Structural features and the history of mud volcanoes in southern Hokkaido, northern Japan

Abstract

There are nine mud volcanoes in Niikappu and Shizunai Towns in southern Hokkaido, northern Japan, with the maximim size of 330m short axis, 750m long axis, and 50m height, aligning in a line for 12.5km along the Nikkappu anticline of Miocene formations where the anticline is tight. These mud volcanoes were active after the latest interglacial age: severe mud volcanism made a caldera-like depression with a diameter of 1km in the north of Nikkappu Town, and four mud volcanoes were subsequently made in the depression. The mud volcanoes were made by releasing abnormal high pore pressures in the depth generated by probable lateral compression accompanied folding and methane formation. The mud volcanism is relatively weakened during the last 100 years, but weak eruption of mud and gas has been continuing in some of the mud volcanoes during large earthquakes with over 5th degree of seismic intensity of the Japan Meteorological Agency. Mud diapir showing a 'flame structure' was also observed with the mud volcanoes.