Quaternary Volcanoes > Active Volcanoes > Iwate

5: Eruptions of historical times / 6: Latest activities

5: Eruptions of historical times
Eruptions in 14th to 15th centuries:
   No documentary records have been found for this period at the moment, although geologically confirmed. There were repeated eruptions of fine-grained scoria from Yakushi crater followed by ejection of Shirishida scoria. Sometime after that, surface layer of eastern Yakushi crater collapsed and deposited Ippongihara debris avalanche on the foot mixed with surface soil and ash beds of flank areas. The collapse left a scar at the eastern rim of Yakushi crater. Small scale eruptions occurred repeatedly within the Yakushidake crater after the collapse and formed Myokodake scoria cone consisting of scoria and lava. Immediately before these magmatic activities a small-scale phreatic explosion occurred at Oojigokudani.

Eruption of 1686:
   This was a summit eruption of Yakushidake Volcano during Yedo period. Documentary record is available. Precursor sound was heard about 10 days preceding the eruption according to the record. However, detail is not clear. In the early morning of March 25th 1686 ( March 2nd of 3rd year of Teikyou ) a large sound was heard in Morioka, and drifts of trees and wreckages of houses were observed floating down Kitakamigawa River suggesting explosive eruption. The next day, March 26th, rising column of smoke from the summit of Yakushidake was visible from Morioka. Eruption bringing ash fall in the foot stopped by early morning of March 27th, but small-scale eruptions with ejections of ash and volcanic bomb near the summit and phreatic activities continued until sometime in December.

   Omuro crater was newly opened up on the side of Myokodake scoria cone within the Yakushi crater and pyroclastic materials and Kariya Scoria were ejected. Big sound on March 25th may possibly correspond to the phreatic explosion at the initial stage of eruption. Kariya Scoria flowed down NE and SE directions of the summit and ash fall was recorded in Morioka area. Mud flow from melted snow was generated by the eruptions of March 25th to 27th causing damage to cattle and four houses in the eastern foot ( near present day Ippongi ).

Eruptios of 1732:
   This occurred in NE flank of Yakushidake Volcano. Preceding the eruption, on January 20th 1732 ( December 23rd of the 16th year of Kyoho ), earthquakes were felt on NE foot areas. From around midnight of January 21st, earthquake activity intensified and multiple craters aligned along a straight line on the flank of the volcano opened up resulting in the formation of scoria cones and lava flows. Lava flow ( Yakehashiri lava ) continued for about a week. Fumarolic action and emission of volcanic gas continued until November of the same year.

Eruption of 1919:
   This was a phreatic explosion at Oojigokudani of Nishi-Iwate Volcano. It was on July 14th 1919 ( 8th year of Taisho ) when the explosion occurred and intensive fumarolic activity continued for a few days. Ash fall was said to have reached about 4 km away in SW direction but the deposits are now recognizable only around Oojigokudani area.

6: Latest activities

Up to 1998:
   After a highly accurate network of seismographs and earth deformation observation system was installed by Tohoku University in 1994, volcanic tremors were detected for about 45 minutes on September 15th 1995. From September 1995 to the end of May 1996, all the tremors observed around Iwate Volcano were originated from about 10 km depth of eastern flank of Yakushidake. The earthquake foci shifted to about 2 km depth directly beneath Yakushidake summit from June 1996 to the end of December 1997. After that, from December 29th 1997, high frequency earthquakes and tremors started to be observed at about 2 to 4 km depths at Onigashiro and its southern foot areas.

After February 1998:
   From about mid February of 1998, minor tremors became frequent in SE flank areas. The foci gradually shifted to the west. Accompanying with the above, crustal stretching in NS direction was observed near Kurokurayama and Ubakurayama and uplift was detected with Mitsuishiyama as its center. Towards the end of June the earthquake foci areas reached near Mitsuishiyama. Earthquakes were active in an area of 13 km extending in ESE-WSW. On September 3rd 1998, Northern Iwate Prefecture Inland Earthquake ( M 6.2 ) occurred, the focus of which was upstream of Kakkonda River ( outside west of map area ), and an NS striking earthquake fault of 800 m long with maximum vertical displacement 40 cm appeared in Shinozaki, Shizukuishi area ( outside south of map area ). After this earthquake the foci area shrunk to Yakushidake-Inukurayama area and the number of daily occurrences decreased.

   From March 1999, geothermal activities intensified at Oojigokudani and surrounding area, and along the ridge of Kurokurayama to Ubakurayama. Especially, after May 1999, the areas of destroyed vegetation increased and the temperature of belched up gas went up and deposition of sulfur around fumaroles increased. On the ridge areas from Kurokurayama to Ubakurayama, fumarolic action along EW aligned faults became vigorous and some of them became visible from the foot areas. However, after March 2000, expansion of the areas of fumarolic activities slowed and in 2003 even the activities turned to become weak.

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