M6.5 subduction earthquake shakes northern Honshu, Japan

Click here to read this post in Japanese (automatically translated by Google).

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At 11:18 PM local time on March 26, 2026, a 6.5 magnitude earthquake occurred off the coast of northern Japan. Shaking was reported over most of northern Honshu and southern Hokkaido.

For people who live in the area, earthquakes are a fairly regular occurrence. In the past six months, we’ve written about two major earthquakes that shook these areas: a M6.8 earthquake on November 9, 2025, and a M7.6 on December 8, 2025, to the north. Many people in the area may have felt a greater number of earthquakes, as each of these produced aftershocks.

This recent M6.5 earthquake appears very similar to the M6.8 earthquake: a similar location along the subduction zone in Japan, with a very similar focal mechanism indicating thrust slip on a low-angle fault trending west-northwest. We can easily conclude that both earthquakes are massive thrust events, reflecting slip on the subduction interface itself.

Figure 1: Focal mechanisms of earthquakes since January 1, 2025, offshore north of Honshu. Earthquakes are also displayed on a timeline on the right. The November 9 M6.8 and March 26 M6.5 earthquakes are highlighted through Bull’s Eyes.

Given the similarity in location and mechanism, it is not surprising that reports of earthquakes are also similar (although the intensity of the tremors is generally slightly lower in this latest earthquake, since it was somewhat smaller).

Figure 2: Shaking intensity from JMA in the 2025-11-09 M6.8 earthquake (left) and the 2026-03-26 M6.5 earthquake (right). Severity on the JMA severity scale.

Here at Earthquake Insights, we typically use the Modified Mercalli Intensity (MMI) scale to talk about shaking intensity, which is the scale used by the US Geological Survey. However, when an earthquake occurs in Japan, the data from the Japan Meteorological Agency (JMA) is much more complete – but on a different intensity scale. Both scales are intended to represent the experience of people who felt the earthquake. Both have numbers that increase in value as the strength of the vibration increases. But the exact meanings of these numbers are different.

The MMI scale uses Roman numerals I through X, where I = not felt and X = extremely disruptive. The JMA scale uses regular Arabic numerals from 0 to 7, where 0 = imperceptible and 7 = extremely destructive. To provide further refinement, the JMA scale also divides intensities 5 and 6 into “upper” and “lower,” so that they actually have a total of 10 different levels, just like the MMI; It’s just that each number means something different.

Not only are the numbers measured differently, but the entire JMA scale is centered around building types in Japan, with impacts on wooden homes versus reinforced concrete buildings. Since most deaths caused by ground shaking are caused by building collapses, it is extremely useful to have a damage guideline that takes into account local building patterns. One of the reasons why the 2024-01-01 Noto earthquake was so devastating in Japan is that it occurred in an area with a large amount of traditional wooden buildings that were built before new building codes were put in place in 1981. These types of structures are vulnerable not only to ground shaking, but also to fires following earthquakes. The JMA measure provides the kind of improvement needed to consider the impact on those structures directly. Meanwhile, the MMI is a more general scale that attempts to provide a global framework for thinking about earthquakes everywhere in the world.

Whenever this issue arises, we like to drop this number. It is a footnote in a paper by David Wald (2020), and to our knowledge it is the best reference for comparing the two measures. Wald credits a book by Monon and Cecic for providing the data supporting the comparison. Note that Wald shows density IX as the highest value. The USGS web page lists this X-density as the top of the MMI scale. The Wikipedia page for the MMI scale lists two higher values ​​(XI and XII), which were apparently added by Adolfo Cancani in 1904, but these are used infrequently or not at all! We have heard that there is debate about whether any values ​​higher than IX should be used at all. If you have a strong opinion about this, let us know!

Figure 3: Comparison of JMA and MMI intensity metrics. Note 12 from Wald (2020).

Whatever the details, the important thing about the recent M6.5 earthquake is that the intensity of the tremors was almost all 1-3, with the occasional 4. On the JMA scale, that intensity may be scary, but it shouldn’t cause damage.

