Dangerous Waves : Scientists Say the Threat of a Tsunami Is Small but Real

Six times in the last 180 years, tsunamis have come ashore on local beaches. But unlike some other places in the Pacific Ocean basin, the waves were comparatively small and the results not catastrophic.

Still, when a major earthquake, followed by seismic sea waves, landslides and fires, caused more than 190 deaths in Japan this month, it raised the question of just how serious the threat of a tsunami coming ashore here is. The National Weather Service, for its part, issued a statement reminding the Los Angeles area of the potential, saying, “Contrary to what some might think, tsunamis do threaten coastal residents of Southern California.”

Indeed, in 1930, a tsunami claimed a life in Redondo Beach, and another at Cabrillo Beach in 1960. Also in 1960, hundreds of boats in the Cerritos Channel were cast loose by surging currents when waves arrived from an earthquake in Chile, and $575,000 in damage was done to docks and boats from tsunamis that arrived here six hours after the catastrophic Alaskan earthquake of 1964.

But local experts say the prospects of a really dangerous tsunami striking here remain slim. For instance, Hiroo Kanamori, director of the Caltech Seismological Laboratory, says the recurrence interval of devastating waves here might range from “once every several hundred to even several thousand years.”

This contrasts with Japan, where there has been serious loss of life five times in the last century, including more than 20,000 dead from tsunamis in 1896.

A major tsunami here is “very unlikely,” says Lucile M. Jones of the Pasadena field office of the U.S. Geological Survey. She adds: “It’s not impossible, but it’s improbable.”

And the state Office of Emergency Services rates the danger so low that when it sought federal funding for a tsunami vulnerability study last year, it confined its request to the Humboldt County coast, where thrust earthquakes in a volcanic subduction zone do pose a threat of major waves at a relatively frequent interval.

“We see the greatest potential there, not on Southland beaches,” explained Paul Flores, the office’s deputy director.

Records of the National Oceanic and Atmospheric Administration, which has published a book, “United States Tsunamis, 1690-1988,” show that since 1812, local beaches have been hit by three locally generated tsunamis and three tsunamis that came from thousands of miles away.

The transoceanic tsunamis came in 1946, 1960 and 1964; the locally generated ones in 1812, 1855 and 1930.

Some of the greatest loss of life worldwide has come from waves generated close to the scene of catastrophe by offshore earthquakes or volcanic eruptions. This was the case in 1883, when 36,000 died from waves caused by the Krakatoa eruption west of the island of Java in the Indonesian archipelago.

But in this area, the worst locally generated tsunami recorded was on Aug. 30, 1930, when a man drowned in Redondo Beach after a magnitude 5.2 earthquake caused undersea landslides in Santa Monica Bay, generating 20-foot waves. Sixteen people had to be rescued that day by lifeguards in Santa Monica.

The most common cause of tsunamis are thrust earthquakes underneath the ocean causing a sudden vertical displacement of a large quantity of water, setting wave motions into effect that race outward from the scene.

Jones points out that, except for a thrust earthquake in the Santa Barbara Channel in 1812, there has not been any example of other thrust faults in the waters immediately off Southern California. And there are no offshore volcanoes here.

That leaves a possibility of landslides from the hills above Malibu, and, as in 1930, underwater landslides from more conventional strike-slip earthquakes, called “slumping.”

“There is reason to believe that elsewhere in the world, slumping has been the cause of a few of the major tsunamis,” Caltech’s Kanamori says. “We really haven’t experienced anything on that scale, but at the same time, our history is very short, and there is some danger in relying on our experience because we don’t have records before the 19th Century.”

As for transoceanic tsunamis, both Jones and Kanamori say that the geometrical alignment of local beaches with the far-off Pacific locales where many tsunamis are generated is not particularly susceptible to the strongest waves hitting here.

“Japan and Chile have a horrible coincidence in terms of their geometry,” Jones says. “The sea waves generated by the Chilean earthquake headed straight into Japan and killed 140 people. Here, the alignment was different and the waves not as devastating. . . . Obviously, we can get waves, we’re sitting right on the ocean, but the geometry might not be quite right” for the effects to be devastating.

