A
typical bimonthly issue of the ISC Bulletin lists more than 10,000
earthquakes and 300,000 associated readings.
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As recently as the
1960's, seismological investigations often began by compiling readings
from several global bulletins and re-computing epicentres, depths and
magnitudes using the combined data. The global seismic bulletins available
in the 1960's included the International Seismological Summary (ISS) by
the British Geological Survey in Edinburgh, the Preliminary Determination
of Epicenters by the US Coast and Geodetic Survey in Washington, the Bulletin
of the Bureau Central International de Seismologie in Strasbourg, and
the Seismological Bulletin of the Soviet Academy of Sciences in Moscow.
The International Seismological Centre (ISC) was founded in 1964 to compile
readings from these and other sources efficiently and routinely to a high
standard, allowing individual seismologists working from the ISC Bulletin
to concentrate on further investigation and interpretation.
There were few international
precedents for organizing such work, and substantial commitments for direct
support were required from the founding participants. Seismologists from
national agencies of the US, UK, SovietUnion, France, Canada, Australia
and New Zealand agreed to provide their data freely to the new Centreno
small feat in an era of growing efforts to monitor nuclear weapon tests.
In addition, each participant made a commitment to help support the ISC
financially so that the new Bulletin could be offered at low cost, thus
promoting wider distribution.
As
the number of seismic networks and regional agencies around the
world has grown, the ISC has always aimed to provide a comprehensive
global summary. The Bulletin now includes more than 60,000 events
annually.
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The result of these
efforts was profound: more than 350,000 phases were used to compute the
locations of 10,770
events in 1964, the first year of ISC Bulletins. This was a better-than
three-fold increase from the number of events reported in the 1963 ISS.
Within a few years, the ISC was more formally organized with a set of
working statutes that UNESCO helped to formulate, and a Governing Council
of representatives from each member agency contributing significant financial
support. Each of the founding members has continued supporting the ISC
for more than 30 years, joined regularly by agencies from other countries.
Today the ISC receives direct financial contributions from nearly 50 countries
and readings from an even broader cross-section of the global seismological
community.
Thanks to this comprehensive
commitment, the ISC now offers a Bulletin comprised of more than 60,000
events annually as illustrated here.
Preparing the ISC Bulletin
requires international collaboration. Seismologists at individual stations
and national or regional agencies read records, associate phase data within
networks, and compute preliminary hypocentres. Thus, there is no "ISC
network" of stations. Instead, the ISC has always aimed to be as
comprehensive as possible, associating and reporting data from otherwise
under-represented regions whenever they are offered. The ISC does maintain
an International Registry of stations but, in order to encourage the broadest
possible reporting, imposes only minimal requirements for registration.
The number of registered stations has grown to nearly 10,000, of which
more than 3,000 have contributed readings in each recent year. The location
of these stations is shown on page 6.
The nine scientists on staff
at the ISC carry out the final work of grouping independent hypocentres
for the same events, re-associating phase readings, and re-computing hypocentres.
The staff is comprised of experts from around the world. Seismologists
working currently or within the last three
years at the ISC have come from New Zealand, Germany, Russia, Canada,
the United States, the Malagasy Republic, Algeria, China and the Philippines.
Britain has offered a variety of in-kind support in addition to its financial
contributions. The ISC was originally housed in the Edinburgh facility
of the British Geological Survey, and moved to southern England in the
early 1970's to take advantage of computing facilities at Rutherford-Appleton
Laboratory. Today the ISC works in a building it purchased in 1987 in
the small town of Thatcham and its home institution is the University
of Oxford, whose faculty members have offered critical programming and
other support in recent years.
