A major earthquake jolted the San Francisco Bay Area on the afternoon of October 17, 1989. It was the largest quake to shake the area since the massive San Francisco earthquake of 1906. The 1989 Loma Prieta quake hit when commuters were stuck in traffic on their way home from work and a World Series game was set to begin at Candlestick Park in San Francisco. Fifteen seconds later, the shaking had ceased. The U. S. Geological Survey and disaster management organizations found that California's infrastructure system and the people who lived in the shadow of the San Andreas Fault had not sufficiently prepared when disaster struck.
San Andreas Fault
Earthquakes result from the sudden movement of faults, or the boundaries between two tectonic plates. The plates bear a similarity to a cracked eggshell surrounding a hard-boiled egg. The San Andreas Fault, the source of the Loma Prieta quake, travels about 800 miles from its northernmost point near Cape Mendocino, California, through the Mexican border and down to the Gulf of California. The San Andreas forms the boundary between the Pacific and North American plates. Geologists categorize the San Andreas as a strike-slip fault, where any movement occurs horizontally as the Pacific plate moves under the North American plate.
Geological Details
The earthquake occurred at 5:04 p.m. and measured 6.9 on the Richter scale, which assigns numbers to earthquakes based on the strength of the shaking. The temblor occurred 11 miles below the surface and affected a 40-mile section of the San Andreas Fault. The seismic activity caused the plates to shift several feet. The U. S. Geological Survey placed the epicenter (the precise point underground where the earthquake occurred) in the Santa Cruz Mountains, specifically under a peak named Loma Prieta.
Casualties
The violent rocking caused a total of 63 deaths and left almost 4,000 people injured. The majority of the deaths and injuries occurred when the Cypress structure, a double-decker section of Interstate 880, collapsed. The upper, southbound tier fell on top of the lower, northbound one, killing 41 people. Many others suffered crippling injuries. Collapsed buildings in the Bay Area killed more than 10 other people.
Damage
The earthquake wrecked more than 18,000 residences and approximately 2,500 businesses. Bridges, highways and freeways also suffered damage. After the Cypress section of Interstate 880 collapsed, it was torn down because it could not be repaired, and traffic was rerouted to other roads. A segment of the top tier of the Oakland-San Francisco Bay Bridge also collapsed onto its bottom section. The total economic loss amounted to an estimated $8 billion dollars: $6 billion dollars in property destruction and almost $2 billion dollars worth of damage to transportation structures.
Liquefaction
Much of the damage to structures during this particular quake resulted from the instability of the ground, which was caused by liquefaction. This geological phenomenon occurs when seismic activity causes certain types of underlying, clay-like soils to become liquid and lose their strength, destabilizing any structures anchored in the soil. Portions of the San Francisco Bay Area, such as the Marina District and areas of Oakland, contain this unstable soil, called "Bay Mud." Although these areas were almost 100 miles from the epicenter, they sustained heavier damage because the ground literally crumbled beneath the edifices.
Tags: Andreas Fault, Loma Prieta, billion dollars, earthquake occurred, Francisco Area, Geological Survey, Interstate collapsed
