The Peninsula portion of the San Andreas Fault poses the largest risk for San Mateo County, passing through the center of the county. The Northern San Gregorio fault also passes through the western edge of the county. The San Andreas Fault has a 21% chance of creating a magnitude 6.7 or greater earthquake in the next 30 years.
The last earthquake with a magnitude over 5.0 with an epicenter in San Mateo County was the 1957 Daly City earthquake with a magnitude of 5.3. While the epicenter of the magnitude 7.8 earthquake in 1906 on the San Andreas Fault was not located within the county, it still caused extreme ground shaking and a similar earthquake in the future will likely do the same, especially in the heavily populated Bayside, much of which is underlain by alluvial deposits, Bay mud and artificial fill. A rupture along the peninsula will cause extremely violent ground shaking throughout the county. The Bay margins will also be likely to experience liquefaction in a major earthquake.