Gauging the Tsunami Risk in Southern California

By Patrick Healy
KNBC-TV Los Angeles

Wednesday, Mar 30, 2011

Grasping the extent of the tsunami risk for Southern California has been a work in progress, much of it done at USC's Tsunami Research Center. Reality is, the more that's learned, the more risk becomes apparent.

We've long taken comfort from the notion that beneath the ocean off Southern California, you do not find the type of earthquake fault associated with the vertical motion that causes large tsunamis. That happens in subduction zone thrust faults, such as off the coast of Japan, in Alaska's Aleutian Islands, or even in the Cascadia subduction zone off the Pacific Northwest.

But not off the Southern California coast, where our strike-slip faults tend to move sideways, not up and down.

Geologist Mark Legg says don't count on it.

"This is a myth which regrettably much of the earthquake and scientific community still believe," said Mark Legg, PhD, an associate of the Tsunami Center. "A very large strike-slip earthquake can push up the seafloor."

The poetential added peril of a tsunami caused by a local quake is not only that the surge may be larger than one that has traveled thousands of miles, but also that there would be less time for warning and evacuation from low-lying coastal areas. And that would be especially problematic if the earthquake had also caused damage on shore.

Worst case scenario?

"Liquefaction will be occuring," Legg said. "Roads will be knocked out. Bridges might be damaged. Landsliding and all that stuff. And then you get the tsunami on top of that. So it's going to be a very complex and challenging event."

What until now has been given greater consideration is the theoretical scenario of a large California quake indirectly causing a tsunami by triggering a massive undersea landslide.

Legg fears a directly triggered tsunami would be worse. He notes that historically, tsunamis have been linked to quakes off the California coast. He noted that a 2 meter surge (about 6 feet) was associated with the 1927 Lompoc quake. The 1812 quake in the Santa Barbara Channel likely produced a bigger tsunami.

"One account says the water got up to 15 feet in Ventura," said Legg.

Historically, the largest tsunamis have originated with quakes in subduction zones. But not all.

"About 10-15 percent" of destructive tsunamis were triggered by quakes on strike slip faults, according to USC Research Prof. Aggeliki Baberopoulou, PhD, also of the Tsunami Research Center.

"This type of fault is less likely to produce a tsunami," Barberopoulou said. "But they do happen."

Nevertheless, a strike-slip generated tsunami was passed over as the mechanism for the next Southern California disaster scenario to be run by the U-S Geological Survey, which previously has envisioned a 7.8 magnitue quake on the southern San Andreas, and "Ark Storm" flooding from continuous rain over several weeks.

The next natural disaster scenario will involve a tsunami generated by an earthquake in the Aleutian Islands, which would be expected to send us a larger tsunami than the 1964 Anchorage quake. The experts agree that tsunamis tend to propagate vertically from the fault line. If you draw a perpendicular vector from the Aleutians, it points right at Southern California.

Source: http://www.nbclosangeles.com/news/local/Gauging-the-Tsunami-Risk-in-Southern-California-118887294.html?__source=Newsletter-Daily

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