Hidden Faults Discovered At A Major US Earthquake Hotspot

Scientists studying swarms of tiny earthquakes have uncovered hidden fault structures beneath Northern California, offering a clearer picture of one of the most complex and dangerous earthquake zones in the United States.

The research focuses on the Mendocino Triple Junction, an area off the coast of Humboldt County where three major tectonic plates meet. This region links the San Andreas fault with the Cascadia subduction zone and has long puzzled scientists because its underground structure does not match what appears on surface maps.

The study, published on January 15 in Science, was conducted by researchers from the US Geological Survey, the University of California, Davis, and the University of Colorado Boulder.

By analysing very small, low-frequency earthquakes-too weak to be felt by people-the team tracked how tectonic plates grind, slide, and sink beneath one another. These microquakes helped scientists map faults that are completely hidden underground.

Researchers found that instead of three tectonic plates interacting in the region, there are actually five moving pieces, including two fragments buried deep beneath the Earth's surface. One broken piece of the North American plate is being dragged down as another plate sinks into the mantle. Another buried rock fragment, known as the Pioneer fragment, is being pulled beneath California by the Pacific plate.

"If we don't understand the underlying tectonic processes, it's hard to predict the seismic hazard," said co-author Amanda Thomas, professor of earth and planetary sciences at UC Davis. 

This new model helps explain why a powerful magnitude 7.2 earthquake in 1992 occurred much closer to the surface than scientists expected. According to the researchers, the main fault surfaces are shallower and positioned differently than previously believed.

Understanding these hidden faults is crucial for improving earthquake hazard forecasts in a region capable of producing devastating quakes.