Japan’s magnitude 7.1 shock triggers megaquake warning. How likely is this scenario?

Temblor

4 months ago

The largest shocks in the Nankai subduction zone range between magnitude 8 and 9 and occur every 100 to 200 years. But, the recent magnitude 7.1 shock occurred in a zone of repeating magnitude ~7 shocks every ~30 years, probably without producing megaquakes. This zone is likely too far from the edge of the historic megathrust rupture area to bring it significantly closer to failure.

Update: The Advisory expired on August 15, 2024. JMA asks people to “please be aware that a large-scale earthquake could occur at any time along the Nankai Trough, and continue to prepare for earthquakes.” (The quote was translated from Japanese to English.)

By Shinji Toda (Tōhoku University), Ross S. Stein and Volkan Sevilgen (Temblor, Inc.)

Citation: 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

Editor’s note: Throughout this article, “magnitude” refers to moment magnitude, which is often abbreviated as Mw by geoscientists. However, the article also references magnitudes reported by the Japan Meteorological Agency, which we abbreviate throughout as Mj.

On 8 August 2024, a magnitude 7.1 shock struck at a depth of 25 kilometers, about five to ten kilometers off the shores of the city of Miyazaki on Japan’s southern island, Kyushu (USGS, 2024). The population of this region is accustomed to large shocks; in 2016 a magnitude 7.0 earthquake struck the city of Kumamoto, in the center of Kyushu, causing extensive damage. Fortunately, shaking in the recent Miyazaki event was not severe on land (Figure 1); some damage was reported. A modest 0.5-meter tsunami was recorded, but no deaths were reported.

Shaking was moderate to strong on land, reaching up to 35% g. This is high enough to cause significant damage in countries less prepared for quakes than Japan. In this blind test, Temblor’s ground motion and site amplification (SiteAmp) models matched the strong shaking well, as seen by comparing the top two panels, or in the scatter plot at bottom. For the prediction, Temblor used a point source with the magnitude, location and depth given by the USGS. Credit: Temblor, CC BY-NC-ND 4.0

The shock likely struck on the Nankai Trough subduction zone interface. This fault is the site of historic events with magnitudes between 8 and 9 whose ruptures extended to within 100 kilometers of the magnitude 7.1 epicenter. The Nankai Trough is much closer to coastal communities than is the Japan Trench, where the 2011 magnitude 9.0 Tōhoku megathrust struck. This means that any tsunami warning will be much briefer, perhaps 5 minutes, rather than the 17 minutes for the Tōhoku quake. And so, the recent event has raised concern that it could be a foreshock for a much larger megathrust event.

The Megaquake Advisory

Prior to the disastrous 2011 Tōhoku event off the coast of Honshu, Japan’s government expected no more than a magnitude 8.6 event to occur in the Japan Trench. Two days before the Tōhoku shock, a magnitude 7.3 earthquake occurred near the future magnitude 9.0 epicenter. When the mainshock occurred, its immense magnitude was unexpected.

In reaction to the foreshock and massive mainshock in 2011, Japan’s government increased the potential maximum size of a Nankai event from magnitude 8.6 to magnitude 9.1 (which translates to five times more energy released), and instituted a policy that any shock greater than or equal to magnitude 7.0 that fell inside the hypothesized magnitude 9.1 rupture zone would henceforth trigger a “mega earthquake caution,” based on the increased probability of a large quake (Figure 2). Large foreshocks are rare but not unprecedented; for example, the 2016 magnitude 7.0 Kumamoto shock was preceded by a magnitude 6.0 event (Mj 6.5) in about the same location about 28 hours prior.

Figure 2. Guided by the searing experience of the 2011 magnitude 9.0 Tōhoku shock, which had a magnitude much larger than anticipated by the Central Disaster Prevention Council (CDPC), the Japanese government greatly expanded the hypothetical maximum rupture area of a Nankai Trough megathrust earthquake, extending its southwestern end to Miyazaki, site of the 2024 magnitude 7.1 shock. The upper left image shows the projected shaking of the largest expected event (magnitude 8.6) as calculated in 2003. The bottom right image shows the expected shaking of the largest expected event (magnitude 9.1) as updated in 2012. Shaking follows the Shindo intensity scale. Credit: Modified from CDPC (2012)

After the August 2024 event, the Japan Meteorological Agency (JMA) issued a bulletin called ‘Nankai Trough Earthquake Extra Information,’ which served as a warning that a megaquake may be imminent. The announcement was issued as a ‘Megaquake Advisory’ (kyodai jishin chūi / 巨大地震注意) to clarify the need for authorities and residents to take measures to prepare for a possible quake (Nagoya International Center, 2024). This Advisory will last nominally for a week.

