Modeling Pn Geometric Spreading and Attenuation by the Viscoelastic Finite-Difference Method with Applications to the North Korean Test Site

We usually simulate the seismic wave propagation using elastic models, but in the real Earth seismic waves are usually attenuated. Thus, it is hard to simultaneously investigate geometric spreading and attenuation by using the synthetic seismograms. For example, Xie and Lay (2017)investigated amplitude changes of Pn waves by using an elastic finite-difference method. They found that the Pn amplitude can be seriously affected by mantle lid velocity gradients and crustal thickness. However, Pn amplitudes are not only affected by these two factors mentioned above, but also related to P-wave attenuation. In this study, we employ the viscoelastic finite-difference (VFD) to simulate regional wave propagation for the North Korean underground nuclear tests, then perform waveform comparisons between synthetic and observed Pn waves, and try to construct both Pn geometric spreading and attenuation models in Northeast China and the Korean Peninsula.

Fan et al.(2016)developed a viscoelastic finite-difference (VFD) method based on a viscoelastic mechanical model consisting of several standard linear solids. They improved the calculation efficiency by separating Q approximation and VFD simulation procedure. We simulate regional wave propagation based on a crust-upper mantle model parameterized with both velocities and Q values. The results indicate that when the distance increases, the Pn amplitudes increase with an increasing mantle lid velocity gradients and/or increasing Q values, which may result from the superposition of different waves reflected from the Moho and weakening attenuation. The frequency-dependent variations are clearly observed for Pn amplitudes. This is consistent with previous studies (Avants et al., 2011; Yang et al., 2007).

Zhao et al.(2015)found that the Pn paths from the North Korean test site (NKTS) include a range of continental and oceanic paths, and the Pn signals traversing oceanic paths are more strongly frequency dependent than continental paths, with much lower high-frequency amplitudes. We obtained the depth of the Moho discontinuity from CRUST1.0 model. The simulation of Pn-wave propagation along paths with crustal thickness variation for NKTS is compared with observations. The results show that deeper the Moho, stronger Pn wave in crust.

(NSFC41674060, 41630210)