|Elevation||1,486 m (4,875 ft)|
|Age of rock||Oldest 500 kyr|
|Mountain type||Complex stratovolcano|
|Last eruption||February to May 1995|
In 1792, the collapse of one of its several lava domes triggered a megatsunami that killed 14,524 people in Japan's worst volcanic-related disaster. The volcano was most recently active from 1990 to 1995, and a large eruption in 1991 generated a pyroclastic flow that killed 43 people, including three volcanologists.
Its highest peaks are Fugen-dake () at 1,359 metres (4,459 ft) and Heisei-shinzan (?) at 1,486 metres (4,875 ft). The latter emerged during the eruptions of the early, eponymous Heisei era (1989-2019).
Mount Unzen (or Unzendake) rises in the central part of the Shimabara Peninsula, Nagasaki Prefecture. It is located on the outer ring of the Chitose Caldera centering on Tachibana Bay in the west of the peninsula. It consists of a total of more than 20 mountains, however, the complexity in the shape of Unzendake was expressed by various numbers (such as Mitake Goho/Mimine Godake as "24th peak", or Yatsuha as "36th peak"). As a result, the usage of only eight mountains (sometimes three mountains) was coined in a narrow sense, but historically it is a name that refers to the entire mountain range towering over the sea. It is often confused with the name of the oldest peak, Mt. Fugen.
Surrounding the highest peak of Heisei Shinzan (1,483 m), are Fugen-dake (1,359 m), Kunimidake (1,347 m), Myokendake (1,333 m), Nodake (1,142 m), Kusenbedake (1,062 m), and Yadake (943 m). Fugendake and Heisei Shinzan derive magma from the Chitose Caldera. In other words, it is supplied from a magma chamber beneath Tachibana Bay off Obama Onsen. The main peak is Fugen-dake, but the volcanic activity from 1990 (Heisei 2) to 1995 (Heisei 7) made Heisei Shinzan, which was higher in elevation. Heisei Shinzan is also the highest peak in Nagasaki Prefecture.
Unzen originally read "onsen" in the notation of hot spring, but it was changed to the current notation when it was designated as a national park. An army radar base was built near the summit of Fugen-dake during the Pacific War, where about 100 people were stationed.
Mount Unzen is part of Shimabara Peninsula, which has seen extensive volcanism over millions of years. The oldest volcanic deposits in the region date from over 6 million years ago, and extensive eruptions occurred over the whole peninsula between 2.5 and 0.5 million years ago.
The origins of the Unzen complex are traced to the formation of a graben through crustal faulting. This caused parts of the peninsula to subside by up to 1,000 metres (3,300 ft) below sea level and may have caused eruptive activity to localize at one site inside the graben. Eruptions of dacitic lava began from a site slightly to the south of today's Mount Unzen and migrated north over time.
The volcano rapidly grew during its first 200,000 years, forming a large cone. Later eruptions over the following 150,000 years filled in much of the graben. Initially, activity was dominated by blocky andesitic lava and ash flows, changing to dacitic pumice flows and airfall deposits from 500,000 to 400,000 years ago. The period from 400,000 to 300,000 years ago saw the emplacement of large areas of pyroclastic flow and lahar deposits; these form the major part of the volcanic fan surrounding the volcano. Beginning 300,000 to 150,000 years ago, thick phreatomagmatic deposits were laid down, suggesting the subsidence of the volcano into its graben was rapid during this period.
Activity from 150,000 years ago to the present has occurred at a number of sites around the volcanic complex, building four main domes at different times: the No-dake (70-150,000 years old), My?ken-dake (25-40,000 years old), Fugen-dake (younger than 25,000 years old) and Mayu-yama (4,000 years old) volcanic peaks. Fugen-dake has been the site of most eruptions during the past 20,000 years and lies about 6 kilometres (3.7 mi) from the center of Shimabara.
In December 1663, Fugen-dake erupted, producing lava flows that covered forest for over an extension of 1 km. In the spring of the following year, there was a flood from the Kujuku Island crater, located on the southeastern flank of Fugen-dake at an altitude of 600 m. More than 30 were killed.
