California earthquake forecast

The 2015 Uniform California Earthquake Rupture Forecast, Version 3, or UCERF3, is the latest official earthquake rupture forecast (ERF) for the state of California, superseding UCERF2. It provides authoritative estimates of the likelihood and severity of potentially damaging earthquake ruptures in the long- and near-term. Combining this with ground motion models produces estimates of the severity of ground shaking that can be expected during a given period (seismic hazard), and of the threat to the built environment (seismic risk). This information is used to inform engineering design and building codes, planning for disaster, and evaluating whether earthquake insurance premiums are sufficient for the prospective losses. A variety of hazard metrics can be calculated with UCERF3; a typical metric is the likelihood of a magnitude M 6.7 earthquake (the size of the 1994 Northridge earthquake) in the 30 years (typical life of a mortgage) since 2014.

UCERF3 was prepared by the Working Group on California Earthquake Probabilities (WGCEP), a collaboration between the United States Geological Survey (USGS), the California Geological Survey (CGS), and the Southern California Earthquake Center (SCEC), with significant funding from the California Earthquake Authority (CEA).

A major achievement of UCERF3 is use of a new methodology that can model multifault ruptures such as have been observed in recent earthquakes. This allows seismicity to be distributed in a more realistic manner, which has corrected a problem with prior studies that overpredicted earthquakes of moderate size (between magnitude 6.5 and 7.0). The rate of earthquakes of magnitude (M) 6.7 and greater (over the entire state) is now believed to be about one in 6.3 years, instead of one in 4.8 years. On the other hand, earthquakes of magnitude 8 and larger are now expected about every 494 years (down from 617). Otherwise the overall expectations of seismicity are generally in line with earlier results. (See Table A for a summary of the overall rates.)

The fault model database has been revised and expanded to cover over 350 fault sections, up from about 200 for UCERF2, and new attributes added to better characterize the faults. Various technical improvements have also been made.

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