Aso94: Aso seismic observation with broadband instruments

We deployed a network of broadband seismometers for one year around the Naka-dake first crater of Aso volcano in Kyushu, Japan, to reveal the mechanism of long period tremors (LPTs) emitted from the volcano. It is observed that LPTs with a dominant period of about 15 s are always emitted regardless of the surface activity of the volcano. A typical LPT has a short duration less than a minute and its spectrum shows mode peaks at 15, 7.5, 5, and 3 s. The particle motion in the frequency band for the lowest two modes at stations within a few kilometers from the crater is rectilinear, pointing in the direction of the crater. A waveform semblance technique to locate sources of LPTs is devised to utilize the rectilinearity of waveforms. The LPT sources are located at depths of 1–1.5 km beneath the bottom of the crater. When the volcano is explosively ejecting steam and mud, on the other hand, a very long period (∽100 s) displacement (VLPD) polarized outward from the crater often precedes an eruptive event by a few minutes. A typical VLPD is accompanied by a few long period pulses, first positively polarized and concurrent with the onset of VLPD, then negatively polarized just before the eruption. The source of VLPDs is inferred to coincide approximately with that of LPT. On the basis of these observations, a qualitative model is constructed for the hydrothermal system beneath the Naka-dake first crater. An explanation for the unusually long period nature of the LPT is discussed in terms of a class of slow waves, which exist in solid–liquid two-phase systems. A possibility of realtime monitoring at Aso volcano using the observed long period seismic signals is also discussed.