A powerful 7.3 magnitude earthquake hit just off the coast of the Fukushima prefecture in northern Japan on Wednesday evening, leaving one dead and dozens injured. More than 2 million households are without power, according to the Tokyo Electric Power Co. (TEPCO). The Japan Meteorological Agency (JMA) issued a 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 seafloor, 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.
Public broadcaster NHK said one person had died, Reuters reported, and the government said that at least 69 were injured.
The main shock took place around 11:36 p.m. local time Wednesday. The JMA maintains a highly dense network of seismometers; moments after shaking began, the agency issued an Earthquake Early Warning that broadcasts 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 observed in Fukushima and parts of Miyagi prefectures, with a lower 6 in central Miyagi and a high 5 across much of Iwate and Yamagata prefectures. Surrounding areas saw lower 5 shakings, 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 since oceanic plates are denser.
Thrust quakes are responsible for producing the most significant tsunamis since the up-and-down shaking of the seafloor 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 it’s worth remembering that 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 spent-fuel pools, which 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 more energy than 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.
