At 5:59AM local time Tuesday morning, a magnitude 7.4 earthquake shook the east coast of Japan. The Japan Meteorological Agency issued tsunami warnings and evacuation orders soon after the quake hit, warning of possible 10-foot waves. The quake may have been responsible for an issue at a local nuclear power plant where a pump responsible for cooling fuel rods shut down.
Japanese authorities lifted the tsunami warnings at 9:45AM local time, around four hours after the shaking started. Prefectures along the country’s Pacific coast had been given tsunami advisory indicators, but a stronger tsunami warning was issued to Fukushima prefecture — one of the regions devastated by the magnitude 9.0 quake and subsequent tsunami that caused the disaster at the Fukushima Daiichi nuclear power plant in 2011.
Even small tsunamis can generate dangerous currents
Earthquakes that occur underwater can cause tsunamis by deforming the sea floor faster than water can move out of the way. That massive, sudden displacement generates waves. In this case, it took about half an hour for the waves to reach the shoreline. A three-foot tsunami hit the coast of Soma, 185 miles up the coast from Tokyo, a little more than an hour after the temblor, and another 4.6-foot tsunami washed ashore at Sendai, another 40 miles north, an hour later. This was the largest tsunami in that region since the 30-foot tsunami of 2011, according to the Japan Times.
As of 7:39AM local time, there were 150 people at five evacuation centers, the Japan Times says. Five minor injuries have been reported. The cooling equipment for a reactor at the No. 2 power plant at Fukushima, Fukushima Daini stopped working, but resumed functioning at 7:47AM local time, according to the paper. (Fukushima Daini is not the plant that caused a nuclear disaster after being damaged by the 2011 quake and tsunami. That’s Fukushima Daiichi, a nuclear power plant a little more than six miles to the north of Daini).
Today’s quake was initially measured as a magnitude 7.3, which the Japan Meteorological Agency has since revised to a magnitude 7.4 (this differs somewhat from the US Geological Survey’s assessment of the quake as a smaller magnitude 6.9).
Even small tsunamis can generate dangerous currents, as Gerard Fryer — a senior geophysicist with the National Oceanic and Atmospheric Administration’s Pacific Tsunami Warning Center — explains. “It’s like a riptide that changes direction every 15 minutes, or maybe faster than that,” Fryer says. You wouldn’t want to be in a car, swimming, or walking anywhere near those currents when they rushed in. The waves and currents caused by big tsunamis can batter a coastline for as long as a day, but this tsunami wasn’t big enough to last that long, Fryer says.
The danger is already dissipating
With the warnings now lifted, and no tsunami hazard noted anywhere other than Japan’s Pacific coastline after this quake, the danger is already dissipating. It’s still early, but so far it seems that today’s quake didn’t cause significant damage. It did, however, provide a useful test of systems established since the 2011 temblor, according to Fryer. Computer programs are constantly monitoring the measurements recorded by seismometers deployed worldwide, and raise a flag when they detect shaking. The Pacific Tsunami Warning Center knew within a minute that there had been a quake, and in Japan they got a message out within three to four minutes, Fryer says.
“Judging from this, they did pretty darned well.”
During the 2011 quake, the Japan Meteorological Agency issued the tsunami warning too early, before the agency had fully completed its measurements. As a result, the initial warning underestimated how big the earthquake and subsequent tsunami would be, and some people returned to their homes before the biggest waves hit. Since then the agency has enacted new analysis procedures based on an earthquake’s maximum possible magnitude and instituted an updated warning system. “Judging from this, they did pretty darned well,” Fryer says.
Efforts in the US and in Japan to incorporate GPS measurements of land movement into seismic calculations could make accurate estimations of earthquake magnitude even faster in the future, according to Fryer. “There’s no reason why anyone should die in a tsunami, we have warning systems,” he says. “And we’re getting better and better all the time.”