1989 Loma Prieta, California Earthquake
Thomas W. Weaver, BAE/MAE, Penn State, 2010


The Loma Prieta Earthquake of 1989 struck the San Francisco Bay Area of California on October 17, 1989 at 5:04 pm Pacific Daylight Time. Located in the southern Santa Cruz Mountains near the summit of Loma Prieta mountain, the epicenter was 16 km (~10 miles) northeast of Santa Cruz and about 30 km (~19 miles) south of San Jose.
The quake lasted for 10 to 15 seconds with an average surface-wave magnitude of 7.1, a moment magnitude of 6.9, and a Richter magnitude of 7.0. Ground shaking was sufficient enough to cause structural damage at distances of 100 km (~62 miles). (EERI 1989).

The earthquake struck the bay area just before the third game of the World Series at Candlestick Park. It knocked out power to San Francisco, and the city was dark for the first time since the 1906 earthquake and fire, however, power was restored by October 20. At least 27 fires broke out across the city, including a major blaze in the Marina District where apartment buildings sank into the bay mud. Emergency telephone service became sporadic because a fire broke out in the 911 telephone equipment room and citizens had to rely on fire alarm boxes for three days. Citizens also formed bucket brigades to help firefighters who were without water from broken water mains. (Museum of the City of San Francisco 1989).
Figure 1: Loma Prieta Shake Map, Credit: U. S. Geological Survey (Zoback 2005)

Approximately 62 deaths were reported, 400 people were severely injured, and roughly 4,000 total injuries were logged. In the aftermath of the earthquake, 3,000 people were left homeless with around 12,000 homes and 2,600 businesses damaged or destroyed. The Loma Prieta Earthquake caused an estimated $6 billion in damage. (Stoffer 2005).

Seismological and Geotechnical Aspects

The earthquake occurred on a section of the San Andreas fault system previously identified as a segment for an event of magnitude 6.5 of larger happening with a relatively high probability of 30 percent within 30 years. This earthquake re-ruptured the southernmost 45 km (~28 miles) of the San Francisco Earthquake of 1906 fault break. More than 2,300 aftershocks were recorded shortly after the quake, all of which took place on the San Andreas fault; however, a cluster of aftershocks triggered by a magnitude 5.0 earthquake 33 hours after the main shock were located in the vicinity of the Zayante fault. The mechanism of the main shock was determined to be a right-lateral strike-slip and reverse faulting. During the main shock, movement occurred at mid-crustal depths of the Pacific plate in a northwestern, and upward direction with respect to the North American plate. (EERI 1989).

Geotechnical effects of the earthquake were considerable and subsurface soil conditions greatly contributed to the severity and distribution of the earthquake damage. Liquefaction occurred to varying degrees at distances up to 113 km (~70 miles) from the epicenter in the form of sand boils, lateral spreading, settlement, and slumping causing significant damage in the Marina District of San Francisco as well as the coastal areas of Oakland and Alameda. Additionally, liquefaction occurred along the Pacific Ocean coastline with significant damage experienced in the Santa Cruz and Monterey Bay areas. More than 500 landslides and rockfalls were also observed in the Santa Cruz Mountains close to the epicenter. (EERI 1989). Figure 1: "Loma Prieta Shake Map" depicts graphically the magnitude of ground motion in the San Francisco Bay area.

Building Performance

Unreinforced Masonry Buildings

In an earthquake, unreinforced masonry bearing wall buildings are generally considered to be one of the most dangerous building types. They have been responsible for significant property damage and loss of life in previous earthquakes, both in he United State and around the world. In the Loma Prieta earthquake, many unreinforced masonry
bearing wall buildings were seriously damaged and the cause of nine deaths. (Celibi 1997).
Figure 2: Unreinforced Masonry Buildings, Pacific Garden Mall, Credit: U. S. Geological Survey (Nakata 1999)

The majority of the extravagant building damage resulting from the earthquake was suffered by pre-code (pre 1930's)
buildings, primarily of the unreinforced masonry type. The majority of the severely damaged buildings were located in the area south of Market Street where soil conditions significantly impacted the extent of the damage. (EERI 1989).

In the nine counties effected by the earthquake, there were at least 4,800 unstregthened unreinforced masonry bearing wall buildings, of which at least 758 were damaged. Of those damaged, at least 384 were vacated, and of those vacated, at least 54 were demolished. (Celibi 1997).

