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The San Andreas Fault may be linked to others – which could make the next earthquake worse

The Imperial Valley with the Colorado River on one side The Imperial Valley with the Colorado River on one side
10 Jul
2019
Models suggest that far from ending at the Imperial Valley, the San Andreas Fault may be linked to two other faults in the region, forming a continuous line from California into Mexico

Earthquake WeekThe Imperial Valley in south-eastern California is more geologically scarred than most. Bordered by the Colorado River to the east and, in part, the Salton Sea to the west, it rests upon the meeting place of several active faults.

New mechanical models produced by scientists at the Seismological Society of America now suggest that rather than existing in isolation, three of the faults in this region are linked, forming a continuous line that runs from the southern section of the San Andreas Fault, down the Imperial Fault (which crosses the border into Mexico) and finally to the Cerro Prieto fault in Baja California. This linkage could increase the likelihood that the faults rupture together – an undesirable situation which the researchers say would cause larger than expected shaking in the surrounding counties of California.

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‘Our work suggests that the San Andreas effectively does not end at the Salton Sea, but is most likely physically connected to the Imperial fault, and then to the Cerro Prieto fault in Mexico,’ said Jacob Dorsett who worked on the study as an undergraduate at Appalachian State University. ‘All other factors equal, if the San Andreas is connected to these other structures, then it makes the chances of a longer rupture – and a larger magnitude – more likely.’

No one knows when a San Andreas earthquake will happen. (Experts have said that the recent quakes in California did not happen along the San Andreas Fault and are unlikely to affect it.) People have been anticipating a major rupture of the fault for many years and seismologists say the ‘Big One’ is well overdue. It was 1906 when the northern portion last ruptured, raising San Francisco to the ground. It’s been even longer – more than 300 years – since the southern portion did the same. But that doesn’t mean it’s been quiet. The Imperial Valley has seen several earthquakes along its smaller fault lines, the largest and most famous being the El Centro earthquake in 1940. At a magnitude of 6.9 it caused widespread damage and the deaths of nine people. In 1979 the same fault ruptured again with a magnitude of 6.2 and in 1989 two more quakes occurred on the Elmore Ranch and Superstition Hills faults.

In the Imperial Valley, additional research is now needed to determine the exact relationship between the linked faults and the slip rate of each (how fast the two sides of the faults are slipping relative to one another). For now, the researchers hope that this new analysis will prove useful to seismologists who work on large-scale computational models which aim to better forecast Californian earthquakes.

Geographical July 2019CRACKING THE EARTHQUAKE CODE
The July 2019 issue of Geographical takes an in-depth look at the ways advancements in technology are improving the scientific art of earthquake detection. But will we ever be able to accurately predict these devastating incidents?
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