Earthquake Report: M 7.1 Japan

A few days ago (as I write this) there was a magnitude M 7.1 earthquake offshore of Japan.

https://earthquake.usgs.gov/earthquakes/eventpage/us6000nith/executive

The plate tectonics of Japan is dominated by compressional forces. Convergent plate boundaries are where plates move towards each other.

In Japan, these convergent plate boundaries are called subduction zones (one plate “subducts,” or dives, beneath another plate).

The Japanese invest heavily in earthquake science because there has been a long history of subduction zone earthquakes.

Earthquake History

In the 20th century, some are of note. The 1923 Great Kantō Earthquake was a magnitude M~8.0 earthquake that devastated Tokyo and the surrounding region.

In 2011 there was the magnitude M 9.1 Tōhoku-oki subduction zone earthquake in eastern Japan. This is where the Pacific plate subducts westward below the Okhotsk plate, forming the deep sea Japan trench.

This 8 August 2024 M 7.1 earthquake was in southwest Japan, offshore of the Island Kyushu. The Pacific Ocean region offshore of Kyushu is called Hyuga-Nada.

Here, the Philippine Sea oceanic plate subducts beneath the Eurasia plate to form the Nankai deep sea trench.

In 1944 and 1946 there were two large megathrust earthquakes along the Nankai trench. The 1944 M 8.1 earthquake is called a Tokai earthquake. The 1946 M 8.1 earthquake is called a Tonankai earthquake. Earthquakes that rupture these segments are given the same names.

These two earthquakes led to geologists subdividing this subduction zone into segments. Then geologists used prehistoric earthquake data (paleoseismic data) to try to understand if the subduction zone breaks along these segments in any patterns.

Do earthquakes always only rupture one or the other of these two segments? Do both segments sometimes rupture at the same time? This is the same question people ask for most all subduction zones globally.

The reason people ask this question is that the length of the earthquake is a primary factor that controls the magnitude of an earthquake. The width (length times width = area), the amount the fault slips, and properties about the elastic nature of the plates, are the other factors that control earthquake magnitude.

So, if both segments rupture at the same time, the earthquake would be larger (like in 1707 and maybe in 1498).

In 1707 there was the M 8.7 Hōei subduction zone earthquake that ruptured both of the 1944 and 1946 segments. Some have hypothesized that the 1707 earthquake slipped as far south as the 8 August 2024 M 7.1 earthquake.

We also want to use paleoseismic data to get an idea about how often these types of earthquakes happen in a given area. This helps us design buildings so that they can “survive” future earthquakes.

Earthquake Advisory

Before the 2011 earthquake, the largest magnitude earthquake along the Japan trench was in the mid M 8 range. So, the seismic hazard and tsunami hazard was based on this size of an earthquake.

Two years before the 2011 earthquake, Japanese geologists found prehistoric evidence of a tsunami that was caused by a much larger sized earthquake. But, this information had not yet been integrated into the seismic and tsunami hazard analyses for east Japan.

When the 2011 earthquake and tsunami happened, it was larger in size than expected. The mitigation that Japan had used was not sized properly and many people suffered greatly.

Preceding the 2011 M 9.1 earthquake, there was a M 7.3 earthquake. Because of this “precursor” earthquake, Japan planned to place their society into an earthquake advisory when earthquakes of a sufficient size happens along the coast of Japan (Toda et al, 2024).

This plan was particularly important for the region along the Nankai trench. The subduction zone earthquakes in Nankai are much closer to the coastline than for the 2011 earthquake (so tsunami arrive faster here). Placing the coast into an advisory is important particularly because of this shorter tsunami travel time.

Following the 8 August 2024 earthquake, the Japanese government placed this part of the country into an earthquake advisory. The advisory is intended to last for a finite amount of time. However, we do know that sometimes earthquakes that are triggered are triggered following time periods longer than a week.

In the social media section below there are some tweets that have links to stories about this advisory. I encourage everyone to read the Temblor article about this advisory.

Below is my interpretive poster for this earthquake

  • I plot the seismicity from the past month, with diameter representing magnitude (see legend). I include earthquake epicenters from 1924-2024 with magnitudes M ≥ 3.0 in one version.
  • I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.
  • A review of the basic base map variations and data that I use for the interpretive posters can be found on the Earthquake Reports page. I have improved these posters over time and some of this background information applies to the older posters.
  • Some basic fundamentals of earthquake geology and plate tectonics can be found on the Earthquake Plate Tectonic Fundamentals page.

    I include some inset figures. Some of the same figures are located in different places on the larger scale map below.

