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We characterize the seismic events that occurred in North Korea on 9 September 2016 and South Korea on 12 September 2016. The 9 September 2016 event was identified as an explosion, and the two 12 September 2016 events were identified as natural earthquakes using the P/S (P-and S-wave) spectral ratios, Pg/Lg, Pn/Lg, and Pn/Sn as discriminants. The explosive event was relocated within the North Korean nuclear test site using a relative location method and the 2006 North Korea underground nuclear test as the master event, and the epicenter was identified at 41.2976 degrees N latitude and 129.0804 degrees E longitude. From the regional Lg and Rayleigh waves, the body-and surface-wave magnitudes for the 9 September 2016 event were calculated as m(b) (Lg) = 4: 8 +/- 0: 2 and M-s = 4: 2 +/- 0: 1. By adopting an empirical magnitude-yield relation for the body-wave magnitude, and assuming that the explosion was fully coupled and detonated at a normally scaled depth, we estimated that the seismic yield was similar to 6 kt, and the uncertainty range was between 3 and 11 kt. If an overburied depth range between 780 and 1200 m was applied, then the yield would be increased to 16-22 kt.

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. Online Material: Epicentral parameters, computed m b( Lg ), and corrected m b( Lg ) for 82 events used in the study.

We investigate the regional seismic signature of the 9 October 2006 North Korean nuclear test. Broadband regional data for the nuclear test and a group of earthquakes close to the test site were obtained between December 2000 and No- vember 2006. Epicentral distances from the stations to the test site are between 371 and 1153 km. We first use these regional events to calibrate the Lg-wave magnitude in the network. Then the network is used to calculate mbLg �� 3:93 for the North Ko- rean nuclear explosion. Using a modified fully coupled magnitude-yield relation, the yield of the North Korean nuclear test is estimated to be 0.48 kt. Because of large uncertainties in the source depth, the estimate is preliminary. The P=S-type spectral ratios Pg=Lg, Pn=Lg, and Pn=Sn are calculated for the nuclear explosion and a group of earthquakes close to the test site. At frequencies above 2 Hz, the network-averaged P=S spectral ratios clearly separate the 9 October 2006 explosion from the regional earthquakes. Our result indicates that a single-blast explosion in the North Korea re- gion shows different seismic characteristics from an earthquake. Any well-coupled single-blast explosion detonated in this region with yield similar to that for the North Korean nuclear test has a large probability of being identified by a regional seismic network such as the one adopted in this study.

The widely distributed Cenozoic sediments in the Bohai Bay Basin give rise to noticeable modification of broadband teleseismic P waveforms. At one station in the west Bohai Bay Basin, the observed amplitude of tangential P-receiver func- tions is significantly above the noise level, and sedimentary reverberations (e.g., the P-type wave PpPp) remain one of the most prominent features in both the radial and tangential components. To investigate the lateral heterogeneity structure under this site, a 3D raytracing technique is used to compute the teleseismic P-wave response, and a fast simulated annealing algorithm is applied to the simultaneous inversion of radial receiver functions for different backazimuths. An upper crustal structure con- sisting of shallow dipping sedimentary layers with low seismic velocities and large Poisson's ratios is proposed to interpret the observed seismic data. The west-dipping interfaces we obtained are consistent with the north-northeast-south-southwest sur- face geology in North China, and Tertiary extensional fault structures may be re- sponsible for the formation of dipping sedimentary layers.