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Asia, Japan, Japan Trench, Kuril Islands, Kuril Trench

A Comparison between the Japan and Kuril Trenches

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In this article, we are travelling northwards from Mount Fiji to look at the subduction of the north west Pacific Plate under the Okhotsk Plate.  Subduction occurs at two trenches: the Japan Trench and the Kuril-Kamchatka Trench.  Here, we look at both the Japan Trench and the Kuril-Kamchatka Trench as far as the end of the Kuril Volcanic Arc.  We will look at the Kamchatka end of the Kuril-Kamchatka Trench when we look at the junction between it and the Aleutian Arc.

Fig 1: Geoscatter plot by the author of earthquakes of the Japan and Kuril Trenches over the past 25 years. For earthquakes of magnitude less than 7.0, colour denotes depth:  purple, 0-33km, blue 33-70km, green 70-150km, yellow, 150-300km. orange, 300-500km, red ≥500km. Yellow stars are earthquakes between 7.0 and 8.0 magnitude.  Red stars are earthquakes with magnitude ≥ 7.0. © Copyright remains with the author, all rights reserved, 2026.

We start by looking at the Japan Trench, then the Kuril Trench, and finish by comparing the two.

Fig 2: Image on the left: Okhotsk Plate by Alataristarion CC BY-SA 4.0 via Wikimedia Commons, Source: OkhotskPlate, Wiki Commons.  Image on the right is a screenshot from Google Maps of the area plotted.  Trenches are annotated by the author.

Japan Trench

The Japan Trench is a submarine trench in the north western Pacific Ocean, stretching roughly 800 kilometres (497 miles) from the Kuril Islands down to the northern end of the Izu Islands. With a maximum depth is 8,046, the Tuscarora Deep was considered the deepest point on Earth prior to the discovery of the Mariana Trench.

The Trench formed from the subduction of the Pacific Plate under the  Okhotsk Plate where the plates converge at a rate of 79 to 92 millimetres per year.  Earthquakes associated with the subducting plate extend down as far as 482.3 km in the period we looked at. It is the site of large earthquakes, which often cause tsunamis, for example, large slip-to-trench events. The presence of ultra-slippery pelagic clay, discovered by recent drilling projects, allows faults to snap violently all the way up to the seafloor, shifting the seabed horizontally by dozens of metres, causing catastrophic tsunamis. The 2011 Tōhoku earthquake, with a magnitude of 9.0-9.1 Mw occurred on the Trench, displacing the seabed by 60 m upwards; the resulting tsunami led to the Fukushima nuclear disaster. 

Fig 3:  Geoscatter plot by the author of earthquakes and volcanoes of the Japan Trench by the author; earthquakes are plotted for the period 1st January 2001 to 30th August 2026 between 34.75°N, 133.0°E to 46.0°N,162.0°E. Key as above. © Copyright remains with the author; all rights reserved. 2026

Subduction at the Japan Trench has created the North East Japan Arc of north eastern Honshū and Hokkaidō . The arc has  four zones: the volcanic front at the inner arc, an uplift zone, the outer arc and the Hokkaidō Collision Zone. The volcanic front forms  the Ōu Mountains (or Backbone Range), the spine of northern Honshū .  The uplift zone is non-volcanic. The outer arc is further east, comprising non-volcanic ranges of eroded pre-Tertiary rock.  The Hokkaidō Collision Zone is where the  North East Japan Arc meets the Kuril Volcanic Zone.

Fig 4: Image of Lake Toya by 663highland, published under  CC BY-SA 3.0 via Wikimedia Commons. Source: 130922 Lake Toya Toyako Hokkaido Japan03s3 – Lake Tōya – Wikipedia

The northern section of the  volcanic front includes calderas, acidic geothermal activity and sulphur springs. Volcanoes include Mount Osore, Mount Hakkōda and, Mount Iwaki.  The central section comprises highly active explosive stratovolcanoes, including Mount Iwate, Mount Hachimantai and Akita-Komagatake. The southern section of the front is made up of volcanoes characterised by phreatic collapses and frequent historic activity, such as Mount Zaō, Mount Bandai and Mount Adatara.

South western Hokkaidō  (The Oshima Zone) represents the northernmost continuation of the North East Japan Volcanic Arc, sharing geological similarities with northern Honshū.  It is home to Mount Yōtei, a dormant stratovolcano, Mount Usu, exceptionally active, its 1943 eruption pushed up a brand-new lava dome out of a flat wheat field, creating the mountain Shōwa-shinzan, and Mount Komagatake (Hokkaidō -Komagatake), a highly active volcano on the southern tip of the island, known for violent, explosive Plinian eruptions that periodically collapse its summit.

Central Hokkaidō  (The Collision Cluster) is situated directly where the North East Japan Arc meets the Kuril Volcanic Zone. This region contains Hokkaidō ‘s highest peaks, rugged alpine terrain, and intense geothermal systems. Volcanoes include the Daisetsuzan Volcanic Group and Mount Tokachi. Mount Yubari is non-volcanic, comprising metamorphic rock generated by the arc collision.

In Eastern Hokkaidō  (The Caldera & Shiretoko Zone) volcanoes are formed by subduction at the Kuril Trench.  Volcanoes include Mount Meakan, one of Hokkaidō ‘s most active volcanoes, frequently venting ash and volcanic gases, Kussharo, Japan’s largest caldera by surface area, Mount Io (Atosanupuri), a sulphur-stained volcano, Mount Iō (Shiretoko-Iōzan), and Mount Rausu.

