Great quake one of the biggest ever in Japan

Magnitude-8.9 tremor will go down in seismology’s record books

The magnitude-8.9 quake that struck off Japan’s coast on March 11 is the largest in the country’s recorded history.

Even if its magnitude is downgraded in the coming days, as sometimes happens as more data are analyzed, the quake will remain a benchmark in a country that has seen many major quakes. It ranks fifth on the list of biggest quakes this past century. The Indonesian earthquake that spawned 2004’s devastating Indian Ocean tsunami was a magnitude 9.1. 

Japan’s monster earthquake struck at 2:46 p.m. local time, about 150 kilometers off the coast of the island of Honshu. Japan is one of the world’s most prepared societies when it comes to earthquakes, and a recently established early warning system broadcast alerts in many areas, including Tokyo, before the shaking began. 

Seismic activity in the region began with a magnitude-7.2 quake on March 3. Major aftershocks continue to rattle the area. The death toll is unknown. 

PACIFIC HEIGHTS Expected wave heights for the tsunami generated by a magnitude-8.9 earthquake in Japan on March 11 are shown in these data from a computer simulation by the U.S. National Oceanic and Atmospheric Administration. The wave decreases in height as it travels across the Pacific Ocean but then gets higher as it hits shallower areas near coasts. NOAA
Japan owes its lively seismic existence to its precarious geologic setting. The islands of Japan formed where one great plate of Earth’s crust, the Pacific plate, dived beneath the Eurasian and Philippine plates. The collision is part of the “Ring of Fire” of earthquake and volcanic activity around the Pacific Ocean. 

Chikyu, a deep-sea drilling vessel operated by the Japan Agency for Marine-Earth Science and Technology, is in the midst of a many-years study drilling into the seafloor off Japan’s coast to study the genesis of big quakes there. 

The deadliest quake in Japan’s history came in 1923, when more than 140,000 people perished in the magnitude-7.9 Great Kanto Earthquake. That tremor was centered southwest of Tokyo Bay. The March 11 quake struck more to the north, offshore from the city of Sendai. 

“Fortunately for Tokyo it’s a bit further north than the great Kanto earthquake was, which means the damage in Tokyo is likely to be much less,” Kevin McCue, a Canberra-based seismologist at the CQUniversity in Australia, said in a statement.

Tsunami warnings spread across the Pacific in the hours after the earthquake; earthquakes generate tsunamis when the ground rupture displaces massive amounts of water. The size of the Japanese quake, plus its relatively shallow depth of 24 kilometers, meant that it was primed to trigger tsunamis. 

Honshu’s east coast had essentially no time to prepare for the waves, but other locations around the Pacific set into gear preparation and evacuation plans polished after the 2004 Indian Ocean tsunami. Hawaii was reporting waves of 1 meter or less.

Alexandra Witze is a contributing correspondent for Science News. Based in Boulder, Colo., Witze specializes in earth, planetary and astronomical sciences.

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