SCEC Award Number 13094 View PDF
Proposal Category Travel Only Proposal (SCEC Annual Meeting)
Proposal Title Wave propagation in complex, nonlinear media
Investigator(s)
Name Organization
Daniel Roten Eidgenössische Technische Hochschule Zürich (Switzerland)
Other Participants
SCEC Priorities 6 SCEC Groups GMP, DRCV, EEII
Report Due Date 10/11/2013 Date Report Submitted N/A
Project Abstract
 A major challenge in seismic hazard assessment consists in the prediction of near-source ground motions resulting from large, rare earthquakes, which are not well represented in observed data. Realistic simulations of the dynamic rupture and wave propagation during such events should account for nonlinear material behavior in the fault damage zone and in soft soils near the surface. Towards this goal we have implemented the Drucker-Prager yield condition based on the return map algorithm in the highly scalable AWP-ODC finite difference code. We use the Shakeout earthquake scenario, using a kinematic source description (Graves et al., 2008), to study how plasticity on and off the fault and near the surface would affect ground motions during a M 7.8 earthquake on the Southern San Andreas fault. Because the plastic material parameters required for the yielding model (i.e., friction angle and cohesion) are poorly constrained, we experiment with different cohesion models, basin our choice on petroleum industry equations and published laboratory results for the formations underlying the Los Angeles basin. We find that long-period ground motions in the downtown Los Angeles area could be significantly reduced as compared to visco-elastic solutions, even for conservative values of cohesion. These reductions are primarily due to yielding near the fault, although yielding may also occur in the shallow low-velocity deposits of the Los Angeles Basin if cohesions are close to zero. Current simulations assuming a linear response of rocks may overpredict ground motions during future large earthquakes on the southern San Andreas Fault.
Intellectual Merit Nonlinear site response represents an important issue in strong ground motion prediction. Although it is now increasingly accepted that soft soils exhibit some degree of nonlinearity during strong shaking, this effect is typically only analyzed for vertically propagating SH-waves. It has long been suggested that the large strains induced by strong long-period surface waves may exceed the strength of shallow sedimentary rocks.
New simulations performed within this project have allowed us to explore the impact of nonlinearity on the strong long-period surface waves predicted in the ShakeOut earthquake scenario.
Our results indicate that both fault zone plasticity and near-surface nonlinearity may significantly reduce long-period ground motions.
Broader Impacts The projects has achieved a first step towards more accurate physics-based seismic hazard assessment, which will account for nonlinear effects on and off the fault and in shallow sedimentary deposits. It has deepened the collaboration between ETH Zurich, a participating institution, and SDSU, on of SCEC's core institutions. As plastic simulations require knowledge of rock strength as well as the absolute initial stress field, the project also fosters collaboration between seismologists and experts in the fields of geology, tectonics, and geodesy.
Exemplary Figure Figure 1: (a) Shear-wave velocity vs at 200 m depth defined from CVM-4. (b) ShakeOut horizontal peak ground velocities obtained for a viscoelastic medium, and (c) an elastoplastic medium using cohesion model 2.