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Earthquake Damage in San Francisco, CA, April 18, 1906 |
The 1906 San Francisco earthquake was the largest event (magnitude 8.3) to
occur in the conterminous United States in the 20th 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 destroyed 28,000 buildings
in a 520-block area of 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. This earthquake apparently triggered far-field
seismic activity throughout the United States until the 21st of April. Subsequent
earthquakes were felt in the Imperial Valley of California, Milwaukee, Wisconsin,
northern New Mexico, Ohio, and South Carolina.The earthquake was associated with the
largest known displacement along a fault plane in the United States. Strike slip faulting
on the San Andreas fault was observed for 317 km (190 miles) from Point Arena southward.
Faulting in Humbol County on the Shelter Cove fault area was also observed. It is
likely this fault was activated at the same time and ruptured separately from the
San Andreas fault.) The greatest amount of horizontal slip (6.5 m) occurred near Port
Reyer. 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 general
horizontal displacement, there was a vertical movement of as much as 1.2 m in Sonoma
and Mendocino counties.The rift, which began at 5:12:21 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. 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 a 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 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, 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 the least damage were
on hard rock beds, chiefly of the Mesozoic Franciscan Formation. 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. The business district at Santa Rosa (30 km from the fault) was
nearly totally demolished. Healdsburg, San Jose, and Agnews also had 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, received less damage. These towns
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. Finally, the fire was stopped by
dynamiting structures in its path. The burned area included 520 city 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 William Randolph Hearst, "The calamity seems overwhelming, and yet the
people are not overwhelmed." The San Franciscans rebuilt their homes and businesses,
and within two years, a new San Francisco arose from the ashes.
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