Jökull - 01.01.2020, Síða 66

The analog seismogram archives of Iceland
Figure 4. Recorders at the Science Institute of telemetered signals from seismographs in Central Iceland,
Grímsfjall, Vonarskarð, Þúfuver, Jökulheimar, Sporðalda, Ljótipollur, and Litla-Hekla. – Merki frá skjálfta-
mælum á Miðhálendinu var sent til Reykjavíkur þar sem skráning fór fram á Raunvísindastofnun Háskólans.
Photo:/Ljósm. Nick Weir.
movements of the ground. Thus, any disturbance of
the ground could be timed by measuring its distance
from the nearest time signal. On most seismographs
a minute mark was considered sufficient, but on the
Landsnet seismograph the second marks were used
as well, in order to increase the timing accuracy. It
was considered important to eliminate the need for
clock corrections, so continuous radio time was used
directly on the seismogram. The 60 kHz time signal
from Rugby in England was used most of the time of
operation. Another time signal was used during lim-
ited periods, a low-frequency navigation signal for
submarines, called Omega, giving only minute marks.
The Rugby time signal was coded after June 1977,
which came in very handy when reading the seismo-
grams. The seconds marks came in two sizes, signi-
fying 0 and 1 in a binary code. The date and time
were indicated every minute of the seismogram. This
made an easily readable pattern on the seismograms
because each line was 10 minutes long, i.e., each hour
contained six lines. If the time marks were properly
aligned the last digit of the time would be the same in
all vertical lines across the seismogram. Reading the
rest of the code requires a bit of practice but makes
the life of the record reader a lot easier. Unnecessary
zeros in the front are dropped. So:
0 0000 or 000 or 00
1 0001 001 01
2 0010 010 10
3 0011 011 11
4 0100 100
5 0101 101
6 0110 110
7 0111 111
8 1000
9 1001
Second marks number 17–24 tell the year, no. 25–
29 the month, no. 30–35 the day of the month, no.
36–38 the day of the week (Monday no. 1), no. 39-44
the hour (in England), no. 45–51 the minute beginning
at the next minute mark. The date and time, Monday
the 4th of July 1977 at 1635 would then appear:
0111 0111 0 0111 00 0100 001 01 0110 011 0101
7 7 0 7 0 4 1 1 6 3 5
JÖKULL No. 70, 2020 63