  • In the upper left corner is a map showing the plate tectonic boundaries (from the GEM).
  • In the lower right corner is a map that shows the M 7.1 earthquake intensity using the modified Mercalli intensity scale. Earthquake intensity is a measure of how strongly the Earth shakes during an earthquake, so gets smaller the further away one is from the earthquake epicenter. The map colors represent a model of what the intensity may be. The USGS has a system called “Did You Feel It?” (DYFI) where people enter their observations from the earthquake and the USGS calculates what the intensity was for that person. The dots with yellow labels show what people actually felt in those different locations.
  • In the upper left center is a plot that shows the same intensity (both modeled and reported) data as displayed on the map. Note how the intensity gets smaller with distance from the earthquake.
  • To the right of this intensity plot is the USGS finite fault slip model. Colors represent the amount of fault slip that their model calculates.
  • In the upper right corner are two maps showing the possibility of earthquake induced liquefaction for these two earthquakes. I discuss these phenomena in more detail later in the report.
  • To the left of the ground failure maps is a low angle oblique cross section showing the plate configuration in three dimensions (from AGU Trembling Earth, Austin Elliot). I place a yellow star in the region of the M 7.1 earthquake. Note how we can see the Philippine Sea plate seismicity diving downwards beneath the Eurasia plate.
  • In the lower right center are plots from three tide gage locations (shown on the map). These individual plots are included later in the report.
  • Here is the map with a month’s seismicity plotted.


Other Report Pages

Tsunami

Tsunami can be caused by a variety of processes, including earthquakes, volcanic eruptions, landslides, and meteorological phenomena.

Earthquakes, eruptions, and landslides cause tsunami when these processes displace water in some way.

We may typically associate tsunami with subduction zone earthquakes because these earthquakes are the type that generate vertical land motion along the sea floor.

  • Here is a great illustration of how a subduction zone earthquake can generate a tsunami (Atwater et al., 2005).


When the fault slipped, it caused the seafloor to deform and move. This motion also displaced the overlying water column.

As the water column is elevated, it gains potential energy. As this uplifted water expends this energy by oscillating up and down, it radiates energy in the form of tsunami waves.

This M 7.1 earthquake generated a tsunami. The tsunami was observed on tide gages along the coast of Japan but was too small to generate a trans-Pacific tsunami.

This is an animated model of the Great East Japan tsunami of 2011. The warmer the colors, the larger the wave. The first surges reached the closest Japan coasts in about 25 minutes. The first surges reached Crescent City in 9.5 hours.

    Here are three tide gage records from the coast of Japan.

    Time is represented by the horizontal axis and elevation is represented on the vertical axis. The darker blue line in this image represents the tidal forecast. The data recorded by the tide gage are represented by the medium blue colored line. The difference between the prediction and the observation is the tsunami signal (plus any background oceanographic conditions like storm swell or wind waves).

    Wave height is the distance measured between the wave crest and trough. Wave amplitude is the level of water above sea level.

    These data came from the Eurpoean Union Water Levels website.

    The locations of these tide gages are shown on the interpretive poster.

  • Aburatsu


  • Kushimoto


  • Tosashimizu


Shaking Intensity

  • Here is a figure that shows a more detailed comparison between the modeled intensity and the reported intensity. Both data use the same color scale, the Modified Mercalli Intensity Scale (MMI). More about this can be found here. The colors and contours on the map are results from the USGS modeled intensity. The DYFI data are plotted as colored dots (color = MMI, diameter = number of reports).
  • In the upper panel is the USGS Did You Feel It reports map, showing reports as colored dots using the MMI color scale. Underlain on this map are colored areas showing the USGS modeled estimate for shaking intensity (MMI scale).
  • In the lower panel is a plot showing MMI intensity (vertical axis) relative to distance from the earthquake (horizontal axis). The models are represented by the green and orange lines. The DYFI data are plotted as light blue dots. The mean and median (different types of “average”) are plotted as orange and purple dots. Note how well the reports fit the green line (the model that represents how MMI works based on quakes in California).
  • Below the map and the lower plot is the USGS MMI Intensity scale, which lists the level of damage for each level of intensity, along with approximate measures of how strongly the ground shakes at these intensities, showing levels in acceleration (Peak Ground Acceleration, PGA) and velocity (Peak Ground Velocity, PGV).


