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landslides, rock falls, and soil failures that occurred as far as 70 kilometers from the epicenter. Fortunately, most of these were in relatively unpopulated areas. Rock falls that choked the ravine bottoms of many canyons in the Santa Susana Mountains presented postearthquake hazards. Had heavy rains fallen, the rock falls could have been saturated and mobilized into debris flows that would have threatened structures near the mouths of the canyons.

The extent of liquefaction caused by this earthquake was much less than what would have been expected, given the historical ground-water levels and the strong levels of shaking that occurred. The reason was probably the lower-than-average water-table levels in the San Fernando Valley. Localized liquefaction and lateral spreading took place in the San Fernando Valley (primarily settling basins along the Los Angeles River) and other areas in Simi Valley, Santa Monica, and Redondo Beach.

Building Safety Net(work)s for Earthquakes

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ontinuing its work to characterize the regions of the Nation where earthquakes are a public safety risk, the U.S. Geological Survey (USGS) awarded 163 grants, valued at $10 million, to colleges and universities to continue building the critically needed information base for applications such as standards for issuing earthquake forecasts, criteria for building codes and seismic safety standards, and national and regional seismic networks. The USGS also signed 15 cooperative agreements, valued at $2.8 million, with universities to support the U.S. National Seismic Network, one of the key components in understanding earthquake risks from a national perspective. Pilot projects are underway with several regional networks to develop a common model for integrating regional and national networks. Such cooperative research under the USGS-administered National Earthquake Hazards Reduction Program underscores the value of a national program to help identify earthquake hazards, assess earthquake risks, and monitor seismic activity across the United States. The close tie-in through State colleges and universities ensures that the USGS can work effectively with State and local governments and industry to inform them of seismic risks in their areas and to develop building codes and seismic safety standards that protect lives and property.

Conclusions

cientists have issued frequent warnings about seismic hazards in the Western United States, and this earthquake tested the level of southern California's preparedness. Some successful strategies were the result of past experiences, and failures pointed out areas where more work was needed. On the positive side, information gained from scientific efforts of the National Earthquake Hazard Reduction Program, combined with some of the better seismic building practices in the United States, helped to limit the loss of life. In other parts of the world where these types of programs do not exist, similar-sized earthquakes (for example, India in 1993 and Armenia in 1988) have caused thousands of deaths. On the negative side, building construction was not adequate to prevent widespread structural failures in many communities, such as Northridge, Simi Valley, Sherman Oaks, North Hollywood, and Santa Monica. The Los Angeles freeways collapsed at 7 sites, and another 170 bridges suffered varying amounts of observable damage. Repair work on the bridges caused traffic problems for many months following the earthquake.

The large amount of damage caused by the Northridge earthquake is a consequence of an active geologic structure existing within

an urban environment. The type of fault that produced the Northridge earthquake is not unique to the San Fernando Valley. Similar structures exist throughout the area, and there is geologic evidence for several blind thrusts in the Los Angeles basin that are capable of producing events even larger than Northridge. Large earthquakes on these faults could present serious problems for densely populated areas, including downtown Los Angeles, which contains many high-rise buildings. However, the problem of populated areas in close proximity to earthquakes is not limited to Los Angeles. Portions of the San Andreas fault are adjacent to San Bernardino and San Francisco. The Hayward fault passes through densely populated areas of Oakland and East Bay communities. Portland, Oreg., Seattle, Wash., and Memphis, Tenn., all are located in earthquake-prone areas. The lessons learned from the Northridge earthquake about the levels of strong ground shaking produced by a moderate earthquake and the subsequent damage to populated areas should be applied to building construction and earthquake preparedness in all of these cities.

This report was compiled from information gathered by many scientists from the USGS, the California Institute of Technology,

Liquefaction is the process by which wet, loosely compacted soils. are transformed by earthquake shaking to a liquid state.

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tributaries. Thirty-eight lives were lost, and estimated damages were between $10 billion and $16 billion.

The Scientific Assessment and Strategy Team

he Administration, recognizing the high

The

cost of repairing levees and other infrastructures, rebuilding and floodproofing homes, reclaiming agricultural lands, and reestablishing the economic system, decided to reexamine the use of floodplains in the upper Mississippi River basin.

On November 24, 1993, the Scientific Assessment and Strategy Team (SAST) was established by the Assistant to the President for Science and Technology Policy, the Associate Director of the Office of Management and Budget, and the Director of the Office of Environmental Policy. The team was made up of members from the U.S. Army Corps of Engineers, the U.S. Environmental Protection Agency (USEPA), the Federal Emergency Management Agency, the U.S. Fish and Wildlife Service, the National Biological Service, the Soil Conservation Service, and the U.S. Geological Survey (USGS). The USGS was directed to lead this interdisciplinary, interagency group.

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mance of existing floodplain management and related watershed management programs, to recommend changes in policies and programs, and to achieve risk reduction, economic efficiency, and environmental enhancement in the floodplain and related watersheds. The IFMRC also identified legislative initiatives that might be proposed by the Administration. As the scientific arm of the IFMRC, the SAST provided more than 150 maps and analyses for IFMRC use. The IFMRC has completed its tasks and been dissolved, but the SAST continues to conduct analyses and to gather and distribute scientific data.

