Introduction Table of Contents Geologic Hazards

USGS Response to an Urban Earthquake -- Northridge '94

Assisting Recovery, Reconstruction, and Mitigation

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The Immediate Response -- Working with Others During the Immediate Aftermath of the Earthquake

All modern disasters in the United States are met by immediate and long-term demands from Federal, State, and local government agencies for practical information during response and recovery efforts. Some of this information is needed to meet requirements for receipt of Federal disaster funds; some of it is needed for the standards, guidelines, and practices to be used during repairs, rebuilding, and land-use planning. All NEHRP agencies, while engaging in rapid emergency response, are tasked with meeting the demands for information and assisting the many entities in interpreting what the information means.

The USGS brings lengthy experience to disaster response that arises from decades of response to and research on a variety of natural disasters, both national and international. Thus, the nature of its response is reflected in a continuously improving, multifaceted program, planned and operated by people with significant training and practice. The scientific response is a major element. In general, responding scientists have considerable foreknowledge of possible earthquake effects and the areas where these effects are likely to be encountered. In southern California, the rapid outputs from vast arrays of scientific instruments also help guide investigators in making efficient logistical decisions. Responders in the field work individually or in small teams, typically in topical areas such as building-damage assessments, liquefaction, ground rupture, or instrument placements. Others, even at sites elsewhere in the country, begin the process of data analysis, coordinating their activities through a central processing facility. Scientists are in constant communication, posting current information on the Internet, and distributing it widely using large E-mail dissemination programs. Moreover, the USGS typically becomes the official entity for organizing scientific work by other government agencies, universities, and private engineering and geotechnical firms following earthquakes. Notably, the USGS works in concert with university scientists of the Southern California Earthquake Center (SCEC) in this coordinating role.

Rapid Deployment of Seismic Instruments
The USGS and its cooperators have become highly skilled and efficient in rapidly deploying a widespread array of seismic instruments following earthquakes. Instrument deployment following the Northridge event proved to be one of the most successful and well coordinated efforts ever produced following a United States earthquake. Led by the USGS and SCEC, the effort included seven universities, four USGS teams, and contributions from several private companies. More than 80 instruments were placed in dense arrays, then relocated as needed in various configurations to address specific seismological and engineering issues.

In the case of the Northridge earthquake, the central processing facility was at the USGS office at Caltech. In addition to the scientific response, the USGS provided communications and liaison responses. Communications following the Northridge quake were handled primarily through USGS/Caltech, the National Earthquake Information Center (NEIC) in Golden, Colorado, and USGS offices in Menlo Park, California, and Reston, Virginia. USGS scientists also shared information and coordinated activities with other agencies and institutions as key participants in the California Division of Mines and Geology/Office of Emergency Services (CDMG/OES) technical clearinghouse in Pasadena. At each site, professionals trained and experienced in media relations, and in interpreting scientific information from the quake, provided a continuous flow of news to a demanding audience. The media relations functions of the USGS were critical to the smoothness of the government’s overall response in the post-earthquake atmosphere of fear and uncertainty. The USGS also provided liaisons to FEMA’s Federal Interagency Disaster Response Team, and to decision-making entities at all levels of government on an as-needed basis. The USGS also hosted liaisons from other government agencies to help meet their response, recovery, and long-term planning needs.

The Southern California Earthquake Center (SCEC)
SCEC actively coordinates research on southern California earthquakes and focuses on applying earth sciences to earthquake-hazards reduction. Founded in 1991, SCEC is a NSF Science and Technology Center with administrative and program offices located at the University of Southern California. It is cofunded by the USGS and FEMA. The core institutions of SCEC are: University of California, Los Angeles; University of California, Santa Barbara; California Institute of Technology; Scripps Institution of Oceanography; University of California, San Diego; University of Southern California; Columbia University; and the USGS.

Database Management -- The Basis for Effective Response, Recovery, and Mitigation

Rehabilitation, reconstruction, and land-use planning following an earthquake are most effective when data for decision making are easily accessible and internally consistent. The important databases—cultural, geological, geophysical, infrastructure, and political, for example—serve decision making best if they have both geographic uniformity and accessibility at a variety of scales. Recognizing these requirements, the USGS adopted two principal avenues of database and information management: (1) collate, analyze, and synthesize data using geographic information systems (GIS) technology; and (2) publish and distribute data, research results, and other information using the Internet, particularly the World Wide Web (WWW). Both avenues were ideally suited to the role of the USGS as one of the Federal agencies using geographic information systems and WWW technologies.

The Computer Graphics Laboratory (CGL) in Golden, Colorado, provided most of the USGS database support for the Northridge earthquake. The CGL provided (1) advanced computer hardware and GIS software; (2) assistance and training for scientific investigators in using GIS; (3) scientific visualization technology; (4) development of a file-transfer protocol (FTP) data server for distributing digital spatial data among USGS scientists; and (5) development of a WWW site for distributing databases, maps, and reports. Most of these provisions were extended to USGS offices in Menlo Park and Pasadena, California, to enhance capabilities there.

Whether as scientists, data analysts, media relations professionals, or agency liaisons, USGS personnel respond immediately to indications of an earthquake disaster. Typically, magnitude and location information on earthquakes worldwide are interpreted by the USGS within minutes of a major quake, thereby setting in motion the response for that event. The USGS routinely practices and updates its earthquake response plans, both internally and in cooperation with other government agencies and private entities.

