On December 5, a significant 7.0 magnitude earthquake occurred offshore of Cape Mendocino, but it did not generate a tsunami. Most tsunamis are caused by major subduction zone earthquakes that involve substantial vertical movement of tectonic plates. The recent event, however, was associated with horizontal movement along the Mendocino Fracture Zone, which does not produce the necessary vertical sea floor displacement to create a tsunami. The history of seismic activity emphasizes the need for tsunami awareness and preparedness in coastal regions.
The 7.0 magnitude earthquake that occurred offshore of Cape Mendocino on December 5 was significant and had the potential to cause destruction. However, within the last century, such earthquakes in that region are not rare, with seven recorded of magnitude 7 or greater. A pertinent question arises—why was there no tsunami following this earthquake? The majority of tsunamis in the Pacific Ocean are attributed to substantial displacements of the sea floor during major subduction zone earthquakes, typically linked to the Aleutian, Japan, Peru-Chile, or Middle America trenches. In northern California, the Pacific Plate descends beneath the North American Plate, resulting in a trench formation.
Over time, the friction between these tectonic plates builds stress until a critical level is reached, leading to ruptures. These ruptures cause the continental plate’s edge to rebound, displacing the water above it, which creates a tsunami that travels both landward and seaward. Tsunamis generally reach shorelines within 10 to 15 minutes but can take several hours to traverse large distances, such as crossing the Pacific to Japan.
Due to the NOAA tsunami warning system, past tsunamis, including that generated by the 2011 Japan earthquake, resulted in minimal fatalities on the California coast, although there was significant localized damage. The subduction zone extending from Cape Mendocino to Vancouver Island has the capability to produce earthquakes of magnitude 9 or greater. In contrast, the December 5 quake was associated with the Mendocino Fracture Zone, a segment of the San Andreas fault system exhibiting strike-slip motion, characterized by lateral movement rather than vertical displacement, hence the absence of tsunami generation.
Research indicates the Cascadia Subduction Zone has historically experienced great earthquakes and associated tsunamis every few centuries. My past research involved analyzing sediment cores from the ocean floor, revealing regular sequences of deposits linked to powerful earthquakes in that region. Since the conceptual framework of plate tectonics emerged in 1968, it has been established that significant subduction zone earthquakes produce these deposits every 300 to 500 years, with the last known event occurring in January 1700; thus, a subsequent earthquake may occur at any unpredictable moment.
In terms of tsunami risk, it is crucial for residents along the Central Coast to understand the areas potentially impacted. I authored a book in 2018 that discusses local natural disasters, including seven significant tsunamis recorded in the last 125 years, particularly emphasizing the destructive tsunamis of 1946 and 1964, triggered by large earthquakes in the Aleutian Trench that primarily affected the Santa Cruz area. Further discussions will continue in future news cycles.
The article explores seismic activity, focusing on the seismic event near Cape Mendocino and its implications for tsunami generation. It identifies the geological processes resulting in earthquakes and tsunamis while explicating the distinction between subduction zone quakes versus strike-slip faults that do not create significant vertical displacements. The narrative underscores the unpredictability of earthquake occurrences and the history of tsunamis that have impacted the Central Coast, emphasizing the importance of awareness and preparation for future seismic events in the region.
In conclusion, while the recent 7.0 magnitude earthquake off Cape Mendocino was substantial, its lack of a resultant tsunami highlights the differing mechanisms of seismic activity in the region. Understanding local geology is crucial for preparedness against potential future tsunamis, as historical evidence suggests significant seismic events in the coastal area occur at irregular intervals. Continuous research into earthquake patterns remains vital for enhancing prediction capabilities and public safety initiatives along the Central Coast.
Original Source: www.santacruzsentinel.com