In Watsonville, five miles from the epicenter, almost 90 percent of the structural damage to buildings was the result of unreinforced masonry building failure and wooden structures not properly bolted to their foundations. (Phillps 1989)

The previously unreinforced masonry buildings that had been retrofitted showed an inconsistent pattern of success. Many structures on Pacific Garden Mall in Santa Cruz had been attractively and expensively rehabilitated but without sufficient strengthening. Twenty of these buildings were demolished due to earthquake damage and total losses in Santa Cruz approached $170 million. (EERI 1989). Figure 2, "Unreinforced Masonry Buildings, Pacific Garden Mall" depicts the devastation to the Pacific Garden Mall in Santa Cruz as a result of insignificant retrofitting.

Engineered Buildings

Reinforced concrete shear-wall buildings performed quite well. One four-story reinforced concrete shear-wall building in Watsonville suffered essentially nonstructural cracking despite experiencing a peak acceleration of 0.6g at ground level and 1.2g at the roof. In the case of steel industrial structures, some instances of damage were reported but most were relatively minor such as cross braces stretching. (EERI 1989).

Steel framed, brick-clad buildings in Oakland and other cities lost portions of the brick work consistent with the relatively flexible frame loading of infill panels. Pounding of adjacent buildings in downtown San Francisco and else-where also resulted in veneers spalling. (EERI 1989).

The vulnerability of structures under construction was confirmed with several cases. In Hollister, a school gymnasium was badly damaged and in Oakland an uncompleted upper high rise steel frame lost several partially connected columns from the top. (EERI 1989).

A consistent pattern of internal ceiling panels dropping were reported including many in the international terminal of the San Francisco airport. This reinforced the need to properly secure suspended ceilings. Similarly, there were several cases of precast-concrete panels falling from high-rise structures. This reminded designers of the importance of connection details. (EERI 1989).

Some of the more interesting aspects of engineered building damage occurred in the area south of the San Francisco airport. Considerable damage to an elevator shaft occurred when a penthouse water tank fell from the top of the Amfac Hotel. Across the road from this, the Hyatt Regency hotel suffered shear cracks in the basement walls and severe vertical spalling of concrete floor slabs in several elevations adjacent to elevator shafts. (EERI 1989).

Newer structures, and those built on firm ground suffered little. Building damage occurred most often to structures with recognized vulnerabilities and those built on soft-soils. (EERI 1989).

Hospitals and Nursing Facilities

Engineered hospital buildings throughout the area performed well in the earthquake and although many did suffer minor system damage, temporary elevator stoppage, and cosmetic damage, there were no operational interruptions. The hospital buildings constructed under the provisions of the Hospital Act, passed in 1972, performed very well with essentially no damage of any kind. However, some hospital buildings constructed prior to the Hospital Act experienced a limited amount of structural damage that required corrective measures. (EERI 1989).

Some of those damages reported included the seven-story tower building constructed 1927 at Peralta Hospital in Oakland which suffered serious damage and was closed. Also due to structural damage, two stories of the Santa Clara Valley Medical Center were evacuated. The fourth floor of the Watsonville Community Hospital, which experienced moderate structural damage, was evacuated due to loss of elevators and exterior windows. The damage estimates to the Palo Alto Veterans Administration facilities were estimated at $30 million and two of its six buildings were evacuated. The entire Stanford Medical Center suffered approximately $4 million in damages. (EERI 1989).

Nursing Facilities, in general, performed fairly well. Typical damage consisted of cracked or spalled plaster and concrete, displaced equipment, occasional broken windows, and ceiling damage. (EERI 1989).

Public School Buildings

There was only minor structural damage to a few public school buildings from the earthquake. Only three schools sustained severe damage. Regulated under the state building code and the Field Act, which establishes procedures for design and construction of public school buildings and was passed following the Long Beach earthquake in 1933, California public school buildings experienced minimal damage and were used as evacuation shelters for victims of the earthquake. (EERI 1989).

The Loma Prieta school buildings in Los Gatos, constructed in the 1950s and 1960s, were built over hidden branches of the San Andreas fault system. At that time there was no legislative mandate for geologic hazard investigations of school sites. Because of this, one classroom wing heaved upward and the other wing suffered large cracks in the walls and sidewalks. (EERI 1989).

Early estimate's set San Francisco's district-wide losses at more than $45 million, a third of them at the districts Van Ness Avenue offices. Oakland's 92 schools fared better with only $1.5 million in damage. (EERI 1989).

An example of the effectiveness of the state school strengthening program, introduced in 1968, is San Francisco's Winfield Scott School in the heart of the Marina District, very close to an apartment building that burned to the ground after the quake. The school suffered only minor cracks in the plaster and some damage to the playground. It was built in 1930 and made earthquake-resistant in the 1970s. Its losses were estimated at less than $100,000. (EERI 1989).

