三原新山 (みはらしんざん) - Mihara Shinzan, Izu Oshima - 3/27/2025

My Experience - After hiking Komagadake at Hakone, I drove to Atami in preparation for the jet ferry ride to Izu Oshima the following morning. I parked for the evening at Atami City No.1 Water Park Parking Lot タイムズ熱海市営第１親水公園 as the area had both public restrooms and a Lawson directly across the street. Parking was about 1760 Yen from the evening until early the time I left around 730 AM. I also found the area convenient as hiking Hakone area, the trail was very muddy and no place to clean boots afterwards however, at the area I parked in Atami, there were a couple areas to clean shoes or for filling dog water bowls etc.

1. Leaving the parking area at around 730 AM with short 2 minute drive to Tokaikisen Atami Agent 東海汽船 熱海代理店 where I booked a roundtrip jet ferry ticket for about 12,000 Yen
2. Departed at 9:05 AM and took a little less than an hour to arrive at Oshima 大島. 
3. Took taxi for 4,000 yen from ferry terminal at Okata Port, Oshima ferry terminal to Mt.Gairin Observation Deck.(Mt. Mihara Top Trailhead.) 外輪山展望台(三原山頂口)
4. Started hiking around 1050 and was able to do close to 10.5 KM in about 2 hours, 20 minutes. My AllTrails hike recording is here: https://www.alltrails.com/explore/recording/afternoon-hike-3191767-718
5. Took 1420 bus back to Okata Port for 900 Yen
6. Boarded jet ferry back to Atami at around 1550, departed at 1640, and arrived back in Atami around 1725
7. 3 hour drive through crazy traffic back to Takasaki, Gunma, arriving back in Takasaki around 2100 (9 PM)

Demographics

• Volcano No. - G01
• Volcano name - Izu Oshima
• Geologic code **Q3b
• Main activity period - Late Pleistocene - Holocene
• Activity period / recent activity ***From 40,000 or 30,000 yBP.
• Latest eruption: AD 1990
• Volcano type - Composite (compound) volcano - Caldera, Pyroclastic cone, Lava flow
• Representative rock type - MaficMain rock type ****Basalt, Andesite
• Height above sea level - 764 m (Mihara Shinzan)
• Location (Prefecture) - TOKYO (IZU ISLANDS) E. Longitude (WGS84)139.3947
• Active volcano name by JMA - Izu-Oshima
• Eruptive History - Extensive...
• Source: Geological Survey of Japan

About

Mount Mihara (三原山, Mihara-yama) is an active volcano on the Japanese isle of Izu Ōshima. Although the volcano is predominantly basaltic, major eruptions have occurred at intervals of 100–150 years.

History

Mount Mihara's major eruption in 1986 saw lava fountains up to 1.6 kilometres (1.0 mi) high. The eruption had a Volcanic Explosivity Index of 3, and involved a central vent eruption, radial fissure eruption, explosive eruption, lava flows, and a lava lake eruption. There was also a 16 km high subplinian plume. All of the island's 12,000 inhabitants were evacuated by dozens of military and civilian volunteer vessels.

The most recent eruption was in 1990.

Suicide

From a vantage point near the top of the cone it was once possible to leap into the crater. As a result, the volcano became a popular venue for suicides. Beginning in the 1920s, several suicides occurred in the volcano every week. The most notable death by suicide was that of twenty-one year old Matsumoto Kiyoko, who jumped into the volcano on February 12, 1933. She and a friend, Tomita Masako, had planned to go to the volcano together as part of a dōsei shinjū (a same-sex lover's suicide), but after climbing the volcano, Tomita was stopped by a guard. It was later revealed that Tomita had witnessed a suicide attempt a short while previously with another girl, and to get to the volcano, she had purchased a round-trip ferry ticket. Newspapers branded her as a suicide guide and ran dramatized stories warning of the dangers of same-sex relationships between women. After the incident, the suicide toll skyrocketed to 945 that year alone. Authorities eventually erected a fence around the base of the structure to curb the number of suicides.

Mihara in popular culture

Mt Mihara has been featured numerous times in fiction. In 1965, Oshima Island and the volcano itself appeared in the climax of Gamera's debut film, where the military lured the giant turtle there as a means to trap it in a giant rocket-ship built on the island.

Mount Mihara was the place where the Japanese government imprisoned Godzilla in the movie The Return of Godzilla. Five years later, in the sequel Godzilla vs. Biollante, bombs placed on Mt. Mihara go off and release Godzilla from his fiery tomb.

In the novel Ring by Koji Suzuki and its subsequent film adaption, Shizuko Yamamura, the mother of Sadako, predicted that Mount Mihara would someday erupt using her psychic abilities. After a failed psychic demonstration which resulted in Sadako psychically murdering a reporter, Shizuko became depressed and ultimately insane and committed suicide by leaping into the crater of Mount Mihara.Source: Wikipedia "Mount Mihara" (URL: https://en.wikipedia.org/wiki/Mount_Mihara)

Geological Summary

Oshima is an active volcano located on the northern tip of the Izu-Bonin volcanic arc. Our last report of activity at Oshima (BGVN 21:08) enumerated a flurry of shallow low-frequency earthquakes beneath the top and W flank of the volcano that started on 5 August 1996.

Since those relatively benign events, the Japan Meteorological Agency (JMA) had not observed any subsequent events worthy of note until May 2010 when land surface inflation was detected. The inflation was registered by a strainmeter, a Global Positioning System (GPS) network (run by the Geospatial Information Authority of Japan, GSI), and a tiltmeter network (run by the National Research Institute for Earth Science and Disaster Prevention, NIED).

In July 2010 seismicity in the shallow parts of and around Oshima began to increase. (High seismicity synchronous with inflation of the edifice was seen earlier, including in 2004 and 2007). These events were considered to be due to magma intrusion into the deeper part of the volcano. There were no remarkable changes in surface phenomenon. In September, the inflation that was detected in May began declining. Seismicity in the shallow parts of and around Oshima continued at a low level with some small earthquakes which temporally increased in the western offshore areas of Oshima on 22 December 2010.

The earthquakes increased in frequency again on 9 February 2011. GPS and strainmeter measurements indicated contraction since January, but the trend reversed to show inflation in October 2011. Seismicity remained at a low level. Very low level gas emissions were sometimes observed by a camera positioned on the NW summit. Based on a field survey on 28 October, no remarkable change in surface phenomena was observed.

No remarkable activity has been noted since October 2011. Throughout the noted activity, JMA held the Alert Level at 1.

Summary. Izu-Oshima volcano in Sagami Bay, east of the Izu Peninsula, is the northernmost of the Izu Islands. The broad, low stratovolcano forms an 11 x 13 km island constructed over the remnants of three older dissected stratovolcanoes. It is capped by a 4-km-wide caldera with a central cone, Miharayama, that has been the site of numerous recorded eruptions datining back to the 7th century CE. More than 40 cones are located within the caldera and along two parallel rift zones trending NNW-SSE. Although it is a dominantly basaltic volcano, strong explosive activity has occurred at intervals of 100-150 years throughout the past few thousand years. A major eruption in 1986 produced spectacular lava fountains up to 1,600 m high and a 16-km-high eruption column; more than 12,000 people were evacuated from the island.

Information Contacts: Japan Meteorological Agency (JMA), Otemachi, 1-3-4, Chiyoda-ku Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/).