Why Japan’s 6.9 Quake Wasn’t 2011 All Over Again

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Sequels rarely live up to the original. And thank goodness for that. Yesterday, a 6.9 earthquake shook the coast of Japan almost exactly where a 9.1 quake hit nearly 6 years ago. Japan is fortified against quakes and tsunamis. But the 2011 quake was so powerful it generated 30 to 60-foot tsunamis, overtopping the island nation’s extensive sea walls and shore protections, killing over 15,000, leaving 228,000 homeless, and causing a meltdown at the Fukushima Daiichi nuclear plant.

Yesterday’s temblor was comparatively tame. Tsunamis rolled in at 4.6 feet. Nobody died. Only 15 people reported injuries—broken bones, at worst. And although the cooling system for spent fuel rods momentarily stopped at a nearby nuclear reactor (Fukushima Daini, not Daiichi), there was no meltdown.

Partly, that’s because this quake was so much weaker. Going straight numerically, 6.9 doesn’t seem like much less than 9.1. But earthquakes are measured logarithmically. “In terms of magnitude, a 6.9 is basically 1,000 times smaller than a 9.0,” says Paul Huang, a seismologist at NOAA’s Tsunami Warning Center in Palmer, AK. And only 1 to 2 percent of a quake’s energy gets transferred into the ocean, so that thousandfold difference becomes fractional at tsunami scales. Yesterday’s quake moved side to side—a so-called lateral slip—and so displaced less water than a vertical thrust quake like in 2011.

Yesterday’s quake wasn’t just further from the fault, it was closer to shore. Some bit of its energy actually went directly into shaking stuff on land, instead of getting transferred to the water.

This quake was also shallower than its predecessor, which, all things being equal, would mean more energy propagated into the water, not absorbed by earthen crust. “If deep, it has not as much energy, so less gets transferred to water and you get smaller waves,” says Huang. But all things are not equal. Remember, thousandfold difference in energy?

Just because the energy was less, and the tsunamis smaller, does not mean the Japanese government’s evacuation orders were for naught. “Remember, a tsunami is the whole ocean, from top to bottom, moving,” says Huang. Even a few feet of swell powered by the whole ocean is enough to sweep a body out to sea. And local residents are wise to stay wary for more. This weaker sequel has already generated many aftershocks, at least three of which were 5 magnitude or higher.

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