Page images
PDF
EPUB

Understanding natural hazards such as earthquakes, volcanic eruptions, landslides, floods, subsidence, and naturally occurring toxic materials is a critical public issue facing the Nation and the world. Part of the mission of the USGS is to help reduce the "disaster tax" that every citizen pays, regardless of where a hazard occurs. Each year for the past several years, natural disasters have cost the Nation about $50 billion—from the devastation of Hurricane Andrew to the long months of flooding in the Midwest to the renewed anxiety caused by the Northridge, Calif., earthquake. The USGS has a particular charge to help the Nation understand, prepare for, and respond to these natural disasters. Our ability to assist the country in coping with natural disasters comes from our unique combination of earth science expertise, as the articles in this section show. By better understanding the mechanisms of natural hazards, public policymakers can plan and manage in ways that will mitigate the severity of these hazards. Through scientific research, societal planning and preparedness, and proper emergency response, the damaging effects of natural hazards will be lessened, and economic and social losses can be reduced.

[graphic]
[graphic]

The Northridge Earthquake

n the morning of January 17, 1994, at

04:30 (PST) a magnitude 6.7 earthquake severely shook the San Fernando Valley and other regions in and around Los Angeles in southern California. The Northridge earthquake was the most costly earthquake in U.S. history, causing estimated losses of $20 billion. Fifty-seven deaths and more than 9,000 injuries to people in the region were attributed to the earthquake, and 20,000 people were displaced from their homes. The earthquake was moderate in size, but, because it occurred directly under the populated San Fernando Valley, its impact on the people and structures of the Los Angeles area was immense. Thousands of buildings experienced significant damage, and more than 1,600 were “red tagged" as unsafe to enter. Another 7,300 buildings were “yellow tagged” (restricted to limited entry), and minor damage was incurred in many thousands of other structures. The 10 to 20 seconds of strong shaking collapsed buildings,

brought down freeway interchanges, and rup

tured gas lines that exploded into fires. But the early-morning occurrence was a fortuitous life saver, because there were only a few people in many of the large buildings and parking structures that collapsed, and traffic was very light on the freeway overpasses that fell.

But the early-morning occurrence was a fortuitous life saver...

U.S. Geological Survey (USGS) scientists responded quickly to the Northridge earthquake, investigating and reporting on both geological and societal effects. In the month following the earthquake, USGS personnel focused on: • Monitoring the current seismic activity and providing information about seismic hazards to local government, media, and the public. • Collecting data on seismic hazards in southern California to help mitigate damage from future earthquakes.

Early on January 17, the USGS/Caltech offices in Pasadena, Calif., became the center for seismic information collection and dissemination. The large seismic network that monitors earthquakes in southern California is operated from this location, and, within minutes of the Northridge earthquake, scientists began analyzing data from the network and broadcasting the location and magnitude to the public. USGS and Caltech scientists kept up a steady flow of information to the public over the next few days as details about the earthquake and its damaging effects were inferred from seismic data and observed by field crews. To maintain good communications with emergency response groups, a USGS liaison was stationed at the headquarters of the Federal Emergency Management Agency a few miles away in Pasadena, and a California Office of Emergency Services liaison was detailed to the USGS operations center in Pasadena. These efforts to provide information about earthquakes continued throughout the next few months as the Los Angeles area was rocked by hundreds of felt aftershocks.

Throughout the mainshock and aftershock sequence, real-time earthquake information was relayed to members of the Caltech-USGS Broadcast of Earthquakes (CUBE) program. This project is a cooperative effort to develop a system of rapid earthquake information dissemination in southern California. Earthquake locations and magnitudes are sent to pagers worn by scientists and emergency personnel and to computer displays throughout southern California and other parts of the country within about 5 minutes of an earthquake occurrence. Governmental emergency response agencies, water and power utilities, railroads, and telephone companies all make immediate use of this vital information.

Seismological Observations

llowing the mainshock, there were

thousands of aftershocks, including 7 magnitude 5 events, 38 magnitude 4 events, and over 300 magnitude 3 events during the subsequent 4 months. The locations of the aftershocks are distributed across an area of about 30×20 kilometers and map out the extent and orientation of the fault plane. These locations clearly show a plane dipping toward the southwest, which is interpreted to be the rupture surface of the thrust fault that

[ocr errors][merged small][merged small][ocr errors][subsumed][ocr errors][ocr errors]

The locations of several thousand aftershocks of the Northridge earthquake are shown on this map by solid circles. The area covered by the

aftershocks roughly indicates the fault that ruptured under the northern San Fernando Valley. The star is the epicenter of the mainshock, where the rupture began. The depth cross section (upper right) shows how the aftershocks outline a fault plane that extends from a depth of 18 kilometers in the south up to 7 kilometers in the north.

