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UNDERSTANDING EARTHQUAKE HAZARDS IN THE BAY REGION Progress Toward a Safer Future Since the 1989 Loma Prieta Earthquake he 1989 Loma Prieta earth- T quake ended decades of tranquillity in the region. It was a wakeup call to prepare for the potentially even more devastating shocks that are inevitable in the future. Since 1989, the work of the U.S. Geological Survey (USGS) and other organiza- tions has improved understanding of the seismic threat in the Bay region, promoted awareness of earthquake hazards, and contributed to more effective strategies to reduce earthquake losses. These efforts will help reduce the impact of future large quakes in the San Francisco This house in the mountains north of Santa Cruz, , was destroyed by the October 17, 1989, Loma Prieta earthquake. Shaking was very intense here, close to the epicenter of the magnitude 6.9 shock. However, Bay region. heavily urbanized San Francisco and Oakland, more than 50 miles to the north, suffered more than 70% of the 63 deaths and the $6 billion to $10 billion in damage caused by the quake.

Minutes before the scheduled start of the killed 63 people and caused an estimated $6 Earthquake probabilities third game of the 1989 in San billion to $10 billion in property loss. It was The year after the 1989 Loma Prieta shock, Francisco, a magnitude 6.9 earthquake rocked the first large temblor to jolt the burgeoning a panel of scientists reassessed the earthquake the California coast from Monterey to San urban region since the Great San Francisco threat to the San Francisco Bay region. Their Francisco. Centered near Loma Prieta peak in Earthquake of 1906 (magnitude 7.8). Although conclusions were published in the USGS report the mountains south of San Jose, the quake the Loma Prieta earthquake struck on the pe- “Probabilities of Large Earthquakes in the San riphery of the region—more than 50 miles Francisco Bay Region, California.” They pro- AND SURROUNDING REGION from San Francisco and Oakland—it exposed jected 2-in-3 odds for one or more destructive the vulnerability of the region to enormous earthquakes (magnitude 7 or larger) to strike the

101 losses in future quakes, some of which will be Bay region in the period 1990 to 2020. Santa Sacramento larger and closer to its urban core. Studies made since that 1990 report have Rosa 80 Since the Loma Prieta shock, the U.S. Geo- added much new information for determining logical Survey (USGS) and cooperating organi- earthquake probabilities. Geologists have uncov- zations have intensified efforts to understand ered new evidence for the dates and amounts of MAP LOCATION earthquake hazards and apply this new knowl- slip of prehistoric earthquakes on the Hayward, edge to reducing future losses. Greater atten- San Andreas, and other active Bay region faults Bolinas Oakland CALIF tion has been focused on the Nation’s urban re- and for the amounts of movement on those faults San Francisco gions threatened by strong earthquakes, over past millennia. Seismologists have reas- because these areas have the most people at sessed the early earthquake history of the region. risk, the largest inventory of structures, and the Many strong earthquakes occurred in the several PACIFIC 280 San Jose densest and most complex infrastructure. Com- decades before the 1906 San Francisco earth- OCEAN munication of earthquake-hazard information quake, whereas in the decades since then, the re- Loma Prieta to the public and to those in business and gov- gion has been rather quiet seismically. Seismolo- earthquake epicenter ernment responsible for decisions and actions gists attribute this to the 1906 quake temporarily N Santa Cruz has also been strengthened. The following relieving most of the stress in crustal rocks in the 0 30 MILES paragraphs highlight advances and accomplish- Bay region. Using satellite-based techniques, ments of these efforts in the San Francisco Bay geophysicists have gained a better picture of the 0 30 KILOMETERS Monterey region since the 1989 earthquake. continuous motions of crustal plates that cause

U.S. Department of the Interior USGS Fact Sheet-151-99 U.S. Geological Survey 1999 237˚ 238˚ 239˚ The San Francisco Bay region lies on the boundary zone between two of the R O D major tectonic plates that G NORTH E R make up the Earth’s outer SAN S C shell. The continuous motion R AMERICAN E E between the Pacific and K PLATE North American Plates, distributed across this zone, is monitored by geophysi- A N cists using the satellite- 38˚ D R E A based Global Positioning S System (GPS). Arrows on this map depict recent (mid HAYWARD to late 1990’s) rates of movement, relative to the interior of the North American Plate, of reference markers anchored in rock or deep in solid ground. This relentless CALAVERAS motion of the plates strains PACIFIC the crustal rocks of the Bay PLATE region, storing energy that In this deep trench dug across the San Andreas Fault 37˚ SAN eventually will be released near Loma Prieta peak, California, U.S. Geological in earthquakes. During the Survey scientists identified soil layers disrupted by the FAULTS time represented in this last great earthquake before the 1906 San Francisco MAJOR FAULTS diagram, most of the faults earthquake (magnitude 7.8). Radiocarbon dating of A N D in the Bay region have been plant remains from the layers in the trench indicates 0 2 inches/year R E A MOTION S “locked,” not producing that the earlier quake occurred about A.D. 1600, before 0 5 centimeters/year earthquakes. written historical records were kept in California.