So, was the warning scientifically and societally wise?

Figure 3. The official Advisory, annotated in English for clarity, issued by the Japanese government, showing that the magnitude 7.1 shock fell within the pre-determined warning zone. Credit: From the press briefing document by the Japan Meteorological Agency (JMA, 2024)

Repeating magnitude 7 quakes off Kyushu

Seismic instruments have recorded all shocks greater than or equal to magnitude 7 off the coasts of Kyushu and Shikoku for the past century. Seismologists describe this catalog as being “complete” for earthquakes greater than or equal to magnitude 7.

Repeating earthquakes, also called ‘repeaters,’ are characterized by similar location, magnitude and focal mechanism; their rupture areas should overlap. Repeaters are common in subduction zones.

The catalog shows a pattern of ‘repeater-like’ shocks of approximately magnitude 7 occurring every 25 to 30 years in roughly the same places in the Hyuga-nada region. Such events struck in 1931, 1961, 1996, and now, 2024, with overlapping rupture areas (Figure 4). A similar pattern might be seen just to the north of the recent magnitude 7.1 event, with shocks in 1941, 1970, and 1987.

Along the Nankai Trough, the Philippine Sea plate is subducting is under Japan at about 43 millimeters per year. In the 30 years between the quakes, about 1.3 meters of slip would be expected to accommodate this rate of subduction. That’s about the mean slip in the magnitude 7.1 quake, suggesting that these repeater-like shocks may be all that’s needed to keep pace with subduction. So, it’s not clear to us that stress is accumulating toward a great quake in this region; the moderate shocks could be doing the full job.

Figure 4. The past century of large earthquakes in the Hyuga-nada region reveals an absence of events greater than magnitude 7.5. Instead we observe repeater-like behavior of clustered magnitude 7 or so shocks. The 2024 rupture area is estimated from the first 24 hours of aftershocks, and so the rupture area is likely smaller than shown. 1996 rupture areas are from Yagi et al. (1999), and the 1968 rupture area from Yagi and Kikuchi (2003). The USGS ANSS catalog has many spurious magnitudes in this region that are too high, so we use JMA magnitudes (Mj), which differ slightly from more widely reported moment magnitudes (Mw). Credit: Temblor, CC BY-NC-ND 4.0

No great prehistoric quakes within 100 kilometers of the magnitude 7.1 event

Even in Japan, with its high seismic rate, a century offers only a snapshot of earthquake occurrence, and so the historic and ‘paleoseismic’ (prehistoric) records of events offer great insight, although the sizes and locations of older events are much less certain. That 1,400-year record shows that magnitude 8 and greater events on the Nankai Trough have struck every 100 to 200 years, with the shorter end of the spectrum being more common. Eighty years have elapsed since the last pair of events in 1944 (magnitude 8.1) and 1946 (magnitude 8.3), time enough for another event, but still less than the average interval.

Equally important, the paleoseismic record contains no evidence of a megathrust event reaching southwestward to the Kyushu coastline (Figure 5, modified from Garrett et al., 2016). The absence of evidence does not prove this area cannot — or even has not — produced great events. But taken together with the century of roughly repeating events of about magnitude 7, and the absence of events greater than magnitude 7.5, this suggests to us that a megathrust rupture is unlikely to reach the Kyushu coastline.

Figure 5. Summary of paleoseismic evidence and inferred rupture areas by Garrett et al. (2016). Although there is evidence for the 1707, 1361, and 684 ruptures reaching into the Hyuga-nada region, none extend to the site of the magnitude 7.1 shock. Credit: After Garrett et al., 2016.

The difference in seismicity between offshore Kyushu and the rest of the Nankai megathrust surface is striking when one looks at the past 70 years of earthquakes. Beginning about 5 years after the 1944 and 1946 magnitude 8.1 and 8.3 (respectively) quakes struck, most areas of high slip have been devoid of quakes. In contrast, the Kyushu coastline has been highly active during this period.

Figure 6. Starting 4 years after the 1946 mainshock, seismicity became sparse in the areas of high slip in the 1944 Tonankai and 1946 Nankai shocks, but remained abundant in the southwest. The recent quake, located in the center of a band of quakes that may keep up with the subduction of the Philippine Sea plate, suggests that great quakes are unlikely to occur there. Prior to 1950, the catalog is not complete for shocks greater than or equal to magnitude 4.5. Credit: Toda and Stein, (2022)

Why would the southwestern portion of the Nankai Trough subduction zone behave differently than the rest? In writing about the magnitude 7.1 shock, Bradley and Hubbard (2024) argue that the subducted seafloor is unique off Kyushu because a long seafloor mountain range, the Kyushu-Palau Ridge, is subducting there. Seamounts and rough ocean-floor terrain tend to decouple a megathrust, reducing its prospect of rupturing in great quakes.

