Geology and vertical displacement along active faults at the northern and western margin of Shizukuishi basin, Iwate prefecture, northeastern Honshu

概要

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The geology at the northern and western margin of the Shizukuishi basin is composed of upper Miocene Yamatsuda Formation, Pliocene Masuzawa Formation, Plio-Pleistocene Tamagawa Welded Tuffs and units from Takakura and Iwate volcanoes, in ascending order. The geological structure of the margin is characterized by the Nishine fault system, which displaces these formations and units. The fault system, a set of west-dipping reverse faults, forms the topographic and geologic boundary between the Ou Backbone Range and the Shizukuishi basin, and it controls the vertical position of underlying units as well as Takakura and Iwate volcanic products. The vertical displacement of the Shibutami Welded Tuff, which is included in the Tamagawa Welded Tuffs and erupted between 0.7and 1.0 Ma, decreases to the north and south from an area where the tuff has been vertically offset some 300-400 m, for an average displacement of about 0.4 mm/yr. Slip occurred on the northern part of the fault system during the Iwate-ken Nairiku-hokubu earthquake of September 3,1998. The tuff is also displaced at least 250 m along the Matsuo fault for an average displacement of about 0.25mm/yr at the north of the Nishine fault system. This reverse fault strikes nearly parallel to the Nishine fault system and can be traced for 5 km.
Japanese Society for Active Fault Studiesの論文