U.S. Department of the Interior
U.S. Geological Survey
Fact Sheet 2014–3092
September 2014
Just minutes before the start of the
third game of the 1989 World Series in San
Francisco, a magnitude 6.9 earthquake rocked
Northern California from Monterey to San
Francisco. Centered near Loma Prieta peak in
the Santa Cruz Mountains south of San Jose, the
quake killed at least 63 people and hospitalized
another 350. It destroyed a freeway viaduct in
Oakland, dropped a span of the Bay Bridge,
collapsed historic buildings in Santa Cruz and
apartment buildings in the Marina District
in San Francisco, severed communications,
and caused an estimated $6 to $10 billion in
property loss. It was the largest temblor to jolt
the Bay Area since the Great San Francisco
Earthquake of 1906 (magnitude 7.9).
Although the Loma Prieta earthquake
struck on the outskirts of the region, it
exposed the vulnerability of the Bay Area to
future earthquakes—a vulnerability that was
reemphasized on August 24, 2014, when a
magnitude 6.0 earthquake occurred near Napa,
California, about 30 miles north-northeast of
San Francisco. At least 200 people were treated
for quake-related injuries, and initial economic
losses are estimated to be at minimum $362
million. Some future earthquakes will certainly
be larger and closer to the Bay Area’s urban
core than the 1989 and 2014 earthquakes.
Since the Loma Prieta earthquake, many
organizations, including the U.S. Geological
Survey (USGS), have redoubled efforts to
understand earthquake hazards in urban areas
and to apply this new knowledge to reduce
future losses. The most hazardous areas have
been extensively mapped and analyzed, and
the most vulnerable structures have been
retrot or rebuilt. The USGS estimates that
Bay Area agencies and businesses have
invested over $30 billion to retrot or replace
bridges, pipelines, hospitals, municipal
buildings, and other infrastructure to make
them more earthquake resilient and to reduce
the time needed to recover from future
Bay Area earthquakes. Communication of
earthquake-hazard information to the public,
to businesses, and to government agencies has
also been strengthened.
Earthquake Likelihood
Even before the 1989 Loma Prieta
shock, panels of scientists regularly reassessed
the earthquake threat to the San Francisco
Bay Area. They currently assign 2-in-3 odds
that one or more destructive earthquakes
(magnitude 6.7 or larger) will strike the Bay
Area in the next 30 years.
Studies conducted since 1989 have
added much new information for determining
earthquake probabilities. Using information
from airborne laser imagery of the Earth (in
which vegetation was digitally removed),
geologists have rened maps of earthquake
faults. They have uncovered new evidence for
the dates and amounts of slip of prehistoric
earthquakes on the Hayward, San Andreas,
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This house in the mountains north of Santa Cruz, California, was destroyed by the October 17,
1989, Loma Prieta earthquake. Shaking here, close to the epicenter of the magnitude 6.9 shock,
was intense, and the house had inadequate support in its first story.
UNDERSTANDING EARTHQUAKE HAZARDS IN THE SAN FRANCISCO BAY AREA
The 1989 Loma Prieta earthquake
interrupted several decades of seismic
tranquility in the San Francisco Bay
Area. It caused damage throughout
the region and was a wakeup call
to prepare for potentially even more
damaging future quakes. Since 1989,
the work of the U.S. Geological
Survey and many other organizations
has improved the understanding of
the seismic threat in the Bay Area,
promoted awareness of earthquake
hazards, and contributed to more
effective strategies to reduce
earthquake losses. These collective
efforts will help reduce the impact of
future large earthquakes in the Bay
Area.
Progress Toward a Safer Future Since the 1989 Loma Prieta
Earthquake
and other active Bay Area faults and
estimated the amount of movement
on those faults over past millennia.
Using hundreds of continuously
monitored GPS receivers and other
space-based tools, geophysicists
have gained a better picture of
the motions of crustal plates that
cause faults to accumulate stress
and rupture in earthquakes. Bay
Area residents felt many strong
earthquakes in the several decades
before the 1906 San Francisco
earthquake, but the region has been
seismically quiet in the decades
following. Seismologists attribute
this difference to the relief of the
stress in rocks in the Bay Area when
the San Andreas Fault ruptured
in 1906. Scientists recognize that
movement on one fault can trigger
other earthquakes on nearby
faults, leading to clustering of
earthquakes in time: geologic studies
indicate that one such earthquake
cluster struck the Bay Area in the
mid-1700s.