Recent shock brings Kyushu and offshore faults closer to failure

Even though the magnitude 7.1 rupture is far from the edge of historical megathrust events, it has increased the stress on the part of the megathrust that ruptured in the great 1946 Nankai shock, bringing it closer to failure, but only by a tiny amount — 0.01 bar. That’s about a tenth of the Coulomb stress increase that is typically accompanied by an increase in seismicity. When we calculate the stress transfer to the likely edge of previous Nankai ruptures that came closest to the magnitude 7.1 epicenter, the stress increase would be about 0.1 bar — small but not insignificant.

And consistent with these calculations, there has been no uptick in seismicity on the 1946 rupture zone, or between the rupture zone and the periphery of the magnitude 7.1 aftershock zone, in the days since the event.

Figure 7. We calculate that the magnitude 7.1 event brought the southwest edge of the 1946 rupture surface just 0.01 bar closer to failure. This is a negligible amount. In an attempt to be as realistic as possible, here we are calculating the stress in the local fault slip direction (‘rake’) in the rupture model of Kato and Ando (1997). The red halo around the 2024 shock results from the stress increases on adjacent thrust faults. Credit: Temblor, CC BY-NC-ND 4.0

How might the magnitude 7.1 shock trigger earthquakes on other nearby faults? The onshore right-lateral Futagawa and Hinagu faults ruptured in 2016, and we calculate that they were subjected to a modest stress increase of about 0.1 bar by the 2024 magnitude 7.1 shock. Since it struck, there has been a distinct increase in seismicity along these faults, including two shocks with magnitude greater than or equal to 3, likely triggered by the magnitude 7.1 event (Figure 8 top panel). Before the magnitude 7.1 event, shocks of magnitude ~3 occurred there every 25 days on average.

We calculate that offshore thrust faults surrounding the magnitude 7.1 event have been brought 0.5 to 1.5 bars closer to failure — a significant increase. The triggered events could occur on the subduction interface, like the magnitude 7.1 apparently did, or on ‘splay’ faults that extend to the surface (mapped in Fig. 8). So, aftershocks or progressive mainshocks immediately to the north or south or east of the magnitude 7.1 are possible. About a half dozen magnitude 3 to 4 shocks have occurred in this zone in the first 24 hours. The 1996 pair of events ruptured in about the same location 44 days apart; so, another offshore shock of similar magnitude remains a distinct possibility.

Figure 8. Calculated Coulomb stress and dilation imparted by the magnitude 7.1 Miyazaki earthquake suggests that onshore right-lateral shocks (top panel), offshore thrust faults (middle panel), and Kirishma volcano (bottom panel) could be sites of increased seismicity in the days and months ahead. Calculations made in Coulomb 3.4 (https://temblor.net/coulomb/). Credit: Temblor, CC BY-NC-ND 4.0

Earth hazard here isn’t limited to only earthquakes: A string of island arc volcanoes drapes across Kyushu. One of these, Mt. Kirishima, resides only 75 kilometers from the magnitude 7.1 epicenter. We calculate that the earthquake dilated (expanded) the crust in the vicinity of the Kirishima magma chamber and magma conduits to the surface. The shaking waves launched by the magnitude-7.1 quake could also have changed the gas and magma ‘plumbing’ of the volcano. Whether these could induce volcanic unrest is unknown, but the seismicity rate surrounding Kirishima has increased by a factor of at least 10 since the magnitude 7.1 (Figure 9), so we must also pay attention to this hazard.

Figure 9. There has been a sustained burst of seismicity near Kirishima volcano since the magnitude 7.1 shock struck. These quakes are quite small in magnitude. Whether this burst of activity was caused by the shaking waves or the dilation of the volcanic system is unknown, but it could hint at volcanic unrest. Credit: Temblor, CC BY-NC-ND 4.0

Weighing Advisory benefits and costs

We conclude that it is highly unlikely — but not impossible — that the magnitude 7.1 shock will prove to be a foreshock for a major megathrust event in the next several days or months. Although foreshocks have preceded several earthquakes greater than magnitude 8 worldwide, they remain a notoriously unreliable precursor. Notably, the historical and paleoseismic record of great earthquakes in the Nankai Trough does not include any that were preceded by foreshocks of magnitude 7 or so at Hyuga-nada. Our view is not entirely different than that of the government, which also acknowledges that a megaquake is unlikely.

