Jökull - 01.06.2000, Blaðsíða 60

Earthquakes in the Mýrdalsjökull area, Iceland, 1978–1985:
Seasonal correlation and connection with volcanoes
Páll Einarsson and Bryndís Brandsdóttir
Science Institute, University of Iceland, Haga, Hofsvallagötu 53, IS-107 Reykjavík
Abstract — The Mýrdalsjökull volcanic area of Iceland has shown persistent high seismic activity for more
than four decades. No volcanic activity has been detected during this time, however, with the exception of
probable small subglacial eruptions in 1955 and 1999. Seismic data of the period 1978-1985 were analysed.
Epicenters of this period fall into two distinct clusters under the Mýrdalsjökull glacier. One cluster is located
within the caldera of the Katla volcano, the other is centered in the Goðabunga area, beneath and west of
the western caldera rim. A majority of the earthquakes originate within the uppermost 5 km of the crust.
Earthquakes of both clusters have distinct characteristics of volcanic earthquakes, i.e. emergent P-waves and
poor S-waves. These characteristics are more pronounced for the Goðabunga cluster. The coincidence of
the Katla cluster with the caldera and an area of S-wave shadowing suggests that it is related to an active
magma chamber of the Katla volcano. Similarly we take the Goðabunga cluster as an indicator of the existence
of a second magma chamber. It is a matter of definition as to whether this chamber represents a separate
central volcano or another magma chamber of the Katla volcano. The two seismic clusters, together with the
Eyjafjallajökull volcano to the west and its fissure swarm, form a pronounced E-W structure and appear to be
mechanically coupled. Two historical eruptions of Eyjafjallajökull are temporally related to Katla eruptions.
Earthquakes under Goðabunga show a clear seasonal correlation. They are several times more likely to happen
in the latter half of the year than in the first. Two factors are likely to be effective here, the load of the glacier
ice and the pore pressure in the crust beneath the glacier. Both factors would tend to reduce the friction on fault
planes in the autumn. We argue that increased pore pressure produces a larger effect than the change in the ice
load when melt water seeps into the crust.
INTRODUCTION
Seismological methods can be used effectively to
study the internal structure and dynamics of volca-
noes. Hypocenters of earthquakes provide informa-
tion on the location of physical events, focal mecha-
nism studies give information on their nature. Seis-
mic waves that have propagated through the roots of
volcanoes carry information on the physical state of
the material there, e.g. the location of magma cham-
bers. Recent eruptions of Icelandic volcanoes such
as Krafla, Hekla, and Grímsvötn have been accompa-
nied by seismic activity, and seismic studies have im-
proved our understanding of the nature of their mag-
matic activity. Several of the Icelandic central volca-
noes, on the other hand, are the source of persistent
seismic activity in spite of no other apparent signs of
volcanic unrest (Einarsson, 1991). Among these are
Hengill, Torfajökull and Katla. The seismicity is usu-
ally localized, with epicentral clusters near the central
part of the volcano. Several physical processes can
cause these earthquakes, such as slow inflation or de-
flation of a shallow-level magma chamber, intrusion
of magma into the shallow crust, or thermal cracking
due to heating or cooling of brittle crustal material.
In this paper we describe the seismicity of the
Katla volcano and its vicinity during the period 1978-
1985. Excluding the unusually active years 1976-
1977, the study period represents characteristic seis-
mic activity during the last several decades.
JÖKULL No. 49 59