San Francisco Earthquakes

One technique to assess the possibility of future major earthquakes is to investigate the previous 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 hit randomly throughout time, the area would anticipate another earthquake of this similar size in the next 30 years with around a 50 percent likelihood.USGS frequency table showing earthquakes and magnitudes from 1836 to 1989.
But scientists know that earthquakes do not necessarily occur randomly across time. In certain locations, such as the Bay Area, big earthquakes are more prevalent at some periods than others. An example of this clustering can be seen in the timeline above: there were 18 earthquakes of magnitude 6 or bigger in the Bay Area over the 75 years between 1836 and 1911; however, there were no incidents of this magnitude during the 68 years between 1911 and 1979. The amount of movement during the enormous San Francisco earthquake of 1906 was substantial enough to lessen tension across the area, such that just one huge earthquake followed.
USGS earthquake probability and magnitude chart.
Since 1979, however, there have been four earthquakes of magnitude 6 or more, building up to the current 7.1 Loma Prieta earthquake. It is probable that in 1979 we entered a new period of significant earthquake activity akin to the age before 1911. Geologists are now afraid that the strain along the faults has built up again and that further major earthquakes are likely. If the level of earthquake activity throughout the next several decades is comparable to activity between 1836 and 1911, then the chance of a magnitude 7 earthquake in the next 30 years is roughly 75 percent.
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