2. Some scientists believe that the estimate may be two low

Some scientists believe that the 67 percent probability estimate may be too low. They have noted several instances of pairs of earthquakes of magnitude 6.5 or larger in northern California, and they are concerned that the Loma Prieta earthquake could be the first quake of such a pair. An earthquake in 1865, similar to the earthquake of October 17, 1989, was followed 3 years later in 1868 by a major earthquake on the Hayward fault. Other pairs struck in 1836 and 1838, in 1892 and 1898, and in 1906 and 1911. Scientists do not understand the reason for such pairing, which may be due only to random chance.

Scientists are also concerned over an increase in the number of magnitude 5 earthquakes along the southern part of the Calaveras fault east of San Jose since 1979. A similar pattern of activity apparently preceded the Hayward fault earthquake of 1868.

The 67 percent probability does not include these additional pieces of information. Nor does it include the information suggesting that other fault segments in northern California might also be capable of producing large earthquakes. Therefore it seems prudent to consider the 67 percent chance of a large Bay Area earthquake within the next 30 years as a minimum estimate.

The increase in estimated probability from 50 percent to 67 percent between 1988 and 1990 was not because of the Loma Prieta earthquake. The increase resulted from new data on rates of strain accumulation on the Hayward fault and from new data that showed that a magnitude 7 earthquake was possible along the Rodgers Creek fault. Ongoing and future studies are also likely to produce additional data that will result in changes in probability estimates (see the 2002 report, in which scientists concluded there is a 62% probability of at least one magnitude 6.7 or greater quake striking the San Francisco Bay region before 2032), and details about calculated probabilities are still being debated by scientists. The major conclusions reached by the panel, however, are not likely to change. Most importantly, scientists agree that:

• Earthquakes of magnitude 7 and larger are highly likely within the Bay Area during the next several decades.
• Each of these events can cause much more damage than the earthquake of October 17, 1989, because each will likely be located closer to densely populated areas.
• Actions can be taken now to reduce the amount of damage and the number of deaths that are likely to result from future major earthquakes