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

&amp;lt;p&amp;gt;. ABSTRACT&amp;lt;br&amp;gt;When monitoring for possible underground nuclear tests,&amp;lt;br&amp;gt;identifying shallow earthquakes from explosive sources can&amp;lt;br&amp;gt;be achieved using the ratio of the body-wave magnitude to&amp;lt;br&amp;gt;the surface-wave magnitude (mb:Ms criterion), with explosive&amp;lt;br&amp;gt;sources producing less energetic surface wave excitation.&amp;lt;br&amp;gt;Current methods for automated surface-wave detection at the&amp;lt;br&amp;gt;International Data Centre (IDC) rely on a dispersion test - a&amp;lt;br&amp;gt;global group-velocity model is used to predict a time window&amp;lt;br&amp;gt;based on event origins in the IDC Reviewed Event Bulletin&amp;lt;br&amp;gt;(REB). The data in the predicted time window are narrowband&amp;lt;br&amp;gt;filtered into eight frequency bands - if the time of the maximum&amp;lt;br&amp;gt;energy of at least 6/8 of the bands sits within a specified error&amp;lt;br&amp;gt;of the expected dispersion curves, a surface wave is said to be&amp;lt;br&amp;gt;detected. Stevens et al. (2001) added phase match filtering to&amp;lt;br&amp;gt;the process to improve the signal-to-noise ratio, and this was&amp;lt;br&amp;gt;implemented into provisional operations at the IDC in 2010,&amp;lt;br&amp;gt;under the name Maxpmf.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;A number of issues can potentially arise with this automatic&amp;lt;br&amp;gt;detection technique, leading to false detections and mis-associations, these include:&amp;lt;br&amp;gt;&amp;amp;#8226; local noise passing the dispersion test and being erroneously associated;&amp;lt;br&amp;gt;&amp;amp;#8226; surface waves detected at close-to-regional distances&amp;lt;br&amp;gt;experience little dispersion and hence impulsive signals&amp;lt;br&amp;gt;can pass the dispersion test;&amp;lt;br&amp;gt;&amp;amp;#8226; since automatic detection is only attempted for REB&amp;lt;br&amp;gt;events, some surface waves may be missed entirely, as&amp;lt;br&amp;gt;they lack an origin from which to calculate an arrival-time&amp;lt;br&amp;gt;window.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;Assuming random noise and that the signals are independent,&amp;lt;br&amp;gt;Stevens (2007) defined parameters that determine the false&amp;lt;br&amp;gt;alarm rate, determined empirically from the network as it was&amp;lt;br&amp;gt;in 2007. Stevens (2007) recommended that these parameters be&amp;lt;br&amp;gt;continually reviewed. Since automated surface wave processing&amp;lt;br&amp;gt;at the IDC was implemented, the number of International&amp;lt;br&amp;gt;Monitoring System (IMS) seismic stations with at least one&amp;lt;br&amp;gt;surface-wave detection in the REB has significantly increased&amp;lt;br&amp;gt;(from around 50 stations in 2002, to around 145 in 2020)&amp;lt;br&amp;gt;without review of the false alarm rate parameters.&amp;lt;br&amp;gt;We have designed interactive software to manually review&amp;lt;br&amp;gt;stages of the IDC automatic surface-wave detection algorithm.&amp;lt;br&amp;gt;We will use this to investigate the false-detection rate and&amp;lt;br&amp;gt;how it has changed over time, and interrogate whether the&amp;lt;br&amp;gt;independence and random noise assumptions this prediction is&amp;lt;br&amp;gt;predicated on are still valid for a larger network.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;&amp;lt;br&amp;gt;REFERENCES&amp;lt;br&amp;gt;Stevens, J. L., 2007. Automatic surface wave processing support&amp;lt;br&amp;gt;and documentation, Tech. rep., CTBTO Vienna International&amp;lt;br&amp;gt;Centre.&amp;lt;br&amp;gt;Stevens, J. L., Adams, D. A., &amp;amp; Baker, G. E., 2001. Improved&amp;lt;br&amp;gt;surface wave detection and measurement using phasematched filtering with a global one-degree dispersion model,&amp;lt;br&amp;gt;Tech. rep., Science Applications International Corp San&amp;lt;br&amp;gt;Diego CA.&amp;lt;/p&amp;gt;