How Big of a Threat are Tsunamis to Orange County?

The last place you'll want to be if one rolls in
The last place you'll want to be if one rolls in
Wikimedia Commons

A slew of devastating earthquakes hit Japan and Ecuador this month, bringing back chilling memories of the massive 2011 Tohoku quake which triggered a catastrophic tsunami that killed thousands and led to the disastrous meltdown of Fukushima Daiichi Nuclear Powerplant's three reactors.

But while tremors are nothing new for Southern California residents, the possibility of a tsunami striking the region does exist, raising questions about the likelihood, frequency, and size. Earlier this year, NPR reported that the Aleutian Quake Zone could generate a tsunami as large as the 2011 one in Japan towards Hawaii and California.

So...will OC ever be Fukushima-ed?

"Virtually anywhere in the world, anywhere you live near a coast, you should be aware of tsunamis, what they are, what their signs are, and there are warning systems now globally," said Brady Rhodes, structural geologist and professor emeritus of natural hazards at Cal State Fullerton. "Now having said that, the risk of coastal areas is not the same everywhere. There's some areas that are much more prone, much more likely to have a tsunami, than others."

Rhodes, along with CSUF geological sciences professor Matthew Kirby, were part of the first research team with the United States Geological Survey (USGS) to examine the history of tsunamis in Southern California's wetlands. Kirby had traveled to the western coast of southern Thailand three times after the 2004 Indian Ocean earthquake to reconstruct its tsunami history over the past 10,000 years, and eventually moved to Southern California to do the same research, this time funded by the Southern California Earthquake Center (SCEC) and National Science Foundation (NSF).

"But here's the kicker—we haven't found anything yet," said Kirby. "Not finding anything doesn't mean we can't have a tsunami nor that tsunamis haven't happened, because in order to preserve a tsunami layer, it has to be almost the perfect environment, and Southern California is far from the perfect environment."

The research program traveled to wetlands undisturbed by human development from San Diego all the way to Santa Barbara, examining long tubes of sediment layers in the mud. Rhodes also agreed that a lack of evidence is not yet strong evidence to confirm a large prehistoric tsunami never hit.

"There's been some very small tsunamis that hit the coast that did very minor damage in harbors where boats were ripped off," said Rhodes.

However, the risk for a damaging tsunami in Orange County is relatively low. Not zero—but low.

Areas of risk near Seal Beach (for full maps visit http://www.conservation.ca.gov/cgs/geologic_hazards/Tsunami/Inundation_Maps/Orange)EXPAND
Areas of risk near Seal Beach (for full maps visit http://www.conservation.ca.gov/cgs/geologic_hazards/Tsunami/Inundation_Maps/Orange)
California Deparment of Conversation

The reason our risk is low is because the faults in California's coastal border area (Catalina Island, Newport Beach, etc.) aren't big enough or long enough to create devastating tsunamis, and SoCal is lucky to have a number of offshore islands and seafloor topography (hills and valleys) that can absorb energy and prevent the the worst from happening.

"But not impossible," Rhodes stresses. "If the circumstances are right, you could have one, but the odds are low."

Teletsunamis (tsunamis that travel from a far distance) such as from Japan, Chile, or the Aleutian Islands, aren't actually the greatest concern for SoCal. What residents of Orange County should be aware of are something called local tsunamis. These are triggered by nearby coastal faults and underwater landslides.

"The bad thing is that there's very little warning for a local tsunami because tsunamis travel very fast," said Rhodes. "Tsunamis travel about the same speed as a jet airliner [at] cruising altitude."

For a local tsunami triggered by an earthquake, you'll have just minutes to prepare as opposed to hours, and for one triggered by a submarine landslide, no warning at all - both of which give virtually no time, especially if you live along the beach.

But Rhodes emphasized that the risk of anything like that happening is still very low, and is not nearly close to the risk for a tsunami in the Pacific Northwest, as a recent, chilling Pulitzer Prize-winning story put it.

The last tsunami advisory issued in Southern California was September 16, 2015, with waves reaching less than a foot high above usual tides according to the National Weather Service. Hardly noticeable.

Click here for more easy to read Orange County tsunami inundation maps for emergency planning.

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