Bay Area Could Face Big Temblor Despite Loma Prieta Quake

Scientists have confirmed suspicions that the 1989 Loma Prieta earthquake that devastated wide areas of the San Francisco Bay Area did not occur on the main San Andreas Fault, suggesting that the chances of a major quake hitting the area again may not be as low as had been thought.

Many experts had thought that the 7.1-magnitude temblor relieved so much strain in that segment of the fault that the chances of another major quake during the next 30 years were considered extremely low.

However, new evidence shows that the 1989 quake did not occur on the main fault, and thus the San Andreas could still be building toward a large quake, David Schwartz of the U. S. Geological Survey reported during the national convention here of the Geological Society of America.

Schwartz and several colleagues dug three trenches across one area of the San Andreas, about nine miles east of the epicenter of the Loma Prieta quake. The trenches cross a part of the San Andreas known to have ruptured during the great quake of 1906, and ample evidence of that major temblor was found.

However, Schwartz said Monday that there was no evidence of the 1989 quake, so that shaker must have occurred on another fault. Thus, strain could still be building in the San Andreas.

He declined to speculate on the likelihood of a major quake in that area during the next 30 years, but he indicated that it could be as high as 25%. After the 1989 quake, the U. S. Geological Survey’s Working Group of California Earthquake Probabilities indicated that the chances over that period were probably near zero.

Schwartz, who is a member of that group, said that projection will probably have to be revised, but he said the new estimate will have to wait until the evidence can be more fully analyzed.

“This raises a lot of questions,” he said.

Some scientists had already speculated that the Loma Prieta quake did not hit on the San Andreas because that fault normally moves horizontally and the 1989 quake moved vertically. The new evidence confirms that the quake occurred on some other fault.

Trenches allow scientists to study previous quakes by dating such things as stream beds that were disturbed by movement along the fault. If researchers can determine when the stream bed was ruptured, through such procedures as carbon-14 dating, they can determine when the fault moved.

In addition to evidence of the 1906 quake, the researchers found the first evidence of what appears to have been a major quake around 1650.

That discovery coincides with other evidence collected by the Geological Survey’s Carol Prentice in the Point Arena area north of San Francisco. Her work also indicates that a major quake hit that area around 1650.

Because the two sites are more than 100 miles apart, if the evidence was produced by the same quake it must have been quite large.

Schwartz suggested that it could have been greater than an 8-magnitude quake, and if so, this is the first evidence of a great quake in that region before the 1906 catastrophe.