Extremely Long Period Oscillations Detected by Tiltmeters at Miyakejima Volcano, Japan

Extremely long period (ELP) oscillation with a predominant period of about 40 min. has been observed by borehole tiltmeters at Miyakejima Island, an active basaltic volcano lying 170 km to the south of Tokyo, Japan, which has been emitting a large amount of volcanic gas from the summit crater since 2000. The oscillation exhibits a sinusoidal wave train with amplitude of about 5 nrad, and was observed at all five stations and fully synchronized each other. Their polarization directions and amplitudes variation indicate a periodic expansion and contraction of the island caused by a periodic pressure change of a magma chamber at the western part of the island 5-10 km deep, which was modeled from crustal deformation data by Ueda et al.(2004, AGU fall meeting). The ELP oscillation firstly appeared in around August 2002, and has continued intermittently up to 2005. The primarily predominant period is 40 min with a variation from 20 min to 60 min. The commencements of ELP oscillation in the many cases are coincident with the high activity of the shallow seismicity (<3km) inside of the summit caldera, suggesting that the beginning of oscillation and the earthquake activity have a common cause. We propose a model that Helmholtz resonance of magma system composed of a magma chamber and a conduit causes the ELP oscillation. The conduit connects the summit caldera to the magma chamber, filling with magma and gases in the lower and upper part, respectively. The pressure change of gas and the repulsion of magma oscillate the magma head in the conduit and pressure in the magma chamber, which may exhibit a sinusoidal wave train with a single predominant period as observed. Gradual pressure increase of the magma system may cause fractures, that is, earthquake swarms inside the caldera and result in an abrupt pressure drop in the magma system, which triggers the ELP oscillation. The long lasting appearance of the ELP oscillation for 3 years suggests the stability of the magma system, which is also suggested by the crustal deformation observation and by the continuous emission of the volcanic gases from a deep magma chamber around 10 km deep (Kazahaya et al, 2004, Geology; Saito et al., 2005, BV). The ELP oscillation is presumably a strong evidence for abundant fluid magma that is connected directly to the summit caldera.