Mount Chokai (Chokaisan) is a 2,236 m active volcano located in the Tohoku region on Honshu, the largest main island of Japan.  It is constructed of a pair of overlapping stratovolcanoes, both of which have breached calderas, horseshoe craters and debris avalanche flows below the calderas.  Post caldera activity in both calderas built domes.  The most recent activity took place in 1974.  Chokai is the second tallest mountain in the Tohoku region, often called Dewa, Akita or Shonai Fuji.

The andesitic, / basalt andesitic volcano is located on the W coast of Honshu overlooking flat coastal plains and the Sea of Japan.  The closest major city is Sakata, some 27 km NE.  There are nearly 190,000 within 30 km (most of them in Sakata) and 2.3 million within 100 km.  The E coast of Honshu was inundated by tsunami from the 2011 Tohoku earthquake.  Sendai and the E coast is perhaps 120 km SE Chokai.

Neighboring Sakata has a population of around 100,000 located on the coastal plain.  Its economy is largely powered by light manufacturing, agriculture and commercial fishing.  The climate is considered a humid continental climate with large temperature variations.  Average highs are around 17° C; average lows 9° C; with record highs of 40° C and record low of -17° C.  Climate is quite wet, with average precipitation nearly 200 cm/year.  The ecosystem on and around Chokai is unique, due to the highest precipitation of any mountain in Japan.  Runoff feeds local rivers, streams, an active spring system, all of which make local agriculture very productive.

The Chokai Quasi National Park established in 1963 is centered around the volcano, though it includes parts of the neighboring coastline.  Chokai is rated as protected landscape, with the park managed by local prefecture government.  It is also known as the Mount Chokai Geopark.  The Hidden Japan published a beautiful, extended review of Mt Chokai that is well worth reading. 

There is active hiking, climbing, back country snowboarding and skiing on the mountain.  While a round trip hike all the way to the top is a vigorous 8 – 10-hour hike, a hike halfway to Mount Chokai Lake is considered a relatively easy hike.  Heavy snowfall makes trails inaccessible from winter to mid spring to all but back country skiers. 

Like many large, prominent volcanoes in Japan, Mount Chokai has been the object of mountain worship since ancient times.  It gradually became a training ground for Shugendo.  Initial pilgrimage paths to the summit started from the S.  The mountain drew many pilgrims during the 1603 – 1867 Edo Period, the final period of traditional Japan, with 33 chapels at its base.  Additional routes to the summit were opened from the N.  There is a tradition that the neighboring island Tobishima was originally a part of the summit of Chokai.  Many of the traditional trails are now used by hikers and other visitors to the volcano.

As with the rest of Japan, activity at Chokai will be reported by the Japan Meteorological Agency (JMA), though it is not actively monitored for volcanic activity.  The entire nation has robust and active seismic coverage that should be sufficient to report activity.  Yahoo Japan Weather and Disaster has an active webcam trained on Chokai. 

Tohoku Region

The following list of neighboring volcanic systems is gathered from the JMA National Catalogue of the Active Volcanoes in Japan (Fourth Edition).  Although inspection of JMA maps of the region show much more extensive volcanism N of Chokai and E and S of Chokai,  these systems are not considered to be active and will be mentioned only in passing.  All of these volcanoes erupt primarily andesites to basaltic andesites with some minor dacites. 

Note the two different numbering systems used by the JMA National Catalog and the Maps.  The National catalog are typically two-digit numbers, while the Maps list volcanoes as DXX, with a two-digit number following a “D” prefix.  There are links to more detailed descriptive JMA products for individual volcanoes in both the catalog and map.  All distances will be measured from Chokai.

Akita – Yakeyama

The 1,366 m Akita – Yakeyama (Catalog 25 / Map D23) is located some 113 km NNE from Chokai.  It is the most recently active of a group of coalescing volcanoes.  It is immediately W of Hachimantai volcano.  The main volcano, Yakeyama has a small dome in its 600 m wide summit crater.  The Tsugamori volcano to the E is a stratovolcano of similar height with a 2 km crater breached to the NE.  The Kuroshimori dome is 4 km S of Yakedake.  There are several thermal areas.  One of them, Tamagawa Spa on the W foot is strongly radioactive.  The last magmatic eruption formed the Onigajo lava dome in the summit crater 5 ka. 