While most volcanoes on the North East Japan Arc are primarily andesitic, there is one rhyolitic volcano, Naruko, comprising a caldera and lava domes, which last had a small eruption in 837 CE; two dacite volcanoes on Honshū, the Omanago Group, which last erupted in 3050 BCE, and, Numazawa, 3400 BCE; two basaltic volcanoes on Honshū, Akita-Koagatake, which last erupted in 1971 CE and Iwatesan, whose last major eruption was in 1732 CE, with a smaller one in 1919 CE; and, three basaltic volcanoes on Hokkaidō , Oshima – Oshima, which last erupted in 1790 CE, Toya, 2001 CE and Kuttara, 1820 CE.  While Naruko is relatively close (148 km away) to the site of Fukushima Nuclear Disaster, it does not share the same fault zones; Naruko is on the volcanic front in the Tōhoku Backbone Range, and Fukushima is on the coastal forearc / marine slope.

Kuril Trench

The Kuril-Kamchatka Trench, stretching c. 2,100 to 2,900 km,  is a direct extension of the Japan Trench. Its deepest point is 9,600 – 9,717 m. At the Trench, the Pacific Plate subducts under the Okhotsk Plate, with convergence rates from c.75 mm per year in the north to c.83 mm per year  in the south.  Earthquakes associated with the subducting plate extend down as far as 631.9 km in the period we looked at. The Trench  lies off the southeast coast of Kamchatka, running  parallel to the Kuril Island chain, with its northern end is at the triple junction with the Ulakhan Fault and the Aleutian Trench,  and its southern end meeting the Japan Trench east of Hokkaidō .  The Trench is the site of earthquakes with magnitudes over 8.0.

The Kuril-Kamchatka Trench tends to rupture over long distances. Because it is a relatively unobstructed line, single earthquakes can unzip huge stretches of the boundary, for example, the 600 km rupture zone from the 8.8 magnitude 2025 Kamchatka earthquake.

Fig 5: Geoscatter plot by the author of earthquakes and volcanoes of the Kuril Trench by the author; earthquakes are plotted for the period 1st January 2001 to 30th August 2026 between 46.0°N, 133.0°E to 53.0°N,162.0°E. Key as above.  © Copyright remains with the author; all rights reserved. 2026.

The Kuril Islands, themselves, stretch 1,300 kilometres from Hokkaidō  to the southern end of  Kamchatka.  The islands are the summits of some of stratovolcanoes on the submarine volcanic ridge. There are c.100 volcanoes on the Kuril Islands archipelago 200km west of the Trench.  The highest point on the islands is Alaid Volcano, at 2,339 m, on Atlasov Island at the northern end of the chain.

Fig 6:  Image of Alaid Volcano by Eugene Kaspersky, published under CC BY-SA 4.0, via Wikimedia Commons. Source: Atlasov Island1 – Alaid volcano – Wikipedia

 Some of the better known volcanoes include Tao-Rusyr Caldera, with its large 7.5-kilometre-wide caldera, Lake Koltsevoye, Kudryavy (Iturup Island), with high-temperature fumaroles that deposit rare chemical elements, including indium and rhenium, Sarychev Peak, one of the most active volcanoes in the Kuril chain, and Tyatya, with its nested caldera.

As with the Japan Trench most volcanoes have andesite as their primary lava.  However, there are 14 basaltic volcanoes, Chachadake [Tiatia], which last erupted in 1981 CE, Prevo Peak, 1825 CE, Raikoke, 2019 CE, Chikurachki, 2023 CE, Alaid, 2022 CE, Kambalny, 2017 CE, Koshelev, 1690 CE, Yavinsky, 4050 BCE, Zheltovsky, 1923 CE, Mutnovsky, 2000 CE, Gorely, 2010 CE, Opala, 1776 CE, Tolmachev Dol, 300 CE, Barkhatnaya Sopka, 3550 BCE; and, three dacite volcanoes, Kussharo, last erupted in1320 CE, Diky Greben, 350 CE, Kurile Lake, 6440 BCE.

Comparison of Seismicity at Both Trenches

Both the Japan and Kuril Trenches are seismically very active and have produced magnitude 9.0 + megathrust earthquakes.  The Japan Trench also produces more slower earthquakes and shallow ruptures that create large tsunamis.  The Kuril Trench ruptures in regular, massive, deep-fault-coupled blocks that segment from Hokkaidō  to Kamchatka.

When we look at our plots of seismicity below, notably the 3D scatter plots, we see that there are considerably more earthquakes on the Pacific Plate east of the Japan Trench than the Kuril Trench.  This is due to the angle at which the Pacific Plate meets each trench. As the Pacific Plate approaches the Japan Trench it experiences flexural bending.  Before the crust can subduct it curves sharply downwards.  c. 120 to 150 km east of the plate buckles upwards under pressure.  The top layer of the crust cracks under tensional forces as it progresses from the bulge to the steep downwards curve, generating frequent shallow normal faulting earthquakes.  The stress from bending creates a pattern of parallel faults known as a horst and graben structure. This also occurs on the Kuril Trench, however, it is greater at the Japan Trench because the west north west moving Pacific Plate meets the north south oriented Trench at a c. 90° angle, whereas at the southwest to northeast oriented Kuril Trench the Pacific Plate hits at an oblique angle so the plates slide slightly along the Trench spreading some of the energy into horizontal faulting in the overriding plate.  The Pacific Plate is, therefore, more heavily fractured at the Japan Trench than at the Kuril Trench.

