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EARTHQUAKE IN SAN FRANCISCO, APRIL 18, 1906

Facts:

The 1906 San Francisco earthquake was the largest event (magnitude 8.3) to occur in the continental United States in this century. Recent estimates indicate that as many as 3,000 people lost their lives in the earthquake and ensuing fire. In terms of 1906 dollars, the total property damage amounted to about $24 million from the earthquake and $350 million from the fire. The fire alone destroyed 28,000 buildings over a 520-block area in San Francisco. The earthquake, centered near San Francisco, caused damage over a 640-km north-south area. It was felt as far north as Coos Bay and as far south as Los Angeles. The total felt area (971,000 km2) included most of California and parts of Nevada and Oregon.

The earthquake was associated with largest known displacement along a fault plane in the contiguous United States. The observed extent of displacement on the San Andreas fault extended from San Juan in San Benito County to Telegraph Hill in Humboldt County--a distance of more than 430 km. The greatest amount of horizontal slip (6.5 m) occurred in Marin County. The area to the southwest of the rift moved northwest relative to the area on the northeast of the rift (right-lateral strike-slip faulting). Besides the generalh orizontal displacement, there was a vertical movement of as much as 1.2 m in Sonoma and Mendocino counties.

As the rift, which began at 5:13 A.M. local time, came on shore 320 km north of San Francisco forests were demolished, and fences, roads, stream courses, and conduits which crossed the rift were dislocated. In addition to the great rift fissures, there were branch fissures, and considerable cracking of alluvial surfaces. Avalanches and earth slumps occurred, and where the ground contained excess water, earth flows were observed. Springs and artesian wells either increased or decreased in flow. Jets of water spurted through holes or fissures creating mud or sand craterlets.

Buildings on or near the surface fault were destroyed and trees were uprooted or snapped off. Pipelines, broken along the many fissures, caused the water shortage that allowed raging fires to destroy the city.

Generally, there was decrease in intensity from the southwest toward the northeast, as the distance from the zone of faulting increased. However, the character of the ground also determined to a large extent the amount of damage. The areas that incurred the most damage were those upon filled ground. In these areas brick and frame houses were severely damaged or destroyed. Next in intensity to areas of filled land were those upon incoherent sands. There was a large amount of damage done in the cemeteries, which were built on sand. Movement in more than one direction was suggested by monuments twisted upon their bases. The damage in sandy areas was due to the shaking and to settling and sliding. The building destruction and the disturbance of railway roadbeds and rails was much more severe throughout the area covered by the inconsistent beds than on the older hard rock in the adjoining areas. Areas that had least damage were on rock beds.

Although buildings throughout the City and County of San Francisco were damaged, most earthquake damage in this area was only moderate. Toppled chimneys, and cracked plaster on walls and ceilings were common. Santa Rosa, 30.4 km from the fault, was the most severely shaken town in the state. Healdsburg, San Jose, and Agnews also incurred a large amount of damage. All these places were located on the valley floor on loose or only slightly coherent geological formations. The portions of Berkeley and Oakland which were located on the alluvial slope, incurred more damage than the foothills where the buildings were built on rock. In the Los Banos region of the western San Joaquin Valley (48 km from the rift) the shaking intensity was also unusually great. Petaluma and San Rafael, though nearer the rift than Santa Rosa, incurred less damage for they were for the most part on, or close to, rocky ground.

In San Francisco the fire continued unchecked for three days leaving 12.2 km2 burned. The burned area included 520 blocks; 418 km of streetcar and cable were destroyed. Thousands of businesses were destroyed, and hundreds of thousands of people were homeless. There were 350,000 who badly needed food. However, the disaster was unable to subdue the spirit of the people of the city. According to Willaim Randolph Hearst, "The calamity seems overwhelming, and yet the people are not overwhelmed." The San Franciscans rebuilt their

homes and businesses, and a new San Francisco arose from the ashes.