Historic Earthquakes

This earthquake is one of the most devastating in the history of California. The earthquake and resulting fires caused an estimated 3,000 deaths and $524 million in property loss. Damage in San Francisco resulting only from the earthquake was estimated at $20 million; outside the city, it was estimated at $4 million. The sensible duration of the shaking in San Francisco was about 1 minute.

The earthquake damaged buildings and structures in all parts of the city and county of San Francisco, although over much of the area, the damage was moderate in amount and character. Most chimneys toppled or were badly broken. In the business district, which was built on ground made by filling in the cove of Yerba Buena, pavements were buckled, arched, and fissured; brick and frame houses of ordinary construction were damaged extensively or destroyed; sewers and water mains were broken; and streetcar tracks were bent into wavelike forms.

On or near the San Andreas fault, buildings were destroyed (one was torn apart), and trees were knocked to the ground. The surface of the ground was torn and heaved into furrow-like ridges. Roads crossing the faultline were impassable, and pipelines were broken. One pipeline that carried water from San Andreas Lake to San Francisco was broken, shutting off the water supply to the city. The fires that ignited soon after the onset of the earthquake quickly raged through the city because of the lack of water to control them. They destroyed a large part of San Francisco and intensified the loss at Fort Bragg and Santa Rosa.

This earthquake caused the most lengthly rupture of a fault that has been observed in the contiguous United States. The displacement of the San Andreas Fault was observed over a distance of 300 kilometers from San Juan Bautista to Point Arena, where is passes out to sea. Additional displacement was observed farther north at Shelter Cove in Humbolt County, and, assuming the rupture was continuous, the total length of rupture would extend 430 kilometers. The largest horizontal displacement - 6.4 meters - occurred near Point Reyes Station in Marin County.

In areas where dislocation of fences and roads indicated the amount of ground movement, motions of 3 to 4.5 meters were common. Near Point Arena, in Mendocino County, a fence and a row of trees were displaced almost 5 meters. At Wright's Station, in Santa Clara County, a lateral displacement of 1.4 meters was observed. Vertical displacement of as much as 0.9 meters was observed near Fort Ross in Sonoma County. Vertical displacement was not detected toward the south end of the fault.

Although Santa Rosa lies about 30 kilometers from the San Andreas fault, damage to property was severe, and 50 people were killed. The earthquake also was severe in the Los Banos area of the western San Joaquin Valley, where the MM intensity more than 48 kilometers from the fault zone was IX. Santa Rosa lies directly inland from the region of greatest motion on the San Andreas fault.

Trees swayed violently, and some were broken off above the ground or thrown down. The water in springs and artesian wells either increased or decreased its flow. A few sand craterlets formed in areas where water was ejected through cracks or fissures.

The region of destructive intensity extended over a distance of 600 kilometers. The total felt area included most of California and parts of western Nevada and southern Oregon. The maximum intensity of XI was based on geologic effects, but the highest intensity based on damage was IX. Several foreshocks probably occurred, and many aftershocks were reported, some of which were severe.

View southwest from the corner of Geary and Mason streets, San Francisco. Taken April 20, 1906.

Abridged from Seismicity of the United States, 1568-1989 (Revised), by Carl W. Stover and Jerry L. Coffman, U.S. Geological Survey Professional Paper 1527, United States Government Printing Office, Washington: 1993.