Subsequent earthquake advisory issued after M7.4 event, elevated large-earthquake risk, Japan
Japan Meteorological Agency (JMA) issued an official advisory warning of an elevated risk of a larger earthquake along its Pacific coast on April 20, 2026, after a M7.4 quake near the Japan Trench at 07:53 UTC (16:53 local time). The agency estimates about a 1% chance of a Mw7.8 or greater earthquake occurring within seven days, based on historical statistics.
Modeled impact extent for potential earthquakes along the Japan Trench, showing expanding affected areas for Mw7.0 to Mw9.0 scenarios. Credit: JMA
The advisory was triggered after analysis showed the earthquake met established criteria for events that may be followed by a larger earthquake within the same tectonic system.
The probability of a subsequent large earthquake, defined in this context as Mw7.8 or greater, has increased relative to normal background conditions, JMA said. Based on global statistical analysis, the likelihood is estimated at approximately 1% within seven days and within a radius of about 500 km (311 miles) from the initial event. This represents roughly a tenfold increase compared to typical baseline probabilities, although the absolute probability remains low.
Residents in affected coastal areas are advised to review earthquake preparedness measures and ensure readiness to evacuate immediately in the event of strong ground shaking or tsunami warnings. Authorities recommend maintaining awareness of official updates while continuing normal daily activities.
The warning covers Pacific coastal areas along Hokkaido and northeastern Japan — regions directly above where the Pacific Plate collides with and slides beneath the North American and Okhotsk Plates. This subduction zone boundary has produced some of the world’s largest earthquakes, including the devastating 2011 M9.1 quake.
Historical sequences cited in official materials include the March 2011 Tohoku earthquake, where a Mw7.3 event occurred two days before the Mw9.0 mainshock, as well as a 1963 sequence near the Kuril Islands in which a Mw7.0 earthquake was followed within approximately 18 hours by a Mw8.5 event.
JMA added that this advisory does not constitute an earthquake prediction and is intended as a risk-awareness measure. While historical patterns show that some large earthquakes have been preceded by smaller events in the same region, most qualifying earthquakes are not followed by larger ones.
According to the latest available data, at least 4 people were killed, and more than 200 were injured in today’s M7.4 quake.
Power outages affected approximately 2.2 million households across multiple prefectures, including the Tokyo metropolitan area, according to Tokyo Electric Power Company (TEPCO). Transportation infrastructure was also impacted, with bullet train services suspended in parts of Aomori Prefecture.
The largest recorded wave from today’s earthquake was 80 cm (2.6 feet) at Kuji port in Iwate Prefecture. Smaller 40 cm (1.3 feet) waves were recorded at other coastal ports in the affected region.
References:
1 北海道・三陸沖後発地震注意情報について – JMA – April 20, 2026
2 Major M7.4 earthquake hits near east coast of Honshu, Japan, tsunami waves observed along Tohoku coast – The Watchers – April 20, 2026
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