I know that the Hawaii tsunami is now officially old news, but it was an important geological lesson. Why does it take amateur video to adequately explain what was happening? As I watched cable news yesterday, the activity was hidden by screen banners and obscured by blathering newsreaders who didn't have enough knowledge to realize what was happening on the screen in front of them. I've been complaining about it for the last 24 hours, so I thought I should mention what incredible things we did see yesterday.
It boils down to the fact that there was a very thin line between a geologic curiosity and a geologic disaster in Hawaii yesterday. Think what a remarkable thing was happening in the video: a normally placid ocean bay protected from wave action by a long sea wall was suddenly full of mud, and was flowing. First one direction, then the other. How extraordinary is this?
Normal waves are a surface phenomena. Anyone playing in the waves at the beach knows they can avoid being mashed by a big wave by diving under it. The waves form up only a few hundred feet offshore and break against the beach. They involve a limited part of the water column, and the energy level, while high at the point of impact, is also limited. The swells that form up into waves may be generated hundreds or thousands of miles away by storm winds, but the swells travel only a few tens of miles an hour, and the turbulence extends only a few tens of feet beneath the surface of the ocean.
Tsunamis are not at all like normal waves. They are generated by a displacement of the water on the ocean floor by large earthquakes, volcanic eruptions or underwater landslides. They travel hundreds of miles an hour (roughly the speed of a jet airliner), and they affect the entire water column from the seafloor to the surface. They don't even really affect boats on the open ocean, as the boats simply rise and fall a few feet over a twenty minute time span. But the seafloor is affected by the passage. The dangerous aspect of a tsunami is what happens when the waves encounter shallow water along a shoreline. In essence, the energy of the wave that has been dispersed across the whole water column is compressed into a smaller and smaller space. The wave builds, and either draws water away from the coast, or surges into the coast without warning.
The activity at Hilo Bay is a wonderful example of the process. All the water in the bay was in motion, from the surface to the base, and this is what kicked up all the mud in the water. The water was surging in and then out again, explaining the strange currents around Coconut Island, seen in the video above. The total height of the tsunami surge was about three feet.
This was a huge earthquake in Chile, the fifth largest ever recorded. It was reasonable to expect a colossal tsunami, but tsunamis are subject to many variables. Smaller earthquakes have generated larger tsunamis in Hawaii (the worst tsunami to strike Hawaii, in 1946, was from a smaller earthquake that took place in Alaska). The largest earthquake ever recorded, from Chile in 1960, produced a devastating tsunami in Hilo, but the second largest, the Alaska quake of 1964 (magnitude 9.3) caused all of $60,000 in damage and killed no one in Hawaii (16 people died in California and Oregon, and 106 in Alaska). In other words, tsunamis are inherently unpredictable. In the worst tsunamis, of 1946 and 1960, the surge reached depths of 30 feet or more.
A warning system has been present in the Pacific Ocean basin since 1946, and the system was upgraded in the last few years, but has only been tested in a few recent large seismic events.
What I really wish is that the newreaders (er, um, excuse me, anchorpersons and reporters) at the cable networks would take the time to truly educate themselves in the basics of geology. As a teacher of geology, I shiver at the thought of standing up in a classroom of philosophy students and lecturing on philosophy without any time to prepare. These newspeople are standing up in front of millions of people and displaying their appalling ignorance of geology. The other problem, of course, is that their audience is not aware of it.
UPDATE: Chris at goodSchist is collecting some of the mainstream media crimes against geologic understanding here. Be sure to contribute if you see something!
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