The 20th anniversary of the 1989 Loma Prieta earthquake serves as an important reminder to all residents of California that the geologic processes responsible for creating the beautiful natural landscape we enjoy can sometimes occur suddenly and violently so it’s important to be prepared. The anniversary of the Loma Prieta earthquake also marks the anniversary of the creation of the Seismic Hazards Zoning Program (SHZP), one of the key programs operated by California Geological Survey (CGS) to aid in preparedness. One year following Loma Prieta, in direct response to the ground failure caused by the earthquake, the State enacted legislation directing CGS to identify and map areas prone to liquefaction, earthquake-induced landslides and amplified ground shaking. The purpose of the SHZP, to minimize loss of life and property through the identification, evaluation and mitigation of seismic hazards, represents just one of the many ways CGS strives to fulfill its mission to provide scientific products and services about the state's geology, seismology and mineral resources including their related hazards, that affect the health, safety, and business interests of the people of California.
Facts About the Loma Prieta Earthquake
WHEN: 5:04 P.M., Tuesday, October 17, 1989. The shaking lasted 20 seconds.
WHERE: The epicenter was on the San Andreas fault roughly 56 miles south of San Francisco and 10 miles northeast of Santa Cruz, near Mt. Loma Prieta in the Santa Cruz Mountains. The focal depth was 11 miles (typical California earthquake focal depths are 4 to 6 miles). Loma Prleta ruptured the southernmost 30 miles of the break that caused the 1906 San Francisco Earthquake.
HOW BIG: Magnitude established at 6.9 after consultation with monitoring stations around the world. The Loma Prieta quake was felt as far away as San Diego and western Nevada.
AFTERSHOCKS: A magnitude 5.2 aftershock occurred approximately 2.5 minutes after the main shock. In the week following Loma Prieta, 20 aftershocks magnitude 4.0 or greater and more than 300 of magnitude 2.5 or greater were recorded. Thousands of aftershocks were recorded. The aftershock zone stretched 25 miles, from north of Los Gatos near Highway 17 to south of Watsonville near Highway 101.
CASULTIES: 63 people were killed, 3,757 were reported injured and 12,053 displaced.
DAMAGE: Damage and business interruption estimates reached as high as $10 billion, with direct damage estimated at $6.8 billion. 18,306 houses were damaged and 963 were destroyed. 2,575 businesses were damaged and 147 were destroyed. The most notable damage included the collapse of the elevated Cypress Structure section of Interstate 880 in Oakland, the collapse of a section of roadbed on the Bay Bridge, and extensive damage to downtown Santa Cruz and San Francisco's Marina District. The Bay Bridge was unusable for a month. Also, the World Series between the San Francisco Giants and Oakland A's was postponed.
SIGNS: Geologists had forecast a major earthquake in this area based on historical data, especially the lack of a major seismic event along the San Andreas fault since 1906 -- the 8.3 San Francisco earthquake. In the 83 years prior to the 1906 quake, seven damaging earthquakes of magnitude 6.5 or greater occurred. Only two have occurred since the San Francisco earthquake. Several 5.0-plus seismic events in the two years preceding Loma Prieta also served as warnings. There is still a 50 percent chance for one or more magnitude 7.0 earthquakes In the San Francisco Bay Area in the next 30 years, and the probability of a repeat of the 1906 quake is significant.
THE FAULT: The San Andreas Fault is the boundary between the North American plate and the Pacific plate. Land west of the fault has been moving to the northwest relative to land on the east at an average rate of 2 inches per year for millions of year. This motion typically occurs in sudden jumps during large earthquakes. The Pacific plate moved 6.2 feet to the northwest and 4.3 feet upward over the North American plate during Loma Prieta.
DEFINED: Magnitude is a measure of an earthquake's size, but rather than being a direct measure of the level of ground shaking, it is a measurement of the strength of the seismic sound waves given off by the earthquake. A magnitude 8 earthquake radiates 30 times more energy of a magnitude 7 and 900 times the energy of a magnitude 6. Strong ground shaking for a magnitude 7 quake typically lasts about 15 seconds. It lasts a minute in a magnitude 8.
WHAT IF: As devastating as Loma Prieta was, a magnitude 7.5 earthquake on the Hayward Fault in the East Bay could do $65 billion in damage.
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