1989 Loma Prieta Earthquake

Primary Keyword: 1989 loma prieta earthquake (18,100)

Key Takeaways

• The 1989 Loma Prieta earthquake struck at 5:04 PM on October 17, 1989, with a magnitude of M6.9, centered in the Santa Cruz Mountains approximately 100 km south of San Francisco.
• 63 people were killed, 3,757 were injured, and damage totaled approximately $6 billion — making it the costliest U.S. natural disaster at that time.
• The deadliest single event was the collapse of the Cypress Viaduct (Interstate 880) in Oakland, which killed 42 of the 63 total victims when its upper deck pancaked onto the lower deck.
• The Marina District of San Francisco experienced severe liquefaction and fires — built on rubble fill from the 1906 earthquake, the ground behaved like liquid during shaking.
• The earthquake occurred during the World Series between the San Francisco Giants and the Oakland Athletics, meaning a national television audience witnessed the disaster in real time.
• Loma Prieta catalyzed major infrastructure upgrades: the Cypress Viaduct was replaced, the Bay Bridge was retrofitted, soft-story building programs were initiated, and early earthquake warning research accelerated.

Introduction

At 5:04 PM on October 17, 1989, as 62,000 fans filled Candlestick Park for Game 3 of the World Series between the San Francisco Giants and the Oakland Athletics, a M6.9 earthquake struck the San Francisco Bay Area. The epicenter was located in the Santa Cruz Mountains near Loma Prieta peak, approximately 100 km southeast of San Francisco, at a depth of about 18 km.

The shaking lasted approximately 15 seconds. In that quarter-minute, a section of the double-deck Cypress Viaduct in Oakland collapsed, killing 42 people. A 15-meter section of the Bay Bridge's upper deck fell onto the lower deck. Fires broke out in San Francisco's Marina District as liquefied soil buckled buildings and ruptured gas lines. Across the region, 63 people died, 3,757 were injured, and approximately 12,000 were displaced from their homes.

The 1989 Loma Prieta earthquake was not the catastrophic San Andreas rupture that seismologists had long warned about — it was a moderate earthquake on an oblique-slip fault segment, considerably smaller than the 1906 event. But it struck a modern metropolitan area of 6 million people during rush hour, on live national television. The result was a turning point in American earthquake awareness and preparedness.

San Andreas Fault overview Bay Area earthquake risk

The Geology

Fault and Rupture

The Loma Prieta earthquake ruptured a segment of the San Andreas Fault zone in the Santa Cruz Mountains, between the towns of Los Gatos and Watsonville. However, the rupture was not a simple horizontal strike-slip motion like the 1906 earthquake. Instead, it occurred on a section of the fault that dips approximately 70° to the southwest, with significant reverse (thrust) motion in addition to the horizontal slip.

The rupture extended approximately 40 km along the fault, at depths between 8 and 18 km. Surface rupture was minimal and ambiguous — unusual for a San Andreas earthquake of this magnitude. The lack of clear surface rupture initially caused debate among geologists about whether this was truly a San Andreas event or a rupture on a nearby subsidiary fault. The current consensus is that it ruptured a deep, oblique section of the San Andreas Fault zone.

Maximum slip on the fault was approximately 2.3 meters, with roughly equal components of strike-slip (horizontal) and reverse (vertical) motion. This combination of motions reflected the compressional geometry of the San Andreas Fault where it bends through the Santa Cruz Mountains.

Shaking Characteristics

Despite being centered 100 km from San Francisco and Oakland, the earthquake produced significant damage in those cities due to a combination of factors. The earthquake generated strong long-period (low-frequency) seismic waves that traveled efficiently through the underlying bedrock of the Bay Area. More critically, these waves were amplified dramatically by soft soils — bay mud, artificial fill, and unconsolidated sediments — that underlie many of the areas that suffered the worst damage.