The angle the Pacific Plate approaches the trenches also influences how the trenches rupture.  When the plate formed at a mid ocean ridge millions of years ago, lines of weakness were created.  At the Kuril Trench, the trench axis is parallel to these lines; it tends to unzip along pre-existing cracks, distributing stress evenly across small, frequent steps (under 400m) with gaps of 4-5km.  Whereas at the Japan Trench, the trench axis is at a high oblique angle to the ancient weakness lines; the plate cannot unzip cleanly both new normal faults rupture and older faults are reactivated, creating competing fault networks. Massive amounts of stress are concentrated into fewer much large faults with structural steps of more than 800m. These steps are part of a series of isolated stronger patches, separated by heavily damaged gaps of more than 15km width, making the interface heterogeneous.  Bend faults are also hydrated by sea water.  Both act as barriers to seismic ruptures; earthquakes tend to be confined to separate smaller, segments.   The smaller steps with narrower gaps of c. 4- 5km width of the Kuril Trench create a smoother subduction interface (it is considered homogeneous), with fewer barriers to stop propagating rupture so it is easier for megathrust earthquakes to unzip larger stretches of the plate boundary along pre-existing faults.

Fig 7 Video of the 3D scatter plot by the author of earthquakes and volcanoes of the Japan Trench by the author; earthquakes are plotted for the period 1st January 2001 to 30th August 2026 between 34.75°N, 133.0°E to 46.0°N,162.0°E. The key is included in the video. © Copyright remains with the author; all rights reserved. 2026.

Large Recent Earthquakes, Japan Trench

29th September 2003, the M 8.16 Tokachi-Oki earthquake occurred at 41.815°N, 143.910°E near Hokkaidō  with 27.0km depth.  A slip occurred over an area of 100 x 120km with a downward dip of the hypocentre. It was  caused by shallow thrust faulting on, or near, the plate boundary between the Pacific and Okhotsk Plates near both the Japan and Kuril Trenches.  Not only does Hokkaidō  get shallow large thrust earthquakes that originate from the plate boundary bit it also gests large earthquakes from the interior of the downward subducting Pacific Plate.  (M 8.2 – 2003 Tokachi-Oki Earthquake).

Previous large earthquakes occurring in the same rupture of the same section of the plate boundary are the 1952 M8.1 Tokachi – Oki earthquake, 42.084°N 143.899°E, with 45.0 km depth,  (M 8.1 – 1952 Tokachi-Oki Earthquake), and, 1968 M8.2 Tokachi – Oki earthquake, 40.890°N, 143.362°E, 26.9 km depth (M 8.2 – 1968 Tokachi-Oki Earthquake). One earthquake occurred inside the Pacific Plate, 1993 M 7.6 – 52 km NE of Otofuke, Japan, 43.300°N, 143.691°E, 102.2 km depth, (M 7.6 – 52 km NE of Otofuke, Japan).

11th March 2011, the M 9.1 Great Tōhoku Earthquake 38.297°N 142.373°E, with 29.0 km depth occurred near the east coast of Honshū on the subduction zone plate boundary.  The fault moved 50 to 60m with a slip over an area of 400 x 150 km with a downward dip. The main shock was preceded by large foreshocks in the preceding 48 hour period.  (M 9.1 – 2011 Great Tohoku Earthquake, Japan).

Previous large earthquakes in the same area of the Japan Trench are 1978, M 7.7 – 68 km ESE of Ishinomaki, Japan, 38.190°N, 142.028°E, with 44.0 km depth (M 7.7 – 68 km ESE of Ishinomaki, Japan), and 1994, M 7.8 – off the east coast of Honshū, Japan, 40.525°N, 143.419°E. with 26.5 km depth (M 7.8 – off the east coast of Honshū, Japan).

More recently there have been two large earthquakes off northern Japan: a magnitude 7.5 earthquake hit the northern area in December 2025 ( M 7.6 – 2025 Aomori Prefecture, Japan Earthquake), followed closely by a magnitude 7.7 (Mw 7.4) Sanriku earthquake on April 20, 2026 (M 7.4 – 102 km ENE of Miyako, Japan).

Fig 8: Video of the 3D scatter plot by the author of earthquakes and volcanoes of the Kuril Trench by the author; earthquakes are plotted for the period 1st January 2001 to 30th August 2026 between 46.0°N, 133.0°E to 53.0°N,162.0°E. The key is included in the video . © Copyright remains with the author; all rights reserved. 2026

Large Recent Earthquakes, Kuril Islands

15th November 2006, the M 8.3 Kuril Islands earthquake occurred at 46.592°N 153.266°E, with a depth of 10km. A slip with an area of 260 x 80km, with an up dip to the north east of the hypocentre.  The main shock was withing 80km of the trench axis  (M 8.3 – 2006 Kuril Islands Earthquake).

13th January 2007, M 8.1  2007 Kuril Islands Earthquake, 46.243°N 154.524°E, with 10.0 km depth. A slip with an area of 150 x 50km occurred with down dip of the hypocentre. This was shallow east striking or south west striking moderately dipping normal faulting within the Pacific Plate.  The epicentre was east of the Kuril Trench, 95 km east south east of the 2006 earthquake (M 8.1 – 2007 Kuril Islands Earthquake).

29th July 2025, M 8.8 – 2025 Kamchatka Peninsula, Russia Earthquake, 52.495°N,160.240°E, with 35.0 km depth.  This was the main shock of a swarm of 4,600 earthquakes which started ten days earlier, which included  a M7.4 on 20th July 2025 and three M6.6s.  The earthquake was shallow reverse faulting with on the subduction zone interface, with a slip area of 390 x 140 km (M 8.8 – 2025 Kamchatka Peninsula, Russia Earthquake).