With very few exceptions, maps and illustrations critical to and developed from the earthquake response were produced by digital means, and all data sets were obtained in or converted to digital form. Such digital information could then be readily transmitted via the Internet to USGS investigators and, when reviewed and released, to the broader public audience having Internet access. In this manner, new data and interpretations on the Northridge earthquake response were rapidly and frequently made available to a wide audience in the months following the main shock. All digital maps and illustrations could then be accessed and duplicated at a variety of scales for different scientific applications.

The USGS rapidly produced GIS-based maps incorporating general levels of ground shaking, local site-amplification values, and areas of potential ground failures. These maps were intended for broad uses, and they were targeted for the lengthy reconstruction period when decisions needed to be made regarding standards for rebuilding and repairs. Other maps were created to display data sets, such as the data set on damaged structures provided by the California OES/FEMA Disaster Field Office. This map showed the geographic distribution of inspected buildings that were “red-tagged” (no occupancy), “yellow-tagged” (limited entry), and “green-tagged” (no restrictions on entry). Such maps and their applications are discussed throughout this report (see p. 55). One of the products of the USGS efforts is a large GIS database of maps, data sets, and thematic data layers. These products, many of which are described in this document, will be publicly available at a WWW site.

Communicating via the Internet

As part of its modern communications systems, the USGS customarily provides digital information for (and accesses data from) others on the Internet using an “anonymous” FTP site on one of its host computers. Following the Northridge earthquake, the USGS acquired and distributed data sets many times by the anonymous FTP method, and assisted in preparations of digital publications. One example is the report, "Inventory of Landslides Triggered by the 1994 Northridge, California, Earthquake," available with the data sets used to produce it via "anonymous" FTP at:

A hypertext version of the same report, including graphics and photographs, is available on the WWW at:

A principal goal was to establish a WWW site for consolidating all relevant linkages to USGS and other research on the Northridge earthquake. Although many individuals and groups established WWW “home pages” to display data, images, and reports, the USGS has unified these with a single “home page” dedicated to the Northridge earthquake. Access this site on the WWW at:

Lessons learnedLessons Learned
Because all relevant spatial data (commonly produced at great cost) ultimately become digital through GIS and other visualization technologies, these data and their interpretations have an added value through being accurate, easily updated, and transportable.

The USGS decided to include applied technology (Internet; GIS) as an integral part of the post-earthquake research program. This action transformed typical map production into integrated scientific collaboration among scientists and the Computer Graphics Laboratory. The new procedures arising from this collaboration achieved superior results in increasing the efficiency of scientific investigations and communicating their findings.

Damage and Intensity Inventories

To determine the intensity and areal extent of earthquake damage, USGS scientists conducted reconnaissance field trips and queried thousands of individuals by telephone during the first week after the main shock. Additionally, they collected information from press reports, mailed questionnaires, E-mail responses to requests for information broadcast over the Internet, reconnaissance reports from other entities such as engineering firms, and from data on the distribution of “red-tagged” buildings and “yellow-tagged” buildings.

The data showed that the earthquake was felt over approximately 214,000 square kilometers of the land area of the United States. The earthquake was also felt in Ensenada, Mexicali, and Tijuana, Mexico. The earthquake was felt 400 kilometers to the northwest in Turlock in California’s Central Valley and 375 kilometers to the northeast in Las Vegas, Nevada. Damage throughout this region was described, tabulated, and entered into a geographic database that was later accessed for a variety of decision- making purposes.

The area including intensities greater than or equal to Modified Mercalli intensity VI extends from the mouth of the Santa Clara river in Ventura County eastward throughout most of the Los Angeles area. Star indicates epicenter of the Northridge earthquake in both figures.

The distribution of Modified Mercalli intensities (MMI) in the epicentral region shows a maximum intensity of IX for three locations in the San Fernando Valley. Intensity IX means “Damage considerable in specially designed structures; well designed frame structures thrown out of plumb; great in substantial buildings, with partial collapse. Buildings shifted off foundations. Ground cracked conspicuously. Underground Pipes Broken.”

The intensity pattern of the Northridge earthquake is broadly similar to that of the 1971 San Fernando earthquake. In detail, the intensity VII and VIII isoseismals of the Northridge earthquake enclose larger areas than the corresponding isoseismals of the San Fernando earthquake. The intensity V and VI isoseismals of the Northridge earthquake enclose smaller areas than those of the San Fernando earthquake. The maximum intensity assigned to the Northridge earthquake was IX, and the maximum intensity assigned to the San Fernando earthquake was XI. These differences perhaps reflect (1) real differences in the level of ground shaking, (2) systematic changes over time in the damage susceptibility of structures, (3) differences in the distribution of observations, and (4) differences in the way specific effects are interpreted in terms of MM intensities.

Real differences in the distributions of ground shaking in the two events result from differences in the sources of the two earthquakes. First, the locations of the energy release in the two shocks were separated by about 20 kilometers. Second, the 1971 fault rupture nucleated at depth beneath a lightly populated mountain range and propagated southward toward a heavily populated valley. The 1994 rupture nucleated at depth beneath a heavily populated valley and propagated northward toward a region of lightly populated mountains and valleys. Third, the primary 1971 fault rupture broke through to the ground surface, whereas the primary 1994 fault rupture terminated several kilometers below the surface (see p.16).

Lessons learnedLessons Learned
Comparing Modified Mercalli intensities of the 1971 San Fernando and 1994 Northridge earthquakes shows broad similarities because of the similar magnitudes and types of faulting of the two events. However, the two events differed significantly in the location of their fault planes with respect to the ground surface, and with respect to the propagation of energy toward or away from major population centers. Thus, the San Fernando quake probably produced higher intensities in some locations because of the orientation of its fault plane, and because rupture on that plane reached the land surface.

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