Jökull - 01.01.2005, Side 88

Heidi Soosalu and Páll Einarsson
The 1991 and 2000 Hekla eruptions are the first
for which digital seismic data exist, facilitating quan-
titative research on patterns of the eruption-related
seismicity. Eruptions in 1970 and 1980–1981 were
similar in size and behaviour (Einarsson and Björns-
son 1976; Grönvold et al. 1983; Brandsdóttir and Ein-
arsson 1992). Qualitative research based on analogue
seismograms conducted on these eruptions point to
very similar seismic characteristics. In order to make
a comparison of seismicity during eruptive and non-
eruptive times we conducted a study of the back-
ground seismicity at Hekla and its vicinity during a
non-eruptive period in 1991-1996 (Soosalu and Ein-
arsson 1997). Together with Hekla, the study area
(63◦42’–64◦18’N and 18◦30’–20◦12’W) covered the
neighbouring volcanoes, Torfajökull to the east and
Vatnafjöll to the south, and the eastern end of the
South Iceland seismic zone, a transform zone of the
mid-Atlantic plate boundary (Figure 1).
In this paper we draw general conclusions about
the seismic nature of the Hekla volcano and its im-
mediate surroundings, based on digital and analogue
data from June 1990 until mid-August 2005. We de-
fine the characteristics related to the relatively small
Hekla eruptions that have been occurring during re-
cent decades. We also describe the nature of seismic-
ity at Hekla during non-eruptive times, which appears
to be unrelated to Hekla as a volcano but rather fol-
lows the pattern of the transform zone to the west.
GEOLOGICAL SETTING OF THE
HEKLA AREA
The central volcano Hekla in south Iceland is a ridge
elongated in the ENE-WSW direction, formed by re-
peated eruptions and reaching an altitude of 1500 m
above sea level. Recent eruptions have had a tendency
to take place along a radial fracture system, as well
as along the main Hekla fissure that splits the vol-
cano lengthwise. Hekla’s fissure swarm extends NE
and SW of the summit. The central volcanoes next to
Hekla, also discussed in this study, are Torfajökull in
the east and Vatnafjöll in the south (Figure 2).
Hekla is located in a tectonically complex area, at
the junction between the transform-like South Iceland
seismic zone and the Eastern volcanic zone, the east
branch of the chain of active volcanic systems cross-
ing the middle of Iceland (Figure 1). North of Hekla
and Torfajökull, the volcanic zone is characterized by
rifting activity, whereas to the south it has the nature
of a non-rifting flank zone. At the location of Torfa-
jökull, rifting is propagating to the southwest (Óskars-
son et al. 1982).
Hekla is not a typical rift zone volcano, due to
its tectonic setting and peculiar petrology. The prod-
ucts of the Hekla volcanic system range from basalts
through basaltic andesites to dacites and rhyolites
(Jakobsson 1979). The more acidic products are is-
sued from the volcanic edifice, while the basaltic
products come from the fissure swarm. Petrologically,
Hekla is more akin to the group of volcanoes in the
volcanic flank zone to the south-east.
Hekla is one of the most active volcanoes in Ice-
land and has erupted at least 18 times since Iceland
was colonized in the ninth century (Guðmundsson
et al. 1992). Since the major eruption of 1104 AD
until the 1947–1948 eruption, the activity was char-
acterized by relatively large eruptions about twice a
century (Þórarinsson 1967). Within the last decades
Hekla has changed its eruptive pattern. Smaller erup-
tions with volumes of about 0.2–0.3 km3 have oc-
curred about every ten years, in 1970, 1980–1981,
1991, and 2000.
Torfajökull is a major rhyolitic complexwith a 12-
km-diameter caldera (Sæmundsson 1972, 1982), an
outstanding high-temperature geothermal field (Mc-
Garvie 1984), and fissure swarms stretching both NE
and SW of the central volcano. The latest eruption
in the Torfajökull area occurred at the end of the fif-
teenth century (Larsen 1984). The Vatnafjöll central
volcano, south of Hekla, does not have any caldera
or geothermal areas. The fissure swarm of Vatnafjöll
is elongated in the NE-SW direction, parallel to the
fissure swarm of Hekla. No eruptions are known to
have occurred in Vatnafjöll during the last 1100 years
(Bjarnason and Einarsson 1991).
The South Iceland seismic zone is a 70–80 km
long and 10–15 km wide zone in the South Iceland
lowland. It acts as an E-W transform, but is char-
acterized by abundant seismicity on N-S right-lateral
88 JÖKULL No. 55