CHIMP - a shaking dataset for historical large earthquakes in California

Because major earthquakes and the resulting strong shaking are rare events in any one area, seismologists know little about how well earthquake hazard maps describe the actual shaking that occurs. Shaking data recorded since probabilistic seismic hazard assessment began span a short time period, and hence rarely include data from the moderate and large earthquakes that control hazard. We have previously used retrospective assessments, using compilations of historical shaking data spanning hundreds of years for Japan and Italy, to address this problem. We are now compiling a historical shaking dataset for California and surroundings termed CHIMP (California Historical Intensity Mapping Project) to show the maximum shaking at points within the region over the past 160 years. The dataset will include USGS "Did You Feel It" data for recent earthquakes, intensities reinterpreted from felt reports since about 1924 when the U.S. Coast and Geodetic Survey began collecting them, and intensities inferred from historical accounts. We will compare the dataset to current USGS seismic hazard maps to explore various aspects of how the maps perform and possible approaches to improving them. Our initial focus is on how the fraction of sites at which shaking exceeds the mapped value compares with that expected. Later goals include exploring how well the data are described by current ground motion models and providing data for non-ergodic models that describe specific source-receiver pairs. CHIMP will be a publicly available resource for investigators conducting various studies and can be updated using data from future earthquakes or additional historical earthquake data.