Understanding Ground
Shaking and Mapping
Hazards
Nearly 70 percent of the loss of
life and property damage due to the
Loma Prieta earthquake stemmed
from strong ground shaking, and
community managers and scientists
alike quickly recognized the need
for a better understanding of the
most hazardous parts of the Bay
Area. In response to the Loma
Prieta earthquake, the California
Seismic Hazards Mapping
Act of 1990 was passed by the
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California Legislature to assist cities and
counties in protecting public health and
safety by considering seismic hazards when
making decisions concerning land use and
development. The act established a statewide
urban mapping program to identify areas
potentially prone to violent shaking and
ground failure. In preparing the ofcial
maps of seismic-hazard zones, the California
Geological Survey (CGS) incorporates the
latest information on ground properties,
faults, and earthquake potential in the state,
compiled from studies by the USGS and
many other groups.
A related series of maps produced by the
USGS and CGS depict geographic variations
in the likely maximum severity of shaking to
be experienced for different time intervals.
Building response to earthquake shaking
depends on the rates of ground oscillation
from earthquake waves and building height,
and the maps include shaking projections for
a range of ground oscillation frequencies.
Statewide earthquake probability
maps prepared by the USGS, CGS, and
the Southern California Earthquake Center
are used by the California Earthquake
Authority to estimate potential damages
and set premiums for residential earthquake
insurance.
Recordings of ground motion collected
during the Loma Prieta earthquake showed
that ground shaking was greater on the soft
soils around the margin of the bay than
on bedrock farther inland. The Bay Area
earthquake hazard maps are part of the USGS
National Seismic Hazard Maps, which are the
basis for signicant changes in provisions of
the forthcoming 2018 international building
code and the national highway-bridge
code. Because earthquake-resistant design
and construction are essential to reducing
earthquake losses, these code revisions are a
major step toward greater earthquake safety.
In this trench dug
across a fault,
U.S. Geological
Survey scientists
identified soil
layers disrupted
by great
earthquakes.
The part of the Cypress freeway structure in Oakland, California, that stood on soft mud (dashed red line) collapsed in the 1989 Loma Prieta
earthquake, killing 42 people. Adjacent parts of the structure (solid red) that were built on firmer ground remained standing. Seismograms (right)
show that the shaking was especially severe in the soft mud. (USGS photograph by H.G. Wilshire)
The San Francisco Bay Area lies on the boundary between
two of the major tectonic plates that make up the Earth’s
outer shell. The continuous motion of the two plates is
monitored by geophysicists using the satellite-based Global
Positioning System (GPS). Arrows on this map depict recent
(mid-1990s to present) rates of movement by reference
markers anchored in rock or deep in firm ground. Relative
to the interior of the North American Plate (upper right part
of map), the Bay Area and the Pacific Plate are moving
northwest about 1–2 inches per year. This motion of the
plates strains the crustal rocks of the Bay Area, storing
energy that eventually will be released in earthquakes.
Ground Failure
Ground failure—rock falls, landslides,
and liquefaction—can locally be more
damaging during an earthquake than shaking
alone. About 2 percent of the total earthquake-
related losses during Loma Prieta were caused
by ground failure. Rocks may fall from
cliffs, steep slopes may slide, earth may ow
downslope, and even at ground may crack
and tilt. Landslides of all shapes and sizes
can block roads and damage buildings and
neighborhoods.
When shaken strongly, unconsolidated
sandy deposits that are saturated with water
can liquefy and form a slurry. Liqueed
sands cannot resist owing downslope even
on nearly at ground, and cannot support
the weight of man-made structures. As a
result, liquefaction may result in sinking, tilt,
distortion, or destruction of buildings and
bridges, rupture of underground gas lines and
water mains, and cracking and lateral spreading
of the ground surface. During the Loma Prieta
earthquake, loosely compacted sandy deposits
and articial lls liqueed at many locations
around the margins of San Francisco and
Monterey Bays and in adjacent riverbeds. Had
the quake been larger or closer to the heavily
developed margin of San Francisco Bay, the
damage from liquefaction would have been far
greater. Since 1989, the USGS has partnered
with Federal Emergency Management Agency,
Pacic Gas and Electric Company, the San
Francisco Public Utility Commission, the City
of Oakland, and other agencies to map areas
where damaging liquefaction can occur.
