Quality assurance of seismological bulletins using waveform cross-correlation

Waveform cross-correlation (WCC) significantly improves detection, regular phase association, and hypothesis generation of seismic events at the International Data Center (IDC) of the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO). In aftershock sequences of small, medium, and very large earthquakes, extra 50% to 80% of events were found that, by their parameters, should have been included in the IDC Standard Event Bulletin (SEB). Several percent of the aftershocks published in the SEB were not detected using cross-correlation, even when signals from all other events in the same sequences were used as templates. In this paper, we study several cases of lack of cross-correlation confirmation of IDC events obtained by experienced analysts in routine interactive analysis. Regional catalogues, which usually include aftershocks with magnitudes down to 2.0, are used as references when comparing the WCC-based and IDC bulletins. Since the resolution of regional networks is at least one magnitude unit higher, the SEB events missed in the corresponding regional catalogues can probably be considered as false or significantly inaccurate. The events are compared by origin time and location, since the regional networks and the International Monitoring System use different seismic stations, and direct comparison of phase arrivals is impossible. Two aftershock sequences are studied: after the earthquake of March 20, 2008 in China with mb(IDC)=5.4 and the event of May 20, 2012 in Italy (L’Aquila) with mb(IDC)=5.3. In general, most events not found in the SEB using the WCC are missing from the corresponding regional catalogues. At the same time, these catalogues confirm most additional IDC-ready events found only using cross-correlation. This observation confirms the conclusions about the high statistical reliability of events constructed using the WCC, as well as the possibility of expanding the SEB by 50 percent or more with events that are correct according to the IDC event definition criteria.