Analysis procedures at the International Seismological Centre

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Scientific paper

Analysis at the International Seismological Centre (ISC) falls into three main categories-association, location and quantification.
Difficulties of association on a global scale are not always appreciated. The ISC currently analyses 40-50 events a day, and as the first arrivals from any particular earthquake may span a time interval of up to 20 min, readings from events occurring in different parts of the world may overlap considerably in time. The present association algorithm depends on time only, and results in many chance mis-associations, or even the synthesis of fictitious events. At present, these mis-associations are rectified by seismologists' intervention, but full use of amplitude and period information could help to detect these errors automatically.
Revision of locations follows standard least-squares procedures, based on Jeffrey's method of uniform reduction and Jeffreys-Bullen travel times. Locations are made from P phases only (including crustal phases), but other first arrivals and secondary phases are identified and residuals calculated. If depth cannot be determined by geometric means a search is made for depth phases, or failing this, the depth is restrained to that given by another agency, or to a conventional value. No provision is made for local variations in travel time.
Body-wave magnitudes are allocated within the distance range 21-100° from reported readings of A and T, or their ratio, using Gutenberg-Richter calibration curves. Surface-wave magnitudes are calculated from the ``Prague'' formula, using reports of long-period A and T in the distance range 5-160°, but only values from stations at distances of 20° or more are used to determine an average Ms for a particular event. There is no provision for the determination of local magnitude other than to reproduce values assigned by local agencies.
Improvements in these procedures could be made through the automatic association of station readings, the introduction of local travel times, and better determination of earthquake size, particularly for local events.

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