A year-long study jointly conducted by the US Geological Survey, California Geological Survey and Humboldt State University has identified several sites along the California coastline that have been confirmed as locations of paleotsunami deposits.
These results not only provide insight into past tsunami events, but help to assess the potential for future ones as well.
Researchers chose more than 20 locations for evaluation along the entire coast of California, from Crescent City in the north to the Tijuana River in the south.
Due to the majority of the state’s coastline being rocky (an unlikely preservation environment for tsunami deposits), the chosen study sites were in coastal marshlands and lagoons, where chances of deposition preservation are much higher.
How Are Tsunami Deposits Identified?
Tsunami deposits form when the wave picks up sand from the beach and then pushes it inland. When deposited in marshes and lagoons, these distinct sand layers stand out amongst the typical dark, silty, organic-rich muds typical of these environments, making them easily distinguishable from one another.
The researchers obtained these deposits by taking core samples of shallow surface sediments in the locations selected. Sites at two of these locations revealed evidence of past tsunamis: Crescent City and Half Moon Bay. They found two separate deposits at Crescent City – one from 1700 and another from 1964; the deposit at Half Moon Bay was left by a 1946 tsunami.
The team dated deposits using cesium-137 and radiocarbon methods, with the ages of the deposits correlating with the dates of large tsunami-producing earthquakes: the massive M9.0 Cascadia event in 1700, the M8.1 Aleutian Island quake in 1946 and the largest earthquake in US history, the M9.2 1964 Alaska quake.
Future Tsunami Threats to California: Local Source Versus Distant Source
The newly obtained information about past tsunami events in California will now help determine which areas along the coast are most vulnerable to a tsunami hit in the future.
Several factors determine which areas of the state are most at risk: the shape of the coastline, bathymetry (ocean floor topography) and population in low-lying areas. Also an obvious factor in assessing the tsunami threat is its source origin.
Because of the San Andreas fault running along the state’s entire length, the whole coastline is locally at risk of being hit by a tsunami generated by an offshore underwater landslide triggered by an earthquake. While these events are rare, they are possible.
The greatest tsunami hazard to California’s coastline is another local threat – the Cascadia Subduction Zone.
When asked by Decoded Science which areas of the California coast are most vulnerable to a tsunami hit, Rick Wilson, Senior Engineering Geologist with the California Geological Survey and one of the study’s lead authors, stated: “The area with the highest tsunami hazard is north of Cape Mendocino because of the existence of the Cascadia Subduction Zone fault offshore. This fault is capable of producing a magnitude 9 earthquake and very large local tsunami similar to the 2011 earthquake and tsunami in Japan.”
To the area south of Cape Mendocino, the threat of a tsunami would likely come from a distant source somewhere else in the Pacific Ocean, such as Alaska or the Aleutian Islands, said Wilson. Wave heights from large distant source tsunamis could reach 15 to 30 feet in places north of Point Conception in Santa Barbara County, and south of Point Conception waves would likely be smaller at ten to 15 feet high.
Smaller wave heights don’t necessarily mean less danger, Wilson warned – because this area has a large coastal population, it is still moderately vulnerable.
Is California “Overdue” for a Tsunami Hit?
Distant source tsunamis can affect the California coastline at any time. However, another source of concern lies to the north – the Cascadia Subduction Zone. It last ruptured in 1700, unleashing a M9.0 earthquake and causing a tsunami that affected areas as far away as Japan. As tsunami deposits along the US west coast indicate this fault produces earthquakes and tsunamis on average every 300 to 500 years, we are currently within the 200-year window of time for another Cascadia event to occur – and are therefore under threat.
Earth Processes are Unpredictable
Although earthquakes and tsunamis are unpredictable, knowing that the state faces the constant threat of both means residents should be vigilant about being prepared for both.
Wilson emphasized the importance of citizens educating themselves about the threat of tsunamis and knowing what to do: “Ultimately, it is up to each member of the public to do their own homework: 1) research if they live, work, or play in a tsunami hazard zone; 2) know what the warning signs are for a tsunami are; and 3) have a plan and know what to do if a tsunami is coming your way. This will help save your life and the lives of the ones you love,” he concluded.