Unzen's deadliest eruption occurred in 1792, beginning with an earthquake in November of the previous year. On February 10th, 1792, eruptions from Fugen-dake's Jigokuato crater began. Lava flows began on March 1 and continued for nearly two months. On March 22, volcanic plumes were produced, and lava also flowed out of the crater. On March 25, fumes rose and lava flowed down the northeastern part of Fugen-dake at a total length of 2.7 km. Eventually, the east flank of the Mayu-yama dome collapsed unexpectedly following a post-eruption earthquake, creating a landslide into Ariake Bay. This caused a megatsunami that reached a height of 100 metres (330 ft), and killed an estimated 15,000 people. As of 2011 it is the worst volcanic related eruption in Japan.
After 1792, the volcano remained dormant until it erupted again in November 1990.
It started with a slowly building active phase in 1968, with first earthquake swarms continuing until 1975. Large amounts of volcanic gas were ejected by Fugen, which affected about thirty cedar trees. In 1975, bird and animal carcasses were found in the surrounding area, and high concentrations of carbon dioxide were detected emitting from rock fractures. This area was also reported to have erupted volcanic gasses during the 1792 eruption. Earthquakes occurred sporadically since 1975, but there were 89 sensitive earthquakes, with strong shaking equivalent to a maximum seismic intensity of 5 from June to September 1979. At Shimabara Onsen, there was a place where the dissolved carbon dioxide concentration rose sharply from 1975 and increased by 30%. From April 1984, swarms occurred in Tachibana Bay A magnitude 5.7 earthquake with a seismic intensity of 5 occurred in August near the south side of Hayama. As a result of this earthquake, uplift of the Shimabara Peninsula was observed, and magma supply from Tachibana Bay began.
In November 1989, an earthquake swarm began about 20 kilometres (12 mi) underneath and 10 kilometres (6.2 mi) west of Fugen-dake. Over the following year, earthquakes continued, and their hypocentres gradually migrated towards the summit. Finally on November 17, 1990 (Heisei 2), the first phreatic, or steam-blast, eruptions began ejecting from two places near a shrine at the summit. These eruptions only allowed vaporized hot water and ash to blow up from the two craters. In December of the same year, the volcano's state lulled and it seemed that it would end, but it erupted again on February 12, 1991, and on April 3, and 4th. The eruption intensified on May 9.
On May 15, the first debris flow caused by accumulated volcanic ash at the summit occurred, and a number of cracks were formed on the west side of the crater, extending in an east-west direction. After this inflation of the summit area, fresh lava was confirmed to be emerging on May 20, 1991. The lava was so viscous that it did not flow out, but instead became a dome which formed in the crater. The lava dome grew like a peach, eventually splitting and collapsing into four pieces under its own weight. After that, the dome continued to grow, continuously supplied with fresh lava from the crater underneath, creating new lobes that hung down from the summit. The 1st to 13th lobes were named in the order they were formed. Collapsing of the lava dome began to occur when the lobes, pushed out by the newly supplied magma, started to give way on the slope, causing a phenomenon known as pyroclastic flows (in which debris and volcanic gasses flow down the mountainside at speeds of 100 km/h).
The threat of further disastrous events prompted authorities to evacuate 12,000 residents from their homes. On June 3, 1991, the volcano erupted violently, possibly as a result of depressurisation of the magma column after a landslide in the crater. A pyroclastic flow triggered by the collapse of a lava dome reached 4.5 kilometres (2.8 mi) from the crater and claimed the lives of 43 scientists and journalists, including volcanologists Katia and Maurice Krafft and Harry Glicken.
The eruption activity continued until March 1995, with a temporary pause during its course. Between 1991 and 1994, the volcano generated at least 10,000 small pyroclastic flows, destroying about 2,000 houses. From 1993 onward, the rate of lava effusion gradually decreased, and eruptions came to an end in 1995.
The 1990-1995 eruptions of Unzen were the first eruptive activities in the world where pyroclastic flows were recorded clearly and continuously on video (in the past, many pyroclastic flows, such as those on Mt. Pelee, were recorded either as photographs or on small-scale videos).
