Uncovering Tsunamigenic Earthquakes Affecting Southern California

Map of regional peak tsunami amplitude in meters resulting from an earthquake on the Pitas Point and Lower Red Mountain fault system. Source: Kenny J. Ryan (UC Riverside).

How well do we understand local-source tsunami hazards in Southern California? That was the question of the night at a town hall in Ventura during California’s Tsunami Preparedness Week (March 27 - 31), attended by more than 250 community members. Rick Wilson (CGS), Kevin Miller (Cal OES), and Kevin McGowan (Ventura OES) engaged residents in discussions about tsunami science and preparedness. During break-out sessions, residents met the people behind the scientific research, hazard mapping, and risk mitigation local to their area and around the state.

SCEC scientists James Dolan, Tom Rockwell, John Shaw, David Oglesby, Gareth Funning, and Scott Marshall launched the Ventura Special Fault Study Area (SFSA) initiative in 2012 to investigate Southern California's earthquake and tsunami hazard in the Ventura region. Their research presented one scenario in which onshore and offshore faults near Ventura County could rupture together. This multi-fault rupture could produce an earthquake up to a magnitude 7.7. A recent update suggested that a magnitude 8.0 earthquake may even be possible. If that kind of energy release originated offshore, a significant, local tsunami would send waves up to 30 feet toward Santa Barbara, and along the city of Ventura and Oxnard coastlines, waves up to 20-25 feet. (See Ryan, K. J., E. L. Geist, M. Barall, and D. D. Oglesby [2015], Dynamic models of an earthquake and tsunami offshore Ventura, California, Geophys. Res. Lett., 42, 6599–6606, doi:10.1002/2015GL064507.) The most up-to-date and significant results from the Ventura SFSA initiative were presented at a workshop during the 2016 SCEC Annual Meeting in September.

"We all need to recognize that the earthquake hazard doesn’t end at the water’s edge," said SCEC scientist David Oglesby (UC Riverside). Graduate student Kenny J. Ryan (UC Riverside) explained: "For our study, the shape of the coastline and seafloor produce the most interesting effects on the tsunami, causing a southward moving tsunami to refract – and therefore rotate – and focus on the Ventura/Oxnard area. Unfortunately, the Ventura/Oxnard area has relatively flat topography along the coast, so a tsunami can inundate that area quite effectively." 

Topographic/bathymetric map of onshore/offshore Southern California, with height and depth in meters. The Lower Red Mountain and Pitas Point faults are considered in this study. Triangles indicate direction of dip; faults without triangles are considered strike-slip. Letters show approximate (central) city locations: SB = Santa Barbara; V = Ventura; O = Oxnard. Inset shows the map boundary in black.

Distant-Source Tsunamis

Most tsunamis are generated by tectonic collision at major subduction zones, where a very dense oceanic plate subducts under a less dense continental plate. This type of convergence occurs over an area of several hundreds of miles and builds stress and strain over time. When that strain is overcome and released in the form of a big earthquake, a lot of water may be quickly displaced, resulting in a tsunami.

While we do not have major subduction zones near Southern California, we can be affected by distant-source tsunamis, known as teletsunamis. Teletsunamis can affect communities all around the Ring of Fire (the geologically active perimeter of the Pacific, where most of the biggest earthquakes, tsunamis, and volcanic eruptions occur). How much they devastate depends on where they originate, how big they are, and if the faulting mechanism is tsunamigenic.

Source: iris.edu

Major subduction zones off the coast of Alaska, Japan, Chile, and the Pacific Northwest are the most likely culprits for generating tsunamis that could affect California. These tsunamis present danger to the people and infrastructure along our beaches, bays, and rivers. The most recent example was that generated by the 2011 Japan (Tohoku) Earthquake and 2010 Chile Earthquake. Compared to thousands local to the source who lost their lives and properties, damage to California was minor overall. The only confirmed death in Southern California from a tsunami was from a drowning, when the 1964 Alaska Earthquake's tsunami surged into the Cerritos Channel near Long Beach's Alamitos Bay, according to the USC Tsunami Research Center.

Other examples of distant-source tsunamigenic earthquakes that could affect California include:

  • "The Really Big One", an earthquake along the Cascadia Subduction Zone, where three dense plates (the Gorda, Juan de Fuca, and Explorer) are subducting under the less-dense North American plate. The last time it ruptured was in 1700, so Cascadia has been building up strain for over 300 years and it will likely rupture again. Tsunami wave heights in Southern California would be on the order of 5-10 feet.
  • In 2014, the U.S. Geological Survey released the SAFRR Tsunami Scenario, where an earthquake off the Aleutian Islands of Alaska could send tsunami waves (ranging from 5-10 feet) to the shores in Southern California. The Ports of Long Beach and Los Angeles would be greatly affected, costing up to $100 million in damage. Repairs may take years, temporarily halting and slowing vital contributions to the local, regional, and national economies.
  • The biggest earthquake on record is the 1960 Chile Earthquake which spawned a Pacific-wide tsunami. Those near the source felt ground shaking on the level of VIII on the Modified Mercalli Intensity Scale. People on the coast were subjected to wave heights of more than 75 feet. Around the Pacific, wave heights peaked at 10-15 feet, with damage and casualties reported as far as Japan and Hawaii. For California, wave heights were minimal but caused damage to ports and marinas.

Source: www.YouTube.com/greatshakeout

General Tsunami Science

Tsunamis are not like normal waves. One tsunami is comprised of multiple waves, or surges of water, coming from near or far sources such as a large undersea earthquake. Each wave is wide, long, and deep, and the waves compress once they meet the natural rise of the seafloor, or shoal, and rise in height.

Additionally, height is one factor, but velocity is another. How fast the waves are traveling has a lot to do with the generation of whirlpools and inundation into rivers and bays too. One can see the strength of low, but fast-moving, currents that tsunami can generate from this footage taken in Long Beach, California as a result of the 2010 Chile Earthquake.

Cindi Preller at the National Tsunami Warning Center in Palmer, AK explains the origin of the word “tsunami”. Japanese fishermen would return home after trips to see their harbors destroyed, but what they observed was not just an unusually large wind-driven wave, tidal wave, or swell from a distant storm. These inexplicable waves were called "tsunami", which means harbor wave.

Tsunamis can also be caused by meteorites crashes, onshore and underwater landslides, and volcanic eruptions and flank collapses. Although rare, big hurricanes can cause meteotsunamis when the wind resonates with pressure change from the storm system.

Staying Safe Where the Waves Break

Recent science discoveries help us to better understand possible tsunami hazards in our region and beyond. Using this science, we can come together – and inspire others – to become better prepared and informed, and increase our community’s resilience. 

One such effort is TsunamiZone.org, led by the National Weather Service, the California Geological Survey, the California Governor’s Office of Emergency Services, and the Southern California Earthquake Center. The website provides support for California’s Tsunami Preparedness Week, a week each March geared to improving our level of tsunami preparedness. In 2017, more than 140,000 Californians participated. For more resources including tsunami safety graphics, evacuation walk guides, and information for boaters go to: www.TsunamiZone.org/resources. Learn more about tsunami hazard in your area by clicking the "Know Your Zone" tab and using the Cal OES MyHazards Tool and CGS Tsunami Inundation maps.

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