The goal of earthquake prediction is to give warning of potentially
damaging earthquakes early enough to allow appropriate response to the
disaster, enabling people to minimize loss of life and property. The
U.S. Geological Survey conducts and supports research on the likelihood
of future earthquakes. This research includes field, laboratory, and
theoretical investigations of earthquake mechanisms and fault zones. A
primary goal of earthquake research is to increase the reliability of
earthquake probability estimates. Ultimately, scientists would like to
be able to specify a high probability for a specific earthquake on a
particular fault within a particular year. Scientists estimate
earthquake probabilities in two ways: by studying the history of large
earthquakes in a specific area and the rate at which strain accumulates
in the rock.
Scientists study the past frequency of large earthquakes in order to
determine the future likelihood of similar large shocks. For example, if
a region has experienced four magnitude 7 or larger earthquakes during
200 years of recorded history, and if these shocks occurred randomly in
time, then scientists would assign a 50 percent probability (that is,
just as likely to happen as not to happen) to the occurrence of another
magnitude 7 or larger quake in the region during the next 50 years.
But in many places, the assumption of random occurrence with time may
not be true, because when the strain is released along one part of the
fault system, it may actually increase on another part. Four magnitude
6.8 or larger earthquakes and many magnitude 6-6.5 shocks occurred in
the San Francisco Bay region during the 75 years between 1836 and 1911.
For the next 68 years (until 1979), no earthquakes of magnitude 6 or
larger occurred in the region. Beginning with a magnitude 6.0 shock in
1979, the earthquake activity in the region increased dramatically;
between 1979 and 1989, there were four magnitude 6 or greater
earthquakes, including the magnitude 7.1 Loma Prieta earthquake. This
clustering of earthquakes leads scientists to estimate that the
probability of a magnitude 6.8 or larger earthquake occurring during the
next 30 years in the San Francisco Bay region is about 67 percent (twice
as likely as not).
Another way to estimate the likelihood of future earthquakes is to study
how fast strain accumulates. When plate movements build the strain in
rocks to a critical level, like pulling a rubber band too tight, the
rocks will suddenly break and slip to a new position. Scientists measure
how much strain accumulates along a fault segment each year, how much
time has passed since the last earthquake along the segment, and how
much strain was released in the last earthquake. This information is
then used to calculate the time required for the accumulating strain to
build to the level that results in an earthquake. This simple model is
complicated by the fact that such detailed information about faults is
rare. In the United States, only the San Andreas fault system has
adequate records for using this prediction method.
[Illustration: _Using a two-color laser to detect movement along a fault
near Parkfield, California._]
Both of these methods, and a wide array of monitoring techniques, are
being tested along part of the San Andreas fault. For the past 150
years, earthquakes of about magnitude 6 have occurred an average of
every 22 years on the San Andreas fault near Parkfield, California. The
last shock was in 1966. Because of the consistency and similarity of
these earthquakes, scientists have started an experiment to “capture”
the next Parkfield earthquake. A dense web of monitoring instruments was
deployed in the region during the late 1980s. The main goals of the
ongoing Parkfield Earthquake Prediction Experiment are to record the
geophysical signals before and after the expected earthquake; to issue a
short-term prediction; and to develop effective methods of communication
between earthquake scientists and community officials responsible for
disaster response and mitigation. This project has already made
important contributions to both earth science and public policy.
[Illustration: _San Andreas fault in the Carrizo Plain, central
California._]
Scientific understanding of earthquakes is of vital importance to the
Nation. As the population increases, expanding urban development and
construction works encroach upon areas susceptible to earthquakes. With
a greater understanding of the causes and effects of earthquakes, we may
be able to reduce damage and loss of life from this destructive
phenomenon.
[Illustration: _A statue of Christ at Cemetery Hill overlooks the ruined
town of Yungay, Peru. It and a few palm trees were all that remained
standing after the May 31, 1970, earthquake._]
U.S. Geological Survey
Information Services
P.O. Box 25286
Denver, CO 80225
★ U.S. GOVERNMENT PRINTING OFFICE: 1996—421-205
[Illustration: US DEPARTMENT OF THE INTERIOR; MARCH 3, 1849]
As the Nation’s principal conservation agency, the Department of the
Interior has responsibility for most of our nationally owned public
lands and natural and cultural resources. This includes fostering sound
use of our land and water resources; protecting our fish, wildlife, and
biological diversity; preserving the environmental and cultural values
of our national parks and historical places; and providing for the
enjoyment of life through outdoor recreation. The Department assesses
our energy and mineral resources and works to ensure that their
development is in the best interests of all our people by encouraging
stewardship and citizen participation in their care. The Department also
has a major responsibility for American Indian reservation communities
and for people who live in island territories under U.S. administration.
[Illustration: (_Cover photographs, clockwise from top left_): _Mexico
City, Mexico, 1985_; _Coalinga, Calif., 1983_; _Northridge, Calif.,
1994_; _Anchorage, Alaska, 1964_; _San Francisco, Calif., 1906_; _Loma
Prieta, Calif., 1989_.]