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SAST data are being made available as they are checked for quality. An interagency clearinghouse is being established to ensure that the most current digital data are distributed as soon as possible. The data will be made available online, nearline, or offline, depending on the size or type of data. Already, some of the data are available on the Internet by using World Wide Web with an XMosaic front end.

SAST data can be accessed by opening the Uniform Resource Locator (URL) and entering:

http://edcwww.cr.usgs.gov/ sast-home.html

The clearinghouse is being used as a prototype for the Federal Geographic Data Committee (FGDC), which is responsible for planning the National Spatial Data Infrastructure, and is testing some FGDC concepts. As such, it is part of the national information infrastructure and the information superhighway.

Computer-generated map showing the maximum flood extent at St. Louis, Mo., during the floods of 1993. Darker shades represent flooded areas and streams; lighter areas are probably clouds.

Maximum Flood Extent

conducted in the field and by using remote sensing, literature was reviewed, results were reported, maps were produced, and a digitaldata clearinghouse was established.

Database

The

The SAST built a database of some 240 gigabytes of digital data, including readily available agency data such as 1:100,000-scale digital line graph files, the USEPA's toxins release inventory, and Thematic Mapper (TM) data for the entire upper Mississippi River basin as well as TM data for the area hardest hit by the floods. It also contains less commonly available data, such as interpretations of sedimentation and scour, and spatial and attribute information on all major levees on the main stems of the upper Mississippi, lower Missouri, and lower Illinois Rivers.

Hydraulic Models

Upland models.-The SAST ran hydraulic and hydrologic models to estimate the effects of various land-treatment practices in the uplands. Four distinct watersheds were tested in different physiographic regions. The results clearly showed that the effects of land treat

ment varied in regions of different physical characteristics.

Floodplain model-A modeling effort was conducted to gain a better understanding of the effects of levees and different land uses on the floodplains. The UNET model, available through the U.S. Army Corps of Engineers, is a one-dimensional, steady-state hydraulic model that treats areas affected by levees as storage cells. This model was run for the main stem of the Mississippi River from Cairo, Ill., to Hannibal, Mo., along the Missouri River from its mouth to Hermann, Mo., along the Illinois River from its mouth to Meredosia, Ill., and along the Salt, Meramec, and

...removing the levees would have lowered the stage of the floods....

Kaskaskia Rivers from their mouths to the first gaging station. Results of running various scenarios after calibrating the model to the 1993 floods as well as two earlier, less intensive floods showed that removing the levees

would have lowered the stage of the floods, but the amount depended on the use to which the overbank lands were put. However, the flood stage would have been reduced only a few feet in most places in a flood the size of the 1993 flood. In addition, for overbank conditions that significantly reduced conveyance, the flood stage could actually rise. For example, if dense forest covered the floodplain, the study showed that the flood would have been higher in some locations.

Analysis

The problem of flooding includes both

upland and floodplain issues. Five current studies are described here.

Regionalization scheme. -The SAST drafted a regionalization scheme for the uplands based on slope variance, which was compared to variables such as topography, soil type, soil moisture holding capacity, and surface-water ponding. Further refinement of this scheme will be useful in determining appropriate upland land-treatment practices for different places.

Dynamic geomorphology.-The flood of 1993 provided a unique opportunity to examine the effects of floods on sedimentation and scour. "Nature's experiment" left a temporary record useful for determining the effect of energy variations within the flood on both the engineered environment and the natural floodplain.

Levees.-The location of levees on the floodplain clearly affects their effectiveness and durability. The sections of levees that are predictably at risk are now known to include areas occupied by coarse surficial materials deposited by one or more channels active in the past, areas along downstream channel banks between the bends of meanders, areas along tributary channels subject to significant crossflow conditions during flooding, and areas along narrow passages of water between islands and the mainland.

Habitat restoration.-Understanding the dynamics of the geomorphology on the floodplain improves the chances of finding suitable habitat restoration sites. Areas where changes in channel and sand bar locations take place are sites where new plant growth takes hold. These sites are necessary for aquatic species that form an important part of the natural food web. As these growths mature, new places must be created naturally for additional new growth.

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Part

art of the SAST's responsibility is to make data available. A number of maps were produced for the IFMRC for its analysis and for use by appropriate agencies. The base maps portrayed roads and watercourses. Mylar overlays showed levees and their ownership, extent of flooding, historical changes in stream channels, floodplains having 1 and 0.2 percent chances of flood occurrence, existing habitat locations, and other information. A more detailed set of maps under consideration will show the above information plus historical and current land-use and landcover data, floodplain geomorphology, surficial geology, and other variables necessary for making decisions on the floodplain. Prototypes will be published in the USGS's Miscellaneous Investigation series of maps and will be put on compact disc-read only memory (CD-ROM) for distribution.

Some of these data and others have already been incorporated into a computerized demonstration capable of answering questions such as:

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