Other previous large earthquakes near the Kuril Trench are: 1915 c.M 8.0 Central Kuril Islands, 48.392°N, 155.000°E, with 30.0 km depth (M 7.8 – 266 km SSW of Severo-Kuril’sk, Russia), 1952. M 9.0 – 89 km ESE of Petropavlovsk-Kamchatsky, Russia, 52.623°N,159.779°E, with 21.6 km depth (M 9.0 – 89 km ESE of Petropavlovsk-Kamchatsky, Russia), 1963 M 8.5 – 1963 Kuril Islands Earthquake, 44.872°N 149.483°E, with 35.0 km depth  (M 8.5 – 1963 Kuril Islands Earthquake).  The 1952 earthquake ruptured 600 km of the subduction zone interface.

Our last video is of the 3D scatter plot of the two trenches together.  In this we can see the collision zone between the North East Japan Volcanic Arc and the Kuril Volcanic Arc at Hokkaidō .  We can also see the plate tear in the Pacific Plate which accommodates the change in direction of subduction between the Japan and Kuril Trenches (shows as an inverted V within the deeper earthquakes).

Fig 9:  Video of the 3D scatter plot by the author of earthquakes and volcanoes of the Japan and Kuril Trench by the author; earthquakes are plotted for the period 1st January 2001 to 30th August 2026 between 34.75°N, 133.0°E to 53.0°N,162.0°E. The key is included in the video. © Copyright remains with the author; all rights reserved. 2026.

We hope you have found our article useful.

Armchair Volcanologist

© Copyright remains with the author; all rights reserved, 2026.

Sources

We have used Google’s AI in compiling this article, in addition to our usual searches.  This has made the trail to original works less clear.  Our apologies, if your work has not been accredited; we will happily correct.

Ikuko Wada, Jiangheng He, Akira Hasegawa, Junichi Nakajima, “Mantle wedge flow pattern and thermal structure in Northeast Japan: Effects of oblique subduction and 3-D slab geometry.”, in Earth and Planetary Science Letters, 15 September 2015, 76-88, Volume 426, doi: 10.1016/j.epsl.2015.06.021

Fujie, G., Kodaira, S., Kaiho, Y. et al. Controlling factor of incoming plate hydration at the north-western Pacific margin. Nat Commun 9, 3844 (2018). doi: 10.1038/s41467-018-06320-z

Azuma, R., Hino, R., Fujie, G., Obana, K., Ohta, Y., & Kodaira, S. (2025). Seismic structure characterizing the large shallow slip zone of the 17th-century Kuril earthquake. Journal of Geophysical Research: Solid Earth, 130, e2024JB030764. doi: 10.1029/2024JB030764

Jean-Paul Cadet, Kazuo Kobayashi, Jean Aubouin, Jacques Boulègue, Christine Deplus, Jacques Dubois, Roland von Huene, Laurent Jolivet, Toshihiko Kanazawa, Junzo Kasahara, Kinichiro Koizumi, Serge Lallemand, Yasuo Nakamura, Guy Pautot, Kiyoshi Suyehiro, Shin Tani, Hidekazu Tokuyama, Toshitsugu Yamazaki, “The Japan Trench and its juncture with the Kuril Trench: cruise results of the Kaiko project, Leg 3”, Earth and Planetary Science Letters, May 1987, 267-284, Volume 83, Issues 1–4, doi: 10.1016/0012-821X(87)90071-9

Wikipedia:

Japan Trench – Wikipedia

Kuril–Kamchatka Trench – Wikipedia

Raw earthquake data: USGS Earthquakes | U.S. Geological Survey

Volcano data: Global Volcanism Program, 2026. [Database] Volcanoes of the World (v. 5.4.0; 7 Aug 2026). Distributed by Smithsonian Institution, compiled by Venzke, E. doi: 10.5479/si.GVP.VOTW5-2026.5.4

References to individual earthquakes mentioned have been included in the text above.

Asia, Japan

Mount Fuji (Fujisan)

Taking a break from eruptions and seismic activity in Iceland, and disasters, in this article, we are taking a look at subduction zone volcanism, with Mount Fuji as our example.

Mount Fuji is stratovolcano located 100km south west of Tokyo in Honshu, Japan, in the Fuji-Hakone national park. The main crater is 780 m diameter at the top, 100 -130 m diameter at its base, and 240 m deep. The highest point, the Kengamine Peak, 3776 m, was built during the Kengamine eruption c.2,300 years ago, the last known summit eruption.  The most recent eruption was the 1707 Hoei eruption.

Fujisan is one of Japan’s three holy mountains, the others being Mount Tate and Mount Haku. It was viewed as a living god and a bridge between Earth and Heaven by ancient peoples.  In the Shinto religion, Konohanasakuya-hime is a goddess representing fire and volcanoes.  There are shrines to her at the summit and foot of the volcano to calm volcanic fire. Buddists climb the volcano for spiritual meditation and enlightenment. 

Mount Fuji is a very popular tourist destination due to its natural beauty and climbing trails.  It was declared a world cultural site in 2013.

Fig 1: Photo by Ichio Moriya (Kanazawa University) of Fujisan with the Tokyo-Osaka highway. Hoeisan, the smaller cone on the SE flank (to the right), is a remant of Kofuji (Old Fuji). Source: Global Volcanism Program | Volcano Photo Collection — Image GVP-08934. This image is made available under the Creative Commons BY-NC 4.0 license terms.

Tokyo, the capital and economic centre of Japan and a major international finance centre, has a population of 14 million; the Greater Tokyo area has one of 33 million.  According to Wikipedia, in 2022, Tokyo’s gross regional product was US$887.9 billion, accounting for 21.2% of the country’s total economic output.  Of the total output, wholesale and retail was 21.5,  real estate (13.5%), professional, scientific and technical (12.2%), information and communications (11.7%), finance and insurance (7.6%), manufacturing (7.0%), and healthcare (6.7%). Agriculture, forestry and fishery, and mining combined accounted for less than 0.1% of the economic output. Lying near the boundary of three tectonic plates (the Okhotsk, Amur Philippine Sea Plates) and also on the Kantō Plain, Tokyo is at risk from earthquake and volcanic activity, and also floods, especially when typhoon rains swell the many rivers on the Plain.

