The Loma Prieta Earthquake struck at 5:04:15 p.m., Pacific Daylight Time. The epicenter of the earthquake was a remote area of the Santa Cruz Mountains, about ten miles northeast of Santa Cruz and 56 miles south-southeast of San Francisco.
The principal regions of destruction were in the Santa Cruz Mountains, coastal communities in Santa Cruz and Monterey counties and in San Benito County. Parts of San Francisco and Alameda counties were also hard hit.
Of both immediate and long-range concern to the San Francisco Fire Department is the excessive damage from earth shaking in the Marina District, South of Market area and portions of the Inner Mission District where, in places, severe liquefaction occurred which damaged water mains and structures. There was also liquefaction on Treasure Island which is, technically speaking, within the city limits of San Francisco. Further, in addition to the collapse of two decks of the San Francisco-Oakland Bay Bridge, there was near- failure of a westerly portion of the structure which also lies within the jurisdiction of the San Francisco Fire Department.
The U.S. Geological Survey's (USGS) initial report on the earthquake said, "The major damage in the Marina [D]istrict was caused by locally amplified shaking and permanent deformation of the ground due to liquefaction of the sand and debris used to fill the former lagoon for the 1915 Panama-Pacific International Exposition. During the 1906 earthquake, locally violent ground shaking was experienced along the margins of the lagoon."
In "Lessons Learned from the Loma Prieta, California, Earthquake of October 17, 1989," the USGS also wrote, "Areas underlain by thick deposits of water-saturated unconsolidated sand and mud were not only strongly shaken but were also affected by compaction and loss of strength in sediment that liquefied during the shaking; many of these same areas experienced similar processes in the 1906 earthquake."
The USGS, in another report titled "The Loma
Prieta Earthquake of October 17, 1989," wrote that, "Events of
magnitude 7 or larger, each with a probability of 20 to 30 percent...
"A magnitude of 7 shock on any one of these fault segments will probably cause considerably more damage than the recent Loma Prieta event because of their proximity to larger population centers," the report said.
On July 20, 1990, the U. S. Geological Survey revised upward the probability factor for a Richter-magnitude 7 event to 67 percent by the year 2020. This latest revision makes another major earthquake an inevitability as far as Fire Department planning is concerned.
Another significant concern to the San Francisco Fire Department is the large number of freeway viaducts which transverse the City, and were damaged during the earthquake.
A report titled, "Competing Against Time, Report to Governor from the Governor's Board of Inquiry on the 1989 Loma Prieta Earthquake" said, "The Loma Prieta earthquake was, for the San Francisco Freeway Viaducts, a minor-to-moderate earthquake. These viaducts (Embarcadero Viaduct, [Transbay] Terminal Separation Viaduct, Central Viaduct, China Basin Viaduct, Southern Freeway Viaduct and Alemany Viaduct) in San Francisco were all built with the same technology used for the Cypress Viaduct and are the only structures in the State of this design. All of the freeway structures, with the exception of the Alemany Viaduct, were damaged during the earthquake and subsequently closed to traffic." The report also said, "Many of the crack patterns are similar to those observed in the collapsed and damaged portions of the Cypress Viaduct."
Most of the damaged San Francisco Viaduct system had not been repaired by October 1990, but had been seismically stabilized with combustible wood bracing.
The earthquake shook for 15 seconds and resulted in at least 67 deaths from direct earthquake causes, 3,757 injuries, more than 12,000 left homeless and property damage in excess of $10 billion throughout the affected zone, according to the California Office of Emergency Services.
In San Francisco, 11 people died as a direct result of the earthquake and hundreds were injured. Thirty buildings either collapsed or were immediately demolished, and 91 others were condemned, and must be either extensively rebuilt before they can be occupied, or must be demolished.
From 5:04 p.m. October 17 to midnight October 19, 36 fires involving structures were reported to the San Francisco Fire Department. Of these, 34 fires were directly or indirectly attributable to the earthquake and subsequent aftershocks.
When the earthquake struck, the electric supply was lost to most of San Francisco.
Initially, failure of electric service may have been beneficial in reducing the number of potential fires because of the loss of an ignition source for hundreds of PG&E gas leaks. As anticipated, natural gas was responsible for some of the fires following this earthquake.
An estimated 500 dispatches were transmitted by midnight of October 17 of which 80 percent were investigations of natural gas odors.
Damage to private and public property in San Francisco is in excess of $3.2 billion.
Fire losses from causes due to earthquake are in excess of $10 million.
The Fire Department suffered $327,000 damage to facilities, $80,000 to equipment that was either damaged or lost during the earthquake emergency. Almost $1 million was expended for earthquake-related labor and overtime.
Costs of repairing the Auxiliary Water Supply
System (AWSS) are in excess of $200,000.