2018 Volume 40 Issue 3 Pages 175-191
Unrecorded boiling-fountains with the discharge temperature 98.0 ºC and pH 7.5 were discovered in the eastern moat of Onikobe caldera, Japan in late November 2016, and are temporarily named “Eaigawa-jyoryu fountains” in this paper. Time-sequential observation of aerial photographs shows that the Eaigawa-jyoryu fountains appeared during the process of the aftershocks of the 2008 Iwate-Miyagi Inland Earthquake between 16 June 2008 and 21 May 2009. Based on the hypocenter distribution of the aftershocks of the 2008 IwateMiyagi Inland Earthquake, the main shock is ascribed to the rupture of a large-scale reverse fault where the western crustal block was thrusting over the eastern crustal block. The bottom of the hypocenter distribution of the aftershocks has, however, distinctive features. It is the deepest between Yakeishidake and Kurikomayama volcanoes as deep as 10 km below the sea level (b.s.l.) and comes to be shallower to the north and south. It comes to be extremely shallower to the south up to the 4-3 km depth b.s.l. near Onikobe caldera. It gradually comes to be shallower from the center to the east like the typical reverse fault rupture but abruptly reaches to be shallower to the west up to the 4-3 km depth b.s.l. near Kurikomayama volcano, Takamatsudake volcano and Onikobe caldera. The earthquake hypocenters were thus compulsorily confined to be shallower by the brittleplastic transition surrounding the sub-volcanic magma systems beneath these volcanoes throughout the events of the main shock and aftershocks. The right-lateral shear strain concentration made the pore pressure increased in an ENE-WSW trending narrow zone between the southern edge of the large-scale reverse fault of the main shock and the northern edge of the plastic ridge at the Katayama-Arayu steaming grounds in Onikobe caldera during the main shock of the 2008 Iwate-Miyagi Inland Earthquake. This right-lateral shear strain concentration might have ruptured a small-scale ENE-WSW trending right-lateral strike-slip fault in the process of the aftershocks presumably initiating the new discharge of the Eaigawa-jyoryu fountains.