The October 17, 1989 Loma Prieta Earthquake
The earthquake caused thousands of landslides along steep slopes, from hills in the epicentral area to atleast as far north as the Pacific Coast just south of San Francisco.
Several residential developments in the Santa Cruz Mountains were badly damaged by these slides.
On Highway 17 two lanes were blocked west of the summit by a large slide. Large fissures opened inroadways throughout the Bay Area due to settlement and/or lurching.
This rupture of a 30-mile-long segment of the San Andreas Fault has not altered the assessment thatthere is a 50% chance for one or more magnitude 7.0 earthquakes in the San Francisco Bay Area in thenext 30 years.
The probability of a repeat of the 1906 magnitude 8.3 earthquake is still significant.
Damage to Buildings
The Loma Prieta Earthquake and its subsequent aftershocks resulted in widespread damage to a varietyof commercial structures. A large geographical area was affected, as is typical for an earthquake of thismagnitude.
In total, building structures experienced damage over an area of approximately 3,000 square miles.
Although damage was widespread, it was also quite sporadic. As would be expected, areas closest to theepicenter experienced the most concentrated damage. Farther away, heavy damage was generally limitedto buildings of very poor construction founded on soft soils that failed or amplified the earthquakeground motions. This is similar to the effects noted in the 1985 Mexico City Earthquake.
Earthquake effects also tended to be highly directional. Most damage occurred within a narrow bandthat extends northwest to southeast, approximately paralleling the San Andreas Fault. Thus manycommunities along the margins of San Francisco Bay escaped serious damage.
Unreinforced Masonry Buildings
As has been observed in past California earthquakes, the most concentrated and severe damage tobuilding structures occurred in unreinforced masonry (URM) bearing-wall buildings.
URM buildings, constructed of wood-frame roof and floor systems supported by thick unreinforced brickwalls, were commonly constructed throughout California until the 1930s, when new building regulationstaking into consideration the need to withstand earthquakes prevented further buildings of this style.
As a result, these older URM buildings are typically found in the crowded central business districts ofolder California cities.
The remote location of the epicentre of this earthquake allowed the San Francisco Bay Area to survivewith relatively few instances of structural collapse. Except for buildings near the epicenter, most casesof severe damage occurred in older buildings with little ability to withstand earthquakes and in areas ofextremely weak soils.
The fact that many inadequate structures in the region experienced little damage indicates that groundmotion in most areas was not severe.
Even so, most businesses experienced at least a week's business interruption and some capital loss. Manybusinesses must relocate to new facilities until their buildings are repaired or replaced.
In future stronger shocks or in earthquakes located closer to the major population centers, much moreextensive damage and commercial loss are likely.
A major and encompassing effect of the Loma Prieta Earthquake is to transportation. Several majorhighways, overpasses, and ground thoroughfares were damaged and rendered useless, some for only ashort time, others for as much as a few years.
Fire Following Earthquake
San Francisco had 22 structural fires and over 500 reported incidents during the seven hours from thetime the earthquake struck until midnight.
During this period over 300 off-duty firemen responded to a general recall, approximately doubling theavailable fire-fighting personnel.
The four Loma Prieta Earthquake Professional Papers, which were published as multiple chapters, comprehensively document the magnitude 6.9 earthquake in California that shook the San Francisco and Monterey Bay regions on October 17, 1989. They contain almost 3000 pages written by 401 investigators of the earthquake. The investigations were funded by a special Congressional appropriation to the U.S. Geological Survey and National Science Foundation after the earthquake to improve understanding of both the complexity of earthquakes and how society can reduce losses in future earthquakes. PDF’s of individual chapters can be downloaded by clicking on the appropriate professional paper below. Paper copies can be purchased at http://store.usgs.gov.
Professional Paper 1550 – Earthquake Occurrence
William H. Bakun and William H. Prescott, editors
Professional Paper 1550 seeks to understand the M6.9 Loma Prieta earthquake itself. It examines how the fault that generated the earthquake ruptured, searches for and evaluates precursors that may have indicated an earthquake was coming, reviews forecasts of the earthquake, and describes the geology of the earthquake area and the crustal forces that affect this geology. Some significant findings were:
- Slip during the earthquake occurred on 35 km of fault at depths ranging from 7 to 20 km. Maximum slip was approximately 2.3 m. The earthquake may not have released all of the strain stored in rocks next to the fault and indicates a potential for another damaging earthquake in the Santa Cruz Mountains in the near future may still exist.
- The earthquake involved a large amount of uplift on a dipping fault plane. Pre-earthquake conventional wisdom was that large earthquakes in the Bay area occurred as horizontal displacements on predominantly vertical faults.
- The fault segment that ruptured approximately coincided with a fault segment identified in 1988 as having a 30% probability of generating a M7 earthquake in the next 30 years. This was one of more than 20 relevant earthquake forecasts made in the 83 years before the earthquake.
- Calculations show that the Loma Prieta earthquake changed stresses on nearby faults in the Bay area. In particular, the earthquake reduced stresses on the Hayward Fault which decreased the frequency of small earthquakes on it.
