Progress Toward a Safer Future since the 1989 Loma Prieta Earthquake

A USGS fact sheet from September 2014; work it describes as planned or under way is as of then.

The 1989 Loma Prieta earthquake broke several decades of seismic calm in the San Francisco Bay Area, caused damage across the region and was a wake-up call to prepare for worse. Since then the U.S. Geological Survey and many other organisations have improved understanding of the Bay Area's earthquake threat, raised awareness of it and helped develop better ways to reduce losses.

A house in the mountains north of Santa Cruz, destroyed on October 17, 1989. Close to the epicentre, shaking was intense, and the house had too little support in its first story.

October 17, 1989

Minutes before game three of the 1989 World Series in San Francisco, a magnitude 6.9 earthquake shook Northern California from Monterey to San Francisco. Centred near Loma Prieta peak in the Santa Cruz Mountains south of San Jose, it killed at least 63 people and put 350 more in hospital. It destroyed a freeway viaduct in Oakland, dropped a span of the Bay Bridge, collapsed historic buildings in Santa Cruz and apartment buildings in San Francisco's Marina District, cut communications, and caused an estimated $6 to $10 billion in property losses — the biggest Bay Area quake since the Great San Francisco Earthquake of 1906 (magnitude 7.9).

It struck on the edge of the region, yet it exposed the Bay Area's vulnerability — underlined again on August 24, 2014, when a magnitude 6.0 quake near Napa, about 30 miles north-northeast of San Francisco, injured at least 200 people and caused initial losses estimated at no less than $362 million. Some future earthquakes will certainly be bigger and closer to the urban core.

Since 1989 the most hazardous areas have been mapped and analysed and the most vulnerable structures retrofitted or rebuilt. The USGS estimates that Bay Area agencies and businesses have invested more than $30 billion to retrofit or replace bridges, pipelines, hospitals, public buildings and other infrastructure, making them more resilient and shortening recovery after future quakes.

How likely is the next one?

Panels of scientists regularly reassess the threat, and currently put the odds at 2 in 3 that one or more destructive earthquakes (magnitude 6.7 or larger) will strike the Bay Area in the next 30 years.

Research since 1989 has sharpened those estimates. Airborne laser imagery with vegetation digitally removed has refined fault maps; trenches across faults have dated prehistoric earthquakes and measured their slip on the Hayward, San Andreas and other faults; and hundreds of continuously monitored GPS receivers show how the plates move and load the faults with stress.

The Bay Area sits on the boundary between the Pacific and North American Plates. Relative to the interior of the North American Plate, the Bay Area and the Pacific Plate move northwest about 1–2 inches a year, straining the crust and storing energy that earthquakes will release.

Bay Area residents felt many strong earthquakes in the decades before 1906 and few in the decades after — which seismologists attribute to the San Andreas rupture of 1906 relieving stress in the region's rocks. Movement on one fault can trigger quakes on others nearby, so earthquakes cluster in time; geology shows one such cluster struck the Bay Area in the mid-1700s.

Shaking and hazard maps

Nearly 70 percent of Loma Prieta's deaths and damage came from strong shaking. Recordings showed shaking was stronger on the soft soils around the bay than on bedrock inland.

The part of Oakland's Cypress freeway that stood on soft mud collapsed, killing 42 people; neighbouring sections on firmer ground stayed up. Seismograms show how much stronger the shaking was in the mud. USGS photograph by H.G. Wilshire.

• The California Seismic Hazards Mapping Act of 1990, passed in response, set up statewide mapping of areas prone to violent shaking and ground failure, so cities and counties could weigh those hazards in land-use decisions. The California Geological Survey prepares the official maps from studies by the USGS and others.
• USGS and California Geological Survey maps show the likely maximum shaking over different time spans, across a range of ground-oscillation frequencies — since a building's response depends on how fast the ground oscillates and on the building's height.
• Statewide probability maps by the USGS, the California Geological Survey and the Southern California Earthquake Center help the California Earthquake Authority estimate damage and set residential earthquake insurance premiums.
• The Bay Area maps are part of the USGS National Seismic Hazard Maps, the basis for major changes to the then-forthcoming 2018 international building code and the national highway-bridge code.