0 30 miles building of structures that can resist earthquakes. cordings of damaging levels of shaking on a 0 30 kilometers In 1996, the USGS and the California Division wide variety of geologic materials, including of Mines and Geology (CDMG) produced a set soft, unconsolidated sand and clay. These faults to accumulate stress and rupture in earth- of State-wide shaking-hazard maps based on cur- records clearly document that ground shaking is quakes. rent knowledge of more than 200 active faults much more violent on the soft sediments around These data are incorporated in a new USGS and of the historical earthquake record. These the Bay margins than on bedrock, confirming report that estimates the locations, sizes, and maps depict geographic variations in the likely previous USGS projections. Most importantly, probabilities of damaging Bay region earth- maximum severity of shaking to be experienced these records provided a firm basis for revising quakes during the next 30 years (2000 to 2030). within a 50-year period. Maps for various fre- building codes to more fully reflect the need for This new assessment will help business, govern- quencies of ground vibration are included because extra strength in structures built on soft ground. ment, and the public assign priorities for various frequencies of shaking affect buildings of This improved understanding of the shaking haz- strengthening weak structures and for other mea- different heights differently. These maps are a prin- ard on soft ground has led to significant changes sures to reduce earthquake impacts. cipal foundation element for the seismic provi- in provisions of the forthcoming national build- sions of a new national building code to become ing code and to recommended changes to the na- Ground shaking and building codes available in 2000. In parts of the San Francisco Bay tional highway-bridge code. Because earth- Most loss of life and damage to property region, requirements under the new code will dif- quake-resistant design and construction are during an earthquake stems from strong ground fer significantly from those under earlier codes. essential to reducing earthquake losses, these shaking. Hence, accurate assessment of the shak- During the Loma Prieta earthquake, the code revisions are a major step toward greater ing hazard is fundamental to the design and the USGS and CDMG obtained the first set of re- earthquake safety.

The part of the Cypress freeway structure in Oakland, California, that stood on soft mud (dashed red line) TO BERKELEY collapsed in the 1989 Loma Prieta earthquake, killing 42 0 3 MILES BEDROCK people. Adjacent parts of the structure (solid red) that were built on firmer ground remained standing. 0 3 KILOMETERS 80 TREASURE Seismograms (upper right) show that the shaking was ISLAND SOFT MUD

especially severe in the soft mud. (Photo, Lloyd S. Cluff.) 010 GE RID SECONDS Y B 580 BA CYPRESS STRUCTURE YERBA BUENA SAND AND GRAVEL 80 ISLAND

