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Mapping the Earthquake Hazards of

the Los Angeles Region

The highly populated Los Angeles region is near one of the world's major active plate tectonic boundaries, the San Andreas fault zone, and lies astride numerous other faults that can generate destructive earthquakes. Since 1800, at least 40 earthquakes large enough to cause damage have shaken parts of the region. It is estimated that a single

future earthquake could result in losses

ranging as high as 21,000 casualities and $62 billion in property damage. Although large earthquakes are inevitable, losses from them can be reduced substantially by careful engineering design, by wise land use, and by emergency preparedness planning. These mitigating actions, however, require estimates of the likely location and severity of hazardous seismic effects.

The Los Angeles region has been selected as a prototype study area for developing earthquake hazard assessment techniques. Funded by the Earthquake Hazard Reduction Program of the U.S. Geological Survey, the research is conducted by scientists and engineers of the Geological Survey, the California Division of Mines and Geology, various universities, and private consulting firms.

These studies are providing an improved basis for delineating geographic variations in the earthquake hazards; specific examples are discussed below.

Fault Hazards

The regional tectonic framework reflects interaction between two families of active faults: northwest-trending faults, such as the San Andreas, with chiefly horizontal movements, and west-trending faults of the Transverse Ranges with chiefly vertical movements.

Geologic evidence of the age and geometry of latest surface displacement along a fault provides a first clue to its future behavior. Nearly 100 fault strands with offsets in late Quaternary time (the past 700,000 years), in Holocene time (the past 11,000 years), or in historical time have been identified in the region. Such faults are likely candidates for generation of future destructive earthquakes and surface-fault rupture (fig. 10). Detailed geologic mapping, recently completed for many of these fault zones,

provides a guide for future (and development under the State of California's AlquistPriolo Special Studies Zones Act, which regulates construction where the potential for surface-fault rupture exists.

Studies now underway emphasize the determination of geologically recent rates of offset along these faults as an index of their relative activity. The most reliable estimates of slip rates, averaged over the past several hundred thousand years, are for the San Andreas fault (0.8-1.2 inches per year) and the San Jacinto fault zone (0.3-0.5 inch per year). In comparison, estimated offset rates for most other faults of the region are less than 0.04 inch per year, except for a belt of Transverse Ranges faults that extend from near Santa Barbara to San Bernardino and have rates of 0.04 to 0.1 inch per year.

Because the 200-year historical record of earthquakes in southern California is inadequate for estimating the probable frequency of damaging earthquakes along individual faults, a search is underway for geologic evidence of prehistoric large earthquakes in disturbed late Quaternary sedimentary deposits preserved along many of the faults. The findings suggest that parts of the San Andreas and San Jacinto fault zones have generated major earthquakes in intervals of several decades to a few centuries. In contrast, the repeat times of seismic events large enough to disturb the ground surface are measured in many hundreds to several thousands of years at points along other active faults in the region.

Earthquake Shaking Hazards

A technique being developed in the Los Angeles region should significantly improve the prediction of geographic variation in shaking response due to geologic factors. The major elements of this approach are (1) systematic geologic mapping and analysis of the late Quaternary sedimentary deposits of the alluvial basins, (2) regrouping of these deposits into seismically distinct units on the basis of their geotechnical properties and shear-wave-velocity measurements, and (3) mapping of relative ground response using amplification factors derived from comparative studies of recorded ground motions.

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Ground motions from nuclear test explosions in Nevada measured at about 100 sites throughout the Los Angeles region show marked variations in amplitude that result from local geologic factors. For example, levels of shaking on alluvium are three to six times greater than on granite. The most pronounced differences in site response are correlated with differences in the amount of pore space in sediments, the thickness of surficial deposits, and the depth to hard basement rocks.

Several of the most important geologic factors that control levels of shaking can be grouped to identify distinctive types of sites. These site types are the basis for mapping future relative shaking response. Figure 11, an example of such a map for part of the Los Angeles basin, shows expected levels of response to ground shaking, relative to granite, that is of significance to 6- to 30-story buildings. Because this method uses information obtained from geologic maps, well borings, and geotechnical measurements gathered in the course of ordinary urban development, it is a versatile and easily transferable means of predicting geographic variations in shaking response from future earthquakes.

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Earthquake Ground-Failure Hazards

Liquefaction, the temporary transformation of saturated granular material from a solid to a liquefied state, during seismic shaking often results in ground failures that can cause significant damage. Liquefaction potential is evaluated by preparing two types of maps-one showing liquefaction susceptibility and the other expressing the chances of critical levels of shaking likely to induce liquefaction.

Sediment known to be susceptible to liquefaction constitutes a narrow range of depositional and hydrologic environments. Generalized maps of liquefaction susceptibility (fig. 12 is an example for the San Fernando Valley northwest of Los Angeles) are prepared by analyzing the physical properties of the late Quaternary alluvial units, by grouping these according to their probable content of liquefiable material such as clayfree sand, and by considering whether these units are saturated with ground water at depths of less than 50 feet beneath the ground surface.

Alluvial materials most likely to contain clay-free sand are those deposited during

Figure 10.—Faults in the Los

Angeles region that may generate damaging earthquakes and associated surface-fault rupture. Major faults identified are

SA, San Andreas, and
SJC, San Jacinto.

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Water Resources Investigations

Location of principal offices of the U. 8. Geological Survey’s Water Resources Division in the conterminous United States. Cities named are those where Regional and District Offices are located. Puerto Rico in included in the Southeastern Region, and Alaska and Hawaii are included in the Western Region.

Mission and Organization

The U.S. Geological Survey has the major responsibility within the Federal Government for assessing the Nation's water resources. It collects basic data and conducts special investigations to provide background information for planners and managers. Demands for water from a wide variety of users increasingly require that planners at Federal, State, and local levels establish priorities for use. Sound judgment in determining such priorities depends on access to accurate hydrologic information and impartial expertise. The increasing pressures associated with developing energy resources in environmentally sound ways are enlarging demands for hydrologic data. Water is an integral element in all energy and environmental problems.

Programs

Water Resources Division programs fall into four categories: the Federal Program,

the Federal-State Cooperative Program, Assistance to Other Federal Agencies, and the Non-Federal Reimbursable Program.

The Federal Program

The data collection, resource investigation, and reseach activities of this program are carried out in areas where the Federal interest is paramount. These include bodies of water in the public domain, river basins and aquifers that cross State boundaries, and other areas of international or interstate concern. Activities include operation of surface- and ground-water quantity and quality measurement stations throughout the country, the Survey’s Central Laboratories System, hydrologic research and analytical studies, and a variety of supporting services.

The Federal-State Cooperative Program

The Cooperative Program is based on the concept that Federal, State, and local

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