SCEC Award Number 18204 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Effect of off-fault inelasticity on near-fault directivity pulses
Investigator(s)
Name Organization
Steven Day San Diego State University
Other Participants Yongfei Wang
SCEC Priorities 2b, 2d, 4a SCEC Groups GM, FARM, CME
Report Due Date 03/15/2019 Date Report Submitted 03/21/2019
Project Abstract
 It is important to understand how directivity-enhanced, fault-normal velocity pulses behave very close to the rupture surface, where plastic yielding is likely to effect their amplitude and waveform. The project assesses the extent to which plastic yielding, which is absent in standard kinematic models, may systematically affect the amplitude, frequency content, and distance scaling of these pulses. We use some simple 2D modeling experiments to gain a general understanding of how rupture kinematics are reflected in near-fault pulses, and then simulate strike-slip ruptures in 3D with and without plastic yielding. Finally, as an initial step toward assessing the sensitivity of the results to rupture complexity and degree of coherence, we repeat the 3D simulations on a fault with geometric roughness. We find that each of the four 3D models (flat and rough faults, with and without off-fault yielding), scaled to approximately magnitude 7, predicts a fault-normal pulse with behavior characteristic of observed pulses (periods in the range 2-5 second, amplitudes increasing with distance in the forward-directivity direction but approaching a limiting amplitude). Plastic yielding systematically reduces pulse amplitude and increases its dominant period, relative to models that neglect off-fault yielding. Yielding also strongly suppresses the otherwise very strong high-frequency acceleration pulses that otherwise appear in the fault-parallel acceleration when local supershear rupture transients occur. The latter result suggests a mechanism (peak motion reduction by plastic yielding) to account for the apparent absence of the expected Mach wave signature in near-field accelerations from supershear ruptures.
Intellectual Merit The project serves SCEC’s explicit SCEC5 commitment to assess the contributions of inelastic rock response to ground motion hazards. In particular, fault-normal pulse dynamics induce nonlinear response under conditions that depart sharply from the traditional engineering understanding of nonlinearity as a shallow site response to a near-vertically propagating incident field. Understanding this behavior is a prerequisite for a number of other SCEC objectives, including defining the limits of the current CyberShake methodology, which fundamentally relies upon linearity.
Broader Impacts The project supported the doctoral research of a student in the SDSU/UCSD Joint Doctoral Program in Geophysics, and, in particular, enabled him to apply his research experience on earthquake source dynamics to a new class of problems of high relevance to hazard mitigation. More broadly, the incorporation of nonlinear material models into the analysis of seismic pulse dynamics contributes to the integration of methods and results from Earthquake Science into Earthquake Engineering.
Exemplary Figure Figure 12. Fault-parallel and Fault-normal acceleration and velocity, and their Fourier amplitude spectra, pseudo
294 spectral acceleration and pseudo spectral velocity at a station 40 km away from the epicenter.