The amplification effect was profound. Areas built on solid bedrock, such as much of the East Bay hills, experienced moderate shaking and sustained little damage. Areas built on soft soils, such as the Cypress Viaduct corridor in West Oakland, the Marina District in San Francisco, and parts of downtown Santa Cruz, experienced shaking two to five times more intense than bedrock sites at similar distances.

The Earthquake at Candlestick Park

The timing of the earthquake — 5:04 PM during the warm-up before Game 3 of the "Bay Bridge" World Series — made it one of the most widely witnessed earthquakes in history. ABC television cameras were broadcasting live from Candlestick Park when the shaking began. The broadcast cut to static, then returned to show the stadium swaying and the stunned crowd.

Remarkably, Candlestick Park itself suffered no structural failure and no spectators were injured. The reinforced concrete stadium, built on relatively firm ground at Candlestick Point, rode out the shaking without significant damage. The crowd initially cheered, many thinking the shaking was simply a California experience, before the scoreboard went dark and the scale of the disaster became apparent.

The World Series was postponed for 10 days while the region dealt with the disaster. When it resumed on October 27, the Athletics completed a four-game sweep. But the games themselves became an afterthought. The Loma Prieta earthquake had turned a sporting event into a national consciousness-raising moment about earthquake risk.

The Cypress Viaduct Collapse

The Structure

The Cypress Viaduct was a 2 km elevated section of Interstate 880 (the Nimitz Freeway) in West Oakland, built in 1957. It consisted of a double-deck design: two levels of roadway stacked one above the other, supported by reinforced concrete columns. The upper deck carried northbound traffic; the lower deck carried southbound traffic.

The viaduct was built according to the engineering standards of the 1950s, which did not account for the seismic forces that modern codes require. The columns connecting the upper and lower decks lacked adequate lateral reinforcement — the spiral steel that would allow the columns to flex without shattering under seismic loads.

The Collapse

When the earthquake struck, the viaduct experienced approximately 15 seconds of strong shaking. The ground beneath the structure — soft bay mud and fill — amplified the seismic waves significantly, producing ground motions far stronger than would have occurred on bedrock.

The inadequately reinforced concrete columns failed in shear, unable to absorb the lateral forces. A 1.2 km section of the upper deck pancaked down onto the lower deck, crushing the vehicles and occupants on the lower roadway. The collapse happened within seconds, giving drivers virtually no time to react.

Forty-two people were killed in the Cypress Viaduct collapse — two-thirds of the earthquake's total death toll. One survivor, Buck Helm, was pulled alive from his crushed car after being trapped for 90 hours, becoming a national symbol of hope. He died of his injuries four weeks later.

The Cypress collapse concentrated nearly all of the earthquake's fatalities in a single location and highlighted two critical vulnerabilities: the dangerous combination of soft-soil amplification with inadequately reinforced concrete structures, and the particular hazard of double-deck transportation infrastructure.

The Lessons

The Cypress Viaduct collapse immediately prompted a statewide review of elevated freeway structures. Caltrans identified hundreds of bridges and overpasses with similar design deficiencies and launched a multi-billion-dollar seismic retrofit program. The collapsed section of I-880 was not rebuilt as a double-deck structure; instead, a new single-level freeway was constructed along a different alignment, opening in 1997.

Earthquake retrofitting

The Marina District: Liquefaction and Fire

Why the Marina Failed

The Marina District, an affluent residential neighborhood along San Francisco's northern waterfront, experienced some of the most dramatic damage of the earthquake — despite being nearly 100 km from the epicenter. The reason was beneath the surface.

The Marina District was built on filled ground. After the 1906 earthquake and fire, rubble from the destroyed city was dumped into a lagoon near the waterfront to create new buildable land. Additional fill was added for the 1915 Panama-Pacific International Exposition. Houses and apartment buildings were then constructed on top of this unconsolidated fill, with shallow foundations that did not reach the firm ground below.

During the 1989 earthquake, this fill liquefied. Liquefaction occurs when saturated, loosely packed soil loses its strength during shaking and behaves like a liquid. In the Marina District, the liquefied ground could no longer support the buildings above it. Structures tilted, sank, and collapsed as their foundations failed.