There were multiple eruptions since the Onigajo eruption.  Five of them 1250 BC – 1678 AD, three more in the 19^th Century, and eight in the 20^th Century.  The most recent of these were a pair of phreatic events in 1997.  All the 20^th Century activity other than the first in 1929 were VEI 1 eruptions.  1929 was a VEI 2.  All the older eruptions listed by Smithsonian GVP were VEI 2 events.  This may be due to the remoteness of the system.

Hachimantai

The Hachimantai volcanic group (Catalog 26 / Map D22) is a field of scattered vents located some 116 km NNE of Chokai thought to be active before the Holocene.  The 1,613 m summit is an undulating plateau of andesitic lavas with steep slopes on its edges.  There are circular craters near Komonomore and Mokkodake in the center of the plateau.  Craters are youthful looking but have not been dated.  Japanese experts believe it to be of possible Holocene age.  While there are no historic eruptions known, there are active solfataras on the W and S flanks.  Smithsonian GVP lists a pair of confirmed eruptions 9.9 and 7.3 ka.   

Iwatesan

Iwatesan (Catalog 27 / Map D31) is located 116 km NE from Chokai.  It is another symmetrical stratovolcano (2,038 m) when viewed from the E.  From the W, an older cone is visible with a 2 x 3 km caldera.  Initial activity built the Nishi-Iwate volcano 700 ka.  Activity migrated E to form Higashi-Iwate volcano. It collapsed at least seven times over the last 230 ka.  The most recent of these are somewhere between 739 and 1615 AD.  Yashuidake is the (mostly) basaltic summit cone of Higashi-Iwate.  It is topped with a 500 m wide crater and rises well above, burying the E rim of the caldera.  That caldera is breached by a narrow gorge on the NW.  The central crater is partly filled with a lake.  A young lava flow from Yashuidake descended into the caldera.  A fresh-looking 1732 lava flow traveled down the NE flank.

Smithsonian GVP lists multiple eruptions from the system, with at least 21 between 8.5 – 2.0 ka.  Only 8 eruptions over the last 2.0 ka, with the most recent listed eruption in 1934.  On the other hand, Bulletin reports list at least six episodes of volcanic tremor and earthquakes 1995 – 2000, with the most recent of these being Nov 1999.  None of these resulted in any increase in reported hydrothermal or volcanic activity, though inflation was observed in 1998. 

Akita – Komagatake

Akita – Komagatake (Catalog 28 / Map D29) is a 1,637 m stratovolcano topped with two calderas located 98 km NE from Chokai.  The N caldera is 1.2 x 1.0 km.  The S caldera is 3.0 x 2.0 km.  They were formed in a pair of explosive eruptions 13.5 – 11.6 ka.  The post caldera Onamedake cone in the N caldera produced lava flows to the N and E.  It has a 100 m summit crater.  There are a pair of post caldera cones, Medake and Kodake in the NE corner of the S caldera.  Minamidake pyroclastic cone is also located in the the S caldera.  The floor of the caldera is covered with lava flows.  Some of this lava flowed down the W flank of the volcano from the SW rim of the caldera. 

The volcano was active 10.8 – 0.5 ka, with 11 confirmed eruptions.  Some of these were powerful, with a pair of VEI 4 and at least three VEI3 eruptions.  There were three eruptions 400 – 1100 AD in the VEI 2-3 range.  Renewed activity began with a VEI3 in 1891, and three more eruptions 1902 (VEI 1), 1932 (VEI 2) and 1970 (VEI 2).  The small historic eruptions took place from cones and fissure vents inside the S caldera.  The 1932 eruption is considered to be phreatic.

