1) The document describes using cross-correlation techniques to analyze seismic data from underground nuclear explosions in North Korea. 2) Cross-correlation was able to identify signals in the 2006 North Korean test that were below the detection threshold, demonstrating this technique's ability to detect smaller events. 3) An analysis of seismic data from sensors in the five days following North Korea's 2009 underground nuclear test found no aftershocks above magnitude 3.0, consistent with the lack of detected radionuclides from smaller explosions.