Jökull - 01.01.2010, Blaðsíða 94

Ólafur Guðmundsson and Bryndís Brandsdóttir
other recognizable noise events have distorted it. This
applies to both spectral amplitude and spectral shape.
It also holds for individual components of motion (not
shown). The spectra generally contain most power
between 3 and 7 Hz. The stacked spectra over the
whole observational period are summarized in Fig-
ures 4 and 5 for individual stations. Individual sta-
tions exhibit somewhat different spectral shapes and
the three components at any given station also differ
in spectral shape. This suggests a complex wave field
probably from a distribution of sources with different
source spectra or scattering effects. Qualitatively, this
is not surprising. The distribution of geothermal ac-
tivity as depicted in Figure 1 is quite distributed in the
area. The terrain of the area is fairly rough on a small
length scale and it is probable that significant velocity
heterogeneity exists in the near surface.
LA0
0 2 4 6 8 10
LA1
LA2
LA3
LA4
LA5
LA6
LA7
LA8
LA9
a
m
p
li
tu
d
e
s
p
e
c
tr
a
l
d
e
n
s
it
y
frequency [Hz]
Figure 4. The average or stacked amplitude spectra
for the seismographs along line A.– Meðalróf (út-
slagsróf) hvers mælistaðar eftir línu A.
LB0
0 2 4 6 8 10
LB1
LB2
LB3
LB4
LB5
LB6
LB7
LB8
LB9
a
m
p
li
tu
d
e
s
p
e
c
tr
a
l
d
e
n
s
it
y
frequency [Hz]
Figure 5. The average or stacked amplitude spectra
for the seismographs along line B. – Meðalróf (út-
slagsróf) hvers mælistaðar eftir línu A.
Figures 2 to 5 reveal one curious feature in some
of the spectra. A very sharp spectral peak is evident at
the distant stations along line A at about 2.3 Hz (Fig-
ure 6). This peak is clear in the pictures for those sta-
tions where the geothermal noise is low in amplitude.
It is clearly discernible at 8 of 10 stations when we
look in detail. This peak is very stable in amplitude
for the whole 40 hour observational period. Some
other sharp spectral peaks are seen, albeit less clearly
or consistently, e.g. at ∼3 Hz at stations LA6 and LA7
and at ∼7 Hz at stations LA7 and LA8 towards the
end of the observational period. Along line B we see
94 JÖKULL No. 60