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Fig. 1
Worldwide distribution of seismic stations (
triangles
) from Schweitzer and Lee (2003)
complemented with data from Merlin and Somville (1910) and magnitude 6
earthquakes (
circles
,
from Gutenberg and Richter 1954), for the years 1909-1910. Around 150 seismological stations
were operative at that time, but, on a first sight, only for approximately fifty of them seismograms
are available (IASPEI Working Group 2006)
+
and different purposes resulted in different recording parameters. Therefore, the
study of an event recorded in old seismograms implies to deal with many different
kinds of records, with different dimensions, diverse recording speeds, and different
instrument transfer functions. Consequently, the recovering and consideration of
related metadata, describing the mode of operation of the recording system, is an
issue as important as the recovering of the seismogram itself. Of main importance
are the free period, the damping, the magnification and the orientation and polarity
of the recording system. Often, these instrument characteristics may be recovered
from contemporaneous bulletins and station topics, or from daily calibration pulses
included on many old seismograms. This kind of signals, the recording of an elec-
tromagnetic or mechanical kick to the oscillating mass, permit to obtain directly
damping from the decay of the calibration pulse, as well as the free period in case
of undercritical damping of the system.
3 Conservation, Digitization and Restitution
of Analog Recordings
The reanalysis of arrival times, polarities and amplitudes contained in old station
bulletins is fundamental to our knowledge on old earthquakes (e.g. Abe 1981,
Dineva et al. 2002), and the qualitative assessment of waveform similarity and a
trivial comparison of the raw amplitudes can even lead to a quite robust estimation
of the relative size of nearby earthquakes recorded at the same (or nearby) stations
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