Aftershocks and Seismic Activity in the Decade Since 3/11

The Japan Meteorological Agency has compiled data on seismic activity over the 10 years since the 9.0-magnitude Great East Japan Earthquake struck the Pacific Ocean side of northeastern Japan, causing a devastating tsunami.

Over the course of one year following the 2011 earthquake, 5,387 aftershocks of magnitude 4.0 or higher occurred along the Pacific coast from Iwate Prefecture to the north of Chiba Prefecture, but in the past year (March 11, 2020, to March 6, 2021) that number has fallen to 208, which is a more than 96% decrease. Nevertheless, this is still a high number of tremors compared to the annual average of 138 over the 10-year period from 2001 to 2010.

On February 13, 2021, a 7.3-magnitude earthquake struck off the coast of Fukushima Prefecture, generating a maximum seismic intensity of upper 6. This was the first time for an earthquake of that seismic intensity to strike in this part of Japan since the 7.2-magnitude earthquake off the coast of Miyagi Prefecture on April 7, 2011.

Number of Earthquakes in the Aftershock Area of the Great East Japan Earthquake

Created by Nippon.com based on data from the Japan Meteorological Agency.

Over the past 10 years, there have been 14,711 tremors of a seismic intensity of at least 1, which is strong enough to be perceived by the human body. Over the past year (March 11, 2020, to March 6, 2021) the number was 469. This is less than one-seventeenth the number of tremors that struck during the year after the 2011 earthquake, but more than the annual average of 306 earthquakes during the 10-year period from 2011 to 2010.

(Translated from Japanese. Banner photo: A road in Sōma, Fukushima Prefecture, blocked by rocks from a landslide triggered by an earthquake on February 13, 2021. © Kyōdō Images.)