Near-Real-Time Earthquake
Information
After the Loma Prieta earthquake,
managers of earthquake-monitoring networks
in California agreed to combine their data
in real time, thereby creating the California
Integrated Seismic Network (CISN). The CISN
reports within minutes earthquake locations,
magnitudes, and ShakeMaps, which show the
patterns of shaking across the region, helping
community leaders organize emergency crews
and relief efforts. ShakeMaps are particularly
valuable in the Bay Area, where the wide
variety of geologic materials—from hard
bedrock to soft clay—causes large differences
in shaking intensity. The USGS also maps
the levels of shaking in different parts of the
Bay Area as reported by online respondents
through the “Did you feel it?” Web site and
can therefore assess the local intensity of an
earthquake independently of ShakeMap. Both
maps help emergency responders to rapidly
identify locations where damage and need are
likely to be greatest.
In 2005, the USGS partnered
with the California Department of
Transportation (DOT) to produce
ShakeCast, an application that uses
ShakeMaps and DOT-supplied
estimates of the likelihood that a
level of shaking will cause damage
to a particular structure to prioritize
DOT’s inspection of bridges
following earthquakes. Many
other agencies also use ShakeCast
to prioritize inspections of their
facilities after earthquakes.
In 2010, the USGS released
PAGER (Prompt Assessment of
Global Earthquakes for Response),
an alert that rapidly estimates
fatalities and economic losses
for earthquakes. To make these
loss projections, PAGER uses
ShakeMaps of the earthquake,
global census data, and estimates
of the likelihood that the building
stock in the impacted region can
withstand strong shaking (http://
pubs.usgs.gov/fs/2010/3036/).
PAGER assigns a color code
to each potentially damaging
earthquake to indicate the level of
emergency response the earthquake
will require.
Signicant strides have
been made in making earthquake
information available faster.
Digital seismic instruments now used in the
networks report their data within a second,
reducing the time for earthquakes to be detected.
In fact, advances in technology now permit
the computation of earthquake locations and
magnitudes within seconds so that notications
can be broadcast to some areas that have not yet
undergone shaking from an earthquake—this
forms the basis for the Earthquake Early Warning
System, which is being developed by the USGS,
the University of California at Berkeley, the
California Institute of Technology (Caltech), and
the University of Washington. It will send alerts
to the public and community managers ahead of
strong shaking, so that a variety of actions can be
taken, such as opening rehouse doors, stopping
trains, and taking cover.
The Marina District of San Francisco was heavily damaged in the 1989 Loma Prieta earthquake (left)
because it was built on uncompacted, sandy ground in an area with a shallow water table. These
conditions caused shaking to be amplified and some areas of ground to liquefy. Shaking collapsed
the first story of many apartment buildings and liquefied the ground beneath the sidewalk, causing it
to buckle. In the weeks following the quake, the U.S. Geological Survey drill rig shown at right was
used to gather subsurface samples so that the causes of liquefaction could be better understood.
(USGS photographs by J.K. Nakata and T. Holzer.)
Example ShakeMap for the Loma Prieta earthquake. This
ShakeMap used data recorded in 1989, but was produced
years after the earthquake. Such ShakeMaps are now
routinely produced within several minutes of a felt
earthquake. Severity of shaking is rated using the Modified
Mercalli Intensity (MMI) scale, which ranges from 1 (not felt)
to 12 (total destruction).
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Earthquake Scenarios and
Preparedness
Although scientists cannot predict exactly
when destructive earthquakes will occur, they
can estimate the damaging effects of a potential
earthquake of a given size and, together with
engineers, assess the expected property damage
and loss of life. Emergency-response managers,
government agencies, corporate planners, and
private citizens use such assessments to reduce
the risk of losses and to plan for response and
recovery after a large quake. USGS scientists
are working with numerous agencies and
organizations to estimate the possible impacts of
future earthquakes at both regional and national
scales, including scenarios for response exercises
for an earthquake on the Hayward Fault and a
repeat of the 1906 San Francisco Earthquake.
In 2009, the USGS founded the Bay
Area Earthquake Alliance, a public-private
partnership that promotes earthquake awareness
and mitigation in the region, and which annually
cosponsors the Great California ShakeOut
earthquake preparedness exercise held on the
third Thursday of October. Nearly 10 million
Californians participate in this annual exercise.
