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Abstract

Monitoring compliance with the Comprehensive Nuclear-Test-Ban Treaty (CTBT) requires reliable seismological methods for event characterisation to identify possible candidate events within a sequence of seismic events, which are possible treaty violations. National Data Centre (NDC) are the national scientific organizations to advise their governments concerning CTBT monitoring and analysing suspicious events. The German NDC (BGR) regularly organises international NDC Preparedness Exercises (NPE), dealing with fictitious treaty violations to practice the combined analysis of CTBT verification technologies. These exercises help to evaluate the effectiveness of analysis procedures applied at NDCs and the quality, completeness and usefulness of IDC products. The last exercise - NPE2019 - was a combined waveform-radionuclide scenario. The source region and time domain of a possible treaty violation activity was determined from atmospheric transport modelling (ATM) in backtracking mode with input data from fictitious particulate radionuclide and radioxenon measurements at stations of the International Monitoring System. An earthquake sequence could be identified within the specified source region and time period from ATM analysis. Due to the untypical shallow focal depth of about 3 km and no mining activities, the earthquake sequence in combination with synthetic data is well suited to assess discrimination methods. Synthetic waveform data, which were modified from real recordings to simulate more explosive like characteristics, are added to the earthquake sequence. A similarity analysis was applied to the combination of fictitious and real waveform data with the objective to identify the fictitious recordings as conspicuous. Various score values were defined to achieve an automatic procedure.