SCEC Award Number 20001 View PDF
Proposal Category Individual Proposal (Data Gathering and Products)
Proposal Title Nonlinear seismic waves with applications to strong ground motions
Name Organization
Norman Sleep Stanford University
Other Participants
SCEC Priorities 4b, 4a, 3e SCEC Groups GM, FARM, Geology
Report Due Date 03/15/2021 Date Report Submitted 03/02/2021
Project Abstract
We continued our purposed work on nonlinear propagation of strong seismic waves, which are central to the SCEC purview of strong ground motions. Nonlinear failure likely occurs within the shallow (meters to tens of meters) depths within the well-known geotechnical layer. We completed study on the 2002 Denali mainshock, which was recorded by station PS10. Basically, the Coulomb stress ratio of dynamic shear stress on horizontal planes to the overburden stress on these planes is the dynamic normalized acceleration in g's. The horizontal acceleration in g's cannot exceed the effective coefficient of friction. The observed normalized acceleration remained just below 0.35, which is an acceptable effective coefficient of friction for wet gravel. We completed a field project on boulders that appear to have been mildly displaced by the Ridgecrest earthquakes. Debris that accumulate within the gaps beneath the boulders may be the basis for a paleoseismology method.
Intellectual Merit Our research constrains the inelastic (rheological) behavior of geotechnical soils and deeper rock masses through which strong-motion seismic waves propagate. We have shown that Coulomb friction is an appropriate rheology for failure of wet gravel near the Denali fault in the Denali Earthquake. Scaling relationships indicate that the near-field velocity pulse of this even brought the uppermost ~2 km into dynamic frictional failure. This feature appears in 3-D dynamic models by the SCEC community. High frequency S waves appear to have been attenuated when they passed through the region of nonlinear failure. We have investigated nonlinear viscous rheology for shallow muds in the past. Our work is intend to aid in formulating and understanding 3-D numerical nonlinear calculations. We also show ways to preliminarily recognize nonlinear effects on digital seismograms.
Broader Impacts We were unable to do outreach field work on the end-Cretaceous asteroid impact layer with junior college students in Montana because of COVID-19. This work had potential global applications to immediate and delayed trigering of earthquakes by strong seismic waves.
We did show that extreme seismic waves from the impact triggered a small M=~6 event in Colorado and that this fault never slipped again over 66 million years. We also found minor faults that slipped within tens of millennia after the impact and did not slip again. This observation relates to hypotheses where the impact triggered many huge intraplate events away from our field areas that reset global intraplate stress and the tens of millennia were needed for these stresses to return to their previous values. We found with further field work in the previous season that many faults in the area continued to remain active for more than tens of millennia. Thus we did not find useful evidence that intraplate stresses were reset by the impact.
Exemplary Figure Use Figure 1.

Figure 1. Example mildly displaced rock from Wagon Wheel Staging Area that was affected by Ridgecrest earthquakes. Organic material is already entering the gap between the rock and its socket. We have archived bursts of our photos with location, facing direction, and time metadata. The field sites thus may be revisited in the future to appraise the potential for paleoseismology. We expect rains have already filled the gap with debris including some organic material of the year. There may also be preserved information that relates to the season of the earthquake and perhaps wet or dry decade. Earthquakes with similar 14C ages then can be further correlated or dis-correlated. Spheroidal granite boulders are very common in semiarid regions of California.

Credit Norman H. Sleep