SCEC Award Number 15069 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Including Scattering in the UCSB Broadband Modeling Method
Name Organization
Ralph Archuleta University of California, Santa Barbara
Other Participants Jorge Crempien
SCEC Priorities 6c, 6d, 6e SCEC Groups GMP, GMSV, CCSP
Report Due Date 03/15/2016 Date Report Submitted 03/15/2016
Project Abstract
The primary objective was to improve the UCSB method for computing broadband ground motion synthetics by adding scattering to the wave propagation. We have done this by generating scatterograms that are added to the Green’s function for the medium. The scatterogram is obtained by using Zeng’s (1991) envelope for multiple scatterers to modify a white noise signal. The advantage of this method is that the parameters for the envelope can be determined from regional, small magnitude earthquakes. Thus the scattering is constrained by empirical data that is germane to the region where the larger magnitude earthquake is simulated. This approach allows the UCSB broadband method to capture the duration of shaking that is due to both the duration of the source process as well as the duration due to scattering in the medium. Using the 1994 Northridge earthquake we have shown that this scattering does not affect the bias when comparing response spectra of spectral acceleration. However, it greatly reduces the bias of Arias intensity—a measure that includes both amplitude and duration of shaking. In earlier work we have shown that the UCSB method has successfully passed the SCEC validation process for western crustal earthquakes.
Intellectual Merit One of the overarching SCEC goals is the development of a seismic hazard model for Southern California. One cornerstone of a seismic hazard model is the prediction of ground motion (or a metric based on ground motion). Over many years SCEC has supported the development of numerical methods by which ground motion from simulated earthquakes can be computed. One such method has been developed at UCSB. The UCSB method differs from others in that the slip rate function (spatially varying on the fault) determines the entire frequency spectrum of the ground motion, i.e., the low-frequency and the high-frequency parts of the ground motion are all determined by the slip rate on the fault. There is no disconnect between the source of low- and high-frequency source. To improve on this method we have modified how high-frequency ground motion is propagated. In the latest modification we have added scattering to the wave propagation. In this approach we can use scattering parameters that are empirically determined from small earthquakes. We have found that this greatly improves estimates of duration of shaking; duration is a critical time-domain ground motion metric.
Broader Impacts Earthquake engineering design depends on having reliable and verifiable estimates of how strong shak-ing will be for a wide suite of earthquake magnitudes and distances. While the database of strong motion records has been increasing with new instrumentation, there is a serious lack of ground motion records at distances less than 20 km for earthquakes with magnitudes greater than 6.0. Rather than wait for more earthquakes to occur, it is possible to estimate ground motion by simulating earthquakes of different magnitudes. Thus one can supplement the empirical database and incorporate region specific geology by computing ground motion using numerical simulations of earthquakes.
Exemplary Figure Figure 1: Comparison between deterministic (red) and scattering (black) Green’s functions at different epicentral distances for a point source (reverse faulting with 45˚ dip) at a depth of 10 km.