Mount Unzen is perhaps most famous for its destructive and fatal eruption on June 3, 1991 at 4:08 pm. This eruption caused the first large-scale pyroclastic flow, unprecedented at the time, which killed 43 people in the evacuation zone. Among these were French volcanologists Maurice and Katia Krafft, as well as American geologist Harry Glicken. The other forty fatalities consisted largely of those involved in the mass media, as well as firefighters, police officers, farmers and taxi drivers.
The first debris flow in the Mizunashi River (located at the foothills of Unzendake) occurred on May 15, and others continued to occur in succession on the 19th, 20th, and 21st. Shimabara City made an evacuation recommendation to the towns in the Mizunashi River basin each time, resulting in smooth evacuation of residents and no human fatalities.
However, on May 20, when the lava dome appeared on Fugen-dake, it continued to grow day by day, and on the 24th part of the dome collapsed and the first pyroclastic flow occurred. Since then, small pyroclastic flows occurred frequently, reaching a distance of 2.5 km east of the lava dome on the 26th, and 3.0 km on the 29th. On the 26th when the tip of a pyroclastic flow approached 500 m from the nearest private houses, Shimabara City issued an evacuation recommendation for Kita-Kamikoba town, Minami-Kamikoba town, Shiratani town, Tenjin Motomachi, and Fuda Motomachi - all located in the Mizunashi River basin. It was done without trouble.
In order to capture the pyroclastic flow activity on film, the mass media chose a select spot within the evacuation advisory area, but 4.0 km away from the lava dome, 200 m away from the Mizunashi River (where debris flows were frequent), and 40 m high. This shooting spot was located on the prefectural road in the village of Kita-Kamikoba, and was favored by the media because they were able to view Fugen-dake directly in front of them. The nickname "fixed point" was established. After the first pyroclastic flow on the 24th, more than a dozen media members were lined up at the "fixed point". In 1991, the news media had started to use color photographs on the paper, and with the Fugen-dake disaster becoming an ongoing topic of coverage, the companies started competing for color photographs. The competition became even more intense as the Mainichi Shimbun succeeded in shooting nightly pyroclastic flows on May 28.
The fact that pyroclastic flows were being recorded in clear images for the first time attracted a great deal of attention from all over the world, and many volcanologists and government officials entered the evacuation advisory area for interviews and photography. On May 28, the Ministry of Construction (at that time officials of the Public Works Research Institute) released a picture taken when entering the path of previous pyroclastic flows, 500 m below the lava dome, and another scholar group on the afternoon of June 1 or 2nd. After entering the tip of the deposit path, they surveyed the site for about an hour, photographed the pattern and released it.
In addition, many visitors had begun to gather around Unzendake to see the plumes. In particular, since June 2 was Sunday, many visitors from other prefectures gathered around the Mizunashi River and observed the pyroclastic flows with binoculars and photographed them with video cameras. Traffic became heavy as a result.
However, on May 26, reporters from the Asahi Shimbun were warned that it was likely for them to be engulfed by the clouds, and in an evacuation recommendation, they were asked to leave the "fixed point" and find another area. According to a photographer who continued aerial photography of the lava dome from the helicopter, ''The bottom fans out from the Sabo dam of the Mizunashi River, and if a large pyroclastic flow occurs, there is a strong possibility of it striking the 'fixed point'.'' The Asahi Shimbun abandoned their position and retreated past the Tsutsuno bus stop, to instead set up a point in Fukae Town outside the evacuation zone on the 28th, and switched to 24-hour shooting from there. At the "fixed point", the coverage in the vicinity was limited to patrols.
Similarly, the Japan Broadcasting Corporation (NHK) stopped shooting from within the evacuated area from the end of May and arranged for an unmanned camera in the Kamikoba area. But until the unmanned camera was ready, the photographer who had been lowered backwards was advanced to the Kamikoba area once again on June 1. The evacuation area then became smaller, since the commercial broadcasters had shot footage of bright red lava on May 30 and 31; the person in charge of the news program (who ordered the initial retreat of the front-line shooting staff) ordered more images shot like it. Kamikoba soon emerged again as a filming location of the press.
