Using real and synthetic waveforms of the Matata swarm to assess the performance of New Zealand GeoNet phase pickers

Abstract

We assess the performance of the P- and S-phase pickers of the New Zealand Seismograph Network (NZSN) by comparing their outputs for real seismo-grams and corresponding synthetic seismograms of the local earthquake sequence in the Matata region, New Zealand, during 2008. We study the effect of the radiation pattern on the pickers' performance by computing synthetic seismograms at 85 hypothetical stations. These are located at distances of 10, 20, 30, 40, and 50 km from an earthquake source represented by a double-couple point source. The picked S-phase onset has larger deviations from the mean picked onset at each distance compared to the P-phase onset. Deviation from the mean P-phase onset at each point does not affect the corresponding deviation from the mean S-phase onset. We confirm that none of the key stations in 2008 are located where the picking errors are large or where the phases are missed due to the radiation pattern. We also study the effect of noise on the pickers' performance at three different signal-to-noise ratio levels (SNR = 20, 10, and 3 dB). No P-phase onset time was picked at 50 km distance when SNR was 3 dB. Both pickers had a consistent performance at stations with 20 km distance from the earthquake source. At larger distances, three sea-based observatory points with azimuth angles 0°, 21°, and 339° show a consistent performance in the presence of the increased noise level.