Jökull - 01.01.2005, Side 101

Seismic characteristics of the Hekla volcano
terial can provide constraints for a size estimate of a
magma chamber. It is generally assumed that only a
fraction of the contents of a reservoir is drained during
an eruption, until the pressure-drop inside the cham-
ber leads to cessation of the eruption. In a theoreti-
cal study on explosive eruptions (andesitic to rhyolitic
magma), Bower and Woods (1998) estimate the max-
imum amount of erupted material to be ∼10% of the
total contents for a shallow chamber and only ∼0.1–
1.0% for a deep chamber. The Hekla eruptions in
1970, 1980–1981, 1991 and 2000 produced lava and
tephra of about 0.2 km3 (Grönvold et al. 1983; Guð-
mundsson et al. 1992; Höskuldsson et al. submitted).
With an assumption of 10%-drainage this suggests a
magma chamber of 2–3 km3, which should be large
enough to be detected by our method.
The Hekla magma chamber may be a network of
interconnected patches of molten material, rather than
a simple voluminous structure. However, geochem-
ical analysis of Hekla lavas shows that the composi-
tion of products during the course of an eruption is
quite uniform (Grönvold et al. 1983; Karl Grönvold
2003, pers. comm.), thus not supporting a complicated
magma chamber structure.
The quick onset of an eruption fed from great
depth sounds problematic and rather unrealistic.
Strain signals show that the dyke started propagating
half an hour before the onset of the Hekla eruptions
in 1991 and 2000 (Linde et al. 1993; Ágústsson et al.
2000). If the magma travels 14 km ormore during half
an hour, it requires at least a velocity of 7.8 m/s for
the ascending magma. Sacks and Linde (2001; Sel-
wyn Sacks 2001, pers.comm.) suggest that the rapid
start of a Hekla eruption is the result of degassing. The
gas phase is released from the magma inside the reser-
voir, and accumulates in the upper part of the reser-
voir, which forces the level of the liquid magma to
sink. The pressure in the magma chamber increases
due to the ascent of gas bubbles until an eruption
starts, first extruding the gases from the upper part of
the chamber. Because the gas phase erupts first, the
eruption can easily commence more rapidly than an
eruption starting with a lava flow. The gas release ex-
planation is in harmony with the observation that the
Hekla eruptions begin with an explosive phase emit-
ting gases and tephra, and subsequently calm down to
lava effusion (Grönvold et al. 1983; Guðmundsson et
al. 1992; Höskuldsson et al. submitted).
Volcanic tremor during the two eruptions of 1991
and 2000 was very similar. It started simultaneously
with the eruption and had a stable frequency-band
during the first hours, although the eruptive activity
and the amplitude of the tremor varied. The charac-
teristic spectral band was about 0.5–1.5 Hz and the
maximum peaks were around 0.7–0.9 Hz. This is at
the lower end of the frequencies generally observed at
active volcanoes in the world, mainly 0.1–8 Hz (Kon-
stantinou and Schlindwein 2002).
A number of possible sources for volcanic tremor
have been proposed in the literature. Somemodels ex-
plain the tremor as the result of resonant effects pro-
duced by the geometry of volcanic conduits. Turbu-
lent motion in the vapour-gas-magma mixture makes
the volcanic pipes oscillate (e.g. Seidl et al. 1981; Fer-
rick et al. 1982), and the frequency content of the
tremor may vary with the length of the conduit. The
characteristic low frequencies of Hekla tremor could
indicate that the magma channel of Hekla is very
large, i.e. the conduit would extend to a considerable
depth and the magma chamber be at a deep level. Al-
though the degassing-related origin of the tremor is
shallow, the resulting vibration can occur in the long
channel and produce the characteristic low frequen-
cies. Other models suggest that volcanic tremor is
produced by vibrations of tensile, fluid-filled, jerkily
or suddenly opening cracks (Aki et al. 1977; Chouet
1981, 1985). In these models the excess pressure and
degassing in the fluid generates the trembling. Ac-
cording to Chouet (1992) volcanic tremor is the re-
sponse of the tremor-generating system to sustained
bubble oscillations in the fluid. Julian (1994) explains
the cause of the volcanic tremor to be nonlinear exci-
tation by fluid flow, analogous to the excitation mech-
anism of musical wind instruments.
Volcanic tremor often begins prior to the actual
surface outbreak of an eruption and may extend be-
yond the duration of surface activity (e.g. Chouet
1981; Montalto et al. 1995). This was not the case at
Hekla, where the tremor started at the same time as the
eruption and also terminated simultaneously with the
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