M6.8 earthquake shakes Kyushu, Japan

Should we expect another megaquake advisory?

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Citation: Hubbard, J. and Bradley, K., 2025. M6.8 earthquake shakes Kyushu, Japan. Earthquake Insights, https://doi.org/10.62481/763dc600

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Edit, 2025-01-14, 16:46 UTC: A few hours after the earthquake, the JMA committee decided not to issue a megaquake advisory, since the magnitude fell below the cutoff of M7.

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A magnitude 6.8 earthquake struck just offshore Kyushu, Japan in the evening hours of January 13, 2025 (21:19 local time, 12:19 UTC). The Japanese Meteorological Agency reported maximum shaking intensities of 5-, equivalent to MMI ~VI, felt in Takanabe Town, Shintomi Town, and Miyazaki City. Most of the rest of Kyushu experienced JMA intensities of 2-4.

Figure 1: Shaking intensities reported by JMA following the January 13, 2025 M6.8 earthquake. https://www.data.jma.go.jp/multi/quake/quake_detail.html?eventID=20250113213800&lang=en

While intensities of 5- can cause objects to fall from shelves and frighten people, in general this level of shaking is not expected to cause significant damage in Japan, thanks in large part to the earthquake-resilient construction here.

Figure 2: Approximate matching of JMA intensity with the MMI scale. Note 12 of Wald (2020).

News reports indicate that the earthquake raised a tsunami up to 1 meter high, reaching land within 30 minutes of the earthquake. At Miyazaki Port, the wave was only 20 cm.

A larger earthquake occurred in nearly the same place just a few months ago: a magnitude 7.0 (NIED reports that event as a magnitude 7.0, while the USGS prefers a magnitude of 7.1). That earthquake, and the more recent M6.8, are both subduction events, reflecting slip on the plate boundary fault. Here, the Philippine Sea Plate is colliding northwestward with, and sinking beneath, Japan. While that motion is mostly incremental, occurring a little bit every year, the slip on the plate boundary itself occurs in fits and starts. Even these pretty-big M6.8-7.0 earthquakes only accommodate a relatively small proportion of the overall convergence; history tells us that the plate boundary slips periodically in truly great (M8-9+) earthquakes, with at least 12 such events recorded over the last 1,340 years.

So, how does this recent M6.8 compare to the M7.0 about five months ago? While the numbers might look similar, the magnitude scale is logarithmic, so the difference is actually pretty significant: a M7.0 should produce about 1.5 times the amplitude of shaking on a seismogram as a M6.8, and release about twice as much energy. These values should be taken with a grain of salt, because uncertainties in the actual magnitudes can strongly affect the comparison.

We wrote about the M7.0 when it occurred.

Figure 3: Comparison of the shaking intensities recorded by JMA in the 2024-08-08 M7.0 earthquake and the 2025-01-13 M6.8 earthquake.

The shaking in the recent M6.8 was clearly lower intensity and more limited spatially than during the M7.0 last August. Keep in mind that the patterns of shaking also depend on the location (depth, distance from shore), as well as specifics of the rupture.

The M7.0 earthquake caused an unusual splash in the news, because the JMA followed the earthquake with a “megaquake advisory”: a warning that the likelihood of a much larger earthquake, M8-9, was elevated. It is always difficult to communicate earthquake risk, since a risk that is elevated above background levels can still be quite low — and indeed, the most probable situation occurred: the advisory expired with no megaquake.

So, how does this new M6.8 earthquake fit in? Let’s take a look at what’s been going on in the area since June 2024. We plotted the earthquakes in the area using the JMA seismicity catalog. The plotted focal mechanism for the M6.8 is from the USGS, because (at the time of writing) the automated Hi-net focal mechanism solution is a fairly unreasonable strike-slip solution that disagrees with all other reported solutions.

Figure 4: Earthquakes offshore Kyushu since June 2024. Thin black line on the cross section is the Slab2.0 model of the plate interface.

