Jökull - 01.01.2020, Page 68

The analog seismogram archives of Iceland
Figure 5. Map of analog seismograph stations at the peak of the network, in the late eighties and nineties. –
Kort af stöðvum Landsnetsins þegar þær voru flestar í gangi, á níunda og tíunda áratug síðustu aldar.
Magnitude scales for the Landsnet seismographs
The dynamic range of the paper records was limited.
The minimum readable amplitude is about 1 mm and
the records were designed to clip at about 40 mm to
avoid damage to the pen assemblage. It was there-
fore not practical to use an amplitude-based magni-
tude scale, such as ML, to measure magnitude of
recorded earthquakes. A scale based on the duration
of the seismic signal, Mτ , turned out to be rather prac-
tical and give a consistent result for a set of stations
recording the same event. Similar scales were used in
many seismically active areas of the world, including
Hawaii and California (Lee et al., 1972). The stations
were calibrated with respect to duration magnitude
by plotting log τ against ML obtained from the Me-
teorological Office, an average magnitude for events
recorded by the older stations in Reykjavík, Akureyri,
Síða (Kirkjubæjarklaustur) and Eyvindará. The du-
ration, τ , was determined as the time in seconds from
the first P-arrival until the peak-to-peak amplitude last
exceeded 1 mm at 36 dB amplification. A best fit-
ting line was then determined for the set of calibration
events, which thus was used as a magnitude scale for
the respective station.
Mτ = a log τ – b
The constants a and b are given for the calibrated
stations in Table 2. These scales were found to be al-
most insensitive to the epicentral distance. The num-
ber of calibration events ranged between 60 and 200,
and the correlation coefficient was above 0.86 for all
stations. A representative plot of duration versus local
magnitude ML is shown in Figure 6. Most magnitude
values in Skjálftabréf are duration magnitudes.
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