Regional Observations of the 2006 and 2009 Declared North Korea Nuclear Explosions

The October 2006 and May 2009 declared North Korean nuclear tests produced regional seismic phases, Pn, Pg, Sn and Lg that were observed at publically available seismic stations such as MDJ and TJN. In comparison with nearby earthquakes, the relatively low amplitudes of the S-wave phases at high frequencies can be used to identify these events as explosion-like, particularly when contrasted with P-waves, as in P/S ratios. However regional phase amplitudes can vary greatly over small distances, particularly as the crust and lithosphere thicknesses and velocity structures change due to tectonic activity. These variations can complicate simple methods to use P/S ratios to discriminate explosions from earthquakes over broad regions, particularly when there are not nearby events for comparison. In addition, when S-waves do not propagate effectively due to strong attenuation or structural variability, all events below a certain magnitude threshold from a source region to a station may have S-wave amplitudes below the P-wave coda level, leading to the large P/S amplitude ratios characteristic of explosions. Here we make use of earthquake spectral source models, and regional propagation models to compare events over the broad region around the Korean Peninsula. In particular we investigate the ability of the multi-phase regional attenuation tomography technique of Pasyanos et al. (2009) to reduce the geographic variability of earthquake P/S values in this region and enhance the ability to identify explosions by their regional phase characteristics.