Jökull - 01.06.2000, Page 3

Ground deformation at Katla:
Results of precision levellings 1967-1995
Eysteinn Tryggvason
Ásgarðsvegi 10, 640 Húsavík
Abstract — The volcano Katla in South Iceland is known to have erupted about twice each century in historic
time. The last large eruption occurred in 1918 but a minor eruption probably occurred in 1955. Three optical
levelling tilt stations were constructecl in 1967 in the vicinity of Katla, to detect and observe any ground
deformation which mightprecede the next Katla eruption. Two oftliese stations were located near the east edge
ofthe Mýrdalsjökull glacier, and one near the south edge, all at a distance of 10 km or more from the volcanic
vent which erupted in 1918. These stations were levelled once eacli year from 1967 to 1973. No observations
were madefrom 1973 to 1986, when the levellings were resumed. The results ofthese repeated levellings at the
optical levelling tilt stations near Katla are that ground tilt caused by annual variation of the glacier load is
observed at the stations near the east edge ofthe glacier. Ground tilt related to the volcano Katla has not been
observed, although uplift towards west or north-west at a rate of 0.1 prad per year at the station Kötlukriki is
vaguely suggested.
INTRODUCTION
The volcano Katla is covered by the glacier Mýrdals-
jökull in South Iceland. During eruptions of Katla,
voluminous floods (jökulhlaup) of melt water flow
over the Mýrdalssandur plain towards the south coast
of Iceland. Eruptions have occurred twice each cent-
ury during the last several hundred years with the
last eruption in 1918, and earlier eruptions in 1860,
1823,1755,1721,1660,1625, and 1580 (Thoroddsen,
1925). Prior to 1580, eruption frequency was probably
similar, but historic records are incomplete. A flash
flood on June 25, 1955 was considered to be caused
by a small sub-glacial eruption which did not melt its
way to the glacier surface (Tryggvason, 1960).
In addition to the jökulhlaup which inundate
most of the Mýrdalssandur plain, each Katla erupti-
on causes serious, sometimes disastrous, ash fall in
the areas surrounding the volcano. The floods, which
arrive at about the same time as visible eruption, flow
over the main route of land transport and travel in the
area. The floods begin very suddenly, causing danger
to those on the road across Mýrdalssandur, especially
if eruption begins during weather of limited visibility.
Description of earlier eruptions of Katla mention
rather strong earthquakes a few hours before the visi-
ble eruption, but otherwise, no signs of the com-
ing eruption are known. However, it is considered
as certain that some underground processes precede
the eruptions, probably lasting for months or years.
If some observations or measurements can be desig-
ned to detect these assumed premonitory processes,
prediction of coming eruptions could be feasible. The
most probable subsurface process to precede erupti-
ons of Katla is pressure increase in a magma chamber
beneth the volcano.
If pressure increases in some volume within an el-
astic solid, the surface of this solid will be displaced in
direction away from this volume. For a source, small
relative to its depth, the flat surface of this elastic
solid will be uplifted according to the Mogi equation
(Mogi, 1958), which says that the surface particles
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