AEROMAGNETIC 3D IMAGING OF TEMPORAL MAGNETIC ANOMALY CHANGES DETECTED BY A REPEAT AEROMAGNETIC SURVEY - AN EXAMPLE OF USU VOLCANO, HOKKAIDO JAPAN

Usu Volcano, Hokkaido, Japan consists mainly of dacitic volcanic rocks underlain by basaltic somma lava and Pliocene-Pleistocene andesitic volcanic rocks, and erupts every 20-30 years. The most recent eruption, in 2000, was the first since 1978. A helicopter-borne high-resolution aeromagnetic survey was conducted by the Geological Survey of Japan almost three months after the start of the eruption to monitor the eruptive activity. Successive repeat aeromagnetic surveys were also planned at first but were not conducted since the major activity of the eruption paused until September 2000.

In 2010 at an interval of 10 years, a repeat aeromagnetic survey was conducted by Hokkaido Univ. along east-west survey lines perpendicular to the previous one (Hashimoto et al., 2011). Temporal magnetic anomaly changes were successfully detected by applying the generalized mis-tie control method (Nakatsuka and Okuma, 2006) to the magnetic anomalies of the both surveys (Nakatsuka et al., 2011; Hashimoto et al., 2011). For instance, positive magnetic anomaly changes are detected over the past eruption areas such as the 2000, 1977-1978 and 1943-1945 eruption areas. The peak values of anomaly changes amount up to +50 nT, +70 nT and + 20 nT for the 2000, 1977-1978 and 1943-1945 eruption areas, respectively. These values are conformable to the changes estimated by repeat magnetic measurements at fixed ground stations (Hashimoto et al., 2011).

Three-dimensional (3D) imaging method (Nakatsuka and Okuma, 2013) was applied to these temporal magnetic anomaly changes to estimate the intruded magma by each eruption. These magnetic anomaly changes were imaged as subsurface positive changes of magnetization, implying the acquisition of magnetization by cooling of the intruded magma. For instance, at the 2000 eruption area, a magnetization high of 0.7 A/m was imaged with its centroid depth of 100-200 m below surface. At the 1977-1978 eruption area, a magnetization high of 0.7 A/m was imaged with its centroid depth of 500 m below surface in the summit crater floor. This high occupies the almost same area as the location of the intruded magma estimated by MT surveys (e.g. Matsushima et al., 2001). At the 1943-1945 eruption area, a magnetization high of 0.7 A/m was imaged for the edifice of the Showa-Shinzan Volcano.