Jökull - 01.12.1978, Page 101

by an E—W trending zone of weakness in the
lower crust.
Large earthquakes (M < 7.5) occur either as
individual main shocks or in sequences, where
a large earthquake in the eastern part of the
epicentral zone is followed by large but smal-
ler earthquakes which migrate westwards
along the seismic zone.
Surface fractures strike N or NE and indi-
cate right lateral movement. The elongated
destruction zones suggest rather shallow
origin of earthquakes, not deeper than 15 km.
Maximum intensity in the epicentral region of
the largest historic earthquakes is estimated
XI on the MM scale. Intensity X may occur
anywhere within the epicentral zone.
Maximum stress is horizontal and has a
direction between N30E and N70E.
Minimum stress is also horizontal, transverse
to the volcanic zones on both sides. This stress
system is present throughout the seismic zone,
even in close proximity to the volcanic zones.
Inside the volcanic zones, however, maximum
stress becomes vertical and results in normal
faulting.
BOILING GEOTHERMAL RESERVOIRS
Sveinbjörn Björnsson,
Science Institute, University of Iceland
In boiling geothermal reservoirs rock tem-
perature is determined by the boiling temper-
ature of the reservoir fluid and follows the
boiling curve with depth. These reservoirs
have generally been looked upon as hot water
systems close to boiling but in reality they are
two—phase systems of water and steam or
even three phase systems of water, steam and
gas.
After boiling is initiated in a rising column
of geothermal fluid, gas and steam ascend
faster than the water. The volume fraction of
steam remains small in the lower part of the
column but may exceed 70% at higher levels
where impermeable rocks impede steam flow
to the surface. Steam condenses under the
impermeable rocks and the condensed water
drains downward along narrower channels
against the ascending steam which dominates
the wider permeable channels in the reservoir
rock. This type of flow has been named “one
dimensional convection”.
SIGALDA HYDRO-POWER SITE,
GROUNDWATER PATTERN AND
IMPOUNDING OF RESERVOIR
Davíd Egilsson,
National Energy Authority
In the Sigalda area the postglacial Tungnaá
lavas flank the móberg ridges. The scoriaceous
zones at the top and bottom of each lava flow
constitute good aquifers. The groundwater
basin upstream from Sigalda is fed by two
main streams, one seeping out of the móberg
bedrock from the east and the other from the
southeast. The reservoir basin is mostly cov-
ered with deposits of lacustrine clays from a
previously existing lake (Krókslón). At the
eastern edge of the clay deposits, there is a
horizon of springs fed from the underlying
móberg. Yet, a part of the seeping water from
the east is suppressed underneath the clayey
sediments and conducted along the interbeds
and scoria of the lavas THb, THC, and TH(
into the lava field west of the reservoir area.
There the two streams are intermixed and at
least partly appear as springs in the Sigalda
canyon. The groundwater pattern described
above is interrupted by the impounding of the
reservoir as it impedes inflow of groundwater
into the basin thereby causing backwater
effect in the móberg. Leakage of reservoir
water out of the basin occurs primarily
through the margin of the THh lava and parts
of the river bottom and is conveyed along
scoriaceous zones and interbeds of the lavas
(such as tephra layer H4). Combined these
factors cause rising of groundwater level west
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