Potential for Ground Failure

  • Below are a series of maps that show the potential for landslides and liquefaction. These are all USGS data products.
    There are many different ways in which a landslide can be triggered. The first order relations behind slope failure (landslides) is that the “resisting” forces that are preventing slope failure (e.g. the strength of the bedrock or soil) are overcome by the “driving” forces that are pushing this land downwards (e.g. gravity). The ratio of resisting forces to driving forces is called the Factor of Safety (FOS). We can write this ratio like this:

    FOS = Resisting Force / Driving Force

  • When FOS > 1, the slope is stable and when FOS < 1, the slope fails and we get a landslide. The illustration below shows these relations. Note how the slope angle α can take part in this ratio (the steeper the slope, the greater impact of the mass of the slope can contribute to driving forces). The real world is more complicated than the simplified illustration below.


  • Landslide ground shaking can change the Factor of Safety in several ways that might increase the driving force or decrease the resisting force. Keefer (1984) studied a global data set of earthquake triggered landslides and found that larger earthquakes trigger larger and more numerous landslides across a larger area than do smaller earthquakes. Earthquakes can cause landslides because the seismic waves can cause the driving force to increase (the earthquake motions can “push” the land downwards), leading to a landslide. In addition, ground shaking can change the strength of these earth materials (a form of resisting force) with a process called liquefaction.
  • Sediment or soil strength is based upon the ability for sediment particles to push against each other without moving. This is a combination of friction and the forces exerted between these particles. This is loosely what we call the “angle of internal friction.” Liquefaction is a process by which pore pressure increases cause water to push out against the sediment particles so that they are no longer touching.
  • An analogy that some may be familiar with relates to a visit to the beach. When one is walking on the wet sand near the shoreline, the sand may hold the weight of our body generally pretty well. However, if we stop and vibrate our feet back and forth, this causes pore pressure to increase and we sink into the sand as the sand liquefies. Or, at least our feet sink into the sand.
  • Below is a diagram showing how an increase in pore pressure can push against the sediment particles so that they are not touching any more. This allows the particles to move around and this is why our feet sink in the sand in the analogy above. This is also what changes the strength of earth materials such that a landslide can be triggered.


  • Here Dr. Bohon demonstrates the phenomena of liquefaction.

  • And, another video demonstration.

  • Below is a diagram based upon a publication designed to educate the public about landslides and the processes that trigger them (USGS, 2004). Additional background information about landslide types can be found in Highland et al. (2008). There was a variety of landslide types that can be observed surrounding the earthquake region. So, this illustration can help people when they observing the landscape response to the earthquake whether they are using aerial imagery, photos in newspaper or website articles, or videos on social media. Will you be able to locate a landslide scarp or the toe of a landslide? This figure shows a rotational landslide, one where the land rotates along a curvilinear failure surface.


  • Below is the liquefaction susceptibility and landslide probability map (Jessee et al., 2017; Zhu et al., 2017). Please head over to that report for more information about the USGS Ground Failure products (landslides and liquefaction). Basically, earthquakes shake the ground and this ground shaking can cause landslides.
  • I use the same color scheme that the USGS uses on their website. Note how the areas that are more likely to have experienced earthquake induced liquefaction are in the valleys. Learn more about how the USGS prepares these model results here.


    Some Relevant Discussion and Figures

    • This map shows the current tectonic configuration of this region, along with some inherited features from the tectonic past (e.g. green lines). This is from NUMO’s report: “Evaluating Site Suitability for a HLW Repository (Scientific Background and Practical Application of NUMO’s Siting Factors), NUMO-TR-04-04.”


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    • Also from the NUMO report, this shows the Niigata-Kobe fold and thrust belt. In addition, this map shows a northwest striking convergent plate boundary along the southeastern boundary of Hokkaido. However, it cannot explain the interesting orientation of the M 6.2 deep (240 km) earthquake.


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    • Here is the cool tectonic map from Liu et al. (2013). We all like cool maps! (right?)


      • Tectonic settings of the study region (black box). The solid sawtooth lines and the black dashed line denote the plate boundaries (Bird 2003). The red triangles denote the active volcanoes. The blue dashed lines and the pink lines denote the depth contours to the upper boundary of the subducting Pacific slab and that of the subducting Philippine Sea slab, respectively (Hasegawa et al. 2009; Zhao et al. 2012). The topography data are derived from the GEBCO_08 Grid, version 20100927, http://www.gebco.net. The ages of oceanic plates are from M¨uller et al. (2008).

