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The stresses are initially zero everywhere in the domain and the gravitational body force is not considered. These conditions are suitable to this study's purpose of estimating only dynamic stress changes. Absorbing boundary conditions are applied to all the boundaries except the top boundary, which is a free surface, and the boundary for explosion.  A nuclear explosion is approximated by time-dependent normal tractions applied on the surface of a 1 km-radius sphere carved out of the crust(Fig. \ref{fig:model_setup}c). The hollow sphere is at the center of the domain and buried at the depth of 2 km. The actual explosion must have occurred at a shallower depth but we used this geometry to prevent the mesh around the explosion site from being refined too much. The magnitude of normal traction is spatially uniform over the spherical boundary but varies in time. The magnitude is 0 Pa from 0 to 0.1 sec, linearly increases to 75 MPa from 0.1 to 2.1 sec and linearly decreases to 0 Pa from 2.1 to 4.1 sec. It remains at 0 Pa afterwards. The maximum magnitude and time variation of the tractions are adjusted such that model outputs can reasonably fit waveforms recorded at the two nearby seismic stations, DNH and YNB, in terms of the magnitude and duration. The monitoring points are set at the locations corresponding to the relative coordinates of these two stations with respect to the explosion site (Fig. \ref{fig:model_setup}b). DNH is at about 210 km NNW and YNB at about 160 km NNE from the test site. Displacement solutions interpolated at the monitoring points are compared with the records of the nuclear test-generated ground motions (Fig. \ref{fig:walkthrough}). \ref{fig:displ_mag}).  \subsection{Results}  The process of wave generation and propagation is shown in Fig. \ref{fig:walkthrough}. Iso-magnitude surfaces for the displacement field reflects the main features included in the models such as the isotropic source, the layered structure composed of crust and mantle and the absorbing boundary conditions. 
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