Earthquake Shaking Intensities for California

Expected levels of earthquake shaking for a region can be calculated by combining knowledge of the likelihood and magnitude of future earthquakes and rupture of specific fault segments with information on how seismic waves propagate through the region and how they affect local soil conditions.

The U.S. Geological Survey and the California Geological Survey have jointly produced seismic hazard maps for California using this knowledge. These maps show the likelihood of exceeding a given level of shaking during a specific time interval. The expected levels of ground motion can be converted to earthquake intensity levels that are directly related to expected damage.

The authors calculated two intensity hazard maps for a 30-year time window. One map shows the probability that an area has a 1 in 10 (10 percent) chance of intensity being exceeded in 30 years. The other map shows areas having even odds (50 percent) of intensity being exceeded in 30 years. Local soil conditions play a significant role in increasing expected intensities in areas of soft sediment and fill.