Strong Seismic Motion Associated with the 1986 Eruption of Izu-Oshima Volcano

Abstract

Izu-Oshima volcano erupted from the summit crater on November 15, 1986. The strong motion accelerograph station in Izu-Oshima recorded felt earthquakes soon after the eruption. We installed two additional temporary accelerograph stations in Izu-Oshima on November 20. The fissure eruption commenced on November 21. The eruption was preceded by intense earthquakes. The largest earthquake with M 6.0 occurred at 0041 UTC (Universal Coordinated Time) on November 22. More than 440 earthquakes were recorded at strong motion accelerograph stations from November 16 to December 7. Hypocenters were determined for about 310 earthquakes. Before the fissure eruption, epicenters were distributed in the northern and western parts of Izu-Oshima Island. Earthquakes prior to the fissure eruption were concentrated within a small area of about 2 km in diameter under the northern caldera rim. Focal depths were less than about 5 km. After the fissure eruption of November 21, epicenters were widespread and hypocenters depths were distributed between 0 km and 15 km. Low frequency earthquakes were observed from November 22 to December 7.