Observed Characteristics of Regional Seismic Phases and Implications for P/S Discrimination in the European Arctic

Abstract

In this paper, we use data from seismic stations operated by NORSAR, the Kola Regional Seismological Centre (KRSC) and IRIS to study the characteristics of regional phases in the European Arctic, with emphasis on the P/S ratio discriminant. While the detection and location capability of the regional station network is outstanding, source classification of small seismic events has proved very difficult. For example, the m _b = 3.5 seismic event near Novaya Zemlya on 16 August, 1997 has been the subject of extensive analysis in order to locate it reliably and to classify the source type. We consider the application of the P/S discriminant in the context of this event and other events observed at regional distances in the European Arctic. We show that the P/S ratios of Novaya Zemlya nuclear explosions measured in the 1-3 Hz filter band scale with magnitude, indicating a need for caution and further research when applying P/S discriminants. Using mainly data from the large NORSAR array, we note that observed P/S amplitude ratios in the European Arctic show large variability for the same source type and similar propagation paths, even when considering closely spaced observation points. This effect is most pronounced at far regional distances and relatively low frequencies (typically 1-3 Hz), but it is also significant on closer recordings (around 10 degrees) and at higher frequencies (up to about 8 Hz). Our conclusion from this study is that the P/S ratio at high frequencies (e.g., 6-8 Hz) shows promise as a discriminant between low-magnitude earthquakes and explosions in the European Arctic, but its application will require further research, including extensive regional calibration and detailed stationsource corrections. Such research should also focus on combining the P/S ratio with other short-period discriminants, such as complexity and spectral ratios.