TO 880 N SAN FRANCISCO OAKLAND

SAN OAKLAND FRANCISCO P AREA OF MAP A C IF IC EARTHQUAKE O C EPICENTER E AN Ground failure Another earthquake hazard, which locally can be more damaging than shaking alone, is failure of the ground. Rocks may fall from cliffs, steep slopes may slide, earth may flow downslope, and even flat ground may crack and tilt. When shaken strongly, unconsolidated sandy deposits that are saturated with water can liquefy and form a slurry. This process is called “lique- faction.” Slurries have little ability to support the weight of man-made structures or to resist flow- ing downslope, even on nearly flat ground. Liq- uefaction may result in sinking, tilt, distortion, or The Marina District was the destruction of buildings and bridges, rupture of part of San Francisco most underground gaslines and water mains, and crack- heavily damaged in the 1989 Loma Prieta earthquake because it was built on uncompacted, sandy ground in an area with a shallow ing and spreading of the ground surface. water table. These conditions caused shaking to be amplified and some areas of ground to “liquefy.” Along In the Loma Prieta earthquake, loosely com- this street in the District, shaking collapsed the first story of an apartment house and liquefied the ground pacted geologic deposits and artificial fills lique- beneath the sidewalk, causing it to buckle. Inset photo shows a “sand volcano” created when liquefied sand fied at many locations around the margins of San “erupted” to the surface. In the weeks following the quake, the U.S. Geological Survey drill rig shown at right Francisco and Monterey Bays and in adjacent river was used to gather subsurface samples so that the causes of liquefaction could be better understood. beds. Had the quake been larger or nearer to the heavily developed margin of San Francisco Bay, be a national standard. It is available to all levels These advances have stimulated State and local the damage from liquefaction would have been far of government (Federal, State, and local) and the government initiatives to identify local hazards greater. The USGS and the Pacific Gas and Elec- private sector for estimating potential losses and and require that they be taken into account in de- tric Company (PG&E) initiated a cooperative re- promoting actions to reduce future risk to people cisions about land use and development. search and development agreement in 1997, in part and property. In its initial development, HAZUS An example is the California Seismic Haz- because liquefaction can threaten gas pipelines. has focused on earthquake hazards and risks. ards Mapping Act of 1990, passed by the Cali- As part of this effort, a sophisticated mechanized USGS scientists contributed critical data and fornia Legislature in the wake of the Loma Prieta earth probe is being used to determine whether technical guidance to the development and test- earthquake. Intended to assist cities and counties young sand deposits ringing the Bay may liquefy ing of the software package. in protecting public health and safety, the Act es- during strong shaking. The results of these field tablished a State-wide mapping program to iden- tests will be used to prepare a new, detailed set of Seismic-hazards mapping tify areas subject to violent shaking and ground liquefaction-hazard maps for the Bay region. Since the late 1960’s, the USGS and its failure. The scientific foundation for the Act came many cooperators have pioneered innovative from knowledge and understanding developed by Earthquake scenarios methods for evaluating, mapping, and quantify- the USGS, CDMG, and cooperating research insti- Although scientists cannot predict the time ing geologic hazards related to earthquakes. tutions and consulting firms. In preparing the official of destructive earthquakes, they can project the effects of a postulated temblor and, together with SHAKING INTENSITY MAP AVAILABLE ON THE WEB engineers, the expected property damage and loss of life. Emergency-response managers, gov- ernment agencies, corporate planners, and pri- vate citizens can use such projections to reduce risk of losses and to facilitate response and re- covery after a large quake. USGS scientists are working with numerous agencies and organiza- tions to estimate the impact of future earthquakes at both regional and national scales. In 1995, the USGS and the Association of Bay Area Governments (ABAG) collaborated to pro- duce maps of ground-shaking intensity for sev- eral possible earthquake scenarios. The maps depict how shaking intensity depends on distance from the earthquake source and on local ground conditions. Maps of individual cities are accessible on the Internet (http://www.abag.ca.gov/bayarea/eqmaps/). These maps show the estimated shaking intensities superimposed on street networks that allow resi- dents to locate their neighborhoods and workplaces. On a national scale, in 1997, the Federal Emer- gency Management Agency (FEMA) released HAZUS, a geographically oriented, PC-based soft- Residents of the San Francisco Bay region can view maps of expected ground-shaking intensity for their communities for ware package that estimates losses from natural haz- several possible large earthquakes. These maps, produced by the Association of Bay Area Governments (ABAG) and the ards. Developed under contract by the National In- U.S. Geological Survey, are available online at http://www.abag.ca.gov/bayarea/eqmaps/. This map of north and central stitute for Building Sciences, HAZUS is intended to San Jose is for a magnitude 7.0 shock on the southern segment of the Hayward Fault (northeast is at top of map). maps of seismic-hazard zones, CDMG incorporates ance sector of earthquake threats, the USGS and the latest information on ground properties, faults, IBHS have convened annual workshops on earth- and earthquake potential in the State. quake and insurance issues and have prepared a monograph on these topics. Earthquake information The USGS is also taking other steps to bring Modern computer technology allows wide earthquake information to the public. A 1990 access to authoritative information about an USGS video, “When the Bay Area Quakes,” vividly earthquake immediately after its occurrence. explains the cause and effects of the Loma Prieta Computers at the USGS and the University of shock. USGS Circular 1130, “Look Before You California at Berkeley (UCB) have for some Build,” published in 1995, shows how geologic years automatically posted information for north- information can contribute to safer land develop- ern California shocks on the World Wide Web. ment in quake-prone areas. A series of 13 USGS In 1996, when a magnitude 