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Japan hit by 7.3-magnitude earthquake

A 7.3-magnitude earthquake hit just off the coast of the Fukushima prefecture in northern Japan on Wednesday evening. The ripples in the image show peak ground acceleration, a good hazard index for buildings up to about seven stories tall. (USGS)
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A powerful 7.3-magnitude earthquake hit just off the coast of the Fukushima prefecture in northern Japan on Wednesday evening, leaving two dead and nearly 100 injured, the Associated Press reported, citing the Japanese Fire and Disaster Management Agency. More than 2 million households are without power, according to the Tokyo Electric Power Co., or TEPCO. The Japan Meteorological Agency (JMA) issued a brief tsunami advisory, with waves of less than three feet expected. The U.S. National Weather Service reported no expected tsunami for the West Coast, Alaska, Hawaii or Guam.

The main quake, which was estimated to have been centered about 37 miles below the sea floor, was preceded by a lesser-magnitude 6.4 earthquake two minutes earlier. Ordinarily, a 6.4 would be considered a big event, but it was quickly revealed to be a “foreshock.” Large aftershocks are likely for days, and there’s still a very slight chance — perhaps 1 in 20 or so — that the 7.3 might be a foreshock to a larger quake in the next three days.

A powerful earthquake with a magnitude of 7.3 jolted Japan's northeast coast on March 16, shaking buildings as far away as Tokyo. (Video: Reuters)

The Fire and Disaster Management Agency said two people were killed and 94 others were injured, including four seriously, the AP reported.

The main shock took place around 11:36 p.m. local time Wednesday. The JMA maintains an extremely dense network of seismometers; moments after shaking began, the agency issued an Earthquake Early Warning that broadcast to computers, phones and television sets. Such warnings are transmitted at the speed of light, making it possible for an alert to reach distant communities before damaging surface waves crawling along Earth’s crust arrive.

Shaking in the high Intensity 6 range was recorded in Fukushima and parts of Miyagi prefectures, with lower 6 in central Miyagi and high 5 across much of Iwate and Yamagata prefectures. Surrounding areas saw lower-5 shaking, with levels around a 4 in the Tokyo metropolitan area.

The JMA warned people to pay close attention to seismic activity and rain because of the increased risk of home collapses and sediment-related disasters.

The agency said that the ground may have loosened and that snow covers may have become unstable after the earthquake, increasing the chances of sediment disasters and avalanches.

The earthquake had a “thrust” mechanism. That means one plate slipped downward beneath another. In this case, it was the Pacific Plate sliding beneath the Okhotsk Plate in a sudden release of pent-up stress. Oceanic plates “subduct” beneath continental plates because oceanic plates are denser.

Thrust quakes are responsible for producing the most significant tsunamis, because the up-and-down shaking of the sea floor is effective at displacing water. A tsunami advisory was issued for Fukushima and Miyagi prefectures, but the surge in water levels wasn’t expected to crest above three feet.

That’s still enough to inundate some vulnerable pockets of the shoreline, but Japanese infrastructure is designed around mitigating earthquake and tsunami risk.

TEPCO confirmed that there was a fire alarm at its Fukushima Daiichi plant after the quake, but no fire was located. Electricity was out briefly at pools that store spent fuel from nuclear reactors, the agency announced.

The March 11, 2011, earthquake in Japan registered more than a 9 on the moment magnitude scale — meaning it released about 355 times as much energy as Thursday’s quake. That’s because the moment magnitude scale isn’t linear. Each increase of 1 on the scale represents a tenfold jump in “seismic moment.” The 2011 quake was accompanied by a tsunami up to 130 feet in height.

Kasha Patel contributed to this report.

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