Fig 2: Screenshot from Google Earth (https://earth.google.com/) showing built up areas near Mount Fuji.  Mount Fuji is the snow capped volcano in the left of the image.  Eagle eyed readers will spot that Mount Fuji is not the only volcano in the image.

While Tokyo is the largest conurbation near Mount Fuji, there are several heavily built up areas nearby, including the cities of Yokohama, Fujinomiya, Gotemba, Fujiyoshida, Fuji City, Fujikawaguchiko, Numazu, Kofu, Hakone and Odawara. The Smithsonian Institute National Museum of Natural History Global Volcanism Program (GVP) notes that a population of nearly 1 million live within 30km of the summit and just over 25 million live within 100 km.  At the time of writing, Google’s AI gives a figure of between 30 – 37 million for people living within 100km of the summit (multiple searches gave different answers but all were in this ball park).

Hazards

The main hazards posed by Mount Fuji, itself, which may cause fatalities and injuries, damage health, livestock, crops, buildings, infrastructure and communications are:

  • Eruptive products: ash, cinders, lapilli and lava. 
  • Lahars and flooding from rains, which may include typhoon rains.
  • Debris avalanche flows caused by volcanic or seismic activity.
  • Differing eruptive styles complicating predictions

Other local geological hazards include large destructive earthquakes as the region is tectonically very active. 

It is worth noting that because Mount Fuji is close to densely populated areas, including the capital the site of government and a large international financial market), the impact of any large future eruption could be not only be nationally but also globally significant.

The volcano is closely monitored by several agencies coordinated by the Japan Meteorological Agency.

Evolution

Fig 3: Image by TTL2698 of the geological cross-section of Fuji. Key: N2 (yellow) = Tertiary sedimentary rocks; αN2  (dark pink)  = Tertiary volcanic rocks; αQ1  (orange) = Komitake volcano; α-δQ1 (dark orange) = Ashitaka volcano; βQ2 (green) = Older Fuji volcano; αβQ2 (light green) = Younger Fuji volcano. Source: Geological cross-section of Fuji.png – Wikimedia Commons, published under CC BY-SA 3.0.

Mount Fuji last erupted in 1707-1708 with a large explosive eruption of VEI 5, preceded 49 days earlier by a the magnitude 8.7 Hoei earthquake on the Nankai Trough, a 900 km trough off the southern coast of Honshu where the Philippine Sea Plate subducts under the Amur and Okinawa Plates.  The eruption was Plinian, occurring on the SE flank of the volcano. Erupted products were mostly pyroclastic, starting with dacite pumice, followed by andesitic scoria, and, finally, basaltic scoria.  Ash fall several centimetres thick reached Edo (now Tokyo).  The ashfall damaged crops in the region causing widespread starvation.  Rainfall combined with ash deposits caused multiple lahars and flooding.  Most fatalities were caused by flooding, landslides and famine.  It took nearly 100 years for the region to recover.

While the last eruption was explosive, Mount Fuji has produced effusive eruptions in its past.  Studies of lavas show that Mount Fuji overlies older volcanoes.

Sen-komitake is an andesitic core discovered in 2004.  Andesitic activity started here 270,000 years ago ending 160,000 years ago.

Komitake – Fuji is a basaltic layer several hundred thousand years old.  Large basaltic lava flows started here started 700,000 years ago, ending 100,000 years ago, .

Old Fuji formed over Komitake – Fuji between100,000 to 17,000 years ago, the Hoshiyama stage.  Activity in this period is explosive with Plinian or sub-Plinian eruptions, dominated by ash clouds, heavy pyroclastic flows, mudflows, and massive scoria fall. Komitake – Fuji was nearly completely covered. Around 18,000 BC the edifice collapsed with a large debris avalanche flow to the southwest, forming the Tanukiko debris avalanche deposit.  This was followed by another collapse eastwards c. 12,000 BC forming the Umafusegawa debris avalanche deposit.

New Fuji formed over old Fuji during the Fujinomiya stage from 15,000 BC to 6,000 BC. The new edifice was constructed within the Tanukiko avalanche scar.  The eruptive style switched back to large effusive basaltic lava flows, including the Mishima, Obuchi and Saruhashi lava flows.  The Mishima lava flow is estimated to have a volume of 4.3 km3  and the Obuchi 1.1 km3.  The Saruhashi lava flow is the longest, extending more that 40 km from the summit.

From 6,000 BC to 3,600 BC, the Subashiri – a stage, activity declined considerably.  Only Fuji black soil round the volcano remains.

The Subashiri – b stage, from 3,600 BC to 1,700 BC, marked an increase in activity during which the current cone was built from thin basaltic lava flows up to 2,100 BC.  Effusive flank eruptions also occurred during this period.

The Subashi – c stage, 1,700 BC to 300 BC was marked by initial explosive basaltic Plinian or sub-Plinian eruptions from the summit, 1,700 BC to 1,300 BC, flowed by scoria cone forming flank eruptions, 1,300 BC to 1,000 BC,  The Gotemba debris avalanche flow occurred c.900 BC  removing the remains of the eastern flank of the Hoshiyama stage edifice.  Explosive summit basaltic eruptions became dominant again.

