The earthquake that struck California on April 18, 1906 is considered one of the most significant earthquakes in history. Its significance today stems from the abundance of scientific knowledge it has produced rather than its mere size. The earthquake, which ruptured the San Andreas fault's northernmost 296 miles from northwest of San Juan Bautista to the triple junction at Cape Mendocino, astonished modern geologists with its huge, horizontal displacements and long rupture length.

Indeed, it would take more than half a century for plate tectonics to properly comprehend the fault's significance and the magnitude of its cumulative offset. Reid (1910) developed his elastic-rebound theory of the earthquake source based on the 1906 displacements and strain in the surrounding crust, which is still the most well accepted model of the earthquake cycle today.

At almost exactly 5:12 a.m., local time, a foreshock struck with enough energy to be felt over the San Francisco Bay area. The massive earthquake struck 20 to 25 seconds later, with its epicenter near San Francisco. The powerful shocks were punctuated by severe shaking that lasted 45 to 60 seconds. The earthquake was felt from southern Oregon to south of Los Angeles, and from the interior to central Nevada.

The highest Modified Mercalli Intensities (MMI's) of VII to IX paralleled the length of the rupture, extending up to 80 kilometers inland. The strong association of intensity with underlying geologic conditions was one of the key characteristics of the shaking intensity reported in Lawson's (1908) assessment. The strongest earthquake occurred in areas where land reclaimed from San Francisco Bay washed ashore the earthquake, while areas placed in sediment-filled valleys experienced stronger shaking than surrounding bedrock sites. The changes in seismic danger produced by diverse geology conditions are accounted for in modern seismic-zoning technique.

The Lawson (1908) report remains the official work, as well as possibly the most important study of a single earthquake, as a basic reference about the earthquake and the damage it produced, geologic observations of the fault rupture and shaking effects, and other consequences of the earthquake. The fire it started in San Francisco is probably the most vivid recollection of the earthquake in the public's mind, garnering it the slightly misleading moniker of "San Francisco earthquake.

On the other hand, quaking damage was similarly severe along the fault rupture. The widely reported death toll of 700 from the earthquake and fire is now believed to be three to four times lower than the genuine death toll. The majority of the deaths occurred in San Francisco, with 189 others reported in other cities.