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The impact of 1989 Loma Prieta Earthquake -- 20 years later

Seismological Society of America 2009 Annual Meeting

Session: Advances in science, engineering, public policy and hazard mitigation as a result of the 1989 Loma Prieta earthquake.
Location: DeAnza Ballroom 1, Thursday, April 9, 2009, 1:30 p.m.

The Loma Prieta earthquake transformed the earthquake sciences and engineering and remains a major focus of study, some twenty years later. The 17 October 1989 magnitude 6.9 Loma Prieta earthquake severely shook the San Francisco and Monterey Bay regions and initiated major changes in earthquake science and engineering, disaster response and public policy well beyond California.

The 1989 earthquake epicenter was located near Loma Prieta peak in the Santa Cruz Mountains, approximately 14 km (9 mi) northeast of Santa Cruz and 96 km (60 mi) south-southeast of San Francisco, and it has had perhaps the most profound societal impact of any U.S. earthquake.

Direct observations and instrumental recordings of the earthquake and its damaging effects on the region's infrastructure have led to improved understanding in earthquake processes, including earthquake forecasting, fault interaction and how one large earthquake may trigger another, ground motions, site response, liquefaction and building response as well as significant improvements to building codes and design standards for lifelines. Major programs such as the Caltrans bridge seismic retrofit program, San Francisco's Building Occupancy Resumption Program (BORP), and the California Seismic Hazards Mapping Program came about because of the 1989 earthquake.

In this session, speakers from the public and private sectors will address the advances in science, engineering, public policy and hazard mitigation that resulted directly from the Loma Prieta earthquake.

Two talks provide broad overviews of the earthquake's legacies:

• Thomas L. Holzer and Robert L. Wesson, U.S. Geological Survey, describe how Loma Prieta alerted the nation to its earthquake hazard and prompted new earthquake hazard assessments in the central U.S. and southern California, and the continuance of the program in the Pacific Northwest where the potential for major earthquakes was just starting to be recognized. They also describe how the "successful" forecasting of the earthquake prompted intensive efforts to improve long-term earthquake forecasting, and how changes in the rate of earthquakes and creep on the Hayward Fault confirmed that the release of earthquake strain can affect the earthquake potential of nearby faults. Loma Prieta was one of the best recorded earthquakes in U.S. history and these records led to significant changes in the building code, particularly in how local soil conditions are taken into account. Despite the $4 million investment in studying the earthquake, important questions remain unanswered. For example, the Lake Elsman foreshocks that preceded Loma Prieta remain a mystery and illustrate the challenge of recognizing foreshocks before a large earthquake.

• Chris Poland, Degenkolb Engineers, describes the legacy of the earthquake on state and local public policy. The California Seismic Safety Commission prepared an action plan that led to numerous programs and projects including the Hospital Facilities Seismic Safety Act. The California Geologic Survey initiated mapping of areas with potentially serious seismic hazards from landslides and liquefaction. Images of the earthquake prompted many local cities to recognize and to start mitigating the risks that they faced. Communities also committed to post-earthquake inspection of buildings when the need for quick post-earthquake re-occupancy became apparent.

Two talks address current initiatives:

• FIRE! Are fire and water agencies adequately prepared for a major earthquake today? Fire following the 1989 earthquake caused the greatest U.S. earthquake-related loss since 1906. The recent ShakeOut Scenario in Southern California involved over 5 million people and concluded that approximately 1,600 ignitions following a M7.8 earthquake on the San Andreas Fault would destroy about 200 million sq. ft. of building floor area accompanied by hundreds to perhaps a thousand deaths, a loss of $40 to $100 billion dollars and hundreds of thousands homeless. This Invited Speaker will discuss reducing this "fire-following" problem via improvements to protect densely built neighborhoods. He will depart the meeting immediately following his talk (DeAnza Ballroom 1, Thursday April 9, 2pm) in order to address the San Francisco Fire Commission.

• The San Francisco Department of Building Inspection is assessing anew the City's seismic risk – the likely impact of future earthquakes on buildings citywide. Debra Walker, member of the San Francisco Building Inspection Commission, will address pending policy recommendations regarding seismic risk to soft story, wood-frame buildings, which typically have large openings on the ground floor, making the first story weak or "soft" and the building vulnerable to falling over during an earthquake. The recommendations were recently submitted to Mayor Newsom for consideration.

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