The Subashi – d stage runs from 300 BC to present. This stage is marked by frequent basaltic flank eruptions within 13.5 km of the summit. The last summit eruption, the Kengamine eruption, occurred c.2,300 years ago, after which activity has switched to flank eruptions, notably the Yufune 2 eruption 2,000-2,2000 years ago.  Both eruptions produced highly fluid basaltic lavas that generated extensive scoria fallouts. 

 The two most recent largest eruptions are the 864 AD – 866 AD Jogan eruption and also the 1707 Hoei eruption that we have already mentioned.  The Jogan eruption was a VEI 3, occurring on the northwest flank near Nagaoyama, discharging 1.5 km3 of basaltic Aokigahara Lava Flow. The lava flowed into Lake Senoumi, splitting it into two smaller lakes, Lake Saiko and Lake Shōjiko.  Ash and lapilli emitted from the summit, with fallout reaching as far as Edo Bay and Kai Province.  The eruption buried villages, destroyed homes and caused a number of fatalities.

Magmas

Mount Fuji lies on the north end of the Izu-Bonin-Mariana Arc, near the junction with the South Western Japan Arc and North Eastern Japan Arc.  Unlike other arc volcanoes with intermediate and felsic lavas, basaltic lavas dominate.

It is thought that Mount Fuji has two magma reservoirs: the first, a basaltic one, with a depth of c.20 km at the bottom of the granitic middle crust of the Philippine Sea Plate, and the second, a more silicic one with a depth of  8 -9 km.  The lower magma reservoir is deeper than that of other arc volcanoes whose reservoirs tend to be less than 10 km deep.

Analysis of the chemical composition of Mount Fuji’s lavas showed that new activity in the Subashiri period was driven be an injection of basaltic magma 5,600 years ago.  The eruptive style was stable with effusive eruptions until 3,450 years ago when the style changed from effusive to explosive.  The transition from effusive to explosive is thought to be driven by a change in the water content of the parental magmas.

Tectonic Setting

Mount Fuji lies near the triple junction of the Philippine Sea Plate, Amur Plate and Okhotsk Plates. The Philippine Sea Plate subducts under the Amur Plate.  The Pacific Plate also subducts under the region. 

Fig 4: Plates near Mount Fuji.  Image extracted from File:Tectonic plates boundaries detailed-en.svg – Wikimedia Commons, published under CC BY-SA 4.0.  Plate names in black added by the author where the screenshot has excluded them.

Seismic Activity

Our focus here is tectonic activity in the region, not magmatic activity (we do not have access to the data for the latter).  We looked at the area 39°N,133.5°E to 30°N, 147°E from 1st January 2006 to 24th August 2026. 

Our plots show the dominance of the subduction of the Pacific Plate in the area. In the geoscatter plot we can also just about see the triple junction near Mount Fuji delineated in shallow (purple) earthquakes.

What we cannot see clearly in any of our plots is the subduction of the Philippine Sea Plate; however, this is visible in our plot of the area to the west, 39.63°N, 153.14°E to 28.60°N, 124.75°E, where we can see subduction under the Okinawa Plate (Shallow 6.8 magnitude Earthquake South of Kumamoto, Japan, 28th July 2026). This implies that the Philippine Sea Plate may not subduct under the Amur Plate but has a different type of boundary, which may. or may not. contribute to the basaltic magmas of Mount Fuji, or the subduction is aseismic – something we will look into at a future date.

Fig 5: Geoscatter plot by the author of earthquakes between 39°N,133.5°E and 30°N, 147°E from 1st January 2006 to 24th August 2026.  Colour of earthquakes with magnitude less than 6 denotes depth: purple, 0-33km, blue 33-70km, green 70-150km, yellow, 150-300km. orange, 300-500km, red ≥500km. Yellow stars are earthquakes between 6.0 and 7.0 magnitude.  Red stars are earthquakes with magnitude ≥ 7.0. The cyan triangle is the rough location of Mount Fuji.  © Copyright remains with the author, all rights reserved, 2026.
Fig 6: Scatter plot by the author of earthquakes between 39°N,133.5°E and 30°N, 147°E from 1st January 2006 to 24th August 2026.  Colour of earthquakes with magnitude less than 6 denotes depth: purple, 0-33km, blue 33-70km, green 70-150km, yellow, 150-300km. orange, 300-500km, red ≥500km. Yellow stars are earthquakes between 6.0 and 7.0 magnitude.  Red stars are earthquakes with magnitude ≥ 7.0. The cyan triangle is the rough location of Mount Fuji. © Copyright remains with the author, all rights reserved, 2026.

Armchair Volcanologist

© Copyright remains with the author, all rights reserved, 2026.

Sources

USGS for raw earthquake data: Earthquake Hazards Program | U.S. Geological Survey

Yamamoto, T., Nakano, S. & Ishizuka, Y. Temporal variations of magma composition, eruption style and rate at Fuji Volcano, Japan. Earth Planets Space 73, 169 (2021). https://doi.org/10.1186/s40623-021-01505-1

Yosuke Aokia, Kae Tsunematsu , Mitsuhiro Yoshimoto, Recent progress of geophysical and geological studies of Mt. Fuji Volcano, Japan, Earth-Science Reviews

Volume 194, July 2019, Pages 264-282. https://doi.org/10.1016/j.earscirev.2019.05.003

Yamamoto, T., Nakada, S., Extreme Volcanic Risks 2: Mount Fuji, Volcanic Hazards, Risks and Disasters, 2015, Chapter 14, ISBN 978-0-12-396453-3

Wikipedia: Mount Fuji – Wikipedia

Global Volcanism Program, 2026. Fujisan (283030) in [Database] Volcanoes of the World (v. 5.4.0; 7 Aug 2026). Distributed by Smithsonian Institution, compiled by Venzke, E. https://doi.org/10.5479/si.GVP.VOTW5-2026.5.4 . Specific link to Fujisan: Global Volcanism Program | Fujisan

Wikipedia: Tokyo – Wikipedia

Asia, Japan

Shallow 6.8 magnitude Earthquake South of Kumamoto, Japan, 28th July 2026

1 Comment

On 28th July 2026 a 7.1 magnitude (USGS 6.8 magnitude) earthquake struck Uto, south of Kumamoto, Kyushu.  At the time of writing, 34 fatalities had been reported, with 123 injured and 9,000 people displaced.  The earthquake was widely felt, including as far afield as China. In 2016 the area was hit by three large earthquakes: a 6.2 and 6.0, followed by a 7.0 the following day. Our thoughts are with the victims, their families and friends, and everyone else impacted.