produced the earthquake. This plane extends
upward from the mainshock hypocenter at
18 kilometers toward (but not reaching) the
surface. Preliminary inversions of seismic
waveform data indicate slips of as much as

4 meters on the fault plane at depths below 5
kilometers; little slip occurs on the shallow
portions of the fault.
The location of the fault plane as
inferred from the aftershock distribution does

[graphic]
[graphic]
[graphic]
[graphic]
[graphic]
[graphic]
[graphic]
[graphic]

The body of a Los Angeles Police Department officer lies near his motorcycle after plunging off the Antelope Valley Freeway overpass, which collapsed onto the Golden State Freeway during the Northridge earthquake.

not correspond to any mapped geologic fault. The earthquake occurred, however, within a system of known thrust faults that extends along the northern edge of the San Fernando Valley. Like the fault that produced the Northridge earthquake, many of these faults are “blind thrusts,” which do not extend to the surface and therefore are difficult to recognize prior to large earthquakes. The extensive damage caused by the earthquakes was mainly a result of intense shaking. The large-amplitude motions were recorded on many strong-motion instruments within the Los Angeles area, producing one of the best data sets of strong ground motions. The recordings showed peak accelerations of

0.5 to 1.0 g in the aftershock area and decreas

ing to 0.1 g at distances of about 50 km. Several sites close to the epicenter area recorded accelerations over 1 g These high levels of ground motion and the resultant widespread damage emphasized the need for a better understanding of how local geology affects the levels of ground shaking. A coordinated effort by USGS and university seismologists deployed more than 75 portable seismographs for recording aftershocks to study the complicated wave propagation and local site effects. Instruments were placed in many

severely damaged areas of Northridge, Sherman Oaks, and Santa Monica, as well as the

Like the fault that produced the Northridge earthquake, many of these faults are “blind thrusts,” which do not extend to the surface and therefore are difficult to recognize prior to large earthquakes.

A g of acceleration is equivalent to the force of gravity. A few sites near the Northridge earthquake recorded more than 1 g of vertical acceleration. These ground movements would have been capable of throwing objects (of any mass) into the air.

Support for the Southern California Earthquake Center

A. a practical example of how its national earthquake research expertise can be applied locally, the U.S. Geological Survey (USGS) has added $1.2 million to a cooperative agreement with the University of Southern California (USC) for the operation of the Southern California Earthquake Center. Since 1991, the USGS has obligated over $6 million in funds in support of this vital local center, which has been charged with educating the public about earthquake activity in southern California. Through activities such as a public realtime seismic data center, an educational outreach program for high school and elementary school pupils, and workshops for professors of engineering and seismology, USC and USGS scientists carry out comprehensive data collection and public outreach efforts in support of the national earthquake research effort.

[graphic]

Strong shaking from the Northridge earthquake collapsed this parking structure on the campus of the California State University at Northridge. Because the earthquake occurred

at 4:30 a.m., no people were in the structure.

A ruptured gas main along Balboa Boulevard in Granada Hills caused this fire. Several homes in the area burned down, but, because there was no wind, most fires were easily

contained.

collapsed freeway sites at the interchange of Interstate 5 and Highway 14, Highway 118 near Woodley, and Interstate 10 near La Cienega.

Geodetic Observations

enchmarks resurveyed by using the

Global Positioning System (GPS) showed significant permanent ground deformation caused by the earthquake. Vertical uplifts of 40 to 50 centimeters and horizontal movements of 2 to 20 centimeters occurred in the aftershock region. These movements are consistent with the fault geometry derived from seismological observations. Preliminary modeling of the data indicates that slip of 2.5 to 3.5 meters occurred on a 10×10-kilometer patch of the fault. The motion was primarily thrust faulting, and most of the slip occurred at depths greater than 6 kilometers. Following the Northridge earthquake, a number of GPS receivers were permanently installed to provide continuous monitoring of deformations in the San Fernando Valley and Los Angeles basin.

Geological Observations

he lack of evidence of tectonic surface

ruptures suggests that the Northridge fault did not extend to the surface. This observation is consistent with seismologic and geodetic data, which show that all slip occurred at depths below 5 kilometers. However, there were observed regions of surface cracking and deformations, which were thought to be the result of strong shaking rather than direct fault ruptures. The most extensive area of ground deformation was in Portrero Canyon on the northern side of the Santa Susana Mountains, where a series of discontinuous tension cracks and normal faults had displacements of as much as 60 centimeters. None of the deformation was associated with any previously mapped surface fault. Another system of small cracks was studied in Granada Hills, where ground deformations caused numerous water- and gas-main ruptures. These features were also caused by ground shaking rather than tectonic faulting.

Extensive landslides occurred in the

younger sediments of the western Santa Susana Mountains, Oak Ridge, and Big Mountain areas. There were thousands of

[graphic]
[graphic]
« PreviousContinue »