4.7 shock Fact Sheets, published from 1995 to 1997 under struck the San Francisco Bay region, the great the overall title “Reducing Earthquake Losses demand for information overwhelmed the ca- Throughout the United States,” explains earth- pacity of the Web site. The site has been con- quake hazards in nontechnical language. In addi- tinually upgraded, and its capacity expanded. tion to posting information on quake activity on the The U.S. Geological Survey, as part of a public-private Currently, a location map is usually posted on partnership, operates a network of seismic instru- Web, the USGS routinely provides such data to the site within 1 minute of a widely felt earth- ments at many sites in Oakland, California. The news organizations. Maps based on these data ap- quake, and the magnitude within 5 minutes instruments, such as this one (see inset) in Fire Station pear regularly in the media. In other efforts to di- (http://quake.usgs.gov/recenteqs/). When a 20, measure and report levels of ground shaking. This rectly disseminate information and answer the magnitude 5.0 quake occurred beneath Bolinas network will help guide emergency response to areas public’s questions about earthquakes, USGS scien- most in need in future quakes. in mid-August 1999, more than 30,000 users tists participate in media events, conferences, and were able to access this Web site in the first ¥ Probing natural and filled ground at the Port of quake-preparedness fairs, and visit local schools. hour after the quake struck. Oakland—These studies investigate how much The USGS, CDMG, UCB, and PG&E are the shaking of the underlying bedrock would be Earthquakes remain a serious threat to the San cooperating to add information on the severity of amplified by the overlying soil during an earth- Francisco Bay region. Large quakes will certainly ground shaking to the Web site. As of fall 1999, quake and whether the ground would liquefy. shake this urban region in coming decades, but the maps showing the pattern of shaking measured ¥ Determining whether slopes within the Oak- impacts of these events can be reduced. Continuing across the region can be posted within tens of land Hills are likely to fail by landsliding dur- advances in science and engineering and improved minutes of broadly felt earthquakes. ing earthquake shaking—Computer analyses practices in construction, land use, emergency re- Quick access to information on ground shak- of the stability of slopes take into account pos- sponse, and disaster recovery will offer new avenues ing will be of particular value in the Bay region, sible levels of shaking, topography, and to limit losses and reduce impacts of future shocks. where the wide variety of geologic materials— strength of slope materials. The efforts highlighted here are part of the from hard bedrock to soft clay—causes large dif- USGS Earthquake Hazards Program—a key ele- ferences in shaking intensity. This information Communication ment of the National Earthquake Hazards Reduction will help emergency-response officials to iden- Since the 1989 Loma Prieta earthquake, the Program. In the ongoing efforts to safeguard lives tify locations where damage and need are likely USGS has increased efforts to communicate and property from future earthquakes, the USGS to be greatest. In past quakes, identifying such earthquake-hazard information and research program not only involves USGS scientists but also locations has too often been delayed by disrupted findings to a broad audience. In 1990, the USGS engages expertise and talent in academia, private communications. As a result, the initial picture of and several cooperators, with financial support companies, and State and local governments. the earthquake’s effects has often been seriously from the American Red Cross and the United incomplete. Way, produced a popular educational pamphlet, Robert A. Page, Peter H. Stauffer, and James W. Hendley II Graphic design by “The Next Big Earthquake in the Bay Area May Sara Boore, Susan Mayfield, and Michael F. Diggles Earthquake preparedness Come Sooner Than You Think.” The pamphlet The Loma Prieta earthquake was a painful re- was published in English, Spanish, Chinese, and COOPERATING ORGANIZATIONS minder of the need to be aware of natural hazards Braille, and as an audio tape through Recordings Association of Bay Area Governments and to take steps to minimize the potential impact of for the Blind. More than 2 million copies were California Division of Mines and Geology threatening events. To reduce losses from natural di- initially distributed as Sunday newspaper inserts. California Governor’s Office of Emergency Services City of Oakland Office of Emergency Services sasters, in 1997 FEMA initiated “Project Impact: Because of continuing demand, the pamphlet Federal Emergency Management Agency Building a Disaster Resistant Community.” A pilot was reprinted in 1994, and a total of nearly 5 Geomatrix Consultants Inc. effort under this program of public/private partner- million copies have now been distributed. The Institute for Business and Home Safety ships is being conducted in the city of Oakland. pamphlet has been emulated in other highly ac- National Institute for Building Sciences Contributing expertise to this effort, the tive seismic regions—, the Pacific Gas and Electric Company San Francisco State University USGS is pursuing a range of projects focused on Cape Mendocino region of northern California, Stanford University earthquake hazards threatening the Oakland the Puget Sound region of Washington, and University of California at Berkeley community: southern Alaska. University of California at Santa Cruz ¥ Demonstrating an economical, automated sys- On another front, the USGS joined in part- URS Greiner Woodward Clyde Federal Services tem for rapidly mapping the intensity level of nership in 1997 with the Institute for Business William Lettis & Associates Many other institutions, organizations, and firms strong shaking throughout Oakland minutes af- and Home Safety (IBHS), an affiliation of insur- ter a large earthquake occurs—Maps are de- ance and reinsurance companies. The mission of rived from measurements of the ground shak- the institute is to reduce deaths, injuries, prop- For more information contact: Earthquake Information Hotline (650) 329-4085 ing at numerous points throughout the city and erty damage, economic losses, and human suf- U.S. Geological Survey, Mail Stop 977 from knowledge of how ground type varies fering caused by natural disasters. To increase 345 Middlefield Road, Menlo Park, CA 94025 with location. awareness and understanding within the insur- http://quake.usgs.gov/