10:15 AM - 10:30 AM
[STT58-06] Can magnetic survey estimate locations of intrusions?
Keywords:intrusion, magma, magnetic survey, 3D imaging, Usu Volcano, Usu 2000 Eruption
This time, we took a different approach to overcome the problem. We thoroughly reexamined the aeromagnetic anomalies observed by helicopter-borne EM and magnetics flown at an altitude of 70 m above terrain. Since the flight altitude of this survey is lower than that of the former one, a dipole of magnetic anomalies with a reverse polarity was found on the southwestern flank of the main edifice of volcano. To confirm the magnetic anomaly, we, then, conducted a ground magnetic survey along some profiles. As a result, a comparable magnetic anomaly was observed on ground. Whereas, the survey area is underlain by basaltic somma lava which shows high NRM intensities (6-10A/m) (Okuma et al., 2014). This suggests the existence of an intrusive body with a magnetization intensity lower than that of the somma lava. There are two possibilities which account for the magnetic anomaly. A hot magma of the recent eruptions might have intruded in the somma lava since some fumarolic activities were observed nearby during the ground magnetic survey. A cooled magma intruded during older eruptions is another possibility. Volcanic activities of the volcano changed from basaltic to dacitic after the formation of the main edifice. Since the NRM intensities of dacite is lower than that of soma lava (Nemoto et al., 1957), an old dacitic intrusion can account for the magnetic anomaly as well. Consequently, a repeat magnetic survey might play a role of judging if which hypothesis is more suitable by observing temporal magnetic changes.