July - September 2001

Gary Gibson Joins Executive Committee

The Governing Council has elected Gary Gibson of the Seismic Research Institute in Melbourne, Australia as a new member of the Executive Committee, replacing Jens Havskov on completion his six-year term. As a Committee member, and as chairman for the last four years, Jens helped guide the ISC through fundamental changes in changes in data distribution, first with the release of the full set of data on CDs and more recently on the Internet.

In his wide experience installing seismic networks in developing countries, Gary has emphasised providing new network operators with the tools and training they require to process data and produce a regional seismic bulletin on their own. He hopes to use his position on the Executive Committee and his rapport with local network operators to encourage ever greater exchange seismic bulletin data among around the world. This is a particularly opportune time for such an initiative since flexibility in the ISC's new data management system makes it easier to re-distribute data that include but are not limited to those re-analysed at the ISC.

Governing Council Meets

At its meeting in August, the Governing Council decided that the ISC should continue computing hypocentres from P arrival times alone, but perhaps only for another year or two.

Using PKP times has proved to be tricky because the wrong branch of the travel time curves is often selected. Since the ranges of some branches of PKP are too limited in the Jeffreys-Bullen tables, the problem could be resolved only by adopting new travel times.

Results using S times were much more encouraging. When S times are used the ISC needs to fix hypocentral depth far less often. For subcrustal earthquakes, S times bring ISC's depths into better agreement with depths reported by regional agencies, even without correcting for the differences between P and S baseline shifts in the J.-B. tables.

Unfortunately, depths for many crustal earthquakes are grossly inaccurate when S times are used. Just as with PKP, the problem is using the wrong travel time curve. But in this case ISC's Sg and Sn travel time tables already cover the required distances, so a more careful choice of which curve to use should fix the problem. The Governing Council instructed ISC staff to look for ways of improving identification of regional S phases and report their results to the Executive Committee in 2002.

The Governing Council also instructed ISC staff to look into advantages and disadvantages of publishing the Regional Catalogue exclusively in electronic form. Almost all Catalogue recipients now prefer the CD version, so unit costs for the printed version have grown tremendously. An electronic version might be distributed by e-mail as often as every month, even while annual CD distribution continues. Savings from not printing might be used to help pay for developing a graphical user interface for the electronic Catalogue.

Bulletin Format Adopted

The IASPEI Commission on Seismic Observation and Interpretation formally adopted IASPEI Seismic Format (ISF) at its meeting during the 2001 Assembly. The agreement concludes a 16-year process seeking consensus on a standard to succeed the "Telegraphic" format that is described in the 1979 edition of the Manual of Seismic Observatory Practice.

Seismologists are familiar with the benefits of standards, which make it easier to exchange data, re-use data collected for past projects, and employ programs developed elsewhere. This wide variety of advantages is also part of the problem, however, since each imposes different and sometimes conflicting requirements for conciseness, transparency, and completeness.

Consensus was reached partly by including many optional items. Optional event parameters such as scalar seismic moment, focal mechanism, and stress drop are likely to be included in many ISF bulletins. In addition, however, ISF includes extensive optional phase data, such as the filter used while reading and asymmetrical uncertainty ranges.

ISF is defined as the parts of IMS1.0 for seismic parameter data, plus a specific set of extensions. IMS1.0 was developed for exchanging data from the International Monitoring System of the Comprehensive Test Ban Treaty (CTBT). By including this format as a subset, ISF immediately gains the CTBT International Data Centre and National Data Centres around the world as users. The ISC and the US National Earthquake Information Center already use ISF for private data exchanges. ISF is one of the formats in which data are available publicly from the NEIC, and ISF is the default format of the Bulletin on the ISC web and AutoDRM servers.

Bulletins from many sources should help ISF to be accepted widely, but software is also important. The ISC is developing FORTRAN and C subroutines that other seismologists can link into their own programs to write and parse lines from ISF bulletins. The subroutines will be included on the ISC 1999 Catalogue CD, which is planned for release this December. In addition, Jens Havskov plans to add translators between ISF and "nordic" format to his SeisAn package, and British Geological Survey is considering development of an ISF version of WizMapII, its freely-available interactive earthquake mapping program for Microsoft Windows.

Unassociated Readings Rescued from Archive

ISC application manager Peter Dawson has recovered unassociated phases in ISC archive files to the on-line database. These readings were reported to the ISC but, because they could not be associated with events in the Bulletin, they have previously not been available readily.

These newly available data include nearly 13 million arrival time picks and 2 million amplitude measurements and extend from 1964 to 1998. From 1999 onward no recovery was required since data were inserted directly to the on-line database from contributed bulletins.

Unassociated readings are expected to be used detailed investigations of seismicity in particular regions, which might use data from local and regional catalogues that were not all available to the ISC when the Bulletin was published. In addition, some of the readings may be later phases or surface waves for Bulletin events that could be used in earth structure studies.

Unassociated readings are not included in an event bulletin, of course. But both associated and unassociated readings can be retrieved by station and time period from the ISC web site at http://www.gxjydl.com/search/arrivals/.

IASPEI Symposium Emphasises Hypocentres

At the IASPEI Assembly, a symposium on Practices and Standards at Seismic Data Centres included talks on new earthquake parameters and waveform measurements, such as S-wave splitting, that could be included in seismic bulletins. But attention focused on the fundamentals: advances in computing hypocentres.

Chen Qi-fu (CSB, Beijing) described modest improvement in locations that resulted from relocating events around the world using 3-D earth models. Andrea Morelli (ING, Rome) described an alternative to tomographic inversion in developing travel times that more readily incorporates newly available data. Paul Richards (LDEO, New York) presented the most radical approach, involving simultaneous inversion for relative locations of widely distributed events using travel time differences.

An extended discussion concluding the symposium reached no consensus on a single best way to locate earthquakes. But the participants agreed that new results are so much better that network operators and data centres everywhere should take advantage of modern models and methods in the near future.

Developing a Magnitude Scale for EuroBull

The ISC continues participating in the EPSI project, which aims to develop a near-real-time seismic bulletin for all of Europe. A critical element is defining a magnitude scale to reliably compare sizes of earthquake across Europe. Challenges arise from differences in both tectonic setting and seismic instrumentation.

For more than 300 reference events representing the range of these conditions, ISC senior seismologist Dmitry Storchak computed new hypocentres from the combined set of arrival times from all sources. Underlining the importance of using all available data, Dmitry found that the best hypocentres were as much as 100 km from locations computed with data from an individual network.

Members of EPSI's magnitude work group from Israel, France, and Germany plan to evaluate alternative scales using magnitude computed from Dmitry's hypocentres.