SCEC Award Number 15094 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Strike-slip Faulting Energy Release and Supershear Rupture
Name Organization
Thorne Lay University of California, Santa Cruz
Other Participants Lingling Ye, currently graduate student at UCSC
SCEC Priorities 4a, 4b, 4d SCEC Groups Seismology, FARM, GMP
Report Due Date 03/15/2016 Date Report Submitted 03/06/2016
Project Abstract
Teleseismic recordings of P waves from 26 large (mostly, MW ≥ 7.5) strike-slip earthquakes from 1990-2015 have been analyzed to determine their radiated energy, ER, exploring the stability of the estimates with respect to the faulting geometry and directivity. The radiated energy estimates demonstrate that moment-scaled radiated energy ER/M0, for strike-slip events is systematically higher than found for large megathrust events. Finite-fault inversions for several of the events have been performed to resolve the spatial extent of faulting and to calculate the stress drops, supplementing published estimates for other events. The effects of rupture directivity (deviation from point-source radiation patterns) for the non-uniform slip models have been assessed for several large ruptures by examining azimuthal patterns of individual station ER estimates. The source directivity effect on single-station ER measurements is generally relatively weak, but systematic variation as a function of directivity parameter can be detected for several events. A factor of ~2-3 azimuthal trend is observed for most supershear events, and bias of the average value can be avoided by azimuthal sampling. This effect appears modest, but one must consider the narrow cone of teleseismic P wave paths from the source and their proximity to the nodal planes of strike-slip focal mechanisms. There are also complexities in each rupture and directivity effects are not the extreme end-member cases expected for simple uniform unilateral ruptures. Further work on estimating finite-fault source parameters has been proposed and funded, with the effort moving to Cal-tech where Lingling Ye is now a postdoctoral researcher.
Intellectual Merit The plate tectonic environment of California produces a risk of very large strike-slip earthquakes like the 1906 San Francisco and 1857 Fort Tejon earthquakes. However, only a few, significantly smaller, strike-slip events have occurred in the region recently with good instrumental recording. As a result, SCEC relies on numerical modeling to anticipate ground motions for future large strike-slip events, with many attendant uncertainties. Globally, other regions have recently experienced very large strike-slip ruptures that are well recorded, and this study uses the recordings of those events to calibrate expected behavior of such events, as they will eventually strike in California.
Broader Impacts The project provided support for the last few months of Ms. Lingling Ye’s Ph.D. thesis work at the University of California Santa Cruz, before she relocated to a postdoctoral position at Caltech in Fall 2015. Thus, it has directly supported a promising young, Asian woman scientist working on earthquake science. The focus of the study is to improve the basis for anticipating the nature of future very large strike-slip ruptures in California, providing empirical constraints on rupture behavior that can be used to evaluate and perhaps constrain dynamical rupture models used for ground motion prediction in the region. The main long-term benefit could be improved confidence in scenario earthquakes and ground motion predictions for the largest class of earthquakes likely to strike in California, with improved input to earthquake engineering, strong-motion modeling, and earthquake warning procedures.
Exemplary Figure Figure 3. (a) Moment-scaled radiated energy estimates and (b) energy-based static stress drop, ΔσE, from finite source slip models versus earthquake magnitude (MW) from gCMT catalog. The measurements for 112 M7+ megathrust earthquakes including 5 notable tsunami earthquakes are from Ye et al. [JGR, 2016]. The cyan stars in (a) show ER/M0 measured with radiated energy averaged from single-station estimates with radiation coefficients larger than 0.3. The red stars in (a) indicate ER/M0 measurements with radiated energy after long-period correction with the moment-rate function from the finite-fault slip models.