UPDATE (April 27): This subsequent earthquake advisory ended as expected on April 27th at 5PM JST
UPDATE: The magnitude was increased to M7.7 from 7.5. Since this quake exceeded the M7.0 for this area (where there are two trenches) JMA has issued an subsequent quake advisory alert for potentially more earthquakes in northern Japan along two deep-sea trenches (Japan Trench and Kuril or Chishima Trench) in the Pacific Ocean. This alert covers 182 cities and towns from Hokkaido to Chiba.
The M9.0 quake in 2011 was preceded by a M7.2 foreshock 2-days earlier on March 9th (video from Kamaishi on March 9th during the news alert). This advisory system was created in 2022 where in the event of a M7.0 or greater quake, an alert could be issued for potentially stronger quakes to occur due to similar instances in the past (like in 2011).
This subsequent quake advisory (for the Sanriku area) is similar to the Nankai Trough Megaquake advisory system and is officially called the “Off the Coast of Hokkaido and Sanriku Subsequent Earthquake Advisory”; an alert is issued following an earthquake in the area with a confirmed magnitude of 7.0 or greater once the agency assesses there is an increased chance that an M8.0 quake or greater could occur within seven days.
This could expire like the previous one (December 2025 after an M7.5 quake off of Aomori) in this area (and similarly like the August 2024 advisory for the Nankai Trough). These advisories are meant to alert citizens in the affected areas and to be prepared during the advisory period (it does not mean an actual quake will occur; just an increased chance).
Tsunami warnings dropped for areas under a warning. Tsunami advisory remains in place (advisories were ended around 11:15pm JST). Tohoku shinkansen services resumed after a 4 hour halt. Damages are still being assessed. Two injuries were reported (one in Iwate and one in Aomori); more will be known once daylight breaks on Tuesday morning JST.
Original News
The Japan Meteorological Agency (JMA) says a M7.5 earthquake at a depth of 10km occurred off the coast of Sanriku, Iwate Prefecture at 16:53 on April 20th JST. The ground shaking intensity registered an upper 5 on the Japanese scale (shindo) of 0 to 7 in the hardest-hit areas.
The epicenter occurred in an area where past Sanriku earthquakes have occurred (the largest being the 1896 Meiji-Sanriku quake that measured an M8.5 and caused two large tsunami waves including one that measured 38.2m/125ft).
A tsunami warning has been issued for Iwate Prefecture and parts of Hokkaido and Aomori while a tsunami advisory exists for Hokkaido, Aomori, Miyagi and Fukushima prefectures.
Shinkansen service has been halted between Tokyo and Aomori.
Initial small tsunami waves has arrived at Hachinohe in Aomori (30cm), Kamaishi in Iwate, Mutsu Ogawara Port in Aomori, Urakawa in Hokkaido (20cm), Ishinomaki in Miyagi (20cm), Miyako in Iwate (40cm), Ofunato in Iwate and Kuji in Iwate (80cm).
Small tsunami waves at around 18:35 JST (the bottom portion of the seawall was exposed around an hour ago).
Evacuation orders had been issued for 171,957 residents residing in inundation zones in Hokkaido, Aomori, Iwate, Miyagi and Fukushima prefectures as of 18:45 JST.
Live broadcast VODs:
Yuuko H. in Hakodate helped contribute to this report (she said the ground motion shaking was fairly strong; it was a shindo 4 in her area)
