DEVELOPING MAPS OF POTENTIAL EARTHQUAKE SHAKING FOR CALIFORNIA

The science of seismic shaking hazard analysis has grown and matured in the years since the California Division of Mines and Geology published Map Sheet 23 “Maximum Credible Rock Acceleration from Earthquakes in California” in 1973. That map showed maximum earthquake magnitudes based on the length of faults and estimates of shaking. Very little was known about the rates of earthquakes on any of the faults beyond what was recorded in the short historic record. Much has changed in the most recent successor to Map Sheet 23: Map Sheet 48 “Earthquake Shaking Potential for California” published by the California Geological Survey in 2008. Most importantly, modern Probabilistic Seismic Hazard Analysis (PSHA) allows us to consider data on the rates of earthquakes estimated from geologic slip-rate and paleoseismic studies on individual faults, geodetic studies of deformation in a region, as well as seismic history. In addition, PSHA allows consideration of uncertainties of the data. The location of faults and their maximum earthquake magnitudes are still key to any evaluation of seismic shaking. Geologic slip rate studies, in concept, are some of the primary data needed for this analysis. These studies can have significant uncertainties and must be consistent with large scale geodetic data: the sum of the fault slip across the plate boundary should equal the relative plate movement. Geodetic rates can provide rates of movement across a fault or region, but there is debate about how a geodetic rate measured over the past decade or two compares with the rates of movement that drive earthquakes over centuries. The Working Group on California Earthquake Probabilities, in which CGS partners with USGS and SCEC, ensures that all the relevant data are considered. Coordination with the USGS National Seismic Hazard Map program ensures consistency with seismic hazard maps and building code design maps of the rest of the country.