Jökull - 01.12.1978, Page 102

of the reservoir and increasing flow of spring
water into the Sigalda canyon.
At a reservoir level lower than 492 m a.s.l.
negligible water is lost from the drainage area
as up to that point a major part of the leakage
water reappears as spring water in the canyon
downstream of the dam.
THE MAIN RESULTS OF
SEISMOGRAPHIC OBSERVATIONS OF
THE KRAFLA EVENTS IN NE-ICELAND
Páll Einarsson,
Science Institute, University of Iceland
The current events in the Krafla caldera
and the associated fault swarm are
accompanied by considerable seismic activity.
Both earthquakes and volcanic tremor have
been observed. Inflation of the caldera region
is associated with earthquakes at the depth of
0—3 km in the area of maximum inflation.
Rapid deflation events are accompanied by
volcanic tremor. The amplitude of the tremor
is correlated with the rate of deflation. An
earthquake swarm starts in the Krafla fault
swarm a few hours after deflation begins. The
distance of the earthquake swarm from the
caldera may be as large as 50 km. The seismic
activity is consistent with the interpretation,
that the inflation is caused by magma that
accumulates at the depth of 3 km under the
caldera floor and is injected horizontally into
the fault swarm during the deflation events.
Prominent S-wave shadows exist in the
Krafla area. The shadows are caused by an
attenuating body in the center of the caldera,
probably some kind of a magma chamber.
The bulk of the body is located at less than 7
km depth. The chamber appears to be divided
into two lobes, at least near the top.
EARTHQUAKES IN THE
VESTMANNAEYJAR AND
MÝRDALSJÖKULL VOLCANIC AREAS
IN S-ICELAND
Páll Einarsson,
Science Institute, University of Iceland
The new, dense seismograph network in
Iceland offers new possibilities to study vol-
canic activity. A large swarm of small earth-
quakes occurred about 30 hours before an
eruption broke out on Heimaey in Vest-
mannaeyjar in January 1973. During the
eruption earthquakes occurred under Heima-
ey at the depth of 15—25 km. This seismic
activity appeared to be correlated with the
rate of lava extrusion at the surface. Several
lines of evidence suggest that the source of the
magma is located within the volume
delineated by the earthquake hypocenters.
Seismic activity has been very low under
Heimaey after the eruption stopped. The
Vestmannaeyjar area does not seem to be in
equilibrium, however. Small earthquakes
persist in an area about 15 km SW of Heimaey
and 5 km NE of Surtsey.
Earthquake activity in the Mýrdalsjökull
area was abnormally high during the period
Sept. 1976—Sept. 1977. Earthquakes that
have been located so far occurred within two
separate areas. One area is near the subglacial
volcano Katla, the other area is located 10—
15 km to the west of the first one. In both areas
the hypocenters are distributed between
depths of 0—30 km. These two chimney-like
features may be feeding channels for magma.
RAISED SHORELINES IN SUDURNES,
REYKJANES PENINSULA
Sigmundur Einarsson,
National Energy Authority
There are three major shorelines in the area,
the heights of which are (at Vogastapi) 70 m,
100 JÖKULL 28. ÁR