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Event detection and location performance of the FINESA array in Finland

INSTITUTE OF SEISMOLOGY UNIVERSITY OF HELSINKI, ET. HESPERIANKATU 4, SF-00100 HELSINKI, Finland

Abstract

The experimental seismic array FINESA in Finland is designed to monitor weak seismic events at regional and teleseismic distances. The array geometry currently comprises 15 short-period vertical seismometers in three concentric rings (A-, B-, and C-rings), with a diameter of the outer ring of about 2 km. In late 1989, the data acquisition system of the array was completely modernized. Signals are now transferred continuously via high-speed telephone lines to the processing centers at the Institute of Seismology in Helsinki and NORSAR in Norway, therefore allowing automatic real-time processing of the recorded data.

In this paper, the detection performance of the array in the current configuration has been evaluated. The results are encouraging: during a 2-week test period, FINESA detected at least one P and one S phase for 84 per cent of the events reported in the regional bulletin of the University of Helsinki, and 99 per cent of the events in the weekly teleseismic bulletins. Many additional events at both distance ranges were also found.

The estimated phase velocities obtained by the broadband frequency-wave-number analysis confidently identify the phase type (teleseismic Pgional PgionalS). However, the resolution of the analysis is not sufficient to separate Pg from Pn and Lg from Sn. The estimated backazimuths are reliable for phase association, the standard deviation of the estimates being 7° for regional P phases, 6° for regional S phases, and 23° for teleseismic P phases.

Finally, preliminary results from FINESA's on-line event location capability showed that the average error in the location estimates is 21 per cent of the true epicentral distance. The greatest error sources are uncertainty in the estimated azimuths and occasional misidentification of secondary phases (Lg, Sn and Rg). The error could be reduced by constructing a regional correction term for the azimuth estimates and "tuning" the phase identification algorithms for FINESA. The characteristics of the Rg-phase need to be especially considered.