The Smithsonian GVP Bulletin Reports on Akita – Komagatake start with the Sept 1970 eruption from the Medake cone, active through Jan 1971.  It built a new cinder cone with a lava flow from the foot of the new cone.  There were hundreds of Strombolian explosions at times daily during this eruption.  Fumaroles were active following this eruption, with a short-lived plume observed above the volcano in Dec 2011.  Seismicity in the area increased late Dec 2011 but quickly returned to normal levels. 

Kurikomayama

Kurikomayama (Sukaawadake / Dainichidake) (Catalog 30 / Map D43) is a 1,627 m stratovolcano topped with a 4 km caldera located some 66 km ESE from Chokai.  JMA describes the stratovolcano as a somma, with only the S part of the somma remaining.  The highest peak, Dainichidake is on the E edge of the somma. 

Initial activity 500 ka erupted lava flows N and S followed by growth of the Higashi – Kurikoma stratovolcano.  Magushadake on the W side of the complex was active until 100 ka.  Final activity constructing the main cone was lava flows E, SE and W.  The post caldera Tsurugidake dome dates 110 – 100 ka and still has a high level of hydrothermal activity (fumaroles).  The Daichiagamori dome and Aguroshiyama pyroclastic cones appear to be most recent structures and are located on the S flank. 

This system has not erupted with quite the frequency of its neighbors, though it has been seismically active over the last half century.  The oldest confirmed eruption was 5.5 ka.  This was followed thy eruptions in 1450, three in the 18^th Century, a pair in 1944 – 1946, and most recent VEI 2 in 1950.  Eruption intensity fluctuates between VEI 1- 2. 

Starting in 1950, JMA reports multiple incidents of rumbling and earthquake swarms in 1950 and 1957.  The volcano was mostly quiet until renewed earthquake swarms in 1985 – 1986, 1992 (increase in fumarole temperature and expansion of fumarole area), and multiple earthquake swarms 1994 – 1999.  A shallow M7.2 earthquake 10 km NW of the summit in 2008 triggered landlides and a debris flow.  There were multiple aftershocks but no increase in volcanic seismicity.

Naruko

The 5.5 x 7.0 km Naruko (Narugo) caldera (Catalog 31 / Map D45) is located 72 km SE from Chokai, NW from Sendai.  The rim of the caldera is poorly defined and populated by post caldera dacite domes.  The caldera was formed by at least two eruptions in 73 and 45 ka.  A group of four post caldera domes surround a 400 m acidic lake in the Katanuma crater.  These domes are low and steep sided.  There are 100 – 400 m craters on top of the domes.  Explosive activity during formation partly destroyed them.  They are covered with tephras dating 18 ka spread over a wide area.  Two of the domes are capped with rocks uplifted during their formation. 

There is an active hydrothermal system in the area, with SO2 and water vapor emissions from Shurado crater on Toyagamori dome W of the lake and from the bottom of the crater lake.  The crater lake is the most acidic in Japan.  Solfataras are located on the W shore of Katanuma.  Sulfur is mined from sediments on the lake floor. 

JMA lists four tephra producing eruptions 6.4, 3.4 – 3.3, and 2.8 ka.  The most recent three of these were phreatic.  The only historic eruption was 837 AD, a phreatic eruption in the Katanuma area. 

Hijiori

Hijiori (Catalog 32 / Map D48) is a small, relatively inconspicuous caldera located some 57 km S from Chokai.  The 2.5 km caldera was formed 10.3 ka.  Tephras and pyroclastic flows from the eruption extend E and reached the Pacific coast.  The eruption built a pyroclastic flow plateau several kilometers to the S and 8 km N.  There is a small post caldera dome.  Small local tephra deposits from post caldera dome building may overlie lake sediments, though there is still some dispute about them. 

While there are no current fumaroles, there is an active hydrothermal system.  Caldera lake deposits have been extensively altered, and lake water has a sulfuric acid level. 

Activity at Hijiori began 12 ka with caldera formation 10.3 ka.  The only subsequent activity was hydrothermal, with hot springs.  There are no currently active fumaroles. 

