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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 proba
bly 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.
S. 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
Building Safety Net(work)s for Earthquakes
C. 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 appli
cations 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, val
ued 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.
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 informa
tion 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.
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
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.
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.
he 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.
Geomorphology and surficial geology—SAST scientists examined satellite images, aerial photographs, and historical maps augmented by field observations to determine the location of high-, medium-, and low-energy floodplain terraces at an initial level. These areas are being mapped as the analyses are refined. Information thus gathered will help identify locations suitable for different land uses, including agriculture, habitat restoration, industry, and commerce.
he SAST contributed significantly to
parts I through IV of the IFMRC report and wrote a scientific document that was published as part V of the report. The report is the first in a series that will include the database report; proceedings of the hydraulic, hydrologic, and ecologic modeling workshop; background reports; and numerous articles in scientific journals.
P. 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: • Where are all of the toxics-release inventory sites that were affected by the 1993 floods? Are any of them in the floodplains?