Researchers say Cascadia and San Andreas faults may be linked

Provocative analysis of sea-floor cores suggests that quakes on the Cascadia fault off California can trigger tremors on the San Andreas.

Two of North America’s most fearsome earthquake zones could be linked.

A controversial study argues that at least eight times in the past 3,000 years, quakes made a one–two punch off the west coast of the United States. A quake hit the Cascadia fault off the coast of northern California, triggering a second quake on the San Andreas fault just to the south. In some cases, the delay between the quakes may have been decades long.

The study suggests that Cascadia, which scientists think is capable of unleashing a magnitude-9 earthquake at any time, could set off quakes on the northern San Andreas, which runs under the San Francisco Bay Area.

Several earthquake scientists told Nature that more work is needed to confirm the provocative idea. Researchers have long considered the two faults seismically separate.

Chris Goldfinger, a geologist and palaeoseismologist at Oregon State University in Corvallis, will present the findings on 13 December at a meeting of the American Geophysical Union in San Francisco. “This is mostly a circumstantial case,” he says. “I don’t have a smoking gun.”

Tantalizing clues
Goldfinger and his colleagues first suggested in 2008 that earthquakes in the southern part of Cascadia could trigger quakes on the northern San Andreas1. The scientists reported finding layers of churned-up, sandy sediment in sea-floor cores drilled offshore. These layers, called turbidites, usually form when earthquakes shake the sea floor, causing underwater landslides. The researchers reported finding turbidites in Cascadia that seemed to form just before similar turbidites near the San Andreas — perhaps as a Cascadia quake triggered a San Andreas one.

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