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3. How Scientists Estimate Earthquake Probability
Probabilities Based on Frequency
One way to determine the likelihood of future large earthquakes is
to study the past frequency of such earthquakes. Since 1836, there
have been five earthquakes in the San Francisco Bay Area with a magnitude
of 6.75 or higher. If earthquakes struck randomly over time, the region
would expect another earthquake of this same magnitude in the next
30 years with about a 50 percent probability.
But scientists know that earthquakes do not always occur randomly
over time. In some regions, such as the Bay Area, large earthquakes
are more frequent at some times than others. An example of this
clustering can be seen in the timeline to the left: there were 18
earthquakes of magnitude 6 or larger in the Bay Area during the
75 years between 1836 and 1911; yet there were no events of this
magnitude during the 68 years between 1911 and 1979. Apparently,
the amount of movement during the great San Francisco earthquake
of 1906 was large enough to reduce strain throughout the region,
so that only one large earthquake followed.
Since 1979, however, there have been four earthquakes of magnitude
6 or greater, leading up to the recent 7.1 Loma Prieta earthquake.
It seems likely that in 1979 we began a new era of major earthquake
activity similar to the era before 1911. Geologists are now concerned
that the strain along the faults has built up again and that more
large earthquakes are possible. If the level of earthquake activity
during the next few decades is similar to activity between 1836
and 1911, then the probability of a magnitude 7 earthquake in the
next 30 years is about 75 percent.
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