The Japan Times, July 18, 1998
Evidence of ancient earthquakes points to 'big one' in central L.A.
WASHINGTON (Reuters) Two giant prehistoric earthquakes that hit near present-day Los Angeles are evidence a similar tremor could strike the heart of the heavily populated city, researchers said on Thursday.
In a study published in the journal Science, researchers said at least two earthquakes ranging between magnitude 7.2 to 7.6 occurred in the region within the past 15,000 years.
This counters suggestions the Sierra Madre fault running 19 km north of the city is unlikely to produce such large-scale earthquakes.
"No one has demonstrated there were large earthquakes in the past like this," said Charles Rubin, a geologist at Central Washington University who led the study. "People should realize there is a potential for larger earthquakes."
The two earthquakes were six times larger than the 1994 Northridge earthquake, which struck in the San Fernando Valley northwest of downtown Los Angeles, killing 60 people and causing billions of dollars in damage, he added.
They also moved the earth much farther than a 1971 San Fernando earthquake that caused $558 million in damage and killed 64 people. That tremor moved the earth about 1.5 meters, but the two ancient earthquakes moved the ground more than an average of 5 meters.
"The damage from this kind of earthquake would be substantially different with higher ground motions over a larger area over a longer time," he said in a telephone interview.
Rubin also pointed out the San Fernando and Northridge earthquakes broke north of the city. But a tremor on the same part of the fault as the prehistoric earthquakes would head straight toward downtown Los Angeles, he added.
"Unlike the Northridge earthquake that ruptured northward away from the metropolitan region," the authors wrote, "a magnitude 7 or greater earthquake on the Sierra Madre fault would rupture southward, directing energy into the densely populated regions."
Rubin searched for active faults not disturbed by development using aerial photographs of the city from the 1920s. These pictures allowed the geologist to pinpoint where previous earthquakes had broken the surface of the earth.