So: Was the M6.5 earthquake an aftershock of the M6.8 earthquake that occurred last year? In fact, the word “aftershock” has always been a bit muddy. Normally, we consider any smaller earthquake that occurs after a larger one to be an “aftershock” as long as the seismicity rate continues to rise in the area. However, there is a catch. If a smaller earthquake is close to the size of the first earthquake, we will sometimes call it a “triggered” earthquake. This phrase can be especially useful if the second earthquake occurs on a different (nearby) fault. Since the size of M6.5 was close to that of M6.8, it is unclear to us whether the appropriate term to use here is “aftershock” or “triggered earthquake.”

To complicate matters further, the term “aftershock” implies that the newer, smaller earthquake was somehow caused by the older, larger earthquake. This makes a lot of sense when there is a clear major shock followed by smaller events. However, when we wrote about the M6.8 last November, we noted that it was preceded by some very complex earthquakes during the previous five days: a M5.2 foreshock followed by several days of aftershocks, and then a “cascade,” with three progressively larger earthquakes of M5-6 on November 8, before the mainshock on November 9. This is the figure we then prepared, using the earthquake catalog from the JMA:

Figure 5: Seismic sequence associated with the M6.8 earthquake that occurred in November 2026 off the coast of Japan.

This type of swarm earthquake behavior indicates that there may be some other process that causes earthquakes (such as fluid movement, or seismic creep of a segment of a fault). If this physical process were still underway, attributing responsibility for M6.5 to M6.8 would be more than a stretch.

With all that in mind, let’s look at a map of all the seismic activity at this location since last October, again using the JMA catalog. What can we learn? Here we’ve zoomed in on the seismic activity and also colored it over time to better see newer activity versus older activity.

Figure 6: Seismic activity since October 1, 2025 around the recent M6.5 earthquake. Earthquakes are colored by time, and are also displayed on the right on a timeline. Below the map is a time series showing seismic activity, and also a graph showing the number of earthquakes per day. Earthquakes above M6 are described. Note that event sizes differ slightly (M6.9 vs. M6.8) from the same events in the USGS catalog.

The first thing to see is that the main decline in seismic activity after the mainshock occurred as usual. After the initial strong decay, there was a loud background “hum”, which is normal: aftershocks decay by 1/time, meaning that the decay is faster at first and then slows down.

Second, this recent M6.5 earthquake is not the only larger earthquake to occur in the past few weeks. On March 8, a 6.1 magnitude earthquake occurred.

What does this mean? Well, this could all be completely normal behavior for aftershocks. Large delayed aftershocks occur regularly. While the largest aftershock of an earthquake is approximately 1.2 magnitude lower than the mainshock, there are plenty of earthquakes that violate this “law” (more of a loose guideline).

It is also possible that there are some fundamental physical processes occurring here that help trigger earthquakes. The front shocks and chainring before the main shock were certainly suggestive. If this is true, the recent resurgence of earthquakes may be related.

We also note that the aftershocks are not completely patchy, and form groups distributed over a wider area. We generally trust the locations in the JMA catalog, because they are derived from a very dense network of seismometers. This type of pattern is somewhat unusual in our experience, although it may simply be the case that most networks cannot resolve this type of detail. If the earthquake is very small, it does not generate aftershocks over a wide area, so we do not see scattering like this. If the earthquake is very large, it causes severe aftershocks everywhere. If an earthquake occurs on a steep fault, earthquakes at different depths will be plotted on top of each other, obscuring details. M6.8 may have been a moderate event – is it of the right magnitude to display this interesting behavior, on a suitably low-angle fault, and be observed by the right grid?

Was that enough hand-waving for one day? We think so. If you have any thoughts about this earthquake sequence, or about the different intensity scales, please let us know in the comments!

Leave a comment

Hubbard, J. and Bradley, K., 2025. The M6.8 earthquake off the coast of Japan is preceded by an ascending series of foreshocks. Earthquake Insights, https://doi.org/10.62481/d6ad8513

Mawson, R.M. and Cecic, I., 2012. Intensity and density scales. In New Manual of Seismic Observatory Practice 2 (NMSOP-2) (pp. 1-41). Deutsches GeoForschungsZentrum GFZ. https://gfzpublic.gfz.de/rest/items/item_43219_11/component/file_916905/content

Wald, D. J., 2020. Practical limitations of earthquake early warning. Earthquake Spectra, 36(3), pp. 1412-1447. https://doi.org/10.1177/8755293020911388

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