The name "tsunami" comes from the Japanese whose experiences with the phenomenon are due to seismic activity mostly, it is presumed, along the Kuril-Kamchatka trench off the island of Hokkaido, or conceivably, the Marianas trench off the Marianas Islands south of Japan and east of the Philippines. Properly qualified, the tsunami might be referred to as a "bottom displacement water wave" because it is generated only by events that cause a water body bottom to be suddenly displaced by flexure or, in the case of a landslide, by augmentation. For example, one might say "sea-bottom displacement ocean water wave," or "lake-bottom displacement lacustrine water wave," or "bay-bottom augmentation water wave," all of which illustrates the usefulness of the term, tsunami, and also helps to correct the erroneous impression that the tsunami is a "tidal wave" having something to do with the tidal fluctuations. On the other hand, there is the entirely predictable tidal bore which is a kind of wave, but one that does not occur in Malibu. Suffice to say, there is a tsunami risk in Malibu, and because it is seismically related, there is no way yet known to accurately predict its occurrence or quantify the degree of risk it presents.

ERRONEOUS WARNING

In March, 2009, signs indicating evacuation routes in the event of a tsunami began to appear along canyon roads in Malibu. Similar signs down coast in West Los Angeles suggest that they have originated with Los Angeles County that in turn may have gotten the idea from the state of Washington which has similar signs. Such signs in Malibu, although well intended, are misleading and give potentially fatal instructions. The issue is one of warning. For distant seismic events, ocean sensors, if and when established, can give sufficient time to evacuate; but for local events, i.e., those originating along the Anacapa fault or others directly offshore from Malibu, there would be very little warning. In the event of a locally generated tsunami, getting to high ground immediately, without recourse to an automobile or other vehicle, may be the only way to survive. On the other hand, the idea to evacuate, presumably by automobile driving up canyon roads, is extremely ill-advised not only because it could lead to panic, traffic congestion, and accidents, but also because in almost all instances, there would not be enough time to escape along any coastal roadway. Most coastal roads in Malibu are no more than about 25 feet above sea level.

PRACTICAL WARNING

Transverse movements along faults directly offshore from Malibu should not cause much if any sea-bottom offset and therefore are unlikely to generate a tsunami. On the other hand, north-south movements are likely to cause sea-bottom offset and therefore tsunami generation. This in turn suggests a simple device that could detect the initial direction of north-south movement. For example, a pendulum actuated in a north-south direction could trigger a special alarm, for example, an air-raid siren such as is used in Hawaii. Developing one or more such devices for Malibu would be a much better way to spend emergency funds than posting evacuation-route signs that give a false and very dangerous sense of security.

SURVIVAL

Those living on beaches are, of course, most at risk. Their safety could be greatly increased by determining, now, any nearby escape route to higher ground, say to an elevation of 40 to 50 feet, that can be reached within a two or three minutes. Such routes are easy to plan beforehand for properties along Malibu Road, the lower stretches of Broad Beach Road, Escondido Beach Road, and Latigo Shore Drive opposite all of which there are relatively gentle climbable slopes. Properties along much of PCH east of Las Flores Canyon Road are not so favored except wherever streets or private driveways on the opposite side of the highway are nearby.

It is to be emphasized that escape should be on foot along a predetermined route, much like a fire drill. For properties along Malibu Cove Colony Road, almost all of the Colony, and much of PCH, for which rapid pedestrian escape to higher elevation is not possible, it would be safest either to get as far inland as fast as possible, or if that is not possible then onto roofs with some sort of flotation device. And don't take time to find the camera. On the bright side, properties high enough above the shoreline, say 50 feet or more, should be safe from all but the most horrendous tsunami event.

The most useful datum is the period to be expected before the arrival of the first tsunami due to a local offshore earthquake. Call it the "escape period." Its determination is a task for the seismologist. The escape period should be considered to begin either when the first seismic shock is felt or when an unusual, rapid retreat of water from the shore is observed. Such retreat is actually the trough of the first tsunami wave that has been generated. It would be followed by the arrival of a wall of white water resulting from the approaching wave crest having broken offshore after touching bottom. Probably, it would take a period of only 30 seconds to a minute or two for the entire sequence to occur, and that would be the escape period.

By way of example, if you live at the beach, say on Malibu Road, and feel a seismic shock, you should look to see if the water is receding from shore. If it is, grab the kids and run like hell across the road to higher ground. You should not attempt to drive away, even in your beloved Ferrari, unless it could get you somewhere that high in a minute or so. The operative word is "escape," not "evacuate." Evacuation, which the recently posted signs imply, has a dangerous time-consuming connotation.

* * *