Very long period seismic signals observed before the caldera formation with the 2000 Miyake-jima volcanic activity, Japan

摘要

Very long period (VLP) seismic signals, whose waveform consists of an initial impulsive signal and a later oscillatory wave with 0.2 or 0.4 Hz in dominant frequency, were observed before the caldera formation in the 2000 activity of Miyake-jima volcano, Japan. The results of waveform inversion show that the initial and later parts can be explained by a northward single-force of 1.5×10^8 N working at a depth of 2 km beneath the summit and a moment tensor solution at a depth of 5 km below and 2 km southwest of the summit with ～10^12 Nm, respectively. A clear positive correlation of the amplitudes between the two sources strongly suggests that the shallow single-force triggers the deeper moment source in spite of the several km distance between the two sources. To analyze the source time functions of the moment tensor that do not always oscillate in phase, we introduce a new method of moment tensor diagonalization which is performed in the frequency domain. According to the analysis, the two principal components have similar amplitudes and are greater than the third principal component, suggesting an axially symmetric oscillation. One of the possible systems is a combination of two cracks intersecting perpendicularly. Our interpretation is that the single-force was generated when magma containing rock blocks suddenly began to move in a choked subsurface magma path, and the resultant pressure waves propagated and excited a resonance oscillation of the two cracks.