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et al. 2000). Time domain moment tensor inversion of regional historical inter-
mediate period waveforms led to useful source approximations for the largest in-
strumentally recorded shallow earthquakes that hit Portugal, Spain and France, re-
spectively (Mw 5.5-6.1, Stich et al. 2003, 2005, Fig. 4), showing good consistency
with modern seismotectonic studies. For later decades, technical advances including
the densification of networks and the deployment of electrodynamic instruments
in addition to existing mechanical systems, permitted moment tensor inversion for
smaller earthquakes in areas of comparably dense station coverage, e.g. the case of
a Mw 5.2 and 5.3 earthquake doublet in 1951 in southern Spain (Batll o et al. 2008,
Fig. 4), or small to moderate (Mw 4.5-5.6) aftershocks of the 1952 Kern County
earthquake in California (Dreger and Savage 1999).
5 Conclusions
In the early XX century, fundamental concepts of seismic source physics, such as the
double couple model for the equivalent body forces of a shear dislocation, were yet
to be discovered, as well as the benefits of computer technology and digital signal
processing. Fundamental advances of scientific theory and methodology should lead
themselves to a reprocessing and reinterpretation of previously obtained data, which
is especially true for analog seismograph recordings. To date, most of the instrumen-
tal era in seismology predates the invention of digital recording systems and pro-
cessing schemes, containing the larger share of all moderate and large earthquakes
for which waveform information may be available. Large earthquakes, either in a
regional or global context, are the rarest events within the seismic cycle, and may
be of particular interest, although sometimes their analysis from historic recordings
may be hampered by the small dynamic range (causing nonlinearity or clipping of
the signal) or the narrow bandwidth (with the instruments being not sufficiently sen-
sitive to long period signals, leading to an underestimation of the total source dura-
tion and seismic moment). The characterization of source properties from recorded
waveforms of early XX century earthquakes may provide key information for very
diverse topics such as regional tectonics and strain accumulation, the identification
and kinematics of individual seismogenic faults, earthquake recurrence and seismic
hazard, tsunami generation, or the benchmarking of contemporaneous magnitude
estimates or earthquake parameters derived from macroseismic observations.
Pointed reasons strongly sustain the need to reanalyze the seismograms of past
conspicuous events. The use of such records for seismic research may expand con-
siderably the instrumental period of earthquake seismology. But such reanalysis is
not straightforward. Especially dedicated procedures should be taken into account,
from restoration of the physical support to the search and recovery of the metadata
accompanying the old seismograms to be processed. It is necessary to recover, at
least, the information concerning the transfer function, orientation, and polarity of
the recording instrument from seismic bulletins, photography and other contem-
poraneous documents. In the previous sections the acquisition of analogue records
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