Overall, Jones says she views the tsunami threat here as so small that “this is not where we should be focusing our preventive priorities. You get back to the whole idea of risk management. You can’t say we’ll never have a tsunami here, but putting a whole lot of our resources, our money, into tsunami management rather than earthquakes is just not merited.”

Southern California Edison studied the tsunami danger when it built its San Onofre nuclear generating plant near the Orange-San Diego county line in the mid-1960s, and decided to construct a 30-foot seawall to protect the plant from the largest waves it could conceive, according to Dennis K. Ostrom, a company engineer.

“We calculated the maximum run-up at 27.5 feet,” Ostrom says. “This would be a six-foot tsunami or storm wave, and a still water level of 15 feet, all occurring simultaneously.”

Even so, Jim Lander, the retired director of the NOAA’s Geophysical Data Center in Boulder, Colo., cautions that even unsettled water or a few large waves emanating from tsunamis have done a lot of damage to boats and docks in the past in Southern California and will probably do so again.

“What causes a lot of it is the very high currents. . . . Boat moorings aren’t made to be so secure from strong currents, and even when a small tsunami comes in, the currents may be measured at more than 20 miles an hour, and boats go shooting around, smacking into each other,” Lander says.

In 1964, for instance, an oil tanker caught in a small tsunami and strong currents in Los Angeles Harbor slammed into Norm’s Landing on the main channel, ripped out 175 feet of dock and capsized a fishing boat.

Part of the phenomenon is also abnormal recessions of water, followed by sudden rises. In 1964, in Ventura’s small boat harbor, the water level dropped eight feet at one point in just 25 minutes as a result of the tsunami caused by the Alaskan earthquake.

Harbor openings that narrow down surges of water may actually amplify the power of the waves. On the same day, hours after the Alaskan earthquake, water suddenly rose more than four feet in the yacht harbor at Marina del Rey and about five feet at the entrance. About $200,000 in damage was done to boats and facilities.

Because the losses have been relatively small in prior tsunamis here--the highest being a total of about $1 million in 1960--some experts fear that beach residents are too complacent about the dangers.

A big offshore quake could prove dangerous, and Edward N. Bernard, director of the NOAA’s Pacific Marine Environmental Laboratory in Seattle, cautions: “Locally generated tsunamis are rapid. . . . Nearby coastal communities have little or no time before the arrival of the first wave.”

In 1986, when there was a transoceanic tsunami warning in the Southland, hundreds of persons flocked to the beaches to see what was going on, rather than staying away from them. Nothing happened that time, but on other occasions, elsewhere, such attitudes have proved fatal.

When deadly waves did strike Crescent City in 1964, several people who came to see what had happened in the wake of the first two waves were killed by the third and fourth.

THE IMPACT OF TSUNAMIS

Scientific experts believe devastating tsunamis will hit local beaches only very infrequently. Less serious ones have occurred here on an average of every 30 years since records began in 1812, damaging boats and docks and resulting in two drownings. The last noticeable tsunami in the Southland was in 1964.

MAJOR CAUSES OF A TSUNAMI:

EARTHQUAKES

The surface of the earth is broken up into separate sections known as plates. When a plate beneath the ocean moves upward in an earthquake, it can displace water causing a seismic wave.

VOLCANIC

Volcanic eruptions such as the one that occurred in Krakatoa in 1883, created a very large wave in the Sunda Straits, with a very long period of an hour or more. But the exact mechanism by which the wave was generated is still not clear.

SLUMPING/LANDSLIDES

Slumping is an undersea landslide that may result from earthquakes along the strike-slip faults that exist off the coasts. During the 1964 Alaskan earthquake, submarine slumping contributed to the tsunamis that surged across the Pacific.

In the case of landslides, a large mass of the mountain from the surface slides into the ocean and causes gravity waves which can splash waters to extreme heights.

THE 1964 ALASKAN EARTHQUAKE:

Concentric arcs record the life of a killer tsunami that raced across the Pacific Ocean in March 1964. Born of a quake near Valdez, Alaska, this seismic sea wave fanned out at speeds of more than 800 kmph (500 mph), killing 122 people as it slammed into shorelines around the Pacific.

HEADLINES FROM 1964:

Southland Coast Areas Alerted for Tidal Wave

TANKER TIPS FISHING BOAT IN L.A. HARBOR

Waves Rip Boats Here, Smash Dock Facilities