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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 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.
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 Park field, 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 Park field earthquake. A dense web of monitoring instruments was
deployed in the region during the late 1980s. The main goals of the ongoing Park field
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.
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. |
