Faults May Give Clues to Next Quake

A group of scientists has a theory: Suppose the major faults along the coast between Los Angeles and Ensenada are all connected. And suppose that over the last several hundred years, there have been a series of quakes along various segments, moving steadily northward.

If this is true, the destructive 1933 Long Beach quake on the Newport-Inglewood fault would be just the latest in a sequence of temblors, with the next one slated to occur along the fault line between Compton and Beverly Hills.

Such a quake could be devastating because it would take place in one of the most densely populated areas of Southern California. The magnitude 6.4 Long Beach quake hit before the area was heavily populated, yet it killed 120 people.

Two geology professors, doing their research under the auspices of the Southern California Earthquake Center, offer this thesis in a publication called Seismological Research Letters. This journal gives researchers freer rein than other scientific publications to speculate on theories that have not been completely proven.

Lisa B. Grant of UC Irvine and Thomas K. Rockwell of San Diego State acknowledge that their thesis is being fine-tuned. One reason is because some of the faults cannot be precisely studied in terms of the timing of past earthquakes because they are under the ocean. Several of the quakes they write about predate the establishment of the Spanish missions and historical records. As a result, their dates are uncertain, based on radiocarbon dating found in sediments and accurate only within 125 years or so.

“We made it clear it was speculative,” Rockwell said in an interview. “But every place we studied has produced an earthquake in the past few hundred years. What we don’t know is when the northern part of the Newport-Inglewood last ruptured.”

Caltech seismologist Egill Hauksson calls the work by Grant and Rockwell “bold,” even though he may agree with their caveats.

“Bold” is a word scientists sometimes use when they talk about basing a thesis on uncertain facts. But Hauksson himself joined in published speculation last year that fits into the theory. His article said that two moderate magnitude 4 quakes near Beverly Hills in 2001 could be precursors of a larger quake on the northern segment of the Newport-Inglewood fault.

“We are suggesting there’s a pattern, and if we’re correct it would be good to consider how to prepare for such a quake,” Grant said, adding that they are not predicting a quake at a specific time. “We should increase our awareness of the hazards associated with the Newport-Inglewood Fault Zone. It could be 1933 is only part of the story. There could be a quake further north.”

Rockwell said that radiocarbon dating indicates the recurrent interval between quake sequences in the coastal faults may be on the order of thousands of years, although the present sequence has taken only a few hundred years to unfold.

“There apparently have been four major quakes on the Rose Canyon fault near San Diego in the last 8,000 to 10,000 years,” he said. “There may have been only five big quakes in the last 10,000 years on the Newport-Inglewood fault in Orange County.”

But, he added, once one of these rare quake sequences begins, only a few hundred years may pass before it is finished.

Sue Hough, a geophysicist with the U.S. Geological Survey office in Pasadena and the editor of Seismological Letters, said she had decided to run the Grant-Rockwell thesis in accord with her policy of being “sort of speculative but in a reasonable way.”

“It’s easy after the fact to say that a quake has occurred,” she said. “But now people are looking to apply perceptions of past experience [before the beginning of historical records]. That kind of science is sort of in its infancy. It’s a brave thing to do, but that’s where the action is.”