    • Here is a map from the recent update of the Japan National Seismic Hazard Maps, resulting from knowledge gained following the 2011 M 9.1 earthquake (Fujiwara et al., 2012). The color represents the chance that a region will experience ground shaking at or greater that Japan Meteorological Agency (JMA) seismic intensity 6 in the next 30 years. JMA intensity is a scale of shaking intensity similar to the Modified Mercalli Intensity (MMI) Scale. The numbers are different, so they are difficult to compare. The JMA intensity 6 is similar to MMI X. Today’s earthquakes are in a region of slightly elevated chance of ground shaking (between 6-26%). Today’s M 6.6 earthquake may have reached
    • This is a fantastic educational video from IRIS that discusses the plate tectonics and mentions some earthquakes in the region of Japan.

    • Here is a USGS poster than summarizes the earthquake history and plate geometry for this region. This is the USGS Open File Report 2010-1083-D (Rhea et al., 2010).


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    • Here is the figure showing the tectonic setting (Kurikami et al., 2009). I include their figure caption as a blockquote.


      • Current tectonic situation of Japan and key tectonic features.

    • Here is the figure showing the historical moment tensors for this region (Chapman et al., 2009). I include their figure caption as a blockquote.


      • Focal mechanism plots for earthquakes in southwest Japan from 1997-2006. Based on CMT solutions from the JMA catalogue (data from http://www.fnet.bosai.go.jp).

      There have been many IODP investigations along the Nankai Trough. These investigations include 3-D seismic and scientific drilling in this region. Here are a couple reports from Moore et al. (2009) and Kinoshita, et al. (2007).

    • Here is an image that shows a 3-D view of the seafloor and seismic data. This comes from Moore et al. (2007). First is a map showing the location of this 3-D seismic survey, then the survey results, then their interpretation of the evolution of this margin. I include their figures caption below as a blockquote.


      • Location map showing the regional setting of the Nankai Trough (upper right inset). PSP, Philippine Sea Plate; KPR, Kyushu-Palau Ridge; IBT, Izu-Bonin Trench; KP, Kii Peninsula. Conver gence direction between the Philippine Sea Plate and Japan is shown at the lower right.


      3D seismic data volume depicting the location of the megasplay fault (black lines) and its relationship to older in sequence thrusts of the frontal accretionary prism (blue lines). Steep sea-floor topography and numerous slumps above the splay fault are shown.


      (A to C) Summary diagram showing the development of the Nankai accretionary prism in the Kumano Basin area. After “normal” in-sequence thrusting and building of an accretionary prism, an out-of-sequence (splay) fault system broke through at the back of the prism, a, b, and c refer to sequential sedimentary sequences.

    • This is an image from Jin-Oh Park (University of Tokyo) that shows the Decollement (the megathrust fault) and the seafloor. I include their figure caption below as a blockquote.


      • 3-D prestack depth migration images (inline slice, crossline slice, and depth slice) of the Nankai accretionary wedge off Shikoku Island. Miocene to Pliocene Shikoku Basin sediments underthrusts the overlying accretionary prism along a decollement as the Philippine Sea Plate subducts beneath the Eurasian Plate. The oceanic crust of the subducting Philippine Sea Plate (PSP) is traceable over the entire inlines. Several imbricate thrust faults are observed in the overlying accretionary wedge. The Décollement steps down on the top of subducting oceanic crust around ~30 km landward from the deformation front.

    • Some think that the 1944 earthquake slipped along a splay fault, a fault that splays off of the megathrust. Here is an image that shows how the megasplay fault is configured. This is the source of this figure. I include their figure caption below as a blockquote.


      • A: Tectonic setting of Nankai Trough subduction zone. Large earthquakes (magnitude 8-class) have repeatedly occurred along the Nankai Trough. The orange shaded segment caused the 1944 Tonankai earthquake. The NanTroSEIZE project is underway along the red line B.
      B: Cross section (red line in A). Detailed seismic profiles illustrate the plate boundary fault and the megasplay fault.
      C: Locations of core samples from Sites C0004 and C0008, taken from hanging wall and the footwall, respectively.

    I will include more figures below.

    Social Media

    References:

