Tuesday, July 28, 2009

Tso, can you Tsurf a Tsunami?

Hokusai's 1831 "Great Wave off Kanagawa" and a picture of the Hilo Pier in 1946. The man (who did not survive) was named Antone Aguiar. According to the story at the Tsunami Museum in Hilo, he cut the ship Brigham Victory free from the dock, allowing it to ride out the disaster. The picture is from the archives at NOAA.

There are two main kinds of waves, those that are produced by winds blowing over the ocean surface, the subject of the last two posts on the huge waves that struck Southern California last week, and those that are produced by massive disturbances of the ocean floor, either by earthquake, volcanic eruption, or landslides. These are tsunamis, often (incorrectly) called tidal waves (there are tidal waves of a sort, called tidal bores, but they cannot be mistaken for a tsunami).

After my adventures this summer, which included a grand tour of the Hawaiian Islands with two dozen of my students, and a mini-vacation in Southern California that coincided with the arrival of unusually large waves along the coast, I am writing a short blog series on the dynamics and geology of waves. Tsunamis have been on my mind, having seen perhaps hundreds of tsunami warning signs all over the coastal areas of Hawaii, and seeing, for the first time, tsunami warning signs in Southern California. I don't know when they started appearing (I suspect in 2004), but they certainly weren't there in the years when I was growing up in the southland.

I used to have a more difficult time teaching about tsunamis, but that changed in 2004 when the Indonesian Earthquake produced the tsunami in the Indian Ocean that killed around 230,000 people. Because it hit tourist beaches, thousands of videos and photographs documented the event, and international media coverage was intense. Prior to that event there were not a great many photographs that documented what a tsunami could do, and the really tragic events, like those in 1946 and 1960 in Hawaii, had receded into ancient history for most people.

Wind-driven waves and tsunamis are very different. Waves travel as swells across the seas, with turbulent water extending only a short distance beneath the surface (about half the wavelength). They travel slowly, a few tens of miles per hour, so they can take a week to cross an ocean, as the Tahitian waves did on their way to California this last week. The energy of the wave is expended only in shallow water at the coast, as the waves build up and fall over as rolling breakers. The amount of energy in a storm or typhoon is immense, but the energy is transferred to the water over time, and is dissipated over a period of days and weeks.

A tsunami is generated in a moment, as an earthquake shifts the ocean floor, or a volcanic caldera collapses. Large landslides off of islands sometimes generate tsunamis as well. The disturbance affects the entire column of ocean water, from surface to seafloor, and all of the energy begins traveling very fast outwards from the point of the disturbance. Very fast. As in the speed of an airliner, around 500 miles per hour. In open water, they may pass without notice, as the wavecrests may be a hundred miles apart, and only rise and fall a few feet. Ships at sea are rarely affected by tsunamis.

At shorelines, the story is vastly different. The oncoming waves hit the shallows at a high rate of speed, and friction slows the forward motion, but the energy is still there and it must be expended. The water surface rises and surges forward, quickly inundated the low coastal areas to depths ranging from a few feet to more than a hundred in some of the most disastrous events. The first indication that something is amiss often is a sudden emptying of shallow bays and a drop in sea level lasting several minutes. The surge follows shortly after. Other times, the sea just suddenly surges forward without warning.

Even when warnings are given, people can make bad choices based on misinformation. They may evacuate as ordered, but may enter the devastated area after the first wave recedes, only to be swept away by the second or third wave they didn't know was coming. Of course the millions of people living along the shoreline of the Indian Ocean didn't even get the chance to make those kinds of mistakes in 2004. Since 1946 there has been a Pacific Ocean-wide warning system that has given people many hours notice that a tsunami was approaching. The poorer countries bordering the Indian Ocean have never had such a system, even though it was well known that tsunamis were a distinct possibility. A horrific injustice...

So, does California have to worry about tsunamis? The good news (sort of): we have big earthquakes on the San Andreas and other faults offshore, but they usually involve lateral motion that does not disturb the sea floor in such a way as to cause tsunamis. The bad news is that the northernmost coast of the state is close to the Cascadia Subduction Zone that is very much capable of producing huge tsunamis, and did in the year 1700. And of course as noted above, tsunamis can cross entire oceans, so large quakes in Mexico, South America, Alaska, Kamchatka, Japan, and all the other major seismic zones in the Pacific Rim can cause tsunamis that could reach the California coast. In 1964, the magnitude 9 Alaska quake produced a tsunami that killed a dozen people in Crescent City on the far north coast.

Southern California is perhaps a bit more protected because of the presence of the Channel Islands, which would tend to break up some of the wave energy. But major tsunamis coming out of the south and west have some potential of reaching parts of the coast. And thus, the tsunami warning signs that have appeared.

I don't advise surfing a tsunami...it's hard to avoid breaking the third-story windows of the building you will be mashing into....

One last comment: when I lived in Santa Barbara many years ago, there was a tsunami warning based on the major quake that had devastated parts of Mexico City in 1985. Of course, people evacuated and headed for high ground, right? Of course not. They went down to the beach to watch. Thank goodness the tsunami was a dud in that case.


John W. Wall said...

We lived in Hilo during the 1960 quake. I was all of two years old, but my father, a jounalist, was working downtown when it hit -- the fourth wave was the Big One and sounded like a freight train. My mom, who'd been watching from our home above Hilo, said it looked like the city exploded when the wave hit, especially when the electric generating plant got hit. After getting news of the quake in Chile that created the tsunami, my dad went to see a seismologist at the Volcano Observatory and was shown the seismograph that recorded the quake. The needle had jumped right off the drum.

DM said...

I, too, am a Hilo boy. As a 16-year-old I went down to see the 1960 tsunami and didn't die, but it was a near thing.

I do believe, though, that the third wave, at about 1:03 AM, was the big one.

And Wall's Mom is right. I was on the bridge over the Wailuku, and had a clear view of the power plant when it blew. Spectacular. Of course that was approximately at the moment I was certain that I was going to die, which lent a certain vividness to the scene.

Garry Hayes said...

Thanks to both of you for your comments. I am really pleased to hear from people who were there in Hilo. Geologists are an odd bunch; most people would choose to be somewhere else, but we would want to see it happen. I guess you have to be very careful what you wish for!

Jerry D. Harris said...

Just out of curiosity, is there any model data on what a bolide impact-induced tsunami might be like? As part of my intro geology class, I show the Atlantic impact snippet from the movie "Deep Impact" and discuss what is good and bad about it; one of the bad things is that I am fairly certain that the New York-destroying tsunami is moving far, far too slow for such a huge impact...I haven't done any math to support it, but I suspect that that wave would move faster than the speed of sound, not to mention be way more destructive than the movie depicted (buildings left standing...?!?) I know of purported impact-induced tsunamites, including those (now disputed) along the southern coast of the U.S. from the Chicxulub end-Cretaceous impact, but never seen any work on what the waves themselves would be like...