The 1989 Loma Prieta earthquake, a powerful M 6.9 tremor, struck the San Francisco Bay Area on October 17, 1989, at 5:04 p.m. local time. Originating in the Santa Cruz Mountains, the event sent intense seismic waves across a densely populated region. This earthquake was the strongest to affect the area since the great 1906 San Francisco earthquake, causing widespread destruction and resulting in 63 fatalities. The sudden, brief episode of ground movement reshaped infrastructure and led to major revisions in California’s seismic safety standards.
Defining the Duration of an Earthquake
The duration of the 1989 earthquake depends on the definition used. From a scientific perspective, the physical rupture of the fault itself was remarkably brief. The segment of the San Andreas Fault system broke for only about six seconds. This short rupture time occurred because the fault broke bilaterally, with the rupture moving in two directions simultaneously.
The duration most relevant to human experience is the time of strong ground shaking, which was notably longer than the rupture. Across the affected region, the period of intense shaking generally ranged between 15 and 20 seconds. This measure records the amount of time that seismic waves created ground acceleration strong enough to damage structures. For example, sites near the epicenter in the Santa Cruz Mountains experienced the heaviest shaking for approximately 15 seconds.
Not every location in the Bay Area experienced the same duration or intensity, a difference largely explained by local geology. Areas built on firm bedrock, such as the hills of San Francisco, generally experienced shorter, less intense shaking. Conversely, sites resting on unconsolidated, water-saturated sediments, like the artificial fill used in parts of Oakland and San Francisco, experienced prolonged and amplified shaking. This site amplification caused the strong motion to be more severe and last longer in these specific locations.
The Immediate Effects of the Strong Ground Motion
The relatively short period of strong ground motion, lasting around 15 to 20 seconds, was all it took to cause catastrophic failure in vulnerable structures. The most devastating single consequence occurred in Oakland with the collapse of the Cypress Street Viaduct, a double-deck section of Interstate 880. This structure, which was built on soft Bay mud and non-ductile concrete, could not withstand the amplified shaking. The upper deck fell onto the lower deck along a 1.25-mile stretch, crushing vehicles and resulting in 42 deaths.
The intense shaking also severely damaged the San Francisco–Oakland Bay Bridge, one of the region’s main arteries. A 50-foot section of the upper deck on the eastern cantilever span sheared off and dropped onto the lower deck. This structural failure resulted in one fatality and rendered the bridge unusable for a month, highlighting the vulnerability of older infrastructure.
Beyond direct structural collapse, the strong ground motion caused widespread liquefaction in areas built on artificial fill near the waterfront. Liquefaction is the process where saturated, loose soil temporarily loses its strength and behaves like a liquid due to the vibrations. This effect was particularly pronounced in San Francisco’s Marina District, which was constructed over rubble and hydraulic fill. The loss of soil solidity led to the collapse and severe damage of many buildings in that neighborhood.
The Tectonic Setting of the Loma Prieta Earthquake
The Loma Prieta earthquake occurred on a segment of the larger San Andreas Fault system, which marks the boundary between the Pacific and North American tectonic plates. The epicenter was located approximately 56 miles south of San Francisco, beneath the Santa Cruz Mountains near Loma Prieta Peak. The focal depth of the earthquake was deeper than typical California quakes, originating about 11 miles below the surface.
This M 6.9 event was distinct from a classic San Andreas event because its mechanism was not a pure strike-slip motion, where plates slide past each other horizontally. Instead, it was classified as an oblique-slip reverse, or thrust, earthquake. During the event, the Pacific plate moved not only horizontally northwest by about 6.2 feet but also upward by approximately 4.3 feet relative to the North American plate.
This significant vertical component of movement suggests the rupture may have occurred on a sub-parallel fault adjacent to the main San Andreas Fault, characteristic of a compressional bend in the fault system. This thrusting motion was responsible for lifting the Santa Cruz Mountains and provided insight into the complex mechanics of the plate boundary in this region. The rupture was contained to a relatively short segment of the fault system.