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Description: North Korea announced a second nuclear test on 25 May 2009, the first having taken place on October 9, 2006. Both tests were detected by the global seismic network of the Comprehensive nuclear Test-Ban-Treaty Organisation. We apply a correlation detector using a 10-s signal template from the 2006 test on the MJAR array in Japan to: 1) assess the potential for automatically detecting subsequent explosions at or near the test site; and 2) monitor the associated false alarm rate. The 2009 signal is detected clearly with no false alarms in a three-year period. By detecting scaled-down copies of the explosion signals submerged into background noise, we argue that a significantly smaller explosion at the site would have been detected automatically, with a low false alarm rate. The performance of the correlator on MJAR is not diminished by the signal incoherence that makes conventional array processing problematic at this array. We demonstrate that false alarm elimination by f-k analysis of single channel detection statistic traces is crucial for maintaining a low detection threshold. Correlation detectors are to be advocated as a routine complement to the existing pipeline detectors, both for reducing the detection threshold for sites of interest and providing automatic classification of signals from repeating sources.