Fig 1: Inside Aeon Mall, 警察庁, CC BY 4.0, via Wikimedia Commons

Shinichi Sakai, from the Earthquake Research Institute, University of Tokyo, has said that the earthquake may have been caused by slip on the southern portion of the Hinagu Fault. During the 2016 Kumamoto earthquakes, only the northern extent of the Hinagu Fault slipped. The Hinagu Fault is located in the southern section of  Futagawa-Hinagu fault zone, the Futugawa Fault to the north.  This fault zone lies at the southern end of the Japan Median Tectonic Line, Japan’s longest fault system, which begins near Ibaraki Prefecture on the Pacific coast of Honshu and  runs parallel to Japan’s volcanic arc to Kyūshū.

Fig 2  Shake map  for  2026 6.8  Kumamoto Earthquake  by USGS, Published in the public domain, via Wikimedia Commons.

The Geospatial Information Authority of Japan reported ground deformation: in Yatsushiro, the Sencho geographic reference point was displaced 0.84 m (2 ft 9 in) northeast, and the Izumi station recorded a 0.17 m (6.7 in) movement southward; in Kumamoto, the Jonan station recorded 0.4 m (1 ft 4 in) of displacement to the north.   Surface ruptures along the Hinagu Fault damaged roads, bridges, and buildings. 

Some of the fatalities and injuries were caused by the collapse of the second floor of Aeon Mall Kumamoto, followed by an explosion; a factory chimney, part of the 19th century Yatsushiro-gūshrine and part of bridge collapsed in Yatsushiro; fires were also reported in Yatsushiro.  Damage to rail infrastructure occurred; a freight train derailed near Yatsushiro Station;  there were more than 200 reports of snapped rails and collapsed sound barriers; bent rails were reported at a train depot in Kumamoto and between Matsubashi and Ogawa stations on the Kagoshima Main Line. Landslides occurred on Mount Mayuyama.  48,300 homes lost power in Kumamoto Prefecture; businesses were also impacted. Kumamoto Castle, which was undergoing restoration work following the 2016 Kumamoto earthquakes, was extensively damaged.

Tectonic Setting

The Japanese Archipelago comprises 5 main islands, Hokkaido, Honshu, Shikoku, Kyushu and Okinawa. and c.14,000 smaller islands. Its geology is dominated by the subduction of the Pacific Plate in the north and the subduction of the Philippine Sea Plate in the south.  Japan is seismically very active and home to  c.100 active volcanoes.

Fig 3:  Amur Plate by Alataristarion, CC BY-SA 4.0 , via Wikimedia Commons.  The Okinawa Plate is the small plate between the Philippine Sea Plate and the Yangtze Plate.

Kyushu Island, itself, lies on the rift boundary between the Amur and Okinawa Plate to the west of the subduction of the Philippine Sea Place under these plates.   

The Amur Plate is a small tectonic plate located on the eastern edge of the Eurasian Plate.  It has clearly defined boundaries on the the south by the Qinling suture zone in central China and the Baikal Rift Zone and Stanovoy Mountains on the north. The eastern boundary is with the Okhotsk Plate. The plate is moving anti-clockwise, according to GPS measurements.

The Okinawa Plate is another small continental tectonic plate. To its north is the Amur Plate.  On its eastern margin the Philippine Sea Plate subducts at the Ryuku Trench.  On its western edge there a divergent boundary with the Yangstze Plate.

The Philippine Sea Plate, comprising oceanic lithosphere, lies beneath the Philippine Sea, to the east of the Philippines.  The Philippine Sea plate, the Amur Plate, and the Okhotsk plate meet near Mount Fuji in Japan.

Seismicity

We plotted earthquakes occurring from 1st January 2006 to 30th July 2026 10:00am between 39.63°N, 153.14°E to 28.60°N, 124.75°E.  Our download from USGS contained 17,396 events. The plots clearly show the subduction zones.

Fig 4: Geoscatter plot by the author of earthquakes occurring from 1st January 2006 to 30th July 2026 10:00am between 39.63°N, 153.14°E to 28.60°N, 124.75°E.  For earthquakes of magnitude less than 6.5, colour denotes depth:  purple, 0-33km, blue 33-70km, green 70-150km, yellow, 150-300km. orange, 300-500km, red ≥500km. Yellow stars are earthquakes between 6.5 and 6.8 magnitude.  Orange stars are earthquakes with magnitude ≥ 6.8. © Copyright remains with the author, all rights reserved, 2026.
Fig 5: 3D scatter plot by the author of earthquakes occurring from 1st January 2006 to 30th July 2026 10:00am between 39.63°N, 153.14°E to 28.60°N, 124.75°E.  For earthquakes of magnitude less than 6.5, colour denotes depth:  purple, 0-33km, blue 33-70km, green 70-150km, yellow, 150-300km. orange, 300-500km, red ≥500km. © Copyright remains with the author, all rights reserved, 2026.