- Geological and geophysical mapping indicate that, although the San Andreas Fault can be mapped as a through going fault in the epicentral region, the southwest dipping Loma Prieta rupture surface is a separate fault strand and one of several along this part of the San Andreas that may be capable of generating earthquakes.
Professional Paper 1551 - Strong ground motion and ground failure
Thomas L. Holzer, editor
Professional Paper 1551 describes the effects at the land surface caused by the Loma Prieta earthquake. These effects: include the pattern and characteristics of strong ground shaking, liquefaction of both floodplain deposits along the Pajaro and Salinas Rivers in the Monterey Bay region and sandy artificial fills along the margins of San Francisco Bay, landslides in the epicentral region, and increased stream flow. Some significant findings and their impacts were:
- Strong shaking that was amplified by a factor of about two by soft soils caused damage at up to 100 kilometers (60 miles) from the epicenter. Instrumental recordings of this ground shaking have been used to improve how building codes consider site amplification effects from soft soils.
- Liquefaction at 134 locations caused $99.2 million of the total earthquake loss of $5.9 billion. Liquefaction of floodplain deposits and sandy artificial fills was similar in nature to that which occurred in the 1906 San Francisco earthquake and indicated that many areas remain susceptible to liquefaction damage in the San Francisco and Monterey Bay regions.
- Landslides caused $30 million in earthquake losses, damaging at least 200 residences. Many landslides showed evidence of movement in previous earthquakes.
- Recognition of the similarities between liquefaction and landslides in 1906 and 1989 and research in intervening years that established methodologies to map liquefaction and landslide hazards prompted the California legislature to pass in 1990 the Seismic Hazards Mapping Act that required the California Geological Survey to delineate areas potentially susceptible to these hazards and communities to regulate development in these zones.
- The earthquake caused the flow of many streams in the epicentral region to increase. Effects were noted up to 88 km from the epicenter.
- Post-earthquake studies of the Marina District of San Francisco provide one of the most comprehensive case histories of earthquake effects at a specific site. Soft soils beneath the Marina amplified ground shaking to damaging levels and caused liquefaction of sandy artificial fills. Liquefaction required 123 repairs of pipelines in the Municipal Water Supply System, more than three times the number of repairs elsewhere in the system. Approximately 13.6 km of gas-distribution lines were replaced, and more than 20% of the wastewater collection lines were repaired or replaced.
Professional Paper 1552 – Performance of the Built Environment
Thomas L. Holzer, editor
Professional Paper 1552 focuses on the response of buildings, lifelines, highway systems, and earth structures to the earthquake. Losses to these systems totaled approximated $5.9 billion. The earthquake displaced many residents from their homes and severely disrupted transportation systems. Some significant findings were:
- Approximately 16,000 housing units were uninhabitable after the earthquake including 13,000 in the San Francisco Bay region. Another 30,000-35,000 units were moderately damaged in the earthquake. Renters and low-income residents were particularly hard hit.
- Failure of highway systems was the single largest cause of loss of life during the earthquake. Forty-two of the 63 earthquake fatalities died when the Cypress Viaduct in Oakland collapsed. The cost to repair and replace highways damaged by the earthquake was $2 billion, about half of which was to replace the Cypress Viaduct.
- Major bridge failures were the result of antiquated designs and inadequate anticipation of seismic loading.
- Twenty one kilometers (13 mi) of gas-distribution lines had to be replaced in several communities and more than 1,200 leaks and breaks in water mains and service connections had to be excavated and repaired. At least 5 electrical substations were badly damaged, overwhelming the designed redundancy of the electrical system.
- Instruments in 28 buildings recorded their response to earthquake shaking that provided opportunities to understand how different types of buildings responded, the importance of site amplification, and how buildings interact with their foundation when shaken (soil structure interaction).
Professional Paper 1553 – Societal Response
Dennis S. Mileti, editor
Professional Paper 1553 describes how people and organizations responded to the earthquake and how the earthquake impacted people and society. The investigations evaluate the tools available to the research community to measure the nature, extent, and causes of damage and losses. They describe human behavior during and immediately after the earthquake and how citizens participated in emergency response. They review the challenges confronted by police and fire departments and disruptions to transbay transportations systems. And they survey the challenges of post-earthquake recovery. Some significant findings were:
- Loma Prieta provided the first test of ATC-20, the post-earthquake review process that places red, yellow, or green placards on shaken buildings. Its successful application has led to widespread use in other disasters including the September 11, 2001, New York City terrorist incident.
- Most people responded calmly and without panic to the earthquake and acted to get themselves to a safe location.
- Actions by people to help alleviate emergency conditions were proportional to the level of need at the community level.
- Some solutions caused problems of their own. The police perimeter around the Cypress Viaduct isolated businesses from their customers leading to a loss of business and the evacuation of employees from those businesses hindered the movement of supplies to the disaster scene.
- Emergency transbay ferry service was established 6 days after the earthquake, but required constant revision of service contracts and schedules.
- The Loma Prieta earthquake produced minimal long-term disruption to the regional economy. The total economic disruption resulted in maximum losses to the Gross Regional Product of $725 million in 1 month and $2.9 billion in 2 months, but 80% of the loss was recovered during the first 6 months of 1990. Approximately 7,100 workers were laid off.