Towards Assessing the Quality of Surface Wave Associations in the Reviewed Event Bulletin

Abstract

<p>. ABSTRACT<br>When monitoring for possible underground nuclear tests,<br>identifying shallow earthquakes from explosive sources can<br>be achieved using the ratio of the body-wave magnitude to<br>the surface-wave magnitude (mb:Ms criterion), with explosive<br>sources producing less energetic surface wave excitation.<br>Current methods for automated surface-wave detection at the<br>International Data Centre (IDC) rely on a dispersion test - a<br>global group-velocity model is used to predict a time window<br>based on event origins in the IDC Reviewed Event Bulletin<br>(REB). The data in the predicted time window are narrowband<br>filtered into eight frequency bands - if the time of the maximum<br>energy of at least 6/8 of the bands sits within a specified error<br>of the expected dispersion curves, a surface wave is said to be<br>detected. Stevens et al. (2001) added phase match filtering to<br>the process to improve the signal-to-noise ratio, and this was<br>implemented into provisional operations at the IDC in 2010,<br>under the name Maxpmf.</p><p>A number of issues can potentially arise with this automatic<br>detection technique, leading to false detections and mis-associations, these include:<br>• local noise passing the dispersion test and being erroneously associated;<br>• surface waves detected at close-to-regional distances<br>experience little dispersion and hence impulsive signals<br>can pass the dispersion test;<br>• since automatic detection is only attempted for REB<br>events, some surface waves may be missed entirely, as<br>they lack an origin from which to calculate an arrival-time<br>window.</p><p>Assuming random noise and that the signals are independent,<br>Stevens (2007) defined parameters that determine the false<br>alarm rate, determined empirically from the network as it was<br>in 2007. Stevens (2007) recommended that these parameters be<br>continually reviewed. Since automated surface wave processing<br>at the IDC was implemented, the number of International<br>Monitoring System (IMS) seismic stations with at least one<br>surface-wave detection in the REB has significantly increased<br>(from around 50 stations in 2002, to around 145 in 2020)<br>without review of the false alarm rate parameters.<br>We have designed interactive software to manually review<br>stages of the IDC automatic surface-wave detection algorithm.<br>We will use this to investigate the false-detection rate and<br>how it has changed over time, and interrogate whether the<br>independence and random noise assumptions this prediction is<br>predicated on are still valid for a larger network.</p><p><br>REFERENCES<br>Stevens, J. L., 2007. Automatic surface wave processing support<br>and documentation, Tech. rep., CTBTO Vienna International<br>Centre.<br>Stevens, J. L., Adams, D. A., & Baker, G. E., 2001. Improved<br>surface wave detection and measurement using phasematched filtering with a global one-degree dispersion model,<br>Tech. rep., Science Applications International Corp San<br>Diego CA.</p>