Jökull - 01.12.1977, Side 62

Gravity Changes in Connection with the Volcanic
and Earthquake Activity in Northern Iceland 1975/1976
W. TORGE AND H. DREWES,
INSTITUTE FOR THEORETICAL GEODESY, TECHNOLOGICAL UNIVERSITY OF HANNOVER,
NIENBURGER STR. 6, D 3000 HANNOVER, REPUBLIC OF GERMANY
ABSTRACT
In December 1975, a fissure eruption occurred
at Leirhnjúkur, northern Iceland, whicli was
followed by earthquake activity in the region
north of Lake Mývatn. In orcler to detect asso-
ciated gravity changes along the high precision
gravity profile Akureyri—Mývatn—Grímsstadir,
being established in 1938 and reobserved in
1965, 1970 and 1975 for the investigation of
secular gravity variations, 48 stations of this pro-
file have been reoccupied in 1976. This gravity
survey, covering the region between Másvatn
ancl Jökulsá á Fjöllum, and accompanied by a
technical levelling, has been supplemented by
the reobservation of about 50 gravity stations in
the area between Mývatn—Grímsstadir and Ax-
arfjördur, established between 1965 and 1970.
As one result, a gravity increase (1976—1975)
of 0.04 mgal lias been found at Námaskard, 8
km soutli of Leirhnjúkur, being correlated with
a ground subsidence of 0.3 m. Gravity variations
increase to + a few tentlxs of a mgal, when
approaching Krafla. Significant changes (—0.1
. . . +0.1 mgal) also occurred at Vestursandur
near the coast line and in tlie southward ad-
joining fissure zone, while in the Gjástykki
fissure system a relative gravity increase of 0.2
mgal has been observed.
INTRODUCTION
Gravity variations in connection with volcan-
ism and earthquakes represent one information
source about the mechanism of these events.
From the numerous research results obtained in
this field, we mention the gravity variations ob-
served at the 1964 Alaska earthquake (Barnes
60 JÖKULL 27. ÁR
1966), during the 1965—1968 Matsushiro earth-
quake swarms (Fujita and Fujii 1973) and be-
fore the 1971 San Fernando earthquake (Strange
and Carroll 1976). The variations found are at
the order of some 0.01 to a few 0.1 mgal, and
from the comparison with levelling results, free-
air- and Bouguer-type relationsships have been
derived.
In the active young volcanic zone of northern
Iceland, gravity measurements have been carried
out since 1938, in order to detect eventual se-
cular gravity variations, the last survey taking
place in 1975 (see section 2). In December 1975,
a fissure eruption occurred at Leirhnjúkur in
the Krafla area, which was followed by earth-
quake activity in the region between Mývatn
and Raufarhöfn. In some parts of this region,
vertical displacements have been observed. West
of Krafla, the subsidence reached more than 2m,
the following compensation process being care-
fully monitored by the Icelandic National En-
ergy Authority, Orkustofnun (personal comm-
unication from Dr. Gudmundur Pálmason,
Reykjavík).
As the distance from Leirhnjúkur to the ex-
isting high precision gravity profile Akureyri—
Grímsstadir—Hof is only 8 km, the December
event might influence future gravity measure-
ments and evaluations, being directed to the
investigation of secular gravity variations, by a
discontinuous gravity cliange. Therefore it was
decided to repeat the gravity observations itt the
profile between Másvatn and Jökulsá á Fjöllum,
and to reoccupy some of the gravity stations in
the region between Mývatn/Grímsstadir and
Axarfjördur. The comparison witli the earlier
observations should give an idea about the