Yield Estimation of the 25 May 2009 North Korean Nuclear Explosion

We collect nine vertical component broadband seismograms from the 25 May 2009 North Korean nuclear explosion for a regional seismic network in which eight stations also recorded the 9 October 2006 North Korean nuclear test. Comparing the observed waveforms and spectra from the two events, we estimate that the amplitudes of the records from the second event are approximately five times those from the first one. Additionally, we use 599 vertical broadband seismograms from 82 regional events recorded on the regional network between December 1995 and May 2009 to calibrate the network Lg-wave magnitude. The calibrated network is used to calculate the Lg-wave body-wave magnitude m(b)(Lg)=4.53 for the 25 May 2009 North Korean nuclear explosion. Based upon 15 first arrivals from the two North Korea nuclear explosions, the regional Pn velocity in the northeast China-North Korea region is calculated to be 8.0 km/s. This result, along with the regional geological structures, suggests that the North Korean test site (NKTS) is located at a relatively stable continental region. We thus use a modified fully coupled magnitude-yield relation to estimate the explosion yield, and the result shows that the yield of the 25 May 2009 North Korean nuclear test is approximately 2.35 kt under the minimum burial depth assumption.