The format of the
printed Bulletin has changed over the years to offer the ever-growing
volume of readings in an affordably compact layout. For events since 1995,
the printed Bulletin is issued bi-monthly and readings are included only
for shallow events larger than mb 5.5 and deeper events
larger than mb 5.0. The ISC has always made its data
available in computer-readable formats, formerly on custom-written tapes
but now on a set of standard CDs. A new CD is issued annually with the
Bulletin data (hypocentres and phase readings) for a new year and Catalogue
data (hypocentres only) for the entire history of
the ISC. For the sake of compatibility with the existing programs of seismologists
around the world, the ASCII files on the CDs continue to be formatted
in "96-column fixed format" used in ISC tapes for several decades.
The ISC also operates a web site (www.isc. ac.uk). In the original spirit
of making the Bulletin available at the cost of reproduction, the complete
set of ISC Bulletin data are freely available from the ISC web site. In
an effort to foster more widespread use of modern standards, the on-line
Bulletin is offered in IMS 1.0 format.
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There is no formal
threshold for including events in the Bulletin. The ISC publishes a location
for each event that it can locate with a reasonable degree of reliability
using all of the reported readings. Because the ISC aims to be comprehensive,
the number of events in the Bulletin has grown over time. Unfortunately,
many agencies reporting to the ISC fail to include amplitude measurements
so the ISC is able to compute mb or MS
for only about 25% of the events it can locate. Based on this minority
of events, the practical threshold of the Bulletin estimated from magnitude-frequency
relations varies from one region to another as illustrated on the map
on the next page. The threshold for including events in the Bulletin without
a computed magnitude is significantly better, but also variable from one
place or one year to another.
Despite the inevitable
changes in coverage over time as seismic stations are opened and closed,
the ISC has maintained a uniformity in its processing in order to make
it as straightforward as possible to compare earthquakes from different
years. Thus, the ISC continues to use the Jeffreys-Bullen travel time
tables and to compute hypocentres exclusively from first-arrival times.
Similarly, magnitudes
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Much of the growth
in reporting through the early 1990's occurred in already well-monitored
regions. Further improvements are required in many parts of the world,
including some areas with moderate or high seismic hazard.
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Senior seismologist
Dmitry Storchak (left) is responsible to uphold editing standards for
the Bulletin. He works with seismologists on 2 to 3 year appointments
at the ISC, including Mamy Andrianarinna (right) of the Malagasy Republic.
The ISC's threshold
for computing mb varies with the density of reporting
stations and on how comprehensively their records are read and reported.
Only about 25% of the events reported to the ISC include magnitude estimates.
The threshold based on this fraction is shown above.
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continue to be computed
from teleseismic (21100 for mb, 21160 for
MS) amplitudes using the Gutenberg-Richter attenuation
tables. The Bulletin in print, on CDs and on the Internetalso continues
to include macroseismic data (intensities, felt and damage reports) and
other widely recognized event parameters, such as the Harvard Centroid-Moment
Tensor solutions.
While maintaining
continuity remains important, advances in seismological practice and information
technology have made it important and practical to begin updating the
Bulletin. Building on the tradition of drawing together efforts around
the world, the ISC is reporting more source parameters computed elsewhere,
such as radiated energy, stress parameters and source time functions.
Starting in 2000, the ISC is testing new location procedures, including
alternative algorithms, additional phases, and more accurate travel times.
The ISC is also working towards making reported readings and preliminary
hypocentres available on its web site before they are analyzed in the
Bulletin. Recognizing that advances in earthquake physics and earth structure
often rely on specialized measurements from raw waveforms, the ISC is
working to integrate its Bulletin with waveform archives, for example
by implementing the AutoDRM software developed at Swiss Seismological
Centre and the NetDC software developed at the IRIS Data Management Center.
Phase picks and event
catalogues are likely to remain the starting point for many seismological
investigations for the foreseeable future. By providing this information
and up to date services, the ISC will continue as an important resource
to seismologists and earthquake engineers for years to come.
Additional
Information
For additional information
about the ISC and their bulletins, see the ISC website: www.isc.ac.uk
and Willemann, R.J., Regional Thresholds of the
ISC Bulletin, Seismol. Res. Lett., 70, 313-321, 1999.
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