    Basic & General References

  • Frisch, W., Meschede, M., Blakey, R., 2011. Plate Tectonics, Springer-Verlag, London, 213 pp.
  • Hayes, G., 2018, Slab2 – A Comprehensive Subduction Zone Geometry Model: U.S. Geological Survey data release, https://doi.org/10.5066/F7PV6JNV.
  • Holt, W. E., C. Kreemer, A. J. Haines, L. Estey, C. Meertens, G. Blewitt, and D. Lavallee (2005), Project helps constrain continental dynamics and seismic hazards, Eos Trans. AGU, 86(41), 383–387, , https://doi.org/10.1029/2005EO410002. /li>
  • Jessee, M.A.N., Hamburger, M. W., Allstadt, K., Wald, D. J., Robeson, S. M., Tanyas, H., et al. (2018). A global empirical model for near-real-time assessment of seismically induced landslides. Journal of Geophysical Research: Earth Surface, 123, 1835–1859. https://doi.org/10.1029/2017JF004494
  • Kreemer, C., J. Haines, W. Holt, G. Blewitt, and D. Lavallee (2000), On the determination of a global strain rate model, Geophys. J. Int., 52(10), 765–770.
  • Kreemer, C., W. E. Holt, and A. J. Haines (2003), An integrated global model of present-day plate motions and plate boundary deformation, Geophys. J. Int., 154(1), 8–34, , https://doi.org/10.1046/j.1365-246X.2003.01917.x.
  • Kreemer, C., G. Blewitt, E.C. Klein, 2014. A geodetic plate motion and Global Strain Rate Model in Geochemistry, Geophysics, Geosystems, v. 15, p. 3849-3889, https://doi.org/10.1002/2014GC005407.
  • Meyer, B., Saltus, R., Chulliat, a., 2017. EMAG2: Earth Magnetic Anomaly Grid (2-arc-minute resolution) Version 3. National Centers for Environmental Information, NOAA. Model. https://doi.org/10.7289/V5H70CVX
  • Müller, R.D., Sdrolias, M., Gaina, C. and Roest, W.R., 2008, Age spreading rates and spreading asymmetry of the world’s ocean crust in Geochemistry, Geophysics, Geosystems, 9, Q04006, https://doi.org/10.1029/2007GC001743
  • Pagani,M. , J. Garcia-Pelaez, R. Gee, K. Johnson, V. Poggi, R. Styron, G. Weatherill, M. Simionato, D. Viganò, L. Danciu, D. Monelli (2018). Global Earthquake Model (GEM) Seismic Hazard Map (version 2018.1 – December 2018), DOI: 10.13117/GEM-GLOBAL-SEISMIC-HAZARD-MAP-2018.1
  • Silva, V ., D Amo-Oduro, A Calderon, J Dabbeek, V Despotaki, L Martins, A Rao, M Simionato, D Viganò, C Yepes, A Acevedo, N Horspool, H Crowley, K Jaiswal, M Journeay, M Pittore, 2018. Global Earthquake Model (GEM) Seismic Risk Map (version 2018.1). https://doi.org/10.13117/GEM-GLOBAL-SEISMIC-RISK-MAP-2018.1
  • Storchak, D. A., D. Di Giacomo, I. Bondár, E. R. Engdahl, J. Harris, W. H. K. Lee, A. Villaseñor, and P. Bormann (2013), Public release of the ISC-GEM global instrumental earthquake catalogue (1900–2009), Seismol. Res. Lett., 84(5), 810–815, doi:10.1785/0220130034.
  • Zhu, J., Baise, L. G., Thompson, E. M., 2017, An Updated Geospatial Liquefaction Model for Global Application, Bulletin of the Seismological Society of America, 107, p 1365-1385, https://doi.org/0.1785/0120160198

    • Specific References

  • Chapman et al., 2009. Development of Methodologies for the Identification of Volcanic and Tectonic Hazards to Potential HLW Repository Sites in Japan –The Kyushu Case Study-, NUMO-TR-09-02, NOv. 2009, 192 pp.
  • 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.
  • Hayes, G.P., Wald, D.J., and Johnson, R.L., 2012. Slab1.0: A three-dimensional model of global subduction zone geometries in, J. Geophys. Res., 117, B01302, doi:10.1029/2011JB008524
  • 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.
  • Kurikami et al., 2009. Study on strategy and methodology for repository concept development for the Japanese geological disposal project, NUMO-TR-09-04, Sept. 20-09, 101 pp.
  • Moore, G.F., Bangs, N.L., Taira, A., Kuramoto, S., Pangborn, E., and Tobin, H.J., 2007. Three-Dimensional Splay Fault Geometry and Implications for Tsunami Generation in Science, v. 318, p. 1128-1131.
  • Rhea, S., Tarr, A.C., Hayes, G., Villaseñor, A., and Benz, H.M., 2010. Seismicity of the earth 1900–2007, Japan and vicinity: U.S. Geological Survey Open-File Report 2010–1083-D, scale 1:6,000,000.
  • 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
  • Van Horne, A., Sato, H., Ishiyama, T., 2017. Evolution of the Sea of Japan back-arc and some unsolved issues in Tectonophysics, v. 710-711, p. 6-20, http://dx.doi.org/10.1016/j.tecto.2016.08.020
  • 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

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