Gassan

The massive Gassan (Map D49) stratovolcano is located some 62 km S of Chokai.  It is 1,984 m high topped with a 2 x 5 km caldera open to the N.  The debris avalanche creating the amphitheater traveled at least 19 km N of the present summit with a volume around 1 km3.  There are two parts of the cone, one flat and the other highly eroded.  The young portion is also divided into two parts, with the main area having moderately preserved lava flows.  The marginal area has lava cliffs.  There are at least nine flow lobes.  The summit has been glacially eroded. 

Initial activity was effusive, erupting relatively fluid andesitic and dacitic lavas.  These flowed radially with the oldest being dated around 740 ka.  This continued until around 600 ka.  The upper lavas started around 680 ka and continued until 550 ka.  The current summit was active 550 – 500 ka, with a shift in eruption center around 500 ka, moving 1 km NW by 450 ka.  Activity stopped by 450 ka.  Note that error bars on these dates are pretty wide.

Flank collapse appears to have been triggered by an active local fault which is still active.  The most recent historic earthquake from this fault was 1894, a M7.0.  The Oizumi, et al 2018 paper suggests that continuing action from this fault will trigger another flank collapse. 

Zaozan (Zaosan)

The Zaozan (Zao Zan, Zaosan) (Catalog 33 / Map D58) volcanic group is considered to be the most active of N Honshu.  It is a complex of stratovolcanoes topping out at 1,841 m, located some 111 km SSE from Chokai.  The Ryuzan volcano forms the W group (Nishi-Zao).  Byobu and Fubo volcanos form the S group (Minami-Zao).  Basement granite is relatively high at 1,500 m and the stratovolcanoes are relatively small.    The 7 km3 Zaozan volcano forms the central group (Chuo-Zan).  The complex is topped by several lava domes and the Goshikidake tuff cone. 

There have been multiple collapse events producing debris avalanches during the Pleistocene.  The currently active Okama crater is located on top of Goshikidake.  It has a strongly acidic crater lake (Goshikinuma).  The post caldera Goshikidake pyroclastic cone has been active over the last 2,000 years. 

Initial activity took place 1 Ma – 700ka.  There was a dormant period of 300 ka.  Renewed activity 400 – 100 ka erupted andesitic lava flows from multiple eruption centers.  Peaks from these centers make up the upper half of the edifice.  Kumanodake and Kattadake are the highest peaks.  Caldera formation took place 30 ka, creating a 2 km caldera at the summit.  At the same time, explosive activity erupting basaltic andesite began and continued intermittently until the present.

Smithsonian GVP lists 43 eruptions over the last 9.6 ka.  There was a single VEI 4 300 AD.  Other than that, most of the listed eruptions are VEI 2-3.  The most recent listed eruption was 1940, a small phreatic eruption.  There have been multiple instances of earthquakes, changes in fumarole activity, rumbling and earthquake swarms since then.  The most recent of these was 1995.

Chokai

Chokai (Chokaisan, Catalog 29 / Map D39) is the largest volcano in NE Honshu at 1,974 m.  It overlooks the Sea of Japan, dominating the coastal plain.  It is constructed of two overlapping stratovolcanoes.  Both summits are breached by debris avalanches, the youngest Higashi – Chokaisan to the N and the oldest Nishi – Chokaisan to the S.  Two post-caldera domes were constructed in the upper SE end of the caldera. Reports of historic eruptions date back to the 6^th Century.  The two major cones are also known as Dewa – fuji and Akita – fuji or East Chokai and West Chokai.  

There are at least three flank collapse amphitheaters on Chokai.  There are at least two post-collapse cones (domes) in the two most recent flank collapse amphitheaters.  The Nishi – Chokaisan N opening amphitheater was described by Onuma in 1963 as a somma, with the highest point Shichikozan.  A third cone (dome) named Gassanmori is located at the W crater rim and overlaps the crater rim of Higashi – Choikaisan. 

