We deployed a network of broadband seismometers for one year around the Nak
a-dake first crater of Aso volcano in Kyushu, Japan, to reveal the mechanis
m of long period tremors (LPTs) emitted from the volcano. It is observed th
at 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 sta
tions within a few kilometers from the crater is rectilinear, pointing in t
he direction of the crater. A waveform semblance technique to locate source
s of LPTs is devised to utilize the rectilinearity of waveforms. The LPT so
urces are located at depths of 1-1.5 km beneath the bottom of the crater. W
hen the volcano is explosively ejecting steam and mud, on the other hand, a
very long period (similar to 100 s) displacement (VLPD) polarized outward
from the crater often precedes an eruptive event by a few minutes. A typica
l VLPD is accompanied by a few long period pulses, first positively polariz
ed and concurrent with the onset of VLPD, then negatively polarized just be
fore the eruption. The source of VLPDs is inferred to coincide approximatel
y with that of LPT. On the basis of these observations, a qualitative model
is constructed for the hydrothermal system beneath the Naka-dake first cra
ter. An explanation for the unusually long period nature of the LPT is disc
ussed in terms of a class of slow waves, which exist in solid-liquid two-ph
ase systems. A possibility of realtime monitoring at Aso volcano using the
observed long period seismic signals is also discussed. (C) 2000 Elsevier S
cience B.V. All rights reserved.