January - March 2002

Melda Banganan Heads Home to Manila

After working for two years at the ISC, Esmeralda Banganan rejoined the Philippine Institute of Volcanology and Seismology in January. While Melda was editing the Bulletin with Dmitry Storchak and Mamy Andrianarina, publication caught up with the nominal schedule and then advanced to nearly two months ahead of the schedule used in the past.

Each seismologist edits the Bulletin in his or her own "style". Melda edited exuberantly. Whether she was singing along with her Walkman or laughing out loud, there was rarely any doubt about whether or not Melda was around.

The ISC's system for preparing the Bulletin requires several passes through the data for each month, first to check the results from automatic processing and later, where the processing was questionable, to test alternative phase identifications and other hypotheses about earthquakes. Melda was particularly adept at the first pass, and provided us with quick feedback on just how many events would require extra attention each month.

Melda didn't continue with the ISC long enough to see us begin using our new editing system, which will reduce the need for manually keying edits and systematically compile edit logs. But her help in developing requirements and testing prototypes for the new system will benefit other seismologists editing the Bulletin for years to come.

Melda reports that she's happy to be doing "real time" seismology again with PHIVOLCS. For such an exuberant seismologist, perhaps the satisfaction from lugging seismometers and data loggers through the steamy jungles of Luzon outweighs the comfort of sipping a hot cup of tea at the ISC while scanning the latest processing results and waiting for another cold English morning to warm up.

IDC Contributes Seismic Parameters to the ISC

The Comprehensive Test Ban Treaty Organisation (CTBTO) has made seismic data from its Reviewed Event Bulletin (REB) for 2000 and 2001 available to the ISC. The release of seismic parameters from the REB was recommended by Working Group B at its meeting in February and later approved by the Preparatory Commission, which sets policies for operation of the CTBTO. The ISC has accepted the data, integrated them into its database, and made them available in the printed and on-line versions of the ISC Bulletin.

The ISC's role, of course, is to integrate data from disparate sources and prepare an analysis of the combined data. In fulfilling this role, ISC seismologists carefully review both the grouping of origin estimates for the same event and the association of IDC phase readings with events in the ISC's more comprehensive catalogue. Thanks to this review, the Bulletin will continue to be a useful resource for seismologists evaluating the REB and developing new methods to produce even more complete and accurate bulletins.

Data from the REB of the prototype IDC has also improved the ISC Bulletin for all other applications. Thanks to the numerous teleseismic amplitudes reported in the REB, mb is computed now for almost every event with a teleseismic arrival time. In addition, results from processing at the ISC suggest that very few large events now go undetected in broad oceans and other regions with few seismic stations.

The CTBTO does not plan to release data from 2002 REBs until the Preparatory Commission agrees on a general data policy. A general policy would cover all CTBT monitoring data and IDC products, including waveforms and non-seismic data in addition to seismic parameters from the REB.

Waveforms Links Added to the On-line Bulletin

After finding events in the ISC Bulletin relevant to their work, the next step for many seismologists is obtain waveforms for further study of the source or of earth structure. Waveform archive centres assemble sets of data for events that they expect to be of interest to many seismologists, and the On-line Bulletin has included links to such sets for some time now. But these pre-assembled sets exist for only a small fraction of all events in the Bulletin.

Recently, the ISC added a new type of link to the On-line Bulletin to help users to compose requests for customised selections from the continuous waveforms at the IRIS DMC. The continuous waveforms include data at the time of every recent event, of course, so custom requests can be made for any event.

The challenge in making this system useful was to provide users with a summary of the data that are available, helping them to avoid a scatter-shot request. An inventory at the ISC of IRIS DMC waveforms would require nearly continuous maintenance and might fail to let users know of valuable data if the ISC's inventory were out of date.

Instead, when a user clicks on one of the new links the ISC obtains an up to date inventory of waveforms. The IRIS DMC lent a hand to make this work by adapting their "SeismiQuery" system to provide the information required by the ISC web server. After obtaining an inventory of data available in the hour after an event, the ISC posts a summary to the user and prompts for further information on which data the user wants. The ISC then composes a draft request, which the user edits or confirms before the ISC e-mails a message to the IRIS DMC on their behalf.

This is not the quickest service available from the ISC, since users wait for the summary of available waveforms while the ISC asks the DMC for an inventory. The system also could be improved to get inventories from several waveform archives and then despatch requests to each of them. But for many events the system already provides an easy way to obtain most of the waveforms that are available on the Internet, and it's an example that can be used in developing co-operative arrangements with other waveform archives.

New at the ISC: Old Data!

One goal in recent development at the ISC has been to archive and re-distribute sets of data that were not available when the Bulletin was published. Our objective is to fully integrate the newly available data into the ISC's data collection. For example, a new contribution may include hypocentres computed from local readings for an event that is already in the Bulletin based on teleseismic readings. If so, the ISC will group the new hypocentres with the existing events, making it easier for users compare the differently computed hypocentres.

The challenge for the ISC was to integrate further data while still being able to retrieve data efficiently just as they were at the time that the Bulletin was published. To accomplish this, each datum inserted after the Bulletin is published is specially marked. For users of the On-line Bulletin who request the "published" version of ISC data, the data inserted later are excluded.

"Old data" already integrated with other data at the ISC include

• hypocentres by Engdahl, van der Hilst and Buland from ISC phase readings.
• epicentres by Fox et al. from hydroacoustic arrival times.
• focal mechanisms by Malone and Qamar from PNSN first motions.
• regional CMT's by Morelli and Pondrelli from MedNet data.
• catalogues by McCaffrey et al. and by Nyblade from PASSCAL deployments.

Integration of further data into the "comprehensive" version of the Bulletin shows one of the advantages of an on-line service over other means of data distribution. Bulletins from PASSCAL experiments show how the on-line service can link different types of data. Each of the hypocentres in the on-line Bulletin from a PASSCAL experiment includes a button that can be used to send a request to the IRIS DMC for related waveform data that were collected during the experiment.

Phase readings and hypocentres from experiments are the intellectual property of the original investigators, so these data are available through the generosity of individual scientists. Of course users generally acknowledge the data sources, cite descriptions of the data collection, and sometimes even collaborate with the original investigators. Thus, in the long run data contributors are likely to benefit from their own generosity.

Fox et al. calculated epicentres (black dots) for more than 39,000 earthquakes from May 1996 through April 2000 from arrivals at six hydrophones near the East Pacific Rise. Nearly 35,000 of these earthquakes were not previously included in the ISC Bulletin. Conversely, the Bulletin includes crustal oceanic earthquakes that were not detected hydroacoustically (red dots).