Jökull - 01.12.1989, Page 38

for calibration lines 1 and 2, respectively. The stan-
dard deviation of the averages was 6 mm for line 1
(11 values) and 5 mm for line 2 (5 values). Inaccu-
racy of the instrument quoted by the manufacturer is
5 mm + 1 mm/km, which corresponds quite well
with the standard deviation of the averages for the
shorter line. For the longer line the quoted inaccu-
racy looks a bit pessimistic if compared with our
standard deviation. If a line length of 7 km can be
measured with a standard deviation of 7 mm (by tak-
ing the average of several readings), then the change
in the length between two measurement campaigns
can be measured with a standard deviation of
1^2 = 10 mm. We point out that this standard devi-
ation includes the effect of variable weather condi-
tions, for example errors induced by only sampling
atmospheric pressure and temperature at the end
points of the lines.We conclude that, for our range of
line lengths, measured changes must exceed 20-
25 mm to be interpreted as the result of tectonic
movements. This corresponds to 1.8 ppm in strain,
or better, for most of our lines.
RESULTS
The results of the remeasurements of the Ölfus,
Flói and Holt networks are presented in Figs. 5, 6,
and 7. By comparing these results visually to the
results from the calibration lines (Figs. 3 and 4) it is
clear that the variability of the lines within the
active zone is larger. It is natural to interpret this
greater variability as a result of tectonic movements.
This is supported by the statistics of the data,
whether one looks at the standard deviation of the
readings for each length determination or takes the
standard deviation of the calibration lines as being
representative of the measurements. The standard
deviations of the differences in the following para-
graphs are found as the square root of the sum of the
squares of the two individual standard deviations
shown as vertical bars in FigS. 5-7.
The Ölfus lines show changes, but they are irrégu-
lar in space and cannot be interpreted as the result of
homogeneous strain across the region. The changes
are largest on line • 1-3 ((38 ± standard deviation
16) mm), line 1-4 ((32+14) mm) and line 1-5
((54±15) mm). These are all extensions, as expected
for a left-lateral motion along the zone. Lines 2-5
and 3-5, however, do not show the expected contrac-
tion. Other changes are insigniíicant as well.
All three lines of the Flói net are extended or
show insignificant change, indicating areal exten-
sional strain. The longest line, Flói 1-2, with NW-SE
orientation shows an insignificant extension
((18±24)mm). The line Flói 1-3, almost N-S
oriented, shows 25 mm extension (±8 mm)
corresponding in strain to 3.2 ppm. The last line,
W-E oriented, shows 17 mm extension (±9 mm)
which corresponds to 2.4 ppm. This is not consistent
with the expected horizontal shear in the seismic
zone.
The Flolt net has been measured three times. Here
the changes show some regularity in space, but the
time behaviour is irregular. The lines Holt 1-2, Holt
1-3, and Holt 1-4 are all directed NE-SW and should
contract. Two of these lines, Holt 1-2 and Holt 1-3
where measured in 1979 and both lines show con-
tractions in the time period 1979-1983 as expected,
but they expand between the years 1983-1984. The
line Holt 1-2 contracts 27 mm (±23 mm) between
the years 1979-1983 and expands 45 mm (±23 mm)
between 1983 and 1984. In total the line Holt 1-2
expands 18 mm (±2 mm) over the whole time period
1979-1984 resulting in 3.2 ppm. in strain. The line
1-3 is shortened 44 mm (±11 mm) in the time inter-
val 1979-1983, but the movement is reversed in
1983-1984, 39 mm (±11 mm). In total, line 1-3
shows no significant change between the years 1979
and 1984. The lines Holt 2-3, Holt 2-4, and Holt 3-4
trend E-W and show little length changes, except
possibly the line Holt 2-3 for the time period 1983-
1984.
CONCLUSIONS AND DISCUSSION
Remeasurements of 19 geodimeter lines in the
active transform zone of South Iceland show that
measureable distance changes occured during the
period 1977-1984. The changes are irregular, both in
time and space, and cannot be interpreted as the
result of a slow and continuous strain build-up as is
sometimes assumed to occur along plate boundaries.
36 JÖKULL, No. 39, 1989