Ground failure

Rockfalls, landslides and liquefaction can do more local damage than shaking alone, though they caused only about 2 percent of Loma Prieta's losses. Strongly shaken, loose water-saturated sand can turn to slurry that flows even on nearly flat ground and cannot hold up structures — tilting and sinking buildings and bridges, breaking gas and water mains, and cracking and spreading the ground. In 1989 loose sands and artificial fill liquefied at many places around San Francisco and Monterey Bays and along nearby riverbeds. A larger or closer quake would have done far more. Since then the USGS has worked with FEMA, Pacific Gas and Electric, the San Francisco Public Utilities Commission, the City of Oakland and others to map where damaging liquefaction can occur.

The Marina District was badly damaged because it was built on uncompacted, sandy ground with a shallow water table, which amplified shaking and let some ground liquefy. In the following weeks a USGS drill rig sampled the subsurface to understand why. USGS photographs by J.K. Nakata and T. Holzer.

Information in minutes — and seconds

• California Integrated Seismic Network. After Loma Prieta, California's monitoring networks agreed to pool their data in real time. Within minutes the network reports locations, magnitudes and ShakeMaps of shaking across the region — especially useful in the Bay Area, where ground from hard bedrock to soft clay makes shaking vary widely. The public's reports on the USGS Did You Feel It? website give a second, independent measure.
• ShakeCast (2005, with the California Department of Transportation) combines ShakeMaps with estimates of how likely each structure is to be damaged, to set the order of bridge inspections; many other agencies use it too.
• PAGER (2010) rapidly estimates deaths and economic losses anywhere in the world from ShakeMaps, census data and building vulnerability, and assigns each damaging quake a colour code for the response it needs.
• Early warning. Digital instruments report within a second, and locations and magnitudes can be computed in seconds — fast enough to alert places the shaking has not yet reached. That is the basis of the Earthquake Early Warning System the USGS was developing with UC Berkeley, Caltech and the University of Washington, so that people can open firehouse doors, stop trains and take cover.

A ShakeMap of Loma Prieta, made years later from 1989 recordings. Such maps now come out within minutes. Shaking is rated on the Modified Mercalli Intensity scale, from 1 (not felt) to 12 (total destruction).

Scenarios, preparedness and retrofits

Scientists cannot say when a destructive earthquake will strike, but with engineers they can estimate what a quake of a given size would do. The USGS works with many agencies on such scenarios, including response exercises for a Hayward Fault quake and a repeat of 1906.

• In 2009 the USGS founded the Bay Area Earthquake Alliance, a public-private partnership that co-sponsors the yearly Great California ShakeOut drill on the third Thursday of October; nearly 10 million Californians take part.
• Loma Prieta's lessons, with USGS hazard models, led several cities to require retrofits of collapse-prone unreinforced masonry and "soft-story" buildings — named for their weak first stories, whose collapse was so visible in the Marina District.
• With the San Francisco Public Utilities Commission the USGS mapped the San Andreas Fault precisely so the Hetch Hetchy Aqueduct and Peninsula water lines could be retrofitted; with BART it estimated how much a Hayward Fault quake would offset the rail system's tunnels across the fault.

Many Bay Area landmarks — San Francisco City Hall, the Ferry Building, the Golden Gate Bridge, San Francisco General Hospital (right) and the Bay Bridge's eastern span (left) — have been replaced or retrofitted to withstand a large quake. USGS photographs by S. Haefner.

Telling people

Four USGS Professional Papers document Loma Prieta — the quake, its shaking and ground failures, how structures performed and how society responded. USGS scientists take part in media events, conferences and preparedness fairs. With the California Earthquake Authority, the American Red Cross and Pacific Gas and Electric, the USGS and partners produced the booklet Putting Down Roots in Earthquake Country, also in Spanish, Chinese, Vietnamese and Korean as Protecting Your Family from Earthquakes, and copied in Anchorage, Salt Lake City and the New Madrid region.

As the South Napa quake showed, the threat remains. Better science and engineering, stronger construction, smart land-use zoning and better emergency preparation can reduce what the next large quake does.

Sources

• Brocher, T.M., Page, R.A., Stauffer, P.H., and Hendley, J.W., II, 2014, Progress toward a safer future since the 1989 Loma Prieta earthquake: U.S. Geological Survey Fact Sheet 2014–3092. https://pubs.usgs.gov/publication/fs20143092 — the maps and USGS photographs come from its PDF; rewritten in hubnx's own words.