Following the M7.0 earthquake, the seismicity rate in the nearby area has remained higher than it was before the earthquake, particularly along the plate interface shallower than the hypocenter of that event. This is what we expect. In general, we count an earthquake as an aftershock as long as the seismicity rate remains elevated above normal, so this recent M6.8 could be considered an aftershock.

However, it is also a bit big for an aftershock. In most earthquake sequences, we expect the largest aftershock to be about one magnitude smaller than the mainshock — so, for a M7.0, we should expect to see one earthquake of ~M6. In fact, until now, the largest aftershock since the M7.0 was a M5.5 on September 14, 2024. So, this M6.8 stands out as unusual in that respect.

When two similar-magnitude earthquakes occur in a sequence, we generally call them a “doublet,” rather than mainshock-aftershock. This M6.8 falls into a gray zone: a little too small for a doublet, a little too big for an aftershock. Whatever the name, it is very likely that stresses from the M7.0 are partially responsible for triggering this event. This earthquake is also large enough to impose significant new stress changes on nearby faults.

So: will this earthquake trigger a new megaquake advisory? (It seems that a new problem has arisen in earthquake science: trying to predict whether an earthquake will trigger a megaquake advisory, or not.)

The default cutoff for an advisory is M7.0; this earthquake falls just below that cutoff. (Note that while the USGS reports the event as M6.8, JMA reports is as M6.9 — still below the limit, but not by much.)

However, we understand that the expert panel is convening to discuss the earthquake, and it will be interesting to see what (if any) recommendations result from that conversation. If this earthquake were to have occurred on its own, we can guess that no advisory would be issued: it falls below the (somewhat arbitrary) cutoff. However, it didn’t occur on its own: it was likely triggered by the earlier M7.

That gives us two ways of looking at it. Either it is less concerning, because it is just part of a typical aftershock sequence. Or, it is more concerning, because it tells us that the original M7 was indeed capable of (fairly delayed) triggering other large earthquakes on the nearby megathrust, a situation which is presumably repeated by the new event.

Deciding which of these two perspectives is more reasonable seems quite difficult, to us at least. It is important to remember that there is no record (to our knowledge — please do correct us if we are wrong) of a M7 earthquake in the Hyūga-nada Sea ever directly preceding a megaquake. Thus, we have little historical basis to work from when trying to evaluate competing models of potential megaquake triggering from Hyūga-nada earthquakes.

As ever, the immediate risk of a mega-earthquake is presumably quite low, but such an earthquake is both inevitable and unpredictable.

And perhaps it isn’t just megaquakes that we should be concerned about. Following the M7 earthquake, Toda et al. (2024) calculated that the stress changes from the earthquake could potentially trigger slip on various nearby faults, and further highlighted a burst of microseismicity near Kirishima volcano, about 75 kilometers from the mainshock. Those interactions might deserve another look following this new, slightly smaller, earthquake.

References:

Bradley, K., Hubbard, J., 2024. M7.1 earthquake strikes southern Japan; megaquake advisory issued. Earthquake Insights, https://doi.org/10.62481/cea4a692

Garrett, E., Fujiwara, O., Garrett, P., Heyvaert, V.M., Shishikura, M., Yokoyama, Y., Hubert-Ferrari, A., Brückner, H., Nakamura, A., De Batist, M. and QuakeRecNankai Team, 2016. A systematic review of geological evidence for Holocene earthquakes and tsunamis along the Nankai-Suruga Trough, Japan. Earth-Science Reviews, 159, pp.337-357. https://doi.org/10.1016/j.earscirev.2016.06.011

Toda, S., Stein, R. S., and Sevilgen, V., 2024, Japan’s magnitude 7.1 shock triggers megaquake warning. How likely is this scenario?, Temblor, http://doi.org/10.32858/temblor.348

Wald, D.J., 2020. Practical limitations of earthquake early warning. Earthquake Spectra, 36(3), pp.1412-1447. https://doi.org/10.1177/8755293020911