Abstract
The seismic structure of the stratovolcano Merapi (Java, Indonesia)
was studied using an active seismic experiment. Three 3 km long seismic
profiles each consisting of up to 30 three-component seismometers
with an interstation distance of 100 m were built up in an altitude
range between 1000 and 2000 m above sea level. The detailed study
of the seismic properties of the propagation media in active volcanic
regions is important to understand the natural seismic signals used
for eruption forecasting. The seismic experiment at Merapi therefore
concentrates on the heterogeneous structure within a radius of 5
km from the active dome, where the sources of most of the natural
volcanic seismic events are located. The cone of Merapi volcano consists
of different materials changing on a small scale due to the layering
of eruptive material. Additionally, the topography of the erosion
valleys leads to an irregular deposition, which cannot be described
by a simple 1-D layering. These inhomogeneities have a strong influence
on seismic signals. The direct P and S waves are attenuated quickly
and show only small amplitudes on seismograms. The energy lost from
the direct waves, however, is not changed into heat but scattered
and can be observed as seismic coda following the direct waves. The
observed seismograms show a spindle-like amplitude increase after
the direct P phase. This shape of the envelope can be explained by
the diffusion model. According to this model there are so many strong
inhomogeneities that the direct wave can be neglected and all energy
is concentrated in multiple scattered waves. Besides the envelope,
the coherence and polarization properties of the wavefield also indicate
strong scattering. Only the first onset shows coherence over a station
spacing of 100 m, whereas the late phases carrying the major part
of the energy are mainly incoherent. The horizontal components of
the seismograms have larger amplitudes than the vertical component,
but within the horizontal plane the polarization is almost arbitrary,
corresponding to waves arriving from scatterers located arbitrarily
in space. As a result of the inversion using the diffusion model
we obtain values of the S-wave scattering attenuation coefficient,
etas, and the S-wave intrinsic absorption coefficient, etai. In the
frequency range of 4-20 Hz used in this study the scattering attenuation
is at least one order of magnitude larger than the intrinsic absorption
(etas>>etai). The mean free path of S waves is as low as 100 m (etas-1\~100
m). The scattering coefficient is independent of frequency (etas-f0.0),
whereas the coefficient of intrinsic attenuation increases with increasing
frequency (etai-f1.6). The natural seismic signals at Merapi volcano
show similar characteristics to the artificial shots. The first onsets
have only small amplitudes and the energy maximum arrives delayed
compared to the direct waves. Therefore, these signals appear to
be strongly affected by multiple scattering also.
Links and resources
Tags