The damage was concentrated in soft-story buildings — structures with large openings on the ground floor (typically garages) and inadequate lateral bracing. These buildings, common throughout San Francisco, collapsed at the ground floor level, with the upper stories dropping down intact.

The Fires

Liquefaction ruptured gas lines beneath the Marina District, and fires broke out in several locations. The scene was eerily reminiscent of 1906: buildings collapsing, fires burning, and broken water mains hampering firefighting. The irony was bitter — the Marina District was literally built on the debris of the 1906 disaster, and it was failing for the same fundamental reason: construction on unstable ground without adequate engineering.

Firefighters used the city's auxiliary water supply system (AWSS) — the high-pressure firefighting system built in response to 1906 — and a portable water system to fight the fires. The AWSS, one of the genuine improvements made after 1906, prevented the Marina fires from spreading into a conflagration.

Understanding liquefaction

The Bay Bridge

The San Francisco–Oakland Bay Bridge, the primary link between the two largest cities in the Bay Area, suffered a dramatic failure when a 15-meter (50-foot) section of the upper deck on the eastern span collapsed onto the lower deck. One motorist was killed when their car drove into the gap.

The eastern span of the Bay Bridge was a cantilever truss structure built in 1936. Like the Cypress Viaduct, it was designed to standards that predated modern seismic engineering. The section that failed was located where the bridge transitioned between different structural systems, a point of particular vulnerability.

The Bay Bridge was closed for a month for emergency repairs. The incident, combined with the Cypress Viaduct collapse, convinced California officials that the eastern span of the Bay Bridge needed to be completely replaced — not merely retrofitted. The new eastern span, a self-anchored suspension bridge, opened in 2013 after a lengthy and controversial construction process that ultimately cost approximately $6.5 billion.

Major Damage Sites

Regional Impact and Response

Near the Epicenter

While the dramatic collapses in San Francisco and Oakland dominated media coverage, areas near the epicenter in the Santa Cruz Mountains suffered severe damage. The town of Santa Cruz lost much of its historic downtown, including the Pacific Garden Mall. Los Gatos, Watsonville, and other communities in the epicentral region experienced widespread damage to older buildings.

In the Santa Cruz Mountains themselves, landslides blocked roads and destroyed homes. The mountain communities were isolated for days. The town of Summit Road, located directly over the rupture zone, experienced the most intense shaking, with ground accelerations exceeding 0.6g.

Emergency Response

The earthquake struck during rush hour, and both the Bay Bridge and several major freeways were immediately closed. The resulting traffic paralysis complicated emergency response significantly. Communication systems were overwhelmed — telephone networks were jammed within minutes.

The emergency response to the Cypress Viaduct collapse was hampered by the structural instability of the remaining sections, which limited rescue access. Local residents from the surrounding West Oakland neighborhood were among the first responders, pulling survivors from crushed vehicles before official rescue teams arrived.

The Loma Prieta earthquake was the first major U.S. urban earthquake of the television age. National media coverage was immediate and sustained, driven by the coincidence of the World Series broadcast. This coverage dramatically increased public awareness of earthquake risk and generated political support for preparedness measures.

Economic Losses

Legacy and Reforms

Infrastructure Retrofit Programs

The Loma Prieta earthquake exposed the vulnerability of California's older infrastructure to seismic forces. The state responded with an aggressive retrofit program:

Caltrans launched a comprehensive seismic retrofit program for state-owned bridges and overpasses. By 2000, the state had spent over $4 billion strengthening more than 2,000 bridges. The program prioritized structures with similar vulnerabilities to the Cypress Viaduct — older reinforced concrete designs on soft soils.

The San Francisco Embarcadero Freeway, a double-deck elevated highway along the waterfront, had been damaged in the earthquake. Rather than retrofit it, the city chose to demolish it entirely — a decision that proved transformative for San Francisco's waterfront, opening up views and access that had been blocked since the 1950s. The Embarcadero Boulevard that replaced it is now considered one of the city's most successful urban spaces.