Since May 15, when the debris flow evacuation advisory was issued, the fire brigade in charge of the Kamikoba district stayed either at the Minami-Kamikoba town fire brigade, or the Kita-Kamikoba town agricultural training center (which was initially an evacuation residence for locals). On May 29, due to the frequent occurrences of pyroclastic flows, they were evacuated from the firefighting station in Minami-Kamikoba town to the Shiratani Public Hall in the lower Mizunashi River, but on June 2, they returned to the Agricultural Training Center in Kita-Kamikoba town. This was because of the following reasons:
After 3:30 PM on June 3, small and medium-sized pyroclastic flows occurred frequently, and the first large-scale pyroclastic flow occurred at 3:57. Although this pyroclastic flow (and its resulting pyroclastic surge) did not reach the "fixed point", visibility in the vicinity deteriorated significantly as volcanic ash from the dispersing cloud covered the surroundings, in addition to the rainfall that had continued from the morning.
At 4:08 PM, the second large pyroclastic flow occurred, descending the Mizunashi River Valley and reaching 3.2 km to the east from the lava dome. Although the main body of the flow continued to follow the river, the resulting pyroclastic surge spread out in a fan shape once it had exited the valley mouth and continued further, hitting Kita-Kamikoba town (4.0 km away from the lava dome) and finally stopping near the Tsutsuno bus stop (5.0 km away). The pyroclastic flow also flowed down the Akamatsu-dani River, but changed direction due to a gust of wind from the south, allowing the residents, firefighters, and the shooting staff (who left their camera behind to capture world-famous footage of the eruption) to immediately escape the trouble.
At the "fixed-point", the media members initially left their chartered taxis and company cars on the road with the engine running, facing south so that they could escape immediately in case of an unforeseen event. However the field of view was poor due to dispersing ash from the previous pyroclastic flow, as well as rain, and it was almost impossible to evacuate even from the windward side, because the pyroclastic surge that flowed from the Akamatsu-dani River would have cut them off. A fire brigade member of the agricultural training station, several hundred meters away from the "fixed point", assumed incorrectly that the roaring of the pyroclastic flow was caused by a debris flow instead, and was engulfed by the pyroclastic surge when he left the training station to check the Mizunashi River. Many members escaped from the evacuation advisory area on their own, but suffered severe burns and airway damage. As a result, 16 media members (including part-time students), 3 volcanologists (the Kraffts and Harry Glicken), 12 firefighters who were on guard, 4 taxi drivers, 2 police officers who came by police car to conduct evacuation guidance, 2 workers who were removing the city council election poster bulletin board, and 4 residents who were working in agriculture were killed. The disaster resulted in 43 dead or missing, and 9 injured. The photographer of the Yomiuri Shimbun newspaper died holding his Nikon F4. The camera recorded seven frames of the pyroclastic flow, although it was discolored by heat. All of the areas surrounding the "fixed point" where many casualties occurred, were within the advised evacuation zone.
A professional video camera used by an NTV cameraman who died in the pyroclastic flow was discovered in June 2005. The camera melted due to the high temperature generated by the pyroclastic flow and was severely damaged, but the tape it contained was playable, despite having been buried under volcanic deposits for almost fifteen years. The video received from the camera shows the reporters observing the first pyroclastic flow, and then continuing to report after having moved their position to the "fixed point". It also displays the police cars that announced evacuation, all unaware of the approach of the second large pyroclastic flow. The footage and sound were recorded and recovered, although heavily damaged and warped (the video ends when the cameraman notices a sound apparently associated with the generating pyroclastic flow, and asks "What's that sound?", whilst turning the camera in the direction of Mt. Fugen). This video was broadcast on October 16 of 2005, and is now exhibited at the Unzendake Disaster Memorial Hall (Shimabara City) alongside the melted camera.