Historic Buildings

Several historic buildings such as churches, cathedrals, department stores, office buildings, and theaters incurred moderate to severe damage in the earthquake. The Paramount Theater in Oakland (completed 1931) and the Geary Theater in San Francisco (complete 1909) were two historic buildings that incurred such moderate to severe damage. A number of architectural finishes were damaged in the San Francisco City Hall (completed 1915) and Oakland City Hall (completed 1914) while a number of churches suffered damage including severe damage to the Stanford University Chapel and St. Partick's Church in Watsonville. (EERI 1989).

Damage to Nonstructural Components and Contents

The importance of damage control relative to nonstructural building elements was shown to be an important consideration. Some large corporations reported up to $50 million in
Figure 3: "Soft Story" Mechanism, Marina District, Credit: U. S. Geological Survey (Nakata 1999)

property damage in nonstructural damages. There was a large amount of damage to gypsum board partitions, glazing, and air-conditioning units, that when taken together,
represented relatively minor damage, but overall, added up to a large sum for area-wide damage. Although glazing and cladding of new structures performed well, signs of
developing distress were visible in many new buildings. (EERI 1989).
Overall, nonstructural damage was difficult to determine because it was largely internal and hidden from the outside. However, observations indicated that most nonstructural damage was the consequence of structural failure. (EERI 1989).

Building Layout and Configuration

Irregularities of plan and elevation, together with irregular location of structural or stiff nonstructural elements contributed to many building failures. This played a crucial part in the collapses that occurred in the Marina District of San Francisco. The first-story collapse of several four-story, wood-frame, corner apartment and condominium buildings featured ground floors pierced by garage doors along two sides and represent characteristic cases of the "soft story" vertical-configuration effect on building performance. (EERI 1989). Figure 3, ""Soft Story" Mechanism, Marina District" shows how the lack of sufficient lateral resistance on the first level around the garage doors resulted in a buckling of the level and overall instability,

Overall Patterns and Lessons Learned

Figure 4: Collapsed Section of Cypress I880, Oakland, Credit: U. S. Geological Survey (Nakata 1999)
Estimates indicated that less than 1% of developed property in the impacted area suffered damage based on estimates of $3.3
billion to residential and commercial property, and $2.3 billion damage to public property. Although damage was wide-spread throughout a six-county area, severe damage in the region varied widely from area to area and seemed primarily concentrated in several distinct pockets. These were districts with heavy concentrations of vulnerable building types and ground conditions such as San Francisco Marina District and south of Market, downtown Oakland and the Cypress Street I-880 overpass area, and Los Gatos and Santa Cruz. (EERI 1989).

The number of totally collapsed structures was very small. In fact, the Cyrpess Street freeway structure was the only total collapse of an engineered structure. This reinforced that in earthquakes, heavily occupied non-ductile reinforced-concrete frame structures represent the greatest threat to life. (EERI 1989). Figure 4, "Collapsed Section of Cypress I-880, Oakland" shows the extent of damage inflicted on the columns of I880 and the pancaking of the highway levels.

Although the number of collapses was very low, the total damage was very high. Even though there was no spectacular damage at the Stanford University Campus, the total damage estimate was $160 million. Of the eight academic buildings closed, all were constructed before seismic codes were adopted. Of the five buildings damaged to the extent that access was limited, four were built before seismic code came into effect. Additionally, serious damage was caused to seven wood framed row houses providing communal dwellings for 160 students. (EERI 1989).

Severe damage to rehabilitated unreinforced masonry buildings emphasized the danger of architectural and interior rehabilitation that did not include seismic strengthening. The need for improved reliability of retrofit procedures was confirmed in this earthquake. (EERI 1989).

Engineered buildings performed quite well in large part due to building code provisions implemented for lessons learned in past earthquakes. Damages to structures under construction as well as damage to nonstructural components reminded designers of the important of temporary bracing details and connection details.

The Loma Prieta earthquake deepened an already existing housing crisis in the Bay Area. Overall, only 40 percent of the housing losses were served through the normal disaster-assistance process. Also, homeowners received proportionally more funding than renters, 62 percent of the funds compared to 40 percent of the damaged units. It was determined that state and Federal government should step in to aid local and county government and individuals in the housing recovery process. It was learned that post-earthquake housing recovery requires pre-earthquake planning. The earthquake taught a valuable lesson in exposing the gaps in housing recovery and provided a road map for thinking about recovery planning. Also, existing recovery programs were in need of streamlining and housing recovery programs were learned to be most effective being administered at the local level. (Celebi 1997).

