Geoscience Reference
In-Depth Information
2.3 The Digital Revolution and the GDSN
Because analog (i.e., non-digitally recorded) seismograms have a low dynamic
range (about 3 orders or less in amplitude) and must be digitized for computer
processing, some seismologists recognized that “digital” instrumentation should
be developed to achieve a much higher dynamic range, and for ease of computer
processing. Many scientists and engineers in other disciplines had already been
making great advances in that direction because of the emerging digital technol-
ogy in the 1970s. Seismologists had long recognized that the tandem use of short-
period and long-period instruments was needed to avoid the natural seismic noise
(see Webb 2002). They realized that a new global seismic network should be built
with (1) broadband, high-dynamic range seismographs, (2) digital electronics, (3)
communication by telemetry or a mass storage medium, and (4) processing by
computers.
The introduction of electronic force feedback to sealed inertial seismometers
(Melton 1976; Wielandt and Streckeisen 1982) together with the application of
high-resolution analog-to-digital converters made it possible to construct broad-
band, large dynamic-range seismograph systems. Many of the WWSSN stations
were replaced by broadband digital systems starting in the 1980s (Hutt et al. 2002).
A global digital seismic network has emerged since the 1980s under the guid-
ance of two effective organizations: the international Federation of Digital Broad-
band Seismographic Networks (FDSN), and the Incorporated Research Institutions
for Seismology (IRIS). Digital seismograms recorded by stations worldwide are
now readily available via the Internet from the IRIS Data Management Center
(DMC) within tens of minutes of a M
5
.
7 (all depths, or M
=
5
.
5 for events
>
100 km depth) or larger earthquake occurring anywhere in the world (Ahern
2003), as well as, for example, through the European ORFEUS center at De Bilt,
the Netherlands, the GEOFON center at the GeoForschungsZentrum, Potsdam,
Germany, the Programme GEOSCOPE at the Institut de Physique du globe de Paris,
France. Large strides have been made in networking these data centers, as well, so
that data can be accessed transparently through web or imbedded interfaces, elimi-
nating the need to know the specific location of waveform data in these distributed
archives.
3 Microfilming Historical Seismograms of the World
Before the digital era (prior to 1980), seismograms were usually recorded locally
on paper (a common size is about 30 cm by 90 cm) every day. There are usually
6 seismograms at a given station: east-west, north-south and vertical components
for both long-period and short-period seismometers. Because of their size and fine
resolution, seismograms were not easily reproducible until the 1960s. Consequently,
seismologists must spend large amounts of time and effort to collect seismograms
for their studies of earthquakes that occurred before the WWSSN era (i.e., before
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