Scientists are uncovering crucial insights about small earthquakes off Northern California’s coast. This region is where the San Andreas fault meets the Cascadia subduction zone, a hotspot for potential earthquakes. A study published in Science by researchers from the U.S. Geological Survey and universities in California and Colorado sheds light on the area’s complex geology.
Coauthor Amanda Thomas, a professor at UC Davis, emphasized the importance of understanding tectonic processes for predicting seismic hazards.
The Mendocino Triple Junction is a fascinating area where three major tectonic plates converge. The Pacific plate slides northwest alongside the North American plate, forming the San Andreas fault. Meanwhile, the Gorda plate moves northeast and sinks beneath the North American plate in a process called subduction.
This isn’t as straightforward as it sounds. For instance, a magnitude 7.2 earthquake in 1992 surprised scientists by striking at a shallower depth than they’d anticipated.
David Shelly from the USGS compares studying tectonic structures to looking at an iceberg. You only see part of it above water; the rest is hidden. To get a clearer picture, Shelly’s team deployed a network of seismometers across the Pacific Northwest. They recorded tiny “low-frequency” earthquakes, which are many times weaker than ones people might feel.
The researchers also examined how these small earthquakes react to tidal forces. Just like how tides are influenced by the gravity of the Sun and Moon, tectonic plates are subtly affected too. When gravitational forces align with the natural movement of the plates, the frequency of these small earthquakes increases.
Remarkably, the study found that five tectonic pieces are at play instead of just three major plates. Two pieces are hidden beneath the Earth’s surface. At the southern end of the Cascadia subduction zone, part of the North American plate is being dragged down with the Gorda plate.
To the south, the Pacific plate is pulling a mass of rock called the Pioneer fragment underneath as it moves north. This fragment was once part of the ancient Farallon plate, which mostly disappeared over time.
This updated model also explains the unexpected location of the 1992 earthquake. According to researcher Materna, the subduction surface is not as deep as previously thought. The assumption was that faults closely follow the subducting slab’s leading edge, but this instance shows a significant deviation.
As we continue to analyze our planet’s hidden structures, understanding these complex interactions is essential for both predicting seismic activity and mitigating risks to communities along the West Coast.
For more insights on tectonic activity and its implications, check out resources from the U.S. Geological Survey and recent studies from Science.
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