By Jonny Lupsha, Wondrium Staff Writer
A police training exercise set off false hazard alarms on Oahu and Maui, NPR reported Thursday. Much like last year’s false ballistic missile alert, Hawaii suffered major public panic last week. Here’s why tsunamis are so terrifying.
Correction: An earlier version of this article listed the alarm and the public’s concern as being limited to only tsunamis. It has been updated to reflect the wider scope of the statewide alarm and its implications.
Tsunamis are often described as enormous and deadly tidal waves. While that certainly brings an effective image of a tsunami’s size—and its wrath—to mind, tsunamis technically have nothing to do with the tides. Instead, they’re caused by earthquakes under the ocean and they can kill tens of thousands of people at a time. When Hawaiian residents heard hazard alarms sounding on Oahu and Maui last week, they had good reason to be concerned. Events like these are terrifying and potentially fatal for anyone living in a coastal community. However, with better understanding of them, we can mitigate some of that fear and be better prepared to evacuate or to help, depending on where we live.
Tsunamis: The What and the How
Where does the word “tsunami” come from and why do we call it that? “It’s a Japanese word, and it just combines two words—tsu, which means harbor, and nami, which means wave,” said Dr. Harold J. Tobin, Professor of Geoscience at the University of Wisconsin-Madison. “A tsunami is a harbor wave, and it was given that name a very long time ago—at least 1,000 years ago in Japan—because these were the waves that overwhelm even the protected harbors, the regions where water doesn’t normally come in. So the tsunami is a wave that inundates the coastline and inundates the harbors.”
Nearly all of the earthquakes that cause tsunamis happen in certain geological areas similar to those near Indonesia and Japan. “Both Indonesia and Japan are ‘subduction zones’—places where the ocean floor plate is being carried beneath the rocks of the continent, or the Indonesian and Japanese islands in these two cases, and the plates are converging on one another,” Dr. Tobin said. “In each of the cases, the plates are moving and one plate is going beneath the other, but at the actual boundaries between the two plates, the plates stick together.”
Imagine touching your index fingers’ tips and pushing the fingernails together. They bow and bend, but eventually the force will cause one to overcome the other. That’s what happens with the ocean floor plates, on a much larger scale of course. Dr. Tobin says eventually the force of one plate moving up and over the other causes the water to be flicked upward, which in turn generates the waves we call tsunamis.
The Japan Earthquake and Tsunami of 2011
On March 11, 2011, one of the worst earthquakes in recorded history struck 80 miles east of the city of Sendai in Japan. It had a 9.0 magnitude and caused 33-foot waves to crash on the beaches of Sendai, reaching as far as six miles inland. Varying estimates place the tsunami’s dead and missing toll between 15,000 and 20,000 people.
“The March 2011 earthquake in Japan seems to have produced, according to seismologists, the largest displacement of the sea floor we’ve ever recorded,” Dr. Tobin said. “The offshore region of Earth’s crust, just down by the Japan Trench, moved outward towards the Pacific Ocean by 80 meters, or 200 feet, in one earthquake, and it moved the sea bed upwards by something like eight meters or so.”
Astonishingly, the earthquake was so big that it also hit Hawaii, although on a smaller scale. “Within an hour or so after the tsunami was initiated, and many hours before it arrived even in Hawaii or California or any place else around the pacific basin, they predicted the exact time of arrival at each island in Hawaii, and they also predicted that the height would be about six to eight feet or so, and that prediction was very accurate,” Dr. Tobin said. “The water did actually do some damage in the Hawaiian Islands, but there wasn’t loss of life, because they were able to make this tsunami prediction.”
When we take into account the 2011 tsunami and tsunamis in the 20th century that caused damage and loss of life in Hawaii, last week’s panic over the false alarm becomes very understandable. Hawaii has braved several natural disasters in the past, tsunamis or not. Advancements in weather science and technology are providing earlier warning for extreme weather conditions but, as the 2011 earthquake proved in Japan, sometimes warnings still occur too little and too late.
Dr. Harold J. Tobin contributed to this article. Dr. Tobin is Professor of Geoscience at the University of Wisconsin-Madison. He earned his B.S. in Geology and Geophysics from Yale University and his Ph.D. in Earth Sciences from the University of California, Santa Cruz.