Why do some earthquakes appear where no fault is shown?
There are several possible reasons. (1) The earthquake occurred on a fault shown on the map, but the location of its epicenter is in error. All epicentral locations have some error. (2) The earthquake occurred at depth on a nearby fault that is shown on the map, but the fault is not vertical. The epicenter will then appear offset relative to the trace of the fault at the surface. (3) The earthquake occurred on a fault not shown on the map. Many unnamed and unmapped faults underlie the San Francisco Bay region.
