Hidden Faults Beneath Northern California Reveal Earthquake Risks

Scientists are gaining new insights into the complex and dangerous region where the San Andreas fault meets the Cascadia subduction zone off the coast of Northern California. Recent research, utilizing data from tiny earthquakes, reveals a more intricate network of tectonic activity than previously understood, potentially impacting earthquake hazard assessments.

A Seismic Crossroads

The Mendocino Triple Junction, located offshore from Humboldt County, is where three major tectonic plates converge. South of this junction, the Pacific plate moves northwest alongside the North American plate, creating the San Andreas fault. North of the junction, the Gorda plate (or Juan de Fuca plate) subducts – slides – beneath the North American plate, descending into the Earth’s mantle USGS.

Uncovering Hidden Structures

Researchers from the U.S. Geological Survey, the University of California, Davis, and the University of Colorado Boulder used a dense network of seismometers to detect extremely modest, low-frequency earthquakes. These events, thousands of times weaker than those felt at the surface, occur where tectonic plates slowly slide against each other UC Davis. By analyzing how these small earthquakes respond to tidal forces – the gravitational pull of the Sun and Moon – scientists were able to model the underlying structure.

Five Moving Pieces

The research revealed that the region involves five moving pieces rather than the previously assumed three. A portion of the North American plate is being dragged downward along with the Gorda plate as it subducts. South of the triple junction, the Pacific plate is pulling a mass of rock called the Pioneer fragment beneath the North American plate. The fault separating the Pioneer fragment is nearly flat and hidden from surface view.

Explaining Past Seismic Activity

This new model helps explain the unexpectedly shallow depth of a magnitude 7.2 earthquake that occurred in 1992. Scientists previously believed the faults followed the leading edge of the subducting slab, but this event suggests the plate boundary is not where it was thought to be UC Davis.

Cascadia and San Andreas: A Potential Link

Recent studies suggest a possible link between earthquakes along the Cascadia Subduction Zone and those along the San Andreas fault. While the exact nature of this connection is still under investigation, evidence indicates that these faults may have ruptured in synchrony at times over the past 3,000 years TVW. Further research is needed to determine if one fault can trigger the other Scientific American.

Implications for Earthquake Preparedness

Understanding the complex interactions between these faults is crucial for improving earthquake hazard assessments and preparedness efforts in the Pacific Northwest. The Cascadia Subduction Zone, extending from northern California to southern British Columbia, has historically experienced fewer earthquakes than expected USGS, but a major rupture could cause significant shaking and a large tsunami.