Mendocino Triple Junction Reveals Five Tectonic Pieces, Challenging Earthquake Predictions

Researchers have identified a more intricate tectonic configuration beneath Northern California's Mendocino Triple Junction than previously understood, discovering five distinct moving pieces where three tectonic plates were thought to interact.

This finding, based on an analysis of low-frequency earthquakes, suggests that existing earthquake prediction models may require updates to better assess future seismic events in the region.

The Mendocino Triple Junction, situated off the coast of Humboldt County, represents a geologically active area where the Pacific Plate, North American Plate, and Gorda Plate converge. Previous geological research and models primarily focused on the interaction of these three major plates. However, a new study, published in the journal Science, presents a revised model indicating a greater level of complexity in the subsurface structure.

The research team, which included scientists from the U.S. Geological Survey (USGS), the University of California, Davis (UC Davis), and the University of Colorado Boulder, concluded that the triple junction effectively consists of five active tectonic pieces.

The Mendocino Triple Junction Re-evaluated

The study's primary findings include the identification and confirmation of two additional tectonic components not explicitly accounted for in earlier models:

• Pioneer Fragment: Observations indicate that the Pacific Plate is dragging a fragment of rock, known as the Pioneer Fragment—a remnant of the ancient Farallon Plate—beneath the North American Plate.
• Subducting North American Plate Section: A section of the North American Plate has broken off and is sinking into the Earth's mantle alongside the Gorda Plate through the process of subduction.

Furthermore, the research suggests that the subducting surface, where the Gorda plate is pushed beneath the North American plate, is situated at a shallower depth than previous assumptions indicated. These discoveries collectively challenge prior understandings of plate boundary locations and the configuration of subsurface faults.

Methodology and Supporting Evidence

To develop this updated model, researchers utilized data from low-frequency earthquakes. These subtle tremors, which are generally not felt by humans, provide valuable information about tectonic plate movement deep below the Earth's surface. Seismometers recorded these events across the Pacific Northwest, and the collected data was then cross-referenced with tidal-sensitivity models. These models helped to confirm how underlying rock formations respond to daily tidal stresses exerted by the Sun and Moon, which in turn validated the interpretation of the low-frequency earthquake signals and aided in refining models of underground activity and stress within the tectonic layers.

The revised understanding of the plate boundary and the shallower subduction zone is also supported by historical seismic data. Notably, a 7.2-magnitude earthquake that occurred in California in 1992 originated at a shallower depth than contemporary models had predicted at the time, aligning with the current research findings.

Implications for Seismic Hazard Assessment

The identification of these additional tectonic pieces and the updated understanding of plate boundary locations are considered crucial for the assessment of seismic hazards in the region.

Scientists involved in the study emphasize that relying solely on observations of surface features is insufficient for comprehending the complex underlying configuration, and that faults may not always follow the leading edge of a subducting slab.

This enhanced understanding is particularly pertinent for predicting earthquakes along significant fault systems in Northern California, including the San Andreas fault, which marks the boundary between the North American and Pacific plates, and the Cascadia subduction zone, where the Gorda and North American plates interact. Both of these zones are known to be capable of generating powerful earthquakes.

Researchers involved in this study include David Shelly of the USGS, Kathryn Materna from the University of Colorado Boulder, and Amanda Thomas from the University of California, Davis.