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Expert Calls Shaking in Northridge No Surprise : Seismology: Scientist from Pasadena warns House panel that other faults appear capable of generating far more destructive quakes.

March 05, 1994|KENNETH REICH | TIMES STAFF WRITER

The intense shaking that marked the Northridge earthquake really was not that surprising or unusual, one of Southern California's most prominent earthquake scientists told a congressional committee in Washington this week.

Thomas H. Heaton, an expert at the U.S. Geological Survey's Pasadena field office, also warned that faults under the San Fernando Valley and other parts of Los Angeles appear capable of generating quakes that would be far more destructive, because they would last up to five times longer and result in perhaps eight times larger ground displacement.

"I have seen some expression of surprise at the intense shaking, or high ground acceleration, that was recorded above the (Northridge) earthquake," Heaton told the House Committee on Science, Space and Technology on Wednesday. A transcript of his testimony was later given to The Times.

"However, I do not find these motions to be surprising," he declared, adding that while new or more comprehensive ground acceleration instruments were in use in the San Fernando Valley and other parts of Los Angeles for this earthquake, precise eyewitness descriptions from past quakes indicate that the same kind of shaking has occurred often before.

"Although I believe that indications of such intense shaking have been with us for some time, we now have better recordings of the phenomena," he said.

More importantly, he added, it is vital for those charged with protecting people and structures to understand that while larger magnitude earthquakes would not necessarily result in higher instantaneous intensities of shaking, they would probably do much more damage.

"First, earthquakes larger than our recent Northridge earthquake will have considerably larger rupture dimensions which will result in larger regions located very close to the earthquake rupture," he said.

"Second, the duration of strong shaking will increase significantly. Buildings that were damaged in the 10 seconds of strong shaking from Northridge would be expected to suffer significantly greater damage in a larger earthquake that may last five times longer.

"Considering the extensive damage caused by the Northridge earthquake, the prospect of future earthquakes with larger areas that will experience even longer durations of intense shaking is quite disturbing," he said.

"While we hear much about the high ground accelerations from the Northridge earthquake, the displacement of the ground was generally less than 30 centimeters (11.8 inches) above the earthquake, and less than 15 centimeters (5.9 inches) in downtown Los Angeles.

"Although we do not expect peak accelerations in a very large earthquake to be much greater than we experienced directly above the Northridge earthquake, we can anticipate that the ground displacements will be much larger, perhaps in excess of 250 centimeters (just over 8 feet)."

In fact, he noted, such larger ground motions have already been recorded in a remote area of the Mojave desert adjacent to the Landers earthquake of June 28, 1992.

Such movements in an urban area could have "important consequences," he said.

"First, very tall buildings, base-isolated buildings and large bridges may experience significantly larger deformation that we have seen for past earthquakes. Second, shorter buildings whose structure elements have been damaged by intense high-frequency motion may be susceptible to collapse when their foundations are displaced large distances."

Heaton told the committee that he felt more research is essential because the consequences of larger earthquakes beneath urban areas have not been adequately considered up to now.

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