It's unclear as to what caused the unprecedented size of the second, large-scale pyroclastic flow on June 3. It's been suggested that the heavy ash-laden column that towered over the main flow, collapsed during its descent, causing the surge. It's also been pointed out that when the dome collapsed, 0.5 million cubic meters of hardened lava broke off, leaving a significant collapse scar afterwards, suggesting that the size of the dome collapse (rather than the column collapse) initiated the surge. Other explanations account the fact that previous lahars and pyroclastic flows filled the Mizunashi Valley, raising the level of the river-bed and making it more likely for the flow to "skip" ridges that would have once stopped it. According to the 2011 documentary Face au Volcan Tueur (French for Facing the Killer Volcano), the unexpected surge could have been caused by phreatic explosions as the main body of the pyroclastic flow proceeded down two waterfalls at the headwaters of the river, fragmenting the ash-particles of the avalanche and expanding the energy of the overlaying gas and ash cloud.
Pyroclastic flows continued over the next few days following the June 3rd eruption, however, they were reportedly smaller and less frequent. Even so, rescue efforts and movements to recover the bodies of the victims, were disrupted and stalled due to the ever-present danger. Authorities eventually managed to recover 27 bodies and brought them to the Anyoyi Shrine. 4 of the victims' bodies were never recovered, and were reported as missing at the time, but presumed dead. Many of the recovered bodies were carbonized due to the extreme heat of the surge, and were identified by either dental records, or personal belongings they carried with them (e.g. wristwatches or cameras). 17 victims on the outer edges of the affected zone lived for a while, before succumbing later to their injuries (burns to their skin and lungs, or inhaled ash in their windpipe and lungs). The final victim reportedly died on August 8.
The collapse of the lava dome itself left a notable scar on the eastern side of the summit, and the pyroclastic flow and surge resulted in much damage in the Kamikoba district. Kita-Kamikoba was almost completely destroyed. Around 170 houses were devastated, many of them set alight by the intense heat of the ash. Pictures have shown that the pyroclastic flow left significant deposits near the mouth of the Mizunashi River Valley, enough to bury houses and the river itself. In contrast, the surge that detached from the flow left deposits of ash between 3 and 30 cm thick, however trees were flattened and many buildings continued burning until the next morning. The force of the surge was enough to push a car 80 m down-road.
Many of the casualties of the June 3rd eruption could have been avoided if the danger of pyroclastic flows had been made more apparent beforehand. While the first pyroclastic flow on May 24 was shocking, many didn't understand just how lethal such events could event. A reporter stated at the time: "I knew they were quite hot, but didn't know they could be accompanied by (pyroclastic surges). I thought we could escape by car, but I didn't know the extent at which it burned."
25 days after the event, the Japan Meteorological Agency revealed that while the first dome collapse was reported, much of the mass media and local residents were in a state of confusion about the danger of pyroclastic flows, and weren't particularly concerned about them overall. The media originally reported that the pyroclastic flows seen on Fugen-dake at the time were "geologically small", which many regarded as "a scale that would cause no human fatalities". On May 26, a construction worker operating a Sabo dam in the upstream area of the Mizunashi River was caught in a pyroclastic flow; he survived but was sent to hospital with burns on his arms. Many thought that such burns could be prevented by wearing a long-sleeved shirt. The danger of the phenomena was not made clear. Furthermore, the number of pyroclastic flows from May 25 to June 2 had increased to 165, and all had stopped near the Sabo dam in the upper reaches of the valley. As a result, much of the media grew accustomed to the eruptions and no longer felt like they were in any danger.
Shimabara City issued an evacuation advisory for the villages near the base of Unzen on May 26, but responded to the residents' request that patrolling police officers allow them into the area to return to their homes (in preparation for prolonged evacuation). From the same day onward, a "district name sticker" was issued, and private cars with such a sticker were given permission to enter the evacuated area. For this reason, there were many residents doing laundry and field work at homes in the zone that had been declared dangerous. In the Kamikoba area, in particular, there were many farmers who made a living by cultivating tobacco leaf, but since they had been forced to evacuate for a long time (since the May 15th debris flows), their work grew stagnant and many of them were concerned. For this reason, on June 4, in collaboration with tobacco farmers in Annaka Town (which had not been evacuated), the residents of the Kamikoba area entered the evacuation zone to perform flower filling work later than usual.
