ezsmic ap Foster Landmark Legislntion '"I ] hen a powerful l' f earthquake strikes an urban region, damage concentrates not only near the quake's source. Damage can also occur many miles from the source in areas of soft ground. In recent years, scientists have developed ways to identify and map these areas of high seis­ mic hazard. This advance has spurred pioneering legislation to reduce earthquake losses in areas of greatest hazard.

Buildings constructed on uncompacted fills and soft soils are especially vulnerable to earthquake Television cameras broadcasting the shaking damage. In this photo, taken four hours after the 1989 Lorna Prieta, , earthquake had struck, homes in start of the 1989 instead re­ 's Marina District still burn out of control from fires started by broken gas lines. The district was built on artificial fill that included rubble from the great quake of 1906. Scientists corded the urban devastation from a major can identify areas where such shaking damage is likely to be especially severe. (Photo by Martin Klimek, earthquake striking Northern California. Four Marin Independent Journal.) hours after the earthquake struck, homes in magnitude 9.2 Alaska earthquake, however, rupture of the ground smface by faulting. San Francisco's prosperous Marina District did the nation direct much attention toward Strong shaking damages or collapses weak still burned out of control from fires started understanding and mapping earthquake haz­ structures over wide areas. It also triggers by broken gas lines; the shock severely dam­ ards. In the late 1960's, the U.S. Geological ground fai lures (fracturing, sliding, and aged or destroyed 70 residential buildings in Survey (USGS) launched a program to de­ slumping), which in turn damage or destroy the district. Across in velop methods for identifying and mapping structures and disrupt utility and transporta­ Oakland, the collapse of the double-decker areas of potential earthquake hazard. tion systems. In the mid-1970's, the USGS Cypress freeway structure trapped more than An early product of this program was a published an innovative map of the ground­ 160 people, 42 of whom died. series of maps showing the locations of ac­ shaking hazard for part of the San Francisco Both of these grim spectacles from the tive segments of the San Andreas Fault in Bay region. This map was used by local and magnitude 7.1 Lorna Prieta, California, earth­ Californi a. These maps demonstrated the fea­ regional government bodies to develop seis­ quake occurred more than 50 miles from the sibility of identifying faults that might rup­ mic safety policies. The map predicted that temblor's source in areas underlain by soft ture the ground surface in future earthquakes. shaking on soft ground would be several soil (loose sediment, uncompacted ftll, and This capability lead to new strategies tore­ times as intense as that on nearby rock. Some mud). ln contrast, structures built on rock and duce losses from such ruptures. In 1972, the engineers and scientists were skeptical of ftrm soil, which underlie most of San Fran­ California Legislature passed a landmark law these predictions, but records of strong shak­ cisco and Oakland, were largely unscathed. requiring the identification of seismic-haz­ ing and patterns of damage in the 1989 Lorna Near the earthquake's epicenter, however, ard zones along faults. In these zones, spe­ Prieta earthquake verifted the predictions. shaking was violent enough to cause consid­ cial geologic studies are required before struc­ The map had correctly shown the Marina Dis­ erable damage even in areas underlain by tures can be built for human occupancy. This trict and the area of the Cypress freeway rock and ftrm soil. law has successfully prevented homes, structure as being subject to violent shaking This localization of severe shaking and schools, and offices from being built across during earthquakes. damage was no surprise. It had been noted in active faults. Faced with the disastrous losses from previous San Francisco-area earthquakes, as The major cause of earthquake damage, the Lorna Prieta shock, the California Legis­ early as 1868. Only after the devastating 1964 however, is strong ground shaking, not the lature realized that stronger measures were Seismic hazard maps further legislation to reduce earthquake losses This map sequence illustrates the shaking hazard in San Francisco for a possible repeat of the great 1906 earthquake. Such Effect of distance on shaking Expected ground shaking maps provide information essential for on bedrock decreases rapidly developing effective seismic safety policies with increasing distance from and laws. the San Andreas Fault, from vety violent (red) to moderate (green).

Effect of ground type on shaking Expected ground shaking Areas of most Intense shaking The capability of ground type to amplify shaking This map combines information from Maps 1 and 2 This map, derived from Map 3, shows in red the varies from vety high for mud and uncompacted to show expected shaking levels throughout San areas of most intense shaking where efforts to fill, to moderate for sandy soil, to low for soft roclc, Francisco. reduce earthquake losses should be focused. and to none for hard rock. needed to combat earthquake hazards. In of predicted ground shaking. ban earthquakes are staggering. For example, 1990, the Legislature passed the California Experience in many states reveals that a repeat of the 1886 Charleston, South Caro­ Seismic Hazards Mapping Act to assist cities seismic hazard maps serve diverse audiences. lina, earthquake today would cause an esti­ and counties in protecting public health and Users of these maps include buyers and own­ mated 2,000 fatalities and $5 billion of dam­ safety against such hazards. This law requires ers of real estate, geotechnical consultants and age. In the central Mississippi Valley region, the State Geologist to make maps of seismic engineers, financial institutions, utility and projected losses from a repeat of an 1811 hazard zones, identifying areas prone to vio­ transportation companies, emergency man­ earthquake are 6,000 lives and $50 billion of lent shaking and ground failure. It also re­ agers, and government planners. damage. quires that evaluation of these potential haz­ Mapping seismic hazards is especially Crucial to reducing these potential losses ards precede approval of construction projects important in urban areas of earthquake-prone is sound geologic knowledge leading to ef­ within defined hazard zones and that buyers regions of the United States. Such areas have fective seismic safety policies and legislation. of real estate be notified when the property large populations and huge investments in lies within such a zone. This act builds on the structures and lifelines that are at risk from Roger D. Borcherdt, Robert B. Brown, success ofboth the 1972 law and the early maps earthquakes. Potential losses from future ur- Robert A. Page, Carl M. Wentworth, and James W. Hendley II The California Seismic Hazards Mapping Act of 1990 declares:

a) The effects of strong ground shaking, liquefaction, landslides, or other ground COOPERATING ORGANIZATIONS failure account for approximately 95 percent of economic losses caused by an earth· Association of Bay Area Governments quake. California Division of Mines and Geology b) Areas subject to these processes during an earthquake have not been identified City of San Francisco or mapped statewide ... c) It is necessary to identify and map seismic hazard zones .. .to reduce and miti­ gate those hazards to protect public health and safety. For more information contact:

It is the intent of the Legislature to provide for a statewide seismic hazard map­ Earthquake Information Hotline (415) 329·4085 U.S. Geological Survey, Mail Stop 977 ping ... program to assist cities and counties in ... protecting the public ... from ... hazards 345 Middlefield Road, Menlo Park, CA 94025 caused by earthquakes. http://quake.wr.usgs.gov

U.S. Geological Survey Fact Sheet-224-95 1995