Soft-Story Building Awareness

The Marina District failures brought national attention to the vulnerability of soft-story buildings — multi-unit residential buildings with weak ground floors, typically due to garage openings. San Francisco eventually enacted a mandatory soft-story retrofit ordinance in 2013, requiring approximately 5,000 buildings to be strengthened. Other California cities followed with similar programs.

Earthquake retrofitting programs

Early Warning Research

The Loma Prieta earthquake demonstrated the potential value of earthquake early warning. Seismographic stations near the epicenter detected the earthquake seconds before the most destructive waves reached San Francisco and Oakland. While no warning system existed in 1989, this observation became a foundational argument for developing the ShakeAlert system, which became operational in California in 2019.

Building Code Improvements

The earthquake prompted updates to California's building codes, particularly regarding soft-soil amplification and the performance of older concrete structures. The 1994 Northridge earthquake in Los Angeles would further accelerate these reforms, but Loma Prieta was the event that shifted the political landscape in favor of stricter seismic standards.

Map Specification

Map: 1989 Loma Prieta Earthquake — Epicenter and Major Damage Sites

• Base map: San Francisco Bay Area, extending south to Santa Cruz
• Epicenter: Marked in the Santa Cruz Mountains near Loma Prieta peak (37.04°N, 121.88°W), depth 18 km
• Fault rupture zone: Approximate 40 km segment of the San Andreas Fault zone highlighted
• Major damage sites (numbered markers):
  1. Cypress Viaduct (I-880), West Oakland — 42 killed
  2. Marina District, San Francisco — liquefaction, fires
  3. Bay Bridge eastern span — upper deck collapse
  4. Downtown Santa Cruz — Pacific Garden Mall collapse
  5. Watsonville — unreinforced masonry damage
  6. Embarcadero Freeway, San Francisco — damaged (later demolished)
  7. Candlestick Park — World Series venue (no structural damage)
• Shading overlay: Areas of soft soil / bay fill (showing correlation with damage sites)
• Inset: Detail of Cypress Viaduct collapse area in West Oakland

Chart Specification

Chart: Loma Prieta Fatalities by Location

• Type: Horizontal bar chart
• Data:
  • Cypress Viaduct (I-880): 42 deaths
  • Marina District, San Francisco: 4 deaths
  • Downtown Santa Cruz: 3 deaths
  • Watsonville: 2 deaths
  • Bay Bridge: 1 death
  • Other locations: 11 deaths
  • Total: 63 deaths
• Source: USGS, California Office of Emergency Services
• Key insight: Two-thirds of all fatalities occurred in a single structure (Cypress Viaduct), illustrating how a few vulnerable structures can dominate the death toll in a modern earthquake

Sources

1. USGS (1999). "The Loma Prieta, California, Earthquake of October 17, 1989." USGS Professional Paper 1551. USGS Loma Prieta Professional Paper
2. Housner, G.W., ed. (1990). "Competing Against Time: Report to Governor George Deukmejian from the Governor's Board of Inquiry on the 1989 Loma Prieta Earthquake." State of California.
3. EERI (1990). "Loma Prieta Earthquake Reconnaissance Report." Earthquake Spectra, Supplement to Vol. 6.
4. Seed, R.B. et al. (1990). "Preliminary Report on the Principal Geotechnical Aspects of the October 17, 1989 Loma Prieta Earthquake." Report UCB/EERC-90/05, University of California, Berkeley.
5. USGS (2014). "UCERF3: A New Earthquake Forecast for California's Complex Fault System." UCERF3 Fact Sheet
6. Caltrans (2000). "Seismic Retrofit Program." California Department of Transportation.
7. Fradkin, P.L. (1999). Magnitude 8: Earthquakes and Life Along the San Andreas Fault. University of California Press.
8. Palm, R. & Hodgson, M. (1992). After a California Earthquake: Attitude and Behavior Change. University of Chicago Press.