Full Moment Tensor Analysis of Nuclear Explosions in North Korea

A seismic moment tensor is a 3 x 3 symmetric matrix that characterizes the far-field radiation from a seismic source such as an earthquake or an explosion. We estimate full (unconstrained) moment tensors and their uncertainties for seven events at the North Korea nuclear test site. Six of the events were declared nuclear tests, and the remaining one, which we interpret as a cavity collapse, was an event that occurred eight minutes after the 2017 declared test. We also analyze two earthquake events that occurred to the south of the site. For each of the nine events, we perform a grid search over the 6D space of moment tensors, generating synthetic waveforms at each moment tensor grid point and then minimizing a misfit function between the observed and synthetic waveforms. We characterize the uncertainty of each inferred moment tensor in three ways: We use the variation in waveform misfit on the eigenvalue lune, we use a probability density function for moment tensor source types, and we use a confidence curve. We find that the source types (eigenvalue triples) for the nine events separate into three distinct groups, with one group for the nuclear tests, one group for the two earthquakes, and one group just for the collapse. We find that each of the six moment tensors for the declared tests is the sum of a double couple and a crack tensor, with the crack plane being nearly horizontal.