Magnitude Scaling of Regional-phase Amplitudes from the DPRK Announced Nuclear Tests

At regional distances (<~1700 km) the phases Pn, Pg and Lg are generally the most prominent arrivals from a crustal seismic source.  Amplitude ratios of Pn or Pg to Lg have been investigated by several authors (e.g. Hartse et al. 1997 Bull. Seism. Soc. Am.) as earthquake/explosion discriminators.  Theory and observation show that explosions generate shear phases less efficiently than earthquakes, hence the amplitude ratio of Pn and Pg to Lg is expected to be higher for explosions, especially at frequencies above ~2 Hz.  Walter et al. (2108 Seismol. Res. Lett. DOI 10.1785/0220180128) showed that amplitude ratios Pg/Lg and Pn/Lg at 2-4 Hz were clear discriminants between the six announced nuclear tests of the Democratic People's Republic of Korea (DPRK) and a population of earthquakes.  We investigate regional-phase amplitudes for stations MDJ (distance ~376 km) and USRK (~406 km). Walter et al. found a weak dependence of Pg/Lg in the 2-4 Hz band at MDJ on the magnitude Mw of the explosion. We find this dependence at USRK also.  We also explore the regional amplitude behaviour at a range of frequencies, and dependence on different magnitude measures, such as network body-wave and surface-wave magnitudes.

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