Monday Earthquake’s Epicenter in Same Area as Other Recent Major Temblors, Same Type as 2011 Quake

The Yomiuri Shimbun

15:14 JST, April 21, 2026

The magnitude 7.7 earthquake that struck off the coast of Iwate Prefecture on Monday was a trench-type earthquake, according to experts, based on its location and depth. This is the same type as the 2011 Great East Japan Earthquake.

Trench-type earthquakes occur at undersea plate boundaries – in this case where the oceanic Pacific plate is subducting beneath the continental North American plate. Tsunami warnings and advisories were issued across a wide area.

Major earthquakes have been occurring frequently in the vicinity of the Japan and Chishima trenches in recent months. On Dec. 8, a magnitude 7.5 earthquake struck off the eastern coast of Aomori Prefecture, prompting the Japan Meteorological Agency to issue its first-ever “Hokkaido and Sanriku Pacific coast subsequent earthquake advisory,” urging preparedness for subsequent earthquakes. Two more earthquakes occurred with epicenters in the same region, a magnitude 6.9 on Nov. 9, and a magnitude 6.7 on March 26 this year.

The Monday earthquake occurred within an area in which there is a pattern of one major earthquake being followed by subsequent quakes of similar, or even higher, magnitude in quick succession, according to the Japan Meteorological Agency. When a magnitude 6.9 earthquake struck in 2015, a magnitude 6.5 quake occurred three days later, followed by a magnitude 6.4 quake the day after.

“The epicenter of the recent earthquake is located in an area where the tectonic plates did not shift significantly during the Great East Japan Earthquake. Strain accumulated from that earthquake has led to increased seismic activity,” said Shinji Toda, a professor of seismology and geology at Tohoku University. He called for heightened vigilance.