Compared to earthquakes in recent history, the 1989 Loma Prieta Earthquake was not nearly as severe. The 2010 Haiti earthquake had over 3,500 times the number of deaths while the 1995 Kobe Japan Earthquake had over 16 times the estimated damage.
Earthquake Comparisons
1989 Loma Prieta
$6 Billion
1994 Northridge
$20 Billion
1995 Kobe
$100 Billion
2010 Haiti
$14 Billion
2010 Chile
$8 Billion

The key lessons learned were that unreinforced masonry buildings pose a high threat as well as the importance of seismic rehabilitation to pre-seismic code structures. The number of soft story collapses in residences primarily of the row-house with garage on lower level type lead to a call for seismic rehabilitation and reinforcement laws. The minimal damage to engineered buildings reinforced the importance of seismic building codes and exposed the need to expanded requirements on other building types.


  1. Celebi, Mehmet, Sharon L. Wood, James C. Anderson, Vitelmo V. Bertero, Bret Lizundia, Weimin Dong, William T. Holmes, Robert Reitherman, Mary Comerio, Jeanne B. Perkins, and Ben Chuaqui. (November 14, 1997). "The Lorna Prieta, California, Earthquake of October 17, 1989-Building Structures." U.S. Geological Survey.
    • This paper investigates the performance of various building structures in the 1989 Loma Prieta Earthquake. Two tilt-up buildings, a six story reinforced concrete building, a 42 story steel building, and a summary of unreinforced masonry building damage patterns are evaluated.
  2. Earthquake Engineering Research Institute and National Research Council. (November, 1989). "Loma Prieta Earthquake October 17, 1989, Preliminary Reconnaissance Report." EERI.
    • Summarizing the information gathered during the early stages of investigation, this report details the emergency response, the geoscience and geotechnical aspects, and building investigations related to the earthquake.
  3. J.K. Nakata, C.E. Meyer, H.G. Wilshire, J.C. Tinsley, W.S. Updegrove, D.M. Peterson, S.D. Ellen, R.A. Haugerud, R.J. McLaughlin, G.R. Fisher, and M.F. Diggles. (1999). "The October 17, 1989, Loma Prieta, California, Earthquake—Selected Photographs." U.S. Geological Survey.
    • This report from the USGS, contains annotated photographs of damage around the San Francisco Bay Area taken the day of and the days following the earthquake.
  4. Phipps, Edward J. (1989). "Overview of Fire Service Responses near the Epicenter of the Loma Prieta Earthquake." The Virtual Museum of The City of San Francisco. <http://www.sfmuseum.org/1989/report.html>. (September 28, 2010).
    • This article is a personal account from a retired chief of the San Francisco Fire Department detailing the fire and rescue operations taken after the initial shock of the earthquake.
  5. "San Francisco Earthquake History 1915-1989." (1989). Museum of the City of San Francisco. <http://www.sfmuseum.net/alm/quakes3.html#1989>. (September 28, 2010).
    • A great overview of the day of and the days following the earthquake of October 1989. Induces details of rescue actions as well as government organization actions in the days, weeks, and months following the earthquake.
  6. Stoffer, Philip W. (2005). "The San Andreas Fault In The San Francisco Bay Area, California." U.S. Geological Survey.
    • A very thorough report from the USGS discussing in detail the San Andreas Fault. In addition, Chapter 1 has a great comparison of the Great San Francisco Earthquake of 1906 and the Loma Prieta Earthquake of 1989.
  7. Zoback, Mary Lou. "Putting Down Roots in Earthquake Country: Your Handbook for the San Francisco Bay Area." (2005). U.S. Geological Survey
    • This handbook provides information about the threat posed by Earthquake in the San Francisco Bay region and explains how you can prepare for, survive, and recover.

Additional Resources

  1. "Remembering the 1989 Loma Prieta Earthquake-->" (1989). Santa Cruz Public Libraries. <http://www2.santacruzpl.org/gallery2/v/1989quake/>. (September 28, 2010).
    • Another collection of photographs depicting the damage from the Loma Prieta Earthquake. This set focuses on the damage in Santa Cruz in the aftermath of the earthquake.
  2. Time Magazine (October 30, 1989). "Earthquake." <http://www.time.com/time/magazine/article/0,9171,958883,00.html>. (September 28, 2010).
    • A magazine article published in Time Magazine shortly after the event, details personal accounts of the earthquake as well as a rough time line of events.