Thank you for visiting.

Armchair Volcanologist

© Copyright remains with the author, all rights reserved, 2026.

Sources:

USGS for raw earthquake data: Earthquake Hazards Program | U.S. Geological Survey

Wikipedia: 2026 Kumamoto earthquake – Wikipedia

Asia, Philippines

Philippine Earthquake, Magnitude 7.8 08/06/2026

On 8th June 2026 at 07:37 (local time) a 7.8 magitude earthquake occurred 32 km south west of Maasim in Sarangi Province, off the coast of Mindanao Island, Philippines, with a depth of 33 km.  Tragically, at the time of writing reports suggest that at least 55  people were killed, 38 missing, 1,120 injured, with 390,000 people impacted.  Severe infrastructure damage has impacted rescue and recovery services.  A tsunami, with waves ranging from a few centimetres to 1.4 metres, was recorded.  Our thoughts are with all those affected now and in the coming months.

Fig 1: Location of the 7.8 Philppines earthquake.  Source PhiVolcs: PRIMER ON THE 08 JUNE 2026 MAGNITUDE (MW) 7.8 OFFSHORE SARANGANI EARTHQUAKE | PHIVOLCS

Tectonic Setting

The Philippines  is an archipelago made up of c.7,641 islands.  It lies on the Philippine Mobile Belt, a zone lying above two opposing subduction zones: the Sunda Plate along the western edge at the Manila, Negros and Cotabato Trenches ; and, the Philippine Sea Plate on the eastern edge along the Philippine Trench and the East Luzon Trough. The Philippine Mobile Belt, itself, is the result of oblique collision between the westward-moving Philippine Sea Plate and the eastern margin of the Sunda Plate. It is rapidly deforming, made up of fragments of continental and oceanic crust.

Fig 2:  Map of Philippine Faults by Gubernatoria, published under CC BY 2.5 AU <https://creativecommons.org/licenses/by/2.5/au/deed.en&gt;, via Wikimedia Commons

The 7.8 earthquake was reverse thrust and attributed to movement at the Cotabato Trench.

The archipelago has three main geological features:

  • The 1,200-kilometer-long Philippine Fault Zone; a  strike-slip fault system which accommodates much of the shearing force caused by the colliding plates.  It is seismically active producing large earthquakes, including the destructive M7.6 Luzon earthquake of 1990.
  • Volcanism from the subduction. The Philippines is home to numerous active volcanoes, e.g., Taal, Mayon and Mount Pinatubo
  • A complex structure containing a mixture of ancient metamorphic rocks (dating back to the Jurassic and Cretaceous periods), early volcanic arcs, and microcontinental blocks.
  • The 7.8 earthquake is attributed to movement at the Cotabato Trench.

All in all, an interesting are to study seismicity.

Our Plots

We looked at the area 4.4150°N, 116.16°E  to  21.1218°N, 137.90°E for the period 1st January 2016 to 11th June 2026 (07:33 am).  Our raw data was downloaded publicly available earthquake data from USGS, a total of 7,357 earthquakes with a  minimum magnitude of 3.7 and maximum depth of c.668 km. 

Fig 3: Geoscatter plot of earthquakes occurring between 4.4150°N, 116.16°E  to  21.1218°N, 137.90°E for the period 1st January 2016 to 11th June 2026 (07:33 am).  Earthquakes with magnitude ≥ 7.0 are shown as yellow stars; the magnitude 7.8 earthquake is shown as an orange star.  Smaller earthquakes are coloured by depth as per the legend; their transparency has been increased so we can see the land. © copyright remains with the author; all rights reserved, 2026.

In our Geoscatter plot, we can clearly see the seismicity on the Manila Trench, west of Luzon, the Philippine Trench east of the Philippines, the Philippine Fault Zone to the west of the Philippine Trench and other trenches. Activity along the Philippine Trench dominates the plot.  Most of the earthquakes with magnitude greater than 7.0 are associated with subduction at the Philippine Trench.

We can also see some deeper activity  in the southern part of the plot.  We think that this is related to the fully subducted Molucca Sea Plate which lies to the south of the Philippine Mobile Belt.  The Molucca Sea Plate lies between the southeastward subducting Sangihe Plate and the northwestward subducting Halmahera Plate; the collision zone is thought to be arc to arc collision rather than the more usual arc to continent collision.    This shows better in our scatter plot below.

Fig  4:  Video of 3D scatter plot of of earthquakes occurring between 4.4150°N, 116.16°E  to  21.1218°N, 137.90°E for the period 1st January 2016 to 11th June 2026 (07:33 am).  Earthquakes with magnitude ≥ 7.0 are shown as yellow stars; the magnitude 7.8 earthquake is shown as a black star.  Smaller earthquakes are coloured by depth as per the legend. © copyright remains with the author; all rights reserved, 2026.

In the video we can see that most of the magnitude seven and over earthquakes occur in the region between 0 to 100 km in the southern part of the region and a few are occurring at a depth of over 600 km.  The former are mostly associated with the Philippine Fault Zone and the Philippine Trench.

We hope you find the earthquake plots useful. 

Once again, our thoughts are with all those impacted by the earthquake.

Armchair Volcanologist

© copyright remains with the author; all rights reserved, 2026.

Sources:

USGS:  https://earthquake.usgs.gov/earthquakes

China Daily Asia: Philippine quake death toll reaches 55, search on for 38 missing

PhiVolcs: PRIMER ON THE 08 JUNE 2026 MAGNITUDE (MW) 7.8 OFFSHORE SARANGANI EARTHQUAKE | PHIVOLCS