Higashi – Chokaisan has the most recent flank collapse, from a 466 BC eruption.  Post collapse activity built multiple cones (domes) in the amphitheater, craters and domes on the flank.  Torinoumi is a small crater partly filled with water on the N part of the crater rim.  A small cone called Inakuradake is located just N of Torinoumi.  Kannonmori is one of three small domes on the W flank. 

Activity at Chokai proceeded in three stages.  The first of these took place 600 – 160 ka, building the main volcanic edifice, Nishi – Chokaisan.  These eruptions from a central vent created a major lava flow field and associated pyroclastic flows and airfalls.  The stratovolcano ended up around 1,000 m high with a volume of 47 km3.  After completion, the N sector of the volcano collapsed, leaving debris avalanche deposits on the N foot.  Rocks are mainly andesites with a small amount of basalt.  

After the collapse, eruptions of lavas and pyroclastics continued from a number of vents in a 14 km E-W line.  This alignment and normal faults cutting the edifice may be related to the regional stress field.  The volcanic products covered the E and W flanks of the volcano, though no prominent cone was built 160 – 20 ka.  Total volcanic material erupted during the second stage was 22 km3.  Part of the W flank collapsed, creating a horseshoe shaped amphitheater.  This stage erupted various andesites.   

Stage three built a small stratovolcano, the conical Higashi – Chokaisan on the E side of the older structure starting 20 ka.  Lava flows also erupted from the Saruna crater on the W flank.   In 466 BC (JMA also lists this eruption as 2.6 ka, a 150 year or so discrepancy), the summit of newly built Higashi – Chokaisan collapsed.  The debris avalanche covered an area N to NW of the volcano and created a horseshoe shaped amphitheater in the summit of Higashi – Chokaisan.  Hummocky terrain of Kisakata and Yurihara were formed by these deposits. 

Post collapse activity at the summit continued after the collapse, with lava flows covering perhaps a third of the caldera.  It also extruded at least two lava domes.  Shinzan (Kyowadake) is one of the two central cones of Higashi – Chokaisan.  It is a lava dome formed by the 1801 eruption.  Chokai erupted 4 km3 of material during this stage.   

Chokai is prone to producing lahars, with multiple significant events.  The first of these events were a series of at 8 – 12 flow units less than a few hundred years after the 466 BC flank collapse.  The initial lahar only reached the lower foot of the volcano.  Subsequent lahars flowed farther over the debris fan from the avalanche reaching far from the foot. 

Sandy lahar events contain fresh volcanic material, taking place shortly after an eruption.  There are three discrete sandy lahar events 100 – 300, 400 – 500, and 900 – 1100 AD.  The deposits contain little altered material.  The middle group of these lahar events suggest lahars were followed by a period of stream flow.  Calibrated age of lahars 800 – 1100 AD overlap a pair of witnessed eruptions in 871 and 939 AD.  Reports on the 871 AD eruption also observed a mud slurry from the volcano. 

The most recent lahar event to inundate the volcanic fan took place in the 17^th Century.  As with earlier events, there were several lahars in succession.  Most of these layers contained altered materials.  Lahar events were documented 1659 – 1663, 1740 – 1747 and 1800 – 1804.  Multiple eruptions did not produce lahars.  The most recent lahar event contained significant altered material rather than juvenile rich like earlier events.  

There was some interest in exploring for petroleum and natural gas in the vicinity of Chokai, though part of the underlying formations of interest are covered by debris from Chokai obscuring the structure. 

Chokai Eruptions

Chokai is dominated by effusive lava eruptions.  Pyroclastic layers are scarce, while the lavas are well preserved.  Tephra deposits are extremely limited, meaning that the extent of these deposits has not yet been determined.  The volcano seems to behave predictably before eruptions, starting with an increase in seismicity, fumaroles activity and snow melting for weeks to months before the eruption.  The eruption starts as either hydrothermally dominant or magmatic, evolving to a magmatic eruption or a hydrothermal eruption until activity ends.  Lahars can occur at any stage of the eruption.  Lava flow volumes over the last 2 ka are generally 10x that of tephras. 

