Testing the Reviewed Event Bulletin of the International Data Centre Using Waveform Cross Correlation: Repeat Events at Aitik Copper Mine, Sweden

The quality of the Reviewed Event Bulletin (REB) issued by the International Data Centre (IDC) of the Comprehensive Nuclear-Test- Ban Treaty Organization (CTBTO) is crucial for the Member States as well as for the seismological community. One of the most efficient methods to test the REB quality is using repeat events having very accurate absolute locations. Hundreds of quarry blasts detonated at Aitik copper mine (the central point of active mining - 67.08N, 20.95E) were recorded by several seismic arrays of the International Monitoring System (IMS), found by IDC automatic processing and then confirmed by analysts as REB events. The size of the quarry is approximately 1 km and one can consider that the uncertainty in absolute coordinates of the studied events is less than 0.5 km as measured from the central point. In the REB, the corresponding epicenters are almost uniformly scattered over the territory 67.0N to 67.3N, and 20.7E to 21.5E. These REB locations are based on the measured arrival times as well as azimuth and slowness estimates at several IMS stations with the main input from ARCES, NOA, FINES, and HFS. The higher scattering of REB locations is caused by the uncertainty in measurements and velocity model. Seismological methods based on waveform cross correlation allow very accurate relative location of repeat events. Here we test the level of similarity between signals from these events. It was found that IMS primary array station ARCES demonstrates the highest similarity as expressed by cross correlation coefficient (CC) and signal-to-noise ratio (SNR) calculated at the CC traces. Small-aperture array FINES is the second best and large-aperture array NOA demonstrating mediocre performance likely due its size and the loss of coherency between high-frequency and relatively low-velocity signals from the mine. During the last five years station ARCES has been upgraded from a vertical array to a 3-C one. This transformation has improved the performance of CC-technique as applied to the Aitik mine events. We have also applied a Principal Component Analysis to estimate the level of variability in the signals as well as to build the best waveform template for effective detection and identification of all blasts conducted at Aitik mine.