SCEC Award Number 17001 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Nonlinear attenuation of strong seismic waves, intact rock as a fragile geological feature, modulation of tectonic strain by strong ground motions, and rupture propagation
Name Organization
Norman Sleep Stanford University
Other Participants
SCEC Priorities 4b, 4a, 2e SCEC Groups Geology, GM, Seismology
Report Due Date 06/15/2018 Date Report Submitted 05/25/2018
Project Abstract
Our objective was to investigate how nonlinear behavior of strong seismic waves affects the amplitude of shaking at the free surface. We concentrated on the nonlinear behavior of muddy soil. This material is both elastic with the uppermost layer supporting resonating body waves and mildly nonlinearly viscous. The onset of viscous creep at low dynamic accelerations damaged the reverberating layer changing its fundamental resonant frequencies. The rate of viscous creep increased slowly with dynamic acceleration. Horizontal accelerations exceeded 1 g and tensional P waves did not suppress strong S waves, as would occur within a frictional material. Accelerations above 1 g from S waves reverberating in muddy soil are not harbingers of extreme accelerations at hard rock and frictional soil sites.

We are examining Pilot Hole Array data for small earthquakes triggered by the 2004 Parkfield mainshock P waves before the main S wave arrived. We have identified 2 candidate events. Confirming these events would show the reality of stress concentrations that can host triggered rupture. Societally, rupture of large earthquakes may jump to stress concentrations allowing supershear propagation or to nearby faults allowing large earthquakes to become even larger.
Intellectual Merit Our work addresses the basic SCEC objective of the amplitudes of future and past shaking in strong earthquakes. We concentrated on strong shaking above muddy soil and obtained a physics-based description of nonlinear behavior. Our mechanical formulation differs from traditional engineering approaches.

We have estimates of the amplitudes of past strong shaking near Parkfield from rock damage in several boreholes. There will be soon SCEC community numerical models of large earthquakes that end in the Parkfield segment of the San Andreas Fault. We have requested the numerical results to compare with our field estimates.

We have begun work on nonlinear behavior in the seismogenic zone. We will begin with the tractable problem of the interaction of high-frequency body waves with slipping faults. We will pay attention to when the interaction strengthens the fault causing it to locally lock.
Broader Impacts We searched for faults triggered by very strong seismic waves from the end-Cretaceous asteroid impact with junior college professor Peter Olds and junior college students as outreach. The students are typically from underrepresented urban groups. We found faults in Colorado and New Mexico that slipped just after the impact and have not slipped again over the last 66 million years. We published short paper to encourage search for similar faults in other localities.

Our paleoseismic method is applicable whenever S-wave well logs are available for the uppermost few hundred meters. We have addressed available sites. We have provided estimates for past shaking in Greater Los Angeles, Parkfield, the Bay Area, and Iran. We have shown that lowering the water table at Whittier Narrows will allow strong Love waves from the San Andreas Fault to continue to Downtown Los Angeles. Fortunately, current water policy maintains the Whittier Narrow water table at the pre-industrial free-surface level.

Our results on shaking above frictional and muddy soils are applicable to geotechnical engineering. We realize that we differ from traditional practice. Still, we have a computer friendly method with explicit physics that can be tested with further data from large earthquakes.
Exemplary Figure Figure 1A. Analysis of the 2016 Kumamoto mainshock. The correlation between observed surface signal and predicted signal from borehole signal by transfer functions obtained from small aftershocks deteriorated when the elastic moduli of the reverberating layer decreased from rock damage at relatively modest shaking. Vertical line is eyeball estimate of onset of nonlinearity from time-domain signal. From work in progress.