Jökull - 01.01.2019, Blaðsíða 49

Einarsson
that precursor times determined from felt earthquakes
are shorter than the times found from instrumentally
recorded earthquakes. The precursory swarms of-
ten begin with small earthquakes and then gradually
culminate (Einarsson, 2018). Remote volcanoes like
Bárðarbunga and Grímsvötn are not considered. In
fact, Bárðarbunga was not known as an active vol-
cano until it was revealed on satellite images of Ice-
land in the early seventies (Thórarinsson et al., 1973).
Major rifting episodes in the last few centuries oc-
curred in the Northern and Eastern Volcanic Zones,
mostly in remote areas, and are therefore difficult
to compare with the instrumentally recorded rifting
episodes of the last decades, at Krafla and Bárðar-
bunga. There were felt earthquakes during the Mý-
vatn Fires at Krafla in 1724–1746, Laki eruption of the
Grímsvötn volcanic system in 1783–1784, and Askja-
Sveinagjá eruptions of 1875–1876. These are consis-
tent with repeated injection of laterally propagating
dikes in connection with these eruptions, similar to
the recent events.
A good part of the historical data are due to two
volcanoes, Katla and Hekla. The reasons are twofold,
both are highly active, have had large eruptions once
or twice per century throughout most of the histori-
cal time, and both are close to populated areas where
their activity has had a large impact. Fairly detailed
contemporary reports are available for Katla eruptions
since 1625. Comparison with instrumental data is
impossible, however, because no large eruption has
occurred within the period of instrumental observa-
tions. The historical data are therefore the only data
available for this hazardous volcano. The hazards
are mainly of two kinds, both related to the exten-
sive glacial cover of the volcano, ash fall-out from the
phreatic eruption column, and glacial outburst floods,
jökulhlaups, caused by the melting of the glacier.
Hazards due to electrical activity within the column
and lava flows from eruption sites outside the glacier
are also known. Seismic activity is documented in
association with the beginning of all seven eruptions
since 1625 (Table 1). The precursor time, i.e. time
from the first felt earthquake until an eruption column
is seen over the glacier is quite variable, mostly from
one (1660) to nine hours (1860). In one case, in 1625,
the time seems to have been even shorter. The „jök-
ulhlaup time“, i.e. time from the first earthquake until
a flood wave on the alluvial plane reaches the inhab-
ited area, is also quite variable, from about two hours
(1918) to more than 12 hours (1755). The size of
the precursory earthquakes is of considerable interest.
The reported intensities range from weak to strong,
even to the extent of arousing fear of houses col-
lapsing. Earthquakes are fairly frequent in the Katla
area (e.g., Einarsson, 1991a; Einarsson and Brands-
dóttir, 2000) and judging from the experience of re-
cent decades the magnitude of an event in the caldera
region has to exceed 4 and even 5 to be felt with this
intensity in the inhabited areas south of the volcano.
The historical data for Hekla give a very differ-
ent picture from that of Katla. Earthquakes were felt
in the areas near the volcano in connection with the
beginning of ten eruptions during the period 1510–
1947 (Table 2). The eruptions appear to fall into two
classes with respect to seismic behaviour. Eruptions
in the central edifice of the volcano (1510, 1597, 1693,
1845, 1947) appear to be generally accompanied by
weak to moderate seismicity with short precursor
times, less than an hour, possibly minutes, whereas
eruptions outside the main edifice (1554, 1725, 1878,
1913) are preceded by rather strong seismicity and
precursor times of a few hours. The eruption of 1766–
1768 is an exception. The eruption was mainly from
the Hekla edifice but it was preceded by rather strong
seismic activity for several hours.
The short precursor times for central eruptions of
Hekla are in accord with instrumental observations of
Hekla eruptions since 1970 (Einarsson, 2018) that re-
veal unusually short times between the first detected
earthquakes and the beginning of eruptive activity
(23–79 minutes). In the eruptions of 1510, 1597, and
1693 it appears as if the first felt earthquake happened
simultaneously with the opening phase of the erup-
tion. In 1947 the first felt earthquake happened nine
minutes after the eruption started.
The difference between Hekla and Katla is not
only expressed in the short-term seismic behaviour.
The long-term behaviour is also very different. Katla
has a high level of background seismicity, earthquakes
have been recorded in the volcano almost every day
48 JÖKULL No. 69, 2019