Scientists lost in a tangle of fault lines

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The Independent Online
SEISMOLOGISTS yesterday tried to locate the epicentre of the Los Angeles earthquake in an attempt to find the geological fault responsible.

They are trying to assess whether ground movements have increased the possibility of the 'Big One' - a giant tremor expected along the huge San Andreas Fault which runs from San Francisco to Los Angeles.

Roger Musson, a seismologist at the British Geological Survey in Edinburgh, said it was difficult to tell whether yesterday's earthquake will increase the chances of a much bigger one. 'The geology in the area is so complex that one earthquake in one fault may or may not throw stresses into another,' he said.

He said initial seismic data indicated that the Santa Monica Fault, which runs from east to west, had moved. Other scientists said the smaller San Fernando Fault, north of the Santa Monica Fault, could have been responsible. The US Geological Survey's National Earthquake Centre in Colorado said the strongest tremor happened 25 miles west of downtown Los Angeles.

Yesterday's quake brought two mountain ranges - the Santa Monica and San Gabriel mountains in the San Fernando Valley - slightly closer together, said Jim Mori, a seismologist with the US Geological Survey. 'Eventually these mountain ranges will hit each other and become one mountain,' he said.

California is rich in faults because it lies on the boundary of the Pacific and North American tectonic plates, which are slowly grinding past each other.

The San Andreas Fault lies directly on top of the boundary between the two plates, and tremors along this line are feared most because of their huge destructive potential. In 1906, a quake on the San Andreas Fault devastated San Francisco, causing a huge fire and killing 700 people.

The last large earthquake on the southern end of the San Andreas Fault was in 1857, and geological research suggests they have occurred every 150 years over the past 1,500 years. The US Geological Survey has on this basis predicted that this section of the fault - which could nudge Los Angeles into the ocean - is due for a big earthquake 'within the next few decades'.