In contrast, had the magnitude 7.1 shock struck on the 1944-1946 rupture surface — a region that has been extremely quiet since about 1950 — we think such an Advisory, or even a stronger Alert, would have been eminently justified.

And soon, the government must decide when to cancel the Advisory, a difficult decision because nothing about the aftershock decay, or the chance that the magnitude 7.1 will prove to be a foreshock, changes after a week. Rather, the probabilities gradually and steadily decline with time. The week-long Advisory is reminiscent of a 2016 California OES Advisory for the southernmost San Andreas.

On the basis of a physical model, Hyodo et al. (2016) considered a scenario similar to what occurred for the magnitude 7.1 event, and argued that “for the next anticipated event, countermeasures should include the possibility of a triggered occurrence of a Nankai Trough earthquake by an Mw 7 Hyuga-nada earthquake.” In our judgment, their model was perhaps more appropriate for an earthquake like the magnitude 7.5 in 1968 than the 2024 magnitude 7.1 event because the 1968 shock was both larger and closer to the greatest southwest extent of past great earthquake rupture zones. Nevertheless, against the backdrop of the enormous consequences of a Nankai megathrust event, the Hyodo et al. study likely influenced the government and JMA decision-makers.

The Advisory that JMA has issued is promoting awareness and mitigation among the population and local prefectural governments, which must be weighed against its disruptions and costs to normal life, transportation, tourism, and commerce. Goltz et al. (2024) found that a majority of jurisdictions had response plans in place for receipt of an alert from the JMA, but they lacked key planning elements considered important from a disaster management perspective. So, this Advisory might lead to the strengthening of those plans.

We have argued that the magnitude 7.1 could trigger other large shocks offshore or onshore, or possibly even stimulate volcanoseismic activity. Should any of these potentialities come to pass, those preparations will still be beneficial.

Science editor: Dr. Alka Tripathy-Lang, Ph.D.
Reviewer: Dr. Judith Hubbard, Ph.D.

References

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

CDPC (2012), Central Disaster Prevention Council of the Japanese Government (in Japanese) https://www.bousai.go.jp/jishin/nankai/taisaku/pdf/1_1.pdf

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

Goltz, J. D., K. Yamori, K. Nakayachi, H. Shiroshita, T. Sugiyama, and Y. Matsubara (2024). Operational Earthquake Forecasting in Japan: A Study of Municipal Government Planning for an Earthquake Advisory or Warning in the Nankai Region, Seismological Research Letters 95, no. 4, 2251–2265, doi: 10.1785/0220230304.

Hyodo, M., T. Hori, and Y. Kaneda (2016). A possible scenario for earlier occurrence of the next Nankai earthquake due to triggering by an earthquake at Hyuga-nada, off southwest Japan, Earth Planet Sp 68, no. 1, 6, doi: 10.1186/s40623-016-0384-6.

JMA (2024), Japan Meteorological Agency Press Briefing Document (in Japanese), https://www.jma.go.jp/jma/press/2408/08e/NT_202408081945sv.pdf

Kato, T., and M. Ando (1997). Source mechanisms of the 1944 Tonankai and 1946 Nankaido earthquakes: Spatial heterogeneity of rise times, Geophysical Research Letters 24, no. 16, 2055–2058, doi: 10.1029/97GL01978.

Nagoya International Center (2024), https://www.nic-nagoya.or.jp/en/disaster- preparedness/backnumber/disaster_preparedness/2022/08261710.html

Toda, S., and R. S. Stein (2022). Central shutdown and surrounding activation of aftershocks from megathrust earthquake stress transfer, Nat. Geosci. 15, no. 6, 494–500, doi: 10.1038/s41561-022-00954-x.

USGS (2024), https://earthquake.usgs.gov/earthquakes/eventpage/us6000nith/executive

Yagi, Y., and M. Kikuchi (2003). Partitioning between seismogenic and aseismic slip as highlighted from slow slip events in Hyuga-nada, Japan, Geophysical Research Letters 30, no. 2, doi: 10.1029/2002GL015664.

Yagi, Y., M. Kikuchi, S. Yoshida, and T. Sagiya (1999). Comparison of the coseismic rupture with the aftershock distribution in the Hyuga-nada Earthquakes of 1996, Geophysical Research Letters 26, no. 20, 3161–3164, doi: 10.1029/1999GL005340.

Copyright

We publish our work — articles and maps made by Temblor — under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0) license.

For more information, please see our Republishing Guidelines or reach out to news@temblor.net with any questions.