The Loma Prieta earthquake struck the San Francisco Bay Area on October 17, 1989, at 5:04 p.m. local time. The sudden shaking occurred just as the third game of the World Series was about to begin, focusing the national spotlight on the catastrophic event. The event served as a severe reminder of the active geology beneath California’s coastal communities.
The Measured Size: Moment Magnitude Scale
The official size of the Loma Prieta event is recorded as a Moment Magnitude (\(\text{M}_{\text{w}}\)) of 6.9. The hypocenter, the point within the Earth where the rupture began, was located at a depth of about 16 to 19 kilometers (10 to 12 miles). This depth is significantly deeper than many typical California earthquakes, which often occur at shallower depths of 6 to 10 kilometers.
Seismologists utilize the Moment Magnitude Scale (\(\text{M}_{\text{w}}\)) because it provides a more accurate measure of the total energy released by large earthquakes compared to the older Richter scale. The Richter scale measures only the amplitude of the seismic waves, which can become inaccurate—or “saturate”—for very large events. The Moment Magnitude Scale calculates the seismic moment, a physical property derived from the area of the fault that ruptured, the average distance the fault slipped, and the rigidity of the Earth’s crustal rock.
This calculation better reflects the true size of significant seismic events. An earthquake with a magnitude of 6.9 \(\text{M}_{\text{w}}\) releases approximately 32 times more energy than a 5.9 \(\text{M}_{\text{w}}\) event, illustrating the logarithmic nature of the scale. The energy output of the Loma Prieta earthquake caused widespread destruction across the region.
Source and Rupture: The San Andreas Fault Segment
The earthquake originated in the Santa Cruz Mountains, with the epicenter located near the peak of Loma Prieta, about 16 kilometers northeast of Santa Cruz. The rupture occurred on a segment of the San Andreas Fault system that had been relatively quiet since the 1906 San Francisco earthquake. This segment was recognized as a “seismic gap,” meaning it had accumulated strain and was considered a likely location for a large earthquake.
The nature of the rupture was complex, involving oblique-slip motion rather than the purely horizontal, or strike-slip, motion typically associated with the San Andreas Fault. Seismological data indicated a combination of right-lateral strike-slip movement and a significant thrust (or reverse) component. The fault slipped an average of about 1.5 meters horizontally and 1.6 meters vertically, indicating a substantial upward push of the western side of the fault.
The rupture propagated along a fault surface estimated to be around 35 kilometers long and lasted for about 10 to 20 seconds. While the movement was confined to a deeper, dipping plane, the event suggests that the earthquake may have occurred on a sub-parallel fault adjacent to the main San Andreas trace. This unique oblique motion contributed to the specific pattern of ground shaking felt across the Bay Area.
Translating Magnitude: Ground Shaking and Immediate Damage
The magnitude of 6.9 \(\text{M}_{\text{w}}\) translated into a maximum ground shaking intensity of IX (Violent) on the Modified Mercalli Intensity (MMI) scale. This scale describes the effects of an earthquake on the Earth’s surface, people, buildings, and infrastructure. The MMI IX intensity level indicates heavy damage to well-built structures and the collapse of less-resilient buildings.
The most severe structural failures occurred in areas where the ground was unstable, such as the Marina District in San Francisco, which was built on loose, sandy fill prone to liquefaction. The shaking led to the collapse of a 1.25-mile section of the double-deck Cypress Street Viaduct (Interstate 880) in Oakland, where the upper deck fell onto the lower deck, accounting for 42 of the total fatalities. A 50-foot section of the San Francisco–Oakland Bay Bridge also failed, causing one death and rendering the bridge unusable for a month.
Overall, the earthquake resulted in 63 fatalities and 3,757 injuries across Northern California. The nearby city of Santa Cruz suffered extensive damage to its downtown area, where many unreinforced masonry buildings were severely damaged or destroyed.