If it feels like you were just reading about earthquakes offshore Japan, there’s a reason for that: it’s been less than a month since we wrote about this area offshore Honshu. That last post addressed a
magnitude 6.5 earthquake on March 26th. In it, we commented that we had already written about two large earthquakes in the six months prior: a
M6.8 on November 9th, 2025, and a
M7.6 on December 8th, 2025. (Those latter two events were two of the earthquakes that led to megaquake advisories.)
Well, we can now add this latest M7.4 to the list. All of these earthquakes are associated with the great subduction zone that underlies northern Japan, where the Pacific Plate is sinking westward beneath the country. Plotting the entire JMA earthquake catalog in this area is interesting: there are around 660,000 events in this map region:
Figure 3: Seismicity reported by the JMA, colored by depth, with a cross-section across the subduction zone.
This is a beautiful profile that shows off a few important features of the subduction zone: a classic double Wadati-Benioff zone, a region of intense crustal deformation in the forearc (between the coast and the trench), and a locus of shallow earthquakes just outboard of the French, where the slab breaks in tension as it begins to bend downward.
Let’s take a look at how the recent large earthquakes fit into the great mosaic of Japan Trench ruptures. The M6.8 in November was part of a complex sequence, with a week of foreshocks that cascaded upward to the mainshock. That mainshock was located far offshore, near the trench, initiating at a shallow depth of 18 kilometers. It was followed in March 2026 by a M6.5 earthquake, an unusually large, delayed aftershock for an earthquake of its size.
In contrast, the M7.6 in December occurred ~200 kilometers to the northwest, near the Shimokita Peninsula. It had no known foreshocks, and at ~41 kilometers depth, reflecting the geometry of the west-dipping slab interface. The mainshock was followed by two large aftershocks above M6.5 (M6.6, M6.7) in the days afterwards, within the expected range for a M7.6. Note that a M7.6 earthquake releases about 16 times as much energy as a M6.8, so this really was much larger than the earthquake in November.
The recent M7.4 occurred between the two previous clusters: in space, in magnitude, in depth (35 kilometers).
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