeles as expected from this process. The water table was near the surface during the 1857 event. Love waves with 3-4 s periods and amplitudes of 1 m/s obtained by SCEC investigators from numerical models would produce nonlinear attenuation. The current water table is ddeper but this has only a modest effect. We developed nonlinear models for vertical S-waves to gain insight into Love waves. Modeling supports the dimensional result that velocity seismograms are saw-toothed with sustained acceleration in g's giving the effective coefficient of friction. We obtained an effective coefficient of friction of ~0.4 from a site near Parkfield, appropriate for shallow water table.
Intellectual Merit There has been considerable effort by SCEC to determine the long period (3-4) ground motion in basins around Los Angeles. These calculations assume that the material behaves linearly. Ambient noise seismology similarly provides strong event seismograms with the assumption of linearity. Our analysis of various sites near Los Angeles indicates that past ground motions of 0.3 to 1.6 m/s damaged the rock and conversely that nonlinear attenuation would sap still larger waves. Improved estimates of parameters needed in our calculations including Love wave phase velocity will become available from other investigators.
Broader Impacts (1) Ambient rock is a ubiquitous fragile geological feature. Our method for past peak shaking may be applied wherever shallow (upper 100s m) S-wave velocity has been measured. It is easily extended to S-waves and Rayleigh waves. (2) We infer that nonlinearity needs to be considered in models of San Andreas earthquakes in southern California. We have simple scaling relationships. (3) As outreach to astrobiologists, we have published paper that tidal regolith is widespread habitable environment on planetary rocky moons. We are also studying Archean seismic regolith from a massive asteroid impact in the Archean as an example of damage by Rayleigh waves. We used biology to show that open cracks suddenly carried oxygen-rich water to kilometer depths near Parkfield. (4) We plan to make our simple methods for Love waves in 2-D generally available so that they can be included in 3-D massive calculations. (5) We have found that drawdown of the water table allows Love wave of higher amplitude to propagate. The Los Angeles drawdown (~60 m) since 1857 does not have big effect, but larger drawdown elsewhere may double the velocity amplitude that propagates.
Exemplary Figure Figure 3. Observed particle velocity west component of Cholame 2WA for the 2004 Parkfield mainshock. Dashed lines show saw-toothed form of wave and sustained acceleration in g’s, which provides estimates on the effective coefficient of friction in the subsurface. Modified from [39] using data from [40]. Zero time is arbitrary. (unpublished).