Waveform data recorded at seven of the International Monitoring System (IMS) seismometer arrays are used to measure the relative times of teleseismic P signals with three methods: analyst picks, and two methods that make full use of the capabilities of seismometer arrays, the cross correlation of array beams and the average of channel-by-channel cross correlations. These times are used to estimate the relative locations of the 9 October 2006 and 25 May 2009 Democratic People’s Republic of Korea announced underground nuclear tests. The 2009 test is found to have occurred about 1.8±0.8 km to the west and 0.4±0.6 km to the north of the 2006 test. The use of cross correlation reduces the standard deviation of the travel-time residuals from ∼0.05 to ∼0.01 sec, enabling the two epicenters to be statistically distinguished with high confidence. This result demonstrates the power of a small number of IMS seismometer arrays at teleseismic distances to detect and relatively locate small explosions with high precision.

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