Working Group on California Earthquake Probabilities

Working Group on California Earthquake Probabilities
Name	Working Group on California Earthquake Probabilities
Abbreviation	WGCEP
Formation	1988
Type	Scientific consortium
Purpose	Seismic hazard assessment
Headquarters	Berkeley, California
Region served	California

Working Group on California Earthquake Probabilities is a scientific consortium convened to quantify seismic hazard and forecast earthquake likelihoods for California. The group synthesizes data from institutions such as the United States Geological Survey, the Southern California Earthquake Center, the California Geological Survey, and academia including University of California, Berkeley, Stanford University, and California Institute of Technology. Its probabilistic assessments have informed agencies like the Federal Emergency Management Agency, the California Office of Emergency Services, and the California Department of Transportation.

Overview

The Working Group on California Earthquake Probabilities brings together researchers from United States Geological Survey, Southern California Earthquake Center, California Geological Survey, United States National Academy of Sciences, University of California, Berkeley, California Institute of Technology, Stanford University, Harvard University, and Massachusetts Institute of Technology to produce statewide seismic forecasts. The group integrates paleoseismology from Paleoseismology teams, geodesy from Plate tectonics studies, and seismicity catalogues maintained by ANSS and USGS National Seismic Hazard Model collaborators. Outputs have influenced building codes administered by the International Code Council and policy debates in the California State Legislature and United States Congress.

History and Formation

Formed in 1988 after high-profile events such as the 1989 Loma Prieta earthquake and research mobilizations at USGS Menlo Park, the group followed initiatives by the National Research Council and recommendations from the Seismological Society of America. Early participation included scientists affiliated with California Institute of Technology seismology programs, UC Berkeley Seismological Laboratory, and the Southern California Earthquake Center consortium. The WGCEP produced iterative reports in 1990, 1995, 2002, 2007, 2015, and later updates, coordinating with agencies like FEMA and standards bodies including the American Society of Civil Engineers.

Mission and Objectives

The group's mission is to estimate probabilities of future earthquakes on mapped faults and within regions of California for use by California Geological Survey, Federal Emergency Management Agency, and engineering stakeholders such as the American Society of Civil Engineers and the International Code Council. Objectives include synthesizing paleoseismic evidence from field studies near the San Andreas Fault, integrating geodetic strain measured by GPS networks funded by the National Science Foundation, and producing probabilistic models that inform the USGS National Seismic Hazard Model and regulations implemented by the California Office of Emergency Services and Caltrans.

Methodology and Models

WGCEP employs probabilistic seismic hazard assessment methods derived from work at USGS and academic centers like Caltech and UC Berkeley. Models combine fault catalogs such as the Quaternary Fault and Fold Database and recurrence-rate estimates from paleoseismic trenches near the Hayward Fault, Rodgers Creek Fault, San Jacinto Fault Zone, and San Andreas Fault. The methodology integrates earthquake rupture forecast frameworks used by the CSEP and statistical approaches popularized by researchers affiliated with the Seismological Society of America and the National Research Council. Inputs include slip-rate measurements from Geological Society of America studies, seismicity catalogs from ANSS, and geodetic strain from SCEC and UNAVCO stations. Models produce time-independent and time-dependent probabilities, including rupture segmentation assumptions debated in literature from Earthquake Engineering Research Institute publications.

Key Reports and Findings

Major WGCEP publications include multi-decade forecasts that revised probabilities for the next 30 years on the Hayward Fault, San Andreas Fault, and San Jacinto Fault Zone. Reports concluded heightened likelihoods of magnitude 6.7+ events on the Hayward Fault and multi-segment ruptures on the Southern San Andreas Fault, findings cited in emergency planning by FEMA and resilience discussions at City of Los Angeles and San Francisco. The group's 2008–2015 assessments influenced updates to the USGS National Seismic Hazard Model and informed seismic provisions in codes by the American Society of Civil Engineers and International Building Code.

Impact on Policy and Planning

WGCEP outputs have informed retrofit programs in jurisdictions like San Francisco, policy proposals in the California State Legislature, and infrastructure resilience projects by Caltrans and utility operators including Pacific Gas and Electric Company and Southern California Edison. Its probabilities underpinned risk analyses used by insurers, municipal emergency managers, and urban planners in Los Angeles, San Diego, and the San Francisco Bay Area. Collaboration with FEMA and inclusion in National Earthquake Hazard Reduction Program activities helped translate scientific forecasts into building-code modifications and public preparedness campaigns promoted by California Office of Emergency Services.

Criticism and Controversies

Critics from groups associated with insurance industry actuarial sections, some academics at Caltech and UC Berkeley, and stakeholders in Bay Area jurisdictions have raised concerns about epistemic uncertainties, rupture segmentation assumptions, and the communication of probability to the public. Debates have appeared in forums such as the Seismological Society of America meetings and publications in journals linked to National Academy of Sciences discussions. Controversies include differing interpretations of paleoseismic data on the Hayward Fault and the risk trade-offs for retrofitting programs funded by state legislatures and utility regulators.

Category:Earthquake engineering organizations