Volcanologists did not talk about the danger of pyroclastic flows initially, because they feared residents would panic, especially those still working in the evacuated area. Residents were warned not to enter the evacuation advisory area by Shimabara City and the media, but since May, no concrete image was given regarding the danger of pyroclastic flows compared to that of debris flows (which had caused significant damage previously), and most residents did not take the warnings seriously. Many misunderstood them to be mere clouds of dust. According to the "Investigation of Disaster Information Transmission and Residents' Responses during the 1991 Unzendake Eruption" conducted in 1992, 75% of local residents recognized debris flows as more dangerous than pyroclastic flows before June 2. Only 15% perceived pyroclastic flows to be more dangerous. Furthermore, only 5% of the residents in the Kamikoba area recognized pyroclastic flows as "very dangerous". As was seen in the survey results, the volcanologist's warnings were not even understood by residents living in the most dangerous areas.
On June 3, a debris flow warning was passed out due to rain that began in the morning, and a party was held in Shiratani to celebrate the winner of the Shimabara City Council election on the 2nd. As a result of those two factors, most residents were withdrawn from the evacuation advisory area, and fortunately resulted in fewer victims.
The eruptive activity caused great damage to the Shimabara Peninsula, especially Shimabara City and Fukae Town. The main factor that led to damage were debris flows and lahars, initiated by heavy rain which destabilized pyroclastic flow and ash debris previously deposited on the slopes. The Mizunashi basin, as well as the Senbongi district and parts of Shimabara were severely damaged. In addition, there was concern about the collapse of Mt. Mayuyama, which previously caused the 1972 disaster. In this eruptive activity, however, Mt. Mayuyama protected the center of Shimabara from the pyroclastic flows.
In areas other than Shimabara and Fukae, there was ash fall depending on the wind direction, and at Kumamoto Airport, it had an impact on the arrival and departure of aircraft.
Since the eruptions, heavy rains have frequently remobilised pyroclastic material, generating lahars. Dikes have been constructed in several river valleys to channel lahar flows away from vulnerable areas, and warning systems and evacuation plans have been developed and deployed.
Mount Unzen was designated a Decade Volcano by the United Nations, in 1991 as part of their International Decade for Natural Disaster Reduction, due to its history of violent activity and location in a densely populated area.
In 1999, an ambitious project began at Mount Unzen to drill deep inside the volcano and sample magma in the 1990-1995 eruption conduit. The project hoped to shed light on some fundamental questions in volcanology, such as why magma repeatedly travels in the same conduits despite the solidification of magma in them at the end of each eruption, and how it can lose enough gas on its ascent to erupt effusively rather than explosively.
Drilling began with test bores to assess the viability of a deep borehole. Two holes were drilled, 750 metres (2,460 ft) and 1,500 metres (4,900 ft) deep, and cores taken from these holes were used to better determine Unzen's eruptive history. One further 350-metre (1,150 ft) deep borehole was drilled to test the methods to be used in the final drilling project.
The main drill began in 2003, starting from the northern flank of the volcano with a 440 mm (17 in) hole at an angle of 25 degrees from vertical. At greater depths, the direction of boring was tilted towards the conduit, reaching an angle of 75 degrees from vertical at a depth of 800 metres (2,600 ft). Drilling reached 1,800 metres (5,900 ft), the original target depth, without reaching the conduit, but in July 2004 at a depth of 1,995 metres (6,545 ft), the conduit was finally reached. The vertical depth below the summit was 1,500 metres (4,900 ft).
The temperature at the conduit was about 155 °C (311 °F), much lower than pre-drill estimations of 500 °C (932 °F) and over. This was attributed to hydrothermal circulation accelerating the cooling of the magma over the nine to ten years since the end of the eruption.
Arie River starts at Mount Unzen and flows to the Ariake Sea.