1989 Loma Prieta Earthquake

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On October 17th, 1989 at 5:04 p.m, a 6.9 magnitude earthquake struck the central coast of California. Mt. Loma Prieta in the Santa Cruz Mountains is located roughly 60 miles from the city of San Francisco and 10 miles from Santa Cruz; the epicenter was here, and the quake was named after the mountain. The focal point, where the seismic waves originated, was 11 miles under the epicenter.

The Earth shook for around 20 seconds, and was felt distantly in western Nevada and San Diego. Loma Prieta broke the Earth approximately 30 miles from the epicenter of the 1906 San Francisco earthquake, and was the first large quake since that event. Following the 1989 quake, there were thousands of aftershocks felt over a 25 mile range. The number of aftershocks with a greater magnitude of 2.5 was more than 300 (California Department of Conservation). This is not the largest earthquake ever recorded in the region, but it is remarked by a large number of people affected, and billions of dollars in damages.

Physical Processes

In California, the Pacific and North American Plates meet along the San Andreas Fault line; as the plates move against each other, earthquakes form. This fault is “more than 800 miles long and extends to depths of at least 10 miles within the Earth” (Schulz). This is a transform boundary, which creates a right-lateral strike-slip fault here. During the Loma Prieta earthquake, “the Pacific plate moved 6.2 feet to the northwest and 4.3 feet upward over the North American plate…” (California). The fault moves slowly over time, unless the tension in the Earth is released, the ground shakes, and the fault lines move a significant distance.

Elevated Risk In Area

The San Andrea Fault predisposes the area to earthquakes, making them a common occurrence, although larger events are significantly rarer. Due to this, high-level earthquakes are expected to happen along an expanded timeline. Between the 1906 San Francisco and 1989 Loma Prieta earthquake, many damaging earthquakes occurred, but several 5.0+ occured in the two years before, serving as a warning (California).

Unfortunately, there is no way of accurately predicting the exact time of an earthquake – all that can be done is monitoring of warning signs, and releasing potential warnings, which have a large potential of being inaccurate. After the 1989 earthquake, the risk was not diminished, but seemed to have increased: “…projected 2-in-3 odds for one or more destructive earthquakes (magnitude 7 or larger) to strike the Bay region in the period 1990 to 2020” (Page). While that timeline is almost up, the placement of the fault means that California will never be completely safe from the natural hazard.

Mitigation After 1906

A great deal of scientific discovery regarding seismic activity was researched in the 20th Century. The 1906 San Francisco earthquake led to knowledge that helped to start mitigation against further earthquakes in the area; without this knowledge, Loma Prieta could have been even more devastating. Research conducted after the 1906 earthquake directly led to H.F. Reid’s presentation of the theory of elastic rebound:

…describes how the Earth’s crust gradually and elastically distorts with accumulating plate motion until it is suddenly returned to its undistorted state by rapid slip along a fault, releasing the years of accumulated strain and, in the process, generating seismic waves that produce shaking. (1906 Marked the Dawn)

This idea changed the study of earthquakes after that, as this became a basic principle of seismic activity. As study advanced, shake maps have been produced for all areas of California. From the maps, information regarding land use and building codes began to be put into place throughout the state to protect loss of life and property (1906).

Damage and Loss of Life

The damages are estimated to have cost ten billion dollars, including the direct damages and repairs. Overall, over 12,000 people were displaced by the event, with 3,757 injuries and 63 deaths. Almost 1,000 houses were completely destroyed (California). This earthquake did carry with it several memorable damages, that are often talked about in connection with the disaster. “…the collapse of a two-level, 1.25-mile-long section of the Cypress Viaduct on Interstate Route 880 in Oakland; 42 people were killed. A section of the Bay Bridge connecting San Francisco and Oakland also collapsed, resulting in one death.” CalTrans had discovered that structures built before 1971 and certain earthquake building codes, had a possible threat to collapse, but did not enact their plan for repair soon enough to prevent the damage done in 1989. Congress gave the state one billion dollars in relief funds to fix the damages made to these structures (Collapse).

The earthquake occured minutes before the beginning of the World Series game between the San Francisco Giants and Oakland’s A. “ABC was live on the air, just over four minutes into its broadcast. As the TV feed flickered out, Al Michaels could be heard saying, ‘I’ll tell you what! We’re having an earthquake!’ (Kurkjian). The game was postponed for 10 days, and was the first earthquake to be broadcast live on television, due to the filming already being done for the game.

