Jökull - 01.01.2020, Side 65

Einarsson and Jakobsson
Figure 3. Left: The pen-motor and translation-motor assemblages of the Landsnet seismograph. The pen was
made at the SI from a drawing pen tip attached to a syringe pipe. A special recording ink was used, available
in different colors, black, red, green and blue. Right: The rotation-motor assemblage of the Landsnet seismo-
graph. It offered three different speeds of rotation. The most used time resolution was 90 mm per minute. –
Vinstri mynd: Pennamótor og færslumótor Landsnestmælisins. Pennarnir voru smíðaðir á Raunvísindastofnun
úr oddi af teiknipenna og pípu úr sprautunál. Sérstakt síritablek var notað, sem var fáanlegt í svörtum, rauðum,
grænum og bláum lit. Hægri mynd: Sérstakur mótor sneri tromlunni og mátti stilla á þrjá mismunandi hraða.
Algengast var að nota hraða sem gaf tímaupplausn 90 mm/mínútu.
mic Zone. The final phase of the original project was
then finished by installing stations around the central
highland in 1977. Several stations were then added
in the following years when opportunities arose, e.g.,
around Hengill in 1979. The permanent and semi-
permanent stations are listed in Table 1, their names,
locations, and operation times. The table will be up-
dated as the scanning project progresses. Figure 5
shows a map of the network stations at the peak of
its operation.
The sheets from the Landsnet stations were sent
to the Science Institute where a preliminary anal-
ysis was carried out. The results were distributed
by Skjálftabréf, a pamphlet containing approximate
locations and other information, now accessible at
https://www.jardvis.hi.is/skjalftabref. The first de-
tailed epicentral maps of earthquakes based on the
results appeared in several papers in the eighties and
nineties (e.g., Einarsson and Björnsson, 1987; Einars-
son, 1978, 1991; Brandsdóttir and Einarsson, 1979;
Einarsson and Brandsdóttir, 1980; Foulger and Ein-
arsson, 1980; Björnsson and Einarsson, 1990).
A new digital seismograph system became oper-
ational in South Iceland in late 1990, called the SIL-
system. This system was expanded in the following
years to cover the whole of Iceland. The operation of
the old analog network was then gradually phased out
as the new, sophisticated network expanded. The last
analog seismograph was taken out of service in 2010.
The seismograms and time signal
All the analog seismograms are written on drum
recorder, i.e., a paper sheet was attached to a rotat-
ing drum and the recording element, pen, needle, or
light beam was moved sideways with respect to the
drum, so that a continuous trace was written on the
sheet (Figure 3). When the sheet was removed from
the drum the trace formed lines, with time in each line
progressing from left to right, like lines in a book.
The time resolution of the recording was determined
by the rotation speed of the drum, one, two, four or
six rotations per hour, producing the same number of
lines per hour on the final seismogram sheet.
A time signal was mixed with the seismic signal,
a small excursion of the trace superimposed on the
62 JÖKULL No. 70, 2020