SCEC Award Number 11132 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Source Processes Causing Frequency Dependence of Radiation Patterns and Directivity Effects
Investigator(s)
Name Organization
Eric Dunham Stanford University
Other Participants Cho, Hyunghoon (undergraduate researcher)
SCEC Priorities B3, A10, B1 SCEC Groups GMP, FARM, CS
Report Due Date 02/29/2012 Date Report Submitted N/A
Project Abstract
 Natural faults are geometrically complex, as evidenced by bends, branches, and fractal surface roughness. During SCEC3, we initiated an effort to understand rupture dynamics on nonplanar faults. Our models include strongly rate-weakening fault friction and off-fault plasticity. Slip on nonplanar faults generates stress perturbations that alter rupture propagation, leading to fluctuations in slip and rupture velocity that are correlated with local fault topography. These fluctuations, together with variations in the local orientation of the fault surface, excite incoherent high frequency ground motion. Our initial study focused on strong ground motion in the near-source region from ruptures on synthetically generated self-similar faults. Synthetic seismograms show flat acceleration spectra at frequencies above the corner frequency, up to a maximum frequency set by the minimum wavelength of roughness included in the model. We also looked at far-field radiation patterns and directivity effects as a function of frequency. Short wavelength variability in the fault orientation causes the radiation pattern to transition from an ideal double couple pattern with pronounced directivity at low frequencies to an isotropic pattern without directivity at high frequencies. The transition frequency for magnitude 6-7 ruptures is around 1 s, consistent with observations of this phenomenon. We also examined the background shear-to-normal-stress ratio $\tau/\sigma$ required for self-sustaining propagation. We found that fault roughness increases the critical stress level, from $\tau / \sigma$ ~ 0.2-0.3 for flat or slightly nonplanar faults, to values around 0.6 for rough faults. This is because geometric complexity introduces a resistance to slip in addition to that from friction. The upper limit (~0.6) is set not by the friction law, but by the strength of the off-fault material. These results offer a particularly intriguing explanation for the range of stress levels in the crust, from the low levels inferred for major plate boundary faults like the San Andreas to the higher levels associated with less mature faults.
Intellectual Merit High frequency ground motion (>1 Hz) remains the least understood part of the seismic wavefield, despite its critical importance for engineering applications. Our work identifies the source processes that are responsible for exciting incoherent high frequency ground motion. While dynamic rupture models were used to generate the results, the lessons learned should be directly transferable to more efficient kinematic rupture generators.
Broader Impacts One undergraduate intern was supported by this project and will serve as lead author on a paper that we are currently writing. He was trained in computational seismology and is presently pursuing an advanced degree in computer science. Our work on high frequency ground motion will likely find application in kinematic rupture generators that are used to predict strong ground motion for earthquake engineering applications.
Exemplary Figure Figure 1: Radiation patterns from a spontaneous rupture on a rough fault,
shown for P and S waves (bottom and top rows, respectively) as a function
of frequency (averaged over frequency bins 0-0.5 Hz, 0.5-1 Hz, etc.).
Blue lines show radiation patterns from simulations; red lines are best-fitting
double couple pattern with directivity. Shown below each subplot is the effective
rupture velocity vr that determines the degree of directivity and the
correlation coefficient r that quantifies the quality of the fit.