New Seismologist to
Help Edit the Bulletin

Esmeralda Banganan joined the ISC in January, returning the editing staff to its strength before Alison Bird completed her appointment early last December.

Esmeralda earned her B.Sc. and M.Sc. degrees in physics and engineering at the University of the Philippines and brings 7 years of experience working in regional seismology and hazard analysis with the Philippine Institute of Volcanology and Seismology. She has had further training in global seismology in Tsukuba, Japan, in instrumentation and station siting with the CTBT Preparatory Commission in Vienna, Austria, and in seismic hazard analysis in courses sponsored by Kanto Gakuin University and GeoForschungsZentrum, Potsdam.

"Esmeralda is quick to learn, dedicated and careful", says senior seismologist Dmitry Storchak, "She spots problems right away, allowing time to investigate contact agencies and make corrections." For her part, Esmeralda is enjoying the ISC, but looking forward to summer, when she hopes that British weather might be a bit more akin to Manila's.

New Applications Developer

Dr Richard Luckett joined the ISC in March. Richard holds a B.Sc. in physics from Bath University and a 1996 Ph.D. in seismology from Leeds University, where he worked with Dr Jurgen Neuberg. He has experience in seismic operations from a survey in Africa and from working at the Montserrat Volcano Observatory since completing his Ph.D. Richard is also an accomplished programmer, with years of experience using Unix software development utilities.

Initially, Richard will work at modifying data collection procedures, so that data are inserted in the ISC's relational database rather than added to our data collection file. The new means of collection is a step towards taking greater advantage of associations between contributed data in automatic processing at the ISC, and making contributed data available on-line in advance of ISC analysis. "This is a vital step," says applications manager Peter Dawson, "Richard will help us develop interfaces between the database and ISC's analysis and association software more quickly."

Richard reckons the ISC to be "a great opportunity" and says that he's looking forward to using advanced computer technology to make ISC operations run more efficiently.

1999 Report Available

The new Director's Report describes analysis of fourteen months of the Bulletin during 1999, with a record number of earthquakes, and re-analysis of 18 months of the Bulletin. The Centre benefited from robust sales of its enhanced CD, and improved access to ISC data over the Internet. Computer related development during the year included initial operation of the ISC relational database and the completion of the computer replacement programme.

The report also reviews ISC finances during 1999. Income and expenditures were close to budget, and the net value of the ISC's assets changed only slightly. The Centre's liquidity improved thanks to fulfilment of commitments by the Russian Academy of Sciences. Virtually all contributions expected for 1998 and earlier have now been paid, but at the year's end 12% of 1999 contributions were not yet in hand. The complete report is available as PDF document from the ISC's web site at http://www.gxjydl.com/doc/report/1999DR.pdf.

Bulletin Keying Alternatives

ISC's data input clerk, Cliff Allen, is trying to put himself out of work! Cliff reckons that he spends nearly 25% of his time keying data from printed bulletins. He has undertaken to contact each source of a printed bulletin and attempt to arrange for electronic contributions.

Already, New Delhi and Pretoria have started submitting bulletins by e-mail. Cliff hopes to finish with keying focal mechanisms from Obninsk and Tokyo in the near future, and has plans for eliminating keying of hypocentres and phase data from the Canary Islands and the Azores over the next few months. Cliff is also looking for ways to obtain more complete electronic contributions from several agencies already sending partial bulletins by e-mail.

Database Documentation

Two new postings on the ISC web site offer details about the organisation of the ISC relational database and how data were inserted in the relations.

The relations and their attributes are described in "ISC1.0 Schema", which is a set of HTML files. An introduction and links to the related files are posted at http://www.gxjydl.com/doc/database/schema.html.

Database administrator James Harris filled the relations by parsing the "FFB" files from ISC's CDs and the Centre's "Historical File" for events before 1964. His parsing procedures are described in http://www.gxjydl.com/doc/database/load_ffb.html, which also describes minor inconsistencies that he discovered in the CD files.

Seminars at
Cambridge
and Leeds

Director Ray Willemann gave seminars and the Universities of Cambridge and Leeds during March, titling his talk "Missing Data, Biased Data and Meta-data at the ISC".

Missing data refers to the threshold for completeness of the ISC Bulletin, which varies from place to place, as well as from year to year. The "roll-off" of the global magnitude-frequency distribution (shown at right) between mb 4.2 and 4.8 shows that the previously recognised improvement in the threshold from 1994 to 1995 was followed by a further improvement in 1996.

At higher magnitudes, however, the 1996 distribution is offset from earlier years, and the 1997 distribution is differs even more. The offset is far too large to be attributed to natural variations in seismicity; it is implausible that the typical number of mb 5.0 earthquakes was nearly 400 through 1995, but that there were fewer than 200 in 1997. The alternative is that the 1996 and 1997 curves are offset not downward, but leftward. Looking at the offset this way suggests that recent mb values are biased. That is, earthquakes are being assigned smaller magnitudes than in the past.

Magnitude calculation procedures have not changed at the ISC, so the most likely cause of a bias may be that the mixture of stations reporting amplitudes to the ISC is changing, with newer stations more likely to report small amplitudes. This could be a result of the growing number of stations located on carefully selected low-noise sites. The same properties that reduce noise at these sites might also turn out to reduce signal amplitudes. Alternatively, the low noise stations may be simply more likely to report amplitudes of small signals that previously would have been lost in the noise.

The ISC Bulletin includes contributed hypocentres, and the authors of these hypocentres are the most common sort of meta-data in the Bulletin. For more than 25% of all 1997 earthquakes, in fact, the ISC was unable to compute a reliable hypocentre from reported readings, and designated a contributed hypocentre as the primary estimate. The fraction of non-ISC prime hypocentres is generally growing, but fluctuates from year to year (shown at right).

Natural variations in seismicity play a role in the fluctuations. Kandilli hypocentres were used as prime estimates for many aftershocks of the October 1995 Dinar earthquake, and Rome hypocentres were used as prime estimates for many earthquakes of the 1997 Umbria-Marche sequence.

But changes in seismological practice are the predominant causes of both the trend and the fluctuations. The total increased sharply from 1991 to 1992 when Jakarta started contributing. Increases in 1993 and 1994 correspond with growth in contributions from JMA, and the total decreased in 1995 when the full effect of thresholding JMA data came into effect. With hypocentres of the prototype IDC included comprehensively from 1996, the total rose again.