Tiny earthquakes reveal potentially death hidden beneath California

By Stephen Beech

Tiny earthquakes have revealed potentially deadly hidden faults beneath California.

By tracking swarms of very small tremors, seismologists are getting a new picture of the complex region where the San Andreas fault meets the Cascadia subduction zone.

It could trigger "devastating" major earthquakes, say scientists.

Study co-author Professor Amanda Thomas, of the University of California, Davis, said: “If we don’t understand the underlying tectonic processes, it’s hard to predict the seismic hazard.”

She explained that three of the great tectonic plates that make up the Earth’s crust meet at the Mendocino Triple Junction, off the Humboldt County coast.

South of the junction, the Pacific plate is moving roughly north-west against the North American plate, forming the San Andreas fault.

To the north, the Gorda plate is moving north-east to dive under the North American plate and disappear into the Earth’s mantle, a process called subduction.

But the researchers say that whatever is going on at the Mendocino Triple Junction is clearly a lot more complex than three lines on a map.

For example, a large 7.2 magnitude earthquake in 1992 occurred at a much shallower depth than expected.

The research by scientists from the US Geological Survey (USGS) and the University of Colorado, Boulder, as well as the University of California, Davis, was published in the journal Science.

First author Dr David Shelly, of the USGS, compared it to studying an iceberg.

He said: “You can see a bit at the surface, but you have to figure out what is the configuration underneath."

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The research team used a network of seismometers in the Pacific to measure very small, “low-frequency” earthquakes occurring where the plates rub against or over each other.

The quakes are thousands of times less intense than any shaking that could be felt at the surface.

The team confirmed their model by looking at how the plates respond to tidal forces.

The gravitational forces of the Sun and Moon pull on tectonic plates just as they do on the waters of the ocean.

Thomas explained that when tidal forces align with the direction in which a plate wants to move, you should see more small earthquakes.

The new model includes five moving pieces, not just three plates – and two of them are out of sight from the Earth’s surface.

At the southern end of the Cascadia subduction zone, the team found a chunk has broken off the North American plate and is being pulled down with the Gorda plate as it sinks under North America.

South of the triple junction, the Pacific plate is dragging a blob of rock called the Pioneer fragment underneath the North American plate as it moves northwards.

The fault boundary between the Pioneer fragment and the North American plate is essentially horizontal and not visible from the surface at all.

The Pioneer fragment was originally part of the Farallon plate, an ancient tectonic plate that once ran along the coast of California but is now mostly gone.

Study co-author Dr. Kathryn Materna, of the University of Colorado, Boulder, says the new model explains the shallowness of the 1992 earthquake, because the subducting surface is shallower than previously thought.

She added: “It had been assumed that faults follow the leading edge of the subducting slab, but this example deviates from that.

“The plate boundary seems not to be where we thought it was.”