Tiny earthquakes are revealing a dangerous secret beneath California
By closely tracking swarms of extremely small earthquakes, scientists are gaining new insight into a dangerous and complicated region
By closely tracking swarms of extremely small earthquakes, scientists are gaining new insight into a dangerous and complicated region off the Northern California coast. This area marks the meeting point of the San Andreas fault and the Cascadia subduction zone, a place capable of producing powerful and destructive earthquakes. The research was carried out by scientists from the U.S. Geological Survey, the University of California, Davis and the University of Colorado Boulder, and was published Jan. 15 in Science.
“If we don’t understand the underlying tectonic processes, it’s hard to predict the seismic hazard,” said coauthor Amanda Thomas, professor of earth and planetary sciences at UC Davis.
A Seismic Crossroads Beneath the Coast
The Mendocino Triple Junction lies offshore from Humboldt County, where three major tectonic plates converge. South of this junction, the Pacific plate moves roughly northwest alongside the North American plate, creating the San Andreas fault. To the north, the Gorda (or Juan de Fuca) plate moves northeast and sinks beneath the North American plate, descending into the Earth’s mantle in a process known as subduction.
Although this arrangement may appear simple on a map, scientists say the real structure below the surface is far more complicated. One striking example came from a large (magnitude 7.2) earthquake in 1992 that struck at a much shallower depth than expected.
Looking Below the Surface
First author David Shelly of the USGS Geologic Hazards Center in Golden, Colo., said the challenge is similar to studying an iceberg.
“You can see a bit at the surface, but you have to figure out what is the configuration underneath,” Shelly said.
To uncover that hidden structure, Shelly and his colleagues used a dense network of seismometers across the Pacific Northwest. The instruments recorded extremely small “low-frequency” earthquakes that occur where tectonic plates slowly slide against or over one another. These tiny events are thousands of times weaker than earthquakes people can feel at the surface.
The team tested their underground model by examining how these small earthquakes respond to tidal forces. Just as the gravitational pull of the Sun and Moon affects ocean tides, it also places subtle stress on tectonic plates. When those forces line up with the natural direction of plate movement, the number of small earthquakes increases, Thomas said.
Five Moving Pieces Beneath Northern California
The researchers found that the region involves five moving pieces rather than just three major plates, with two of them hidden deep below the surface.
At the southern end of the Cascadia subduction zone, the team discovered that a portion of the North American plate has broken away and is being dragged downward along with the Gorda plate as it sinks beneath North America.
South of the triple junction, the Pacific plate is pulling a mass of rock known as the Pioneer fragment beneath the North American plate as it moves northwards. The fault separating the Pioneer fragment from the North American plate lies nearly flat and cannot be seen at the surface.
The Pioneer fragment was once part of the Farallon plate, an ancient tectonic plate that once extended along the California coastline and has since mostly disappeared.
Explaining a Puzzling Earthquake
This updated model helps explain why the 1992 earthquake occurred at such a shallow depth. According to Materna, the surface being pushed beneath North America is not as deep as scientists previously believed.
“It had been assumed that faults follow the leading edge of the subducting slab, but this example deviates from that,” Materna said. “The plate boundary seems not to be where we thought it was.”
The work was supported by a grant from the National Science Foundation.
