Long-lost tectonic fragment could shake Northern California

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An earthquake-generating piece of tectonic plate has been discovered beneath Northern California. It’s attached to the bottom of the North American plate like chewing gum stuck to a shoe.

Using numerous tiny, almost imperceptible earthquakes that can help reveal complex faults beneath the Earth’s surface, researchers have identified this previously hidden danger. The plate may have been the cause of the 7.2-magnitude Mendocino earthquake in 1992, researchers reported Jan. 15. Science.

Beneath the peaceful beauty of Northern California’s Lost Coast lies a complex and turbulent geological jumble, one of the most active tectonic regions in the United States. This is where the San Andreas Fault meet him Cascadia subduction zone. Three sections of the Earth’s crust meet in this region, a clash of titans known as the Mendocino Triple Junction: the North American Plate and the Pacific Plate crush sideways, while the smaller Gorda Plate plunges beneath the North American Slab.

In 1992, 7.2 magnitude earthquake rocks Cape Mendocino regiondamaging buildings and roads and triggering landslides and a small tsunami. Surprisingly for an earthquake of such magnitude, the epicenter turned out to be only 10 kilometers deep, which left scientists perplexed; the subducting slab of the Gorda plate was known to be at least twice as deep.

Some have proposed the existence of a “slab gap”, a shallow space formed by the friction of one plate dragging another, with magma from the mantle gushing into this window and generating earthquakes. But another possibility was that there was something else there: a fragment of a tectonic plate.

Tectonic traffic jam

The Mendocino Triple Junction is a complex jumble of colliding crustal plates. The black arrows represent movements relative to the North American plate: the Pacific plate sweeps across it laterally, creating the San Andreas fault system, while the Gorda plate sinks below into the Cascadia subduction zone. The blue arrows show how the Pacific and Gorda plates are moving laterally relative to each other. Using tiny earthquakes generated near the southern edge of the Gorda Plate (shown in red), researchers discovered the “Pioneer” fragment, a remnant of the ancient Farallon Plate that is dragged beneath the North American Plate.

The declining Gorda Plate is actually one of the last remnants of the ancient Farallon Plate. Most of it descended into the mantle, but a fragment could have been trapped during subduction and stuck to the overlying North American plate as it passed. And now this fragment can be dragged under the plate.

How to see this hidden fragment was the problem — it’s not really visible from the surface, say David Shelly, a U.S. Geological Survey geophysicist based in Golden, Colorado, and his colleagues. The team decided to visualize the region’s complex tectonics using swarms of tiny earthquakes. These earthquakes are imperceptible to humans but detectable by seismometers; they reproduce quickly, forming a long-lasting seismic signal called tremor. By “stacking” the abundant occurrences of these events, researchers can determine a more precise depth and location for each, ultimately delineating fault lines and other subsurface features.

The team zoomed in on a region of tremor near the southern edge of the subducting Gorda plate. The tiny earthquakes, they found, were generated by a sideways-moving piece of crust located about 10 kilometers below the surface. According to the team, this indicates a distinct plate fragment, shallower than the subducting slab. They nicknamed it the Pioneer Fragment.

In identifying this hidden fragment, the team also discovered a buried plate boundary, a nearly horizontal fault line between the Pioneer fragment and the overlying North American plate that can be a source of strong but shallow earthquakes – such as the 1992 Cape Mendocino earthquake.

This would mean that the triple junction is instead a quadruple junction – but in fact, there is a fifth piece of tectonic plate hidden beneath the surface, the researchers say. Beneath the southern end of the Cascadia subduction zone is another buried fragment of crust, a piece of the North American plate that broke away from the main plate and is now being pulled toward the mantle by the sinking Gorda plate.

Shedding light on this region’s subsurface helps identify and prepare for previously unknown seismic risks, says Matthew Herman, a geophysicist at California State University, Bakersfield, who was not part of the new study.

“We often think of triple junction regions as a simple intersection of three styles of simple plate boundaries,” says Herman. This study “is part of a growing body of research showing that we cannot understand the full picture” without understanding how Cascadia subduction interacts with the San Andreas Fault system. “This fragment of Pioneer…may present a significantly different type of seismic risk than we would expect. »