Aso-94: Aso seismic observation with broadband instruments
Kawakatsu, H., S. Kaneshima, H. Matsubayashi, T. Ohminato,
Y. Sudo, T. Tutui, K. Uhira, H. Yamasato, H. Ito, D. Legrand
J. Vol. Geothermal Res., 101, 129-154, 2000.
Abstract
We deployed a network of broadband seismometers for
one year around the Naka-dake 1-st crater of
Aso volcano in Kyushu, Japan, to reveal the mechanism of
long period tremors (LPTs) emitted from the volcano (Sassa 1935).
It is observed that LPTs with a dominant period of about 15\,s
are always emitted regardless the surface activity of the volcano.
A typical LPT has a short duration less than a minute and
its spectrum show mode peaks at 15s, 7.5s, 5s, and 3s.
The particle motion in the frequency band for the lowest two modes
at stations within a few kilometers from the crater is rectilinear,
pointing the direction of the crater.
A waveform semblance technique is devised to utilize
the rectilinearity of waveforms to locate sources of LPTs.
The LPT sources are located at depths of 1km to 1.5km
beneath the bottom of the crater.
When the volcano is explosively ejecting steam and mud,
on the other hand, a very long period ($\sim 100\,s$) displacement (VLPD)
polarized outward from the crater often precedes an
eruptive event by a few minutes.
A typical VLPD is accompanied with 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 approximately coincide with that of LPT.
On the basis of these observations,
a qualitative model is constructed for the hydrothermal system
beneath the Naka-dake 1-st crater.
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 of Aso volcano
using the observed long period seismic signals is also discussed.