The oldest listed eruption in the Smithsonian GVP list took place somewhere before 1000 BC from the Saruana crater.  This produced a lava flow that made it all the way to the Sea of Japan.  Both volcanoes were active long ago, though activity over the last 2.6 ka took place from Higach – Chokaisan. 

The JMA description of Chokai lists a pair of events in 466 BC.  The first of these was the flank collapse creating the Kisakata debris avalanche and the horseshoe shaped caldera on its N flank.  Volume of the debris field was 3.5 km3.  The 2022 Ohba, et al paper considers this flank collapse to be a hot event due to wood fragments embedded in tephra found associated with the collapse.  The directed blast deposit covers a fine tephra, indicating the collapse occurred during a closely spaced series of eruptions. 

This collapse was closely followed by post collapse activity extruding a lava flow (Kojingatake lava) and a dome in the newly formed amphitheater.  The last known eruption associated with the collapse was a phreatic eruption and tephra fall from the summit a few hundred years later. 

There are well dated tephra layers from eruptions 500 BC, 100 BC, 100 AD, 200 AD and 400 AD.  Some lahars at the N base of the volcano correlate nicely with these eruptions along with the 871 eruption. 

Historic eruptions took place in 708 AD, 810, 830 and 871.  All of the subsequent activity was in the last 2,000 years, AD.  The first two of these were phreatic eruptions from the summit.  The 830 summit eruption was also phreatic, producing air fall pyroclastics and at least one lahar.   The 871 eruption was a bit larger, initially producing a tephra fall followed by a lava flow.  A lahar from this eruption fouled a local river, collapsing its banks and killing a large number of fish.  This eruption was described as a VEI2 phreatomagmatic eruption.

The volcano was quiet until 939 with another phreatic eruption from the summit.  The next period of quiet lasted until 1659 – 1663, when multiple phreatic eruptions from the summit damaged local crops.

After another period of quiet, phreatic eruptions resumed 1740 – 1747.  These were once again phreatic eruptions from the summit, though a plume was ejected from a crater on the SE flank of the Kojingatake dome.  Sulfur from the hydrothermal system ended up below the amphitheater to the N, fouling rice paddies and killing fish.

The volcano was quite active in the first part of the 19^th Century, with eruptions 1800 – 1804, an earthquake in 1804, and a pair of phreatic eruptions in 1821 and 1834.  Activity started winter of 1800, with a continuous plume by Mar 1801.  The eruption peaked late Aug, with explosions, ejection of volcanic blocks, ash discharge near Kojingatake at the summit, and extrusion of the new Shinzan (Kyowadake) dome.  Shinzan lavas mostly covered its neighbor. Volcanic blocks killed at least 8 mountain climbers.  The 1804 earthquake was quite strong, killing 330, destroying over 5,500 houses, uplifting land and creating a tsunami.  Both the later phreatic eruptions took place near the newly emplaced Shinzan dome near the summit.  The 1834 eruption also produced a lahar.

The most recent eruption sequence took place Feb – May 1974.  It is described as a small-scale phreatic eruption that produced a tephra fall and lahar.  It began near Shinzan.  Seismic activity began the previous Dec, followed by fumes in Jan.  Rumbling started in Feb, with the first explosion coming from a new crater discovered 1 Mar on the E side of Shinzan.  This was phreatic, producing a plume reported by a passing JAL flight, and tephra fall.  The first lahar from this eruption took place 6 Mar.  Plumes from Shinzan and Kjingatake were observed on 8 Apr.  Eruption intensity ramped up with plume and lava flow on 24 Apr.  By 28 Apr, ash fell 30 km N.  The eruption tailed off with the last plume on 8 May.  Fumarole activity continued for an extended period of time.  This eruption is listed as a VEI 1.  For some reason, Smithsonian FCP describes this eruption as coming from the Tyokai volcano. 

The most recent activity was seismic with a M 3.2 earthquake 5 km SE and a swarm 25 Nov – 1 Dec in Kisakata, 15 km NW.