The regional infrastructure’s poor
performance during the Loma Prieta
earthquake, coupled with USGS earthquake
hazard models, has led several municipalities
to require the mandatory retrot of collapse-
prone unreinforced masonry buildings and of
“soft-story” buildings. So called because of
inadequate support in their rst story, collapses
of soft-story buildings were prominent in San
Francisco’s Marina District during the Loma
Prieta earthquake.
The USGS has also partnered with local
agencies to assess the dangers to the utilities
and transportation corridors around the San
Francisco Bay Area. With the San Francisco
Public Utility Commission, the USGS mapped
the precise location of the San Andreas Fault so
that the retrot of the Hetch Hetchy Aqueduct
and water conveyance systems on the San
Francisco Peninsula were more resilient. The
USGS and Bay Area Rapid Transit (BART),
which operates a public rail transit system,
estimated the amount of slip that would likely
be produced by an earthquake on the Hayward
Fault and how it would affect BART’s tunnels
crossing the fault.
Communicating Earthquake Hazards
Comprehensive studies of the Loma Prieta
earthquake and its impacts were collected in a
set of four USGS Professional Papers that are
available online at http://earthquake.usgs.gov/
regional/nca/1989/papers.php. These reports
describe the earthquake, the ground shaking
and ground failures that it produced, the
performance of buildings and other man-made
structures, and the societal response to the
earthquake.
Since the 1989 Loma Prieta earthquake,
the USGS has increased its efforts to better
communicate earthquake-hazard information
to a broad audience. USGS scientists regularly
participate in media events, conferences, and
earthquake-preparedness fairs. The USGS and
several cooperators, with nancial support from
the California Earthquake Authority, American
Red Cross, and Pacic Gas and Electric
Company, produced a popular educational
booklet called “Putting Down Roots in
Earthquake Country.” The booklet has been
translated into Spanish, Chinese, Vietnamese,
and Korean in editions entitled “Protecting
Your Family from Earthquakes.” All versions
are available online at http://earthquake.usgs.
gov/regional/nca/prepare/index.php. These
booklets have been emulated in other highly
active seismic regions in the country, including
Anchorage, Alaska, Salt Lake City, and the
New Madrid region of southeastern Missouri
and northwestern Tennessee.
As demonstrated by the South Napa
earthquake of August 24th, 2014, earthquakes
remain an ongoing threat. Fortunately, the
impact of large future quakes can be reduced
by advances in science and engineering,
improved construction practices, smart land
use zoning, and better emergency response
preparation.
Since the 1989 Loma Prieta earthquake, many iconic structures around the Bay Area—including San Francisco City Hall, the Ferry Building, the Golden Gate
Bridge, San Francisco General Hospital (above right), and the eastern span of the Bay Bridge (above left; disassembly shown in front banner)—have been
replaced or retrofit with earthquake-resistant support so that they may remain intact in the event of a large earthquake. (USGS photographs by S. Haefner.)
Printed on recycled paper
ISSN 2327–6916 (print)
ISSN 2327–6932 (online)
http://dx.doi.org/10.3133/fs20143092
Thomas M. Brocher, Robert A. Page,
Peter H. Stauffer, and James W. Hendley II
Edited by Claire M. Landowski
Graphic design by Vivian Nguyen
and Jeanne S. DiLeo
Many of the efforts highlighted here are part of the
USGS Earthquake Hazards Program—a key element
of the National Earthquake Hazards Reduction
Program.
COOPERATING ORGANIZATIONS
American Red Cross, Association of Bay Area
Governments, Bay Area Rapid Transit, Bay Area
Earthquake Alliance, California Department of
Transportation, California Department of Water
Resources, California Earthquake Association,
California Geological Survey, California Governor’s
Office of Emergency Services, California Integrated
Seismic Network, City of Oakland Office of
Emergency Services, East Bay Municipal District,
Earthquake Engineering Research Institute,
Federal Emergency Management Agency, Pacific
Gas and Electric Company, Pacific Earthquake
Engineering Research Center, San Francisco
Public Utilities Commission, San Francisco State
University, Southern California Earthquake Center,
SPUR, Stanford University, Structural Engineers
Association of Northern California, University of
California at Berkeley, University of California at
Santa Cruz, URS Greiner Woodward Clyde Federal
Services, Lettis Consultants International, Inc., and
many other institutions, organizations, and firms.
For more information contact: 1-888-ASK-USGS
(1-888-275-8747)
http://earthquake.usgs.gov/
http://ask.usgs.gov
https://www.facebook.com/USGeologicalSurvey
https://twitter.com/USGS
