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The Uniform California Earthquake Rupture Forecast Version 2 (UCERF 2) and its Effect on the National Seismic Hazard maps (Abstract)

by E. Field, T. Dawson, K. Felzer, A. Frankel, V. Gupta, T. Jordan, T. Parsons, M. Petersen, R. Stein, R. Weldon, and C. Wills

E. Field, T. Dawson, K. Felzer, A. Frankel, V. Gupta, T. Jordan, T. Parsons, M. Petersen, R. Stein, R. Weldon, C. Wills (2008). The Uniform California Earthquake Rupture Forecast Version 2 (UCERF 2) and its Effect on the National Seismic Hazard maps (Abstract). SMIP08 Seminar on Utilization of Strong-Motion Data, p. 1 - 2.

Abstract

The Working Group on California Earthquake Probabilities (WGCEP) has developed the Uniform California Earthquake Rupture Forecast version 2 (UCERF 2). This model was developed by the USGS, CGS, and SCEC, with significant support from the California Earthquake Authority. The time-independent model was developed jointly with the USGS National Seismic Hazard Mapping Program (NSHMP). As with past WGCEP and NSHMP efforts, the model depends on accurate fault locations and slip rates. This study updated fault locations based on the SCEC Community Fault Model and slip rates based on recent studies. The overall fault slip across the region was constrained by GPS deformation rates and long term plate rates. The resulting model is consistent with moment rates from GPS, long term plate tectonic models, and historic seismicity. A careful analysis of historical seismicity rates revealed an over-prediction of the rate of M>6.5 earthquakes in previous models, which we reduced to a 30% over prediction in UCERF 2 (within observed 95% confidence bounds).

Our study differs from previous WGCEP efforts by: 1) reporting earthquake probability for the entire state of California; 2) using uniform methodology across all regions; 3) compiling and using updated, uniform, and publicly accessible statewide data; 4) developing new methods to make models more rigorously adherent to observational data; 5) implementing in a modular (object-oriented), extensible framework, so that alternative logic-tree branches can easily be investigated and future updates can be quickly accommodated as new data and methods emerge. Advice and comment from the scientific and engineering communities was sought regularly through open meetings and workshops. Time-dependent probabilities were applied to fault-based sources using an empirical model (where long-term earthquake rates were adjusted according to any recent changes in observed seismicity) and using an elastic-rebound-motivated renewal model on major faults where the date of previous event was known.

Although UCERF2 is more complete and more consistent with geologic and seismic data than previous models, there are aspects of the model that we believe need improvement in the future. These include: assumptions regarding fault segmentation, the lack of fault-to-fault ruptures, and the lack of earthquake triggering and temporal clustering effects.

The UCERF2 served as the input fault model for calculation of the California portion of the National Seismic Hazard Map. The 2008 hazard maps are significantly different from the 2002 maps in many parts of the United States. The new maps for the Western United States indicate about 10-percent reductions for 0.2-s spectral acceleration and peak horizontal ground acceleration and up to 30-percent reductions in 1.0-s spectral acceleration at similar hazard levels. Most of the changes in the new maps can be attributed to the introduction of new attenuation relations for crustal and subduction earthquakes; however, changes to the fault and seismicity parameters also can be significant. In California, the ground motion calculated for the same hazard level increased up to 10 percent near major faults and up to over 15% in the Santa Barbara area and above the Cascadia Subduction Zone for 0.2-s spectral acceleration and peak horizontal ground acceleration. For 1.0-s spectral acceleration ground motions calculated for the same hazard level are lower everywhere except above the Cascadia Subduction Zone, but decrease less in the Santa Barbara area and the Mojave Desert, where the fault model was modified to reflect increased slip rates or new fault models.