Aeromagnetic Constraints on the Subsurface Structure of Usu Volcano, Hokkaido Japan
Abstract
Usu Volcano, Hokkaido, Japan consists mainly of dacitic volcanic rocks underlain by basaltic somma lava and Pliocene - Pleistocene andesitic volcanic rocks and has erupted every 20-30 years. Latest eruption in 2000 took place for the first time after the 1977-1978 eruption. A helicopter-borne high-resolution aeromagnetic survey was conducted to better understand the subsurface structure of the volcano almost three months after the start of the eruption. The survey with a stinger-mounted Cs magnetometer was flown at an altitude of 150 m above terrain along north-south survey lines and east-west tie lines, spaced 200 m and 1000 m apart, respectively. DGPS with an accuracy of 50cm was employed for flight-path recovery. Total magnetic intensity was observed every 0.1 second and anomaly was calculated as residual values after subtracting the IGRF-10 field. Magnetic anomalies on a smoothed observation surface were calculated by the reduction method, assuming equivalent anomalies below the actual observation surface (Nakatsuka and Okuma, 2006a). Preliminary 3-D imaging of magnetic anomalies over Usu Volcano was also conducted to constrain the subsurface structure. The 3-D magnetic model indicates that magnetization highs occupy the main edifice of Usu Volcano, suggesting the subsurface distribution of the Usu Somma Lava with a thickness of 1,000 m at the maximum. While, (negative) magnetization lows lie northwest of the Nishi-Yama Craters Area and on the Higashi-Maruyama Cryptodome, where Pliocene and Pleistocene volcanic rocks are distributed, respectively. Their reverse magnetization can be responsible for the magnetization lows. Although the survey was successful to better understand the surface and subsurface distribution of volcanic rocks which constitute the edifice and basement of Usu Volcano, some limitations exist. Any information about the magmas intruded during recent eruptions such as in 1977-1978 and 2000 has not been obtained by the high-resolution aeromagnetic survey, though some intrusions were suggested by other geophysical data. No large magnetic contrast between the intruded magmas and their host rocks might be one of the most probable reasons. This implies that the intruded magmas were not cool enough to be strongly magnetized. Consequently, it is not easy to get useful information associated with the volcanic activity of a volcano only form a single survey. The repeated high-resolution aeromagnetic survey is a promising way to extract temporal magnetic anomaly changes over the active volcanoes with rugged terrain. The crossover analysis method has been already developed for that purpose (Nakatsuka and Okuma, 2006b). Fortunately, large temporal magnetic changes (>50nT/year) have been observed by repeated magnetic measurements on the ground in the Nishi-Yama Craters Area. Therefore, meaningful magnetic anomaly changes can be detected by the repeated high-resolution aeromagnetic survey with a combination of the appropriate survey specification (low flight altitudes and dense line spacing) and the crossover analysis.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFMNS31A1382N
- Keywords:
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- 0925 EXPLORATION GEOPHYSICS / Magnetic and electrical methods;
- 1517 GEOMAGNETISM AND PALEOMAGNETISM / Magnetic anomalies: modeling and interpretation;
- 1540 GEOMAGNETISM AND PALEOMAGNETISM / Rock and mineral magnetism;
- 8488 VOLCANOLOGY / Volcanic hazards and risks