Modernizing the ISC location procedures
IASPEI General Assembly, Santiago, Chile, 2005
The International Seismological Centre (ISC) has developed and put into operation a new Data Management System. As a result, it is now possible to review and subsequently introduce more up-to-date methods of locating seismic events into the ISC operations. At its last meeting in Sapporo, the ISC Governing Council decided to call for a workshop dedicated to location procedures to be held during the 2005 IASPEI General Assembly in Chile. All suggested improvements to location algorithms should be oriented for daily operation at the ISC, which has to locate globally distributed seismic events based on reported arrival times and other strictly parametric data at both local and teleseismic distances.
To have a common ground for comparison of different location algorithms and their implication on the location accuracy, a list of 156 well-located test events (GT0/GT5) was kindly selected by E. R. Engdahl from the IASPEI collection of Ground-Truth (GT) events (Bondar et al., 2004).
A relevant selection from the ISC bulletin is now available in two formats, familiar to ISC CD and Internet site users: Fixed Field Bulletin (FFB) & International Seismic Format (ISF). It contains the originally published ISC hypocentres (coded ISC1), all reported hypocentres and the reference GT hypocentres (coded as IWREF) followed by the seismic station arrival data. This selection covers a period of over 40 years and consists of geographically well-distributed events.
Alternative download methods are shown at the end of this page.
Later we are planning to compute an up-to-date ISC solution for each test event, based on the technique used by the ISC at present and include it in the list with the agency code ISC.
Those interested in participation in this exercise can download a list of the station coordinates and elevations, based on the International Registry (IR), jointly maintained by both ISC and NEIC.
The main goal of the workshop is to discuss possibilities of improving the location capabilities of the ISC by implementing new features in the ISC software. One important constraint is that a spherical standard Earth model is used.
New location features could be e.g.:
* ray parameter dependent correction for topography at reflection points in order to get smaller residuals for reflections at the Earth's surface (e.g., pP, sP, pwP, swP, PP, SS, etc); with these corrections the above phases may be used more regularly;
* non-static, ray-parameter dependent calculations of station-elevation corrections;
* systematic usage of core or Moho reflections to get better depth constraints;
* using back azimuth and slowness observations from arrays for location according to certain rules to be defined;
* using other phases of P and S type;
* using statistical phase identification as oppose to best residual fit identification.
This list of ideas is, of course, only an example and remains open for further suggestions from the workshop participants.
The organizers of the workshop hope that those testing different location programs will be able to spend some time switching the proposed location features on and off. In this case, during the workshop in Chile, it can be discussed, which additional features are most effective and should be made part of the ISC standard location procedure to achieve higher location accuracy.
Those wishing to participate should contact the conveners as soon as possible so that the workshop can be planned in advance.
Alternatively via anonymous ftp:
ftp isc-mirror.iris.washington.edu Name: anonymous Password: email address cd pub/iwref get iwref.ffb ( iwref.ffb.gz , iwref.isf , iwref.isf.gz )
Johannes Schweitzer, NORSAR, P.O. Box 53, N-2027 Kjeller, Norway. Phone: +47-63805940, Johannes.firstname.lastname@example.org
Dmitry Storchak, ISC, Pipers Lane, Thatcham, Berkshire, United Kingdom, RG19 4NS. Phone: +44-1635-861022 email@example.com