Mitigation After 1989

By 1989, most modern-day practices of studying earthquakes were already in place. From this event, scientists were able to learn additional information about how different earth materials are affected during shaking: “….ground shaking is much more violent on the soft sediments around the Bay margins than on bedrock,” (Page). Building codes were updated again, following the new findings. The efforts to update structures such as highways and bridges were fast-tracked to avoid future damage.

Technology now allows for increasingly accurate simulations, and data can be collected and analyzed to a greater extent. Loma Prieta set a standard for recording data, as it was one of the most recorded earthquakes up to that point. This natural disaster woke everyone up to the dangers of earthquakes, and mitigation proved to be essential to reducing the fear that residents of the area, and of the state, feel (Tucker). There’s still no way to know exactly when the next big one will hit, although warning signs like foreshocks and ground creeping can occur, but preparation is key to reducing damage that has yet to occur.


Earthquakes can be directly related to many natural hazards, but the Loma Prieta earthquake caused a large amount of landslides in the area, adding to the damages. “12800 earthquake-induced landslides were mapped over an area of ~2000 km.” These occurred mostly in the Santa Cruz Mountains, and not in suburban areas (Schmidt). However, debris from these mass wasting events did bring delays to repairing damage done by the earthquake, due to unstable slopes. Two major highways ended up blocked, and the damage from the landslides was more than thirty million dollars (The Loma Prieta). Analyzing the slope stability afterwards help to accurately portray the threat to areas at risk for a loss of life and property.


  1. “1906 Marked the Dawn of the Scientific Revolution.” U.S. Geological Survey, earthquake.usgs.gov/earthquakes/events/1906calif/18april/revolution.php.
  2. California Department of Conservation. “The 1989 Loma Prieta Earthquake.” CA Department of Conservation, www.conservation.ca.gov/cgs/earthquakes/loma-prieta.
  3. “Collapse of the Bay Bridge and the Cypress Viaduct.” U.S. General Accounting Office, www.gao.gov/assets/220/212748.pdf.
  4. Griesedieck, Judy. “The Mercury News.” The Mercury News, https://www.mercurynews.com/2018/10/17/photos-loma-prieta-earthquake-scarred-bay-area-29-years-ago/.
  5. Kurkjian, Tim. “’The Sound of Fear’: Thirty Years Ago, the Loma Prieta Earthquake Shook the World Series — and the World.” ESPN, ESPN Internet Ventures, 17 Oct. 2019, www.espn.com/mlb/story/_/id/27637121/the-sound-fear-thirty-years-ago-loma-prieta-earthquake-shook-world-series-world.
  6. Page, Robert, et al. “Progress Toward a Safer Future Since the 1989 Loma Prieta Earthquake.” United States Geological Survey, pubs.usgs.gov/fs/1999/fs151-99/.
  7. “ShakeMap Showing the Inferred Intensity of Ground Shaking in the 1868 Earthquake (Measured as MMI, or Modified Mercalli Intensity), Compared to a ShakeMap for the 1989 Magnitude 6.9 Loma Prieta Earthquake.” U.S. Geological Survey, https://www.usgs.gov/media/images/shakemaps-1868-hayward-quake-and-1989-loma-priearthquake.
  8. Schmidt, Kevin M., et al. “Deformation from the 1989 Loma Prieta Earthquake near the Southwest Margin of the Santa Clara Valley, California.” Geosphere, vol. 10, no. 6, Dec. 2014, pp. 1177–1202. EBSCOhost, doi:10.1130/GES01095.1.
  9. Schulz, Sandra, and Robert Wallace. “The San Andreas Fault.” United States Geological Survey, pubs.usgs.gov/gip/earthq3/safaultgip.html.
  10. “The Loma Prieta, California, Earthquake of October 17, 1989 – Landslides.” U.S Geological Survey , 1998.
  11. Tucker, Danielle. “Q&A: 30 Years after the Loma Prieta Earthquake.” Stanford Earth, 16 Oct. 2019, earth.stanford.edu/news/qa-30-years-after-loma-prieta-earthquake#gs.5xsmvx.
  12. Weber, Gary. “Collapsed Section of Bay Bridge Caused by 1989 Loma Prieta Earthquake.” KQED, https://www.kqed.org/news/109768/looking-back-at-loma-prieta-from-someone-who-was-on-the-bay-bridge.
  13. “U.S. Geological Survey.” U.S. Geological Survey, https://www.usgs.gov/media/images/cartoon-sketch-pacific-plate-north-american-plate-boundary.

Cite this paper

1989 Loma Prieta Earthquake. (2022, Feb 10). Retrieved from https://samploon.com/1989-loma-prieta-earthquake/

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