1906 San Francisco Earthquake - Analysis

Analysis

For a number of years, the epicenter of the quake was assumed to be near the town of Olema, in the Point Reyes area of Marin County, because of evidence of the degree of local earth displacement. In the 1960s, a seismologist at UC Berkeley proposed that the epicenter was more likely offshore of San Francisco, to the northwest of the Golden Gate. However, the most recent analysis by the United States Geological Survey (USGS) shows that the most likely epicenter was very near Mussel Rock on the coast of Daly City, an adjacent suburb just south of San Francisco. An offshore epicenter is supported by the occurrence of a local tsunami recorded by a tidal gauge at the San Francisco Presidio; the wave had an amplitude of approximately 3 in (8 cm) and an approximate period of 40–45 minutes.

The most important characteristic of the shaking intensity noted in Lawson's (1908) report was the clear correlation of intensity with underlying geologic conditions. Areas situated in sediment-filled valleys sustained stronger shaking than nearby bedrock sites, and the strongest shaking occurred in areas of Bay where landfill failed in the earthquake (earthquake liquefaction). Modern seismic-zonation practice accounts for the differences in seismic hazard posed by varying geologic conditions.

The USGS estimates that the earthquake measured a powerful 7.9 on the moment magnitude scale. The earthquake caused ruptures visible on the surface for a length of 470 kilometers (290 mi). Modified Mercalli Intensities of VII to IX paralleled the length of the rupture, extending as far as 80 kilometers inland from the fault trace.