Tectonics

Tectonics of N Honshu are driven by the subduction of the Pacific Plate beneath the Okhotsk Plate at the Japan Trench E of Honshu.  The Okhotsk Plate is a wedge-shaped minor tectonic plate that extends N to include all of the Kamchatka Peninsula, and W to mainland Siberia.  It S end is in the vicinity of Tokyo, bounded by the Philippine Sea Plate offshore and the Amur Plate to the S and W.  In many places, this is described as the North American Plate.  As tectonics here is driven by subduction, the region is prone to megathrust earthquakes, the most recent of these being the M9.0 2011 Tohoku earthquake.

One of the interesting things about this particular subduction margin is the distribution of volcanic centers.  Normally, volcanic systems are randomly distributed along active plate margins.  Volcanoes here have formed in conspicuous groupings.  Recent volcanic systems are about 50 km wide and spaced with 80 km gaps.  This line is parallel with the Japan Trench.  The descending Pacific Plate is perhaps 150 km beneath the volcanic front.

The low velocity zones beneath Honshu are not continuous.  Rather, it is divided into what was described by Tamura, et al in 2002 as Hot fingers in the mantle wedge.  Localized hot melting bodies may exist and extend into the back-arc side where temperatures are 200 – 300° C higher than that of neighboring mantle.  Low velocity zones do not exist in the gaps between volcanic centers.  They do exist under all the volcanic centers, along with some locations where volcanic activity has not yet begun.  Magma in these locations is thought to be accumulating near the Moho depths as it makes its way to the surface.  The total volume of magma supplied from the mantle likely exceeds the volume of materials erupted so far. 

One of the responses to magma addition to the crust is vertical uplift.  Honshu has an undulating topography at about the 80 km interval.  This correlates strongly with the thermal structure of the underlying mantle wedge.  The volcanoes are located at topographic highs and over the low velocity zones, leading to describing these zones as fingers. 

Conclusions

Chokai continues to be active, with a vigorous hydrothermal system, lahar and flank collapse history.  Recent eruptions have been mostly phreatic, though dome extrusion and lava flows were emplaced during the 1800 – 1804 sequence.  The volcano is dangerous, unstable, and has a healthy magma supply.  It should be treated as such. 

Additional information

1974 activity of Chokai volcano, Japan, Ui, et al, Dec 1977

Lahar record during the last 2500 years, Chokai volcano, northeast Japan:  Flow behavior, sourced volcanic activity, and hazard implications, Minami, et al, Dec 2019

Volcanic hazard maps of Japan, second edition, Nakamura, et al, Mar 2013

Discretization of deep low-frequency earthquakes associated with different temporal activity patterns beneath Japanese volcanoes, Kurihara & Abara, Oct 2021

Geological map of volcanoes:  research of recent eruption history, Hoshizumi & Nakano, 2005 Spring

Active Japanese volcanoes, C Gottsche, Apr 1883

Spatiotemporal characteristics of relocated deep low-frequency earthquakes beneath 52 volcanic regions in Japan over an analysis period of 14 years and 9 months, Kurihara & Obara

Geology of Chokai volcano, S Hayashi, 1984

Petrology of Chokai volcano, Part I, Petrography and major element compositions, S Hayashi, 1984

Late Holocene tephrostratigraphy at Chokai volcano, northern Japan, and contribution to hazard assessment, Ohba, et al, May 2022

Geology and petrology of Chokai volcano, northeastern Japan, Part I, geology and petrography, K Onuma, 1963

Petrology of Chokai, volcano, Part III, Trace elements and petrogenesis, S Hayashi, 1986

Geology and petrology of Chokai volcano, northeastern Japan, Part II, petrochemistry and petrogenesis (1), K Onuma, 1963

Petroleum geology of the eastern foot of Chokai volcano, Taguchi & Abe

Long-term changes in distribution and chemistry of middle Miocene to Quaternary volcanism in the Chokai – Kurikoma area across the northeast Japan arc, Kondo, et al, Feb 2004

Location map of active volcanoes, JMA

National catalogue of the active volcanoes in Japan (the fourth edition), JMA

Volcano by region (index map of quaternary volcanoes), JMA