SCEC Award Number 18001 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Shallow and deep nonlinear seismic waves with application to strong ground motions, paleoseismology, and rupture propagation
Name Organization
Norman Sleep Stanford University
Other Participants
SCEC Priorities 4a, 2e, 2d SCEC Groups Geology, FARM, GM
Report Due Date 03/15/2019 Date Report Submitted 03/15/2019
Project Abstract
The amplitude of past and future seismic shaking is central to the SCEC purview. We made significant progress on the topics of strong shallow seismic waves, nonlinear attenuation of these waves, and the closely related topic of nonlinear behavior in the uppermost 2 km with reference to the 2002 Denali mainshock. We are continuing work on the nonlinear failure of shallow viscous muddy soil. With regard to triggered and induced earthquakes, we studied well-known outcrops in Colorado and New Mexico of the K-Pg boundary. The impact triggered earthquake faults that have not slipped again. We continued fieldwork in Montana. We modeled thermal weakening at asperity tips during rapid fault sliding, obtaining a convenient self-consistent formulation.
Intellectual Merit A key purview of SCEC is to estimate levels of past and future shaking. We concentrated on using seismic records of large earthquakes to search for nonlinear effects that are important during strong shaking. With the Denali earthquake, we searched for effects associated with rock failure in the uppermost 2 km of hard rock from the near-field velocity pulse. S waves passing through the failed region were strongly attenuated. We modeled dynamic weakening of rapidly sliding faults from heating of asperity tips and obtained a conveniently parameterized self-consistent formalism. We are also investigating shallow nonlinear behavior during strong shaking. Near-fault dissipation of energy, fault rheology, and near surface nonlinearity are essential input to numerical models of dynamic earthquake rupture.
Broader Impacts Our work helps estimate the strength of past and future earthquake shaking. In particular, our work on stress concentrations helps in understanding when large earthquakes jump from one fault trace do another causing even larger earthquakes.

We worked with junior college students in Montana documenting small earthquakes triggered by extreme seismic waves from the end-Cretaceous asteroid impact. There are implications to intraplate stress as some of the events were delayed by tens of millennia.

Our work on thermal weakening of asperity tips on sliding faults may be applicable to faults in cold ice in the outer solar system.
Exemplary Figure Figure 2: High frequencies in 16-26.24 s interval at PS10 for the 2002 Denali earthquake: Exponential curves are proportional to exp(-t_0 f), where f is frequency and t_0 = 0.5 s, and, exp(-t_0 f)+ 0.1 exp(t_1 f) where t_1 = 1/30 s, are shown of comparison. The former curve represents the predicted effect of nonlinear suppression of S waves by the near-field pulse within the hard rock. The latter includes a small contribution from S waves that evaded the pulse. The objective is to establish that long-period stresses from the gross earthquake interact nonlinearly to suppress high-frequency S waves.