A YEAR ago this week, the city of Northridge near Los Angeles was rocked by an earthquake of magnitude 6.7 that killed more than 60 people. Now scientists have come up with disturbing evidence that an earthquake with 15 times as much energy could strike greater Los Angeles at any time, wreaking havoc on the millions of people who live there.
This alarming news comes from a team led by earthquake geologist James Dolan of the University of Southern California in Los Angeles. The researchers examined earthquake records for the Los Angeles area going back 200 years and estimated the strain building up in the region鈥檚 geological faults. They conclude that 鈥渕oderate鈥� earthquakes such as last year鈥檚 Northridge event are too puny and infrequent to release the region鈥檚 accumulating strain. Instead, it is more likely that much larger and even less frequent earthquakes release the tension (Science, vol 267, p 198).
California is prone to earthquakes because one of the Earth鈥檚 large tectonic plates, the Pacific plate, is scraping northwards past the North american plate at the rate of roughly a millimetre a week, carrying the Californian coast along with it. The two plates, however, tend to stick against one another, causing strain to build up in the region鈥檚 faults.
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In their study, Dolan and his colleagues studied the six riskiest fault systems in the Los Angeles area and considered two different scenarios. First, they calculated how many moderate, Northridge-sized earthquakes should have occurred in the past 200 years if most of the region鈥檚 strain is released by tremors of this size. They answer they came up with was 17. But the record shows just two moderate quakes in this period: Northridge, and another in 1971.
The strain could, however, be released by larger quakes of magnitudes 7.2 to 7.6. If the lion鈥檚 share of the Los Angeles area鈥檚 strain was released in this way, say the researchers, a Big One should happen roughly once every 140 years. No such earthquakes have occurred since records began, but excavations through some of the faults suggest that quakes of this size might have occurred in the past.
There is a more reassuring explanation for the area鈥檚 earthquake deficit. Much of the region鈥檚 stresses could be released by tens of thousands of tiny tremors. Dolan, however, thinks that this 鈥渁seismic creep鈥� is extremely unlikely. It should, he says, produce visible signs at the surface such as cracked roads and bent railway tracks.
Dolan concludes that the LA region must get ready for the worst. 鈥淚 think we have to 鈥� prepare for the possibility of earthquakes occurring in the future in LA that are much, much, larger than anything we鈥檝e experienced thus far.鈥