Mini Earthquakes Expose Hidden Tectonic Features Buried Beneath Earth’s Surface

The hidden processes that drive earthquakes cause a lot of commotion that we don’t ever see. Tectonic plates are always on the move, colliding and sliding into each other. In Northern California, a bustling scene where the San Andreas fault meets the Cascadia subduction zone shows how the action happening underground is a lot more complex than it seems.

A new study published in Science has put forward an updated perspective on the Mendocino triple junction, the point where the Gorda, North American, and Pacific tectonic plates meet. Here, swarms of small earthquakes that aren’t perceptible to people have revealed the ways that tectonic plates move and the role of other moving pieces that are hidden beneath Earth’s surface.

Read More: The Hidden Layers of Earth and Tectonic Plate Movements

Seismic Unrest in the Pacific

Off the coast of Cape Mendocino in northern California, tectonic plate movement stirs up some of the most significant seismic activity in the U.S. This is where the nearly 800-mile-long San Andreas Fault — the boundary between the North American and Pacific plates — intersects with the southern edge of the 600-mile-long Cascadia subduction zone.

The meeting of these two juggernauts could bring about massive earthquakes, which is why researchers are intent on figuring out what’s happening beneath the surface.

“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 the University of California, Davis, in a statement.

The main culprits are the three major plates in the area. The Pacific plate is currently moving northwest against the North American plate, forming the San Andreas Fault. North of this, the Gorda plate is slipping under the North American plate and sinking into Earth’s mantle, which is an example of subduction.

Tracking Tiny Earthquakes

The Mendocino triple junction may be a hotbed for earthquakes, but researchers don’t even know the whole picture of what’s going on beneath the Pacific coast. One mystery that they’ve never been able to answer is why the magnitude 7.2 earthquake that struck Cape Mendocino in 1992 occurred at a shallower depth than expected (shallow earthquakes tend to do the most damage).

The researchers involved with the new study aimed to see what was really happening beneath the triple junction and its surroundings, hoping to gain insight into hidden tectonic processes.

“You can see a bit at the surface, but you have to figure out what is the configuration underneath,” said first author David Shelly, a geophysicist with the USGS Geologic Hazards Center.

To accomplish this task, the researchers measured small, “low-frequency” earthquakes at the triple junction with a network of seismometers. These earthquakes, the product of plates rubbing against or over each other, are thousands of times less intense than the shaking produced by larger quakes that we would feel at the surface, according to the researchers.

Finding a Tectonic Fragment

The earthquake observations found that tidal forces — the gravitational forces of the sun and moon — pull on plates in ways that could possibly spur more small earthquakes.

In addition, the researchers noticed additional features concealed beneath Earth’s surface. One of them is a chunk at the southern end of the Cascadia subduction zone that has broken off of the North American plate. The chunk is now being forced underneath North America alongside the Gorda plate.

Experts observed another feature south of the triple junction, where a blob of rock called the Pioneer fragment is being dragged underneath the North American plate by the Pacific plate. This fragment, the researchers say, was once part of an ancient tectonic plate called the Farallon plate, which has mostly disappeared by now.

Lastly, the observations have helped clear up the mystery of the unexpectedly shallow 1992 earthquake. They’ve shown that the subducting surface at the triple junction is shallower than previously thought, redefining where the plate boundary exists.

Read More: Colliding Tectonic Plates Are Making the Iberian Peninsula Rotate Clockwise

Article Sources

Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article: