The Effect of Steep Topography on Explosion-Generated Seismic Signals

Abstract:

Nuclear monitoring requires a detailed understanding of the physical factors that affect seismic signals. Nuclear tests have frequently been performed in locations with strong topography, and we are performing a series of calculations to determine the effects of topography on seismic signals. We examined the effect of depth beneath a mountainside or other steep slope on seismic waves by performing three 3D nonlinear calculations. The first is placed beneath a mountain; the second is higher and inside the mountain; the third is beneath a flat surface.

We find that generation of long period surface waves is strongly dependent on location relative to the slope. Long period surface waves are generated almost entirely by the static horizontal displacement in the upper few hundred meters below the surface. If the explosion is inside the mountain and close enough to the free surface that stress is relieved by the sides of the mountain, then subsurface horizontal displacement is reduced and surface wave amplitudes are reduced dramatically. However, for explosions that are below the mountain, long period surface waves are increased in amplitude.

In contrast, regional P and S phases are slightly larger for the explosion in the mountain than beneath it, especially for paths that go across the mountain. Otherwise, there is little difference in regional phases between the flat and topographic calculations.

Far field P body waves are slightly enhanced by the presence of the mountain due to reduction of the pP interference. Far field SV body waves are reduced in amplitude relative to the flat case because the pS phase which is very strong for a flat surface is reduced in amplitude for the mountain. Far field SH waves which do not exist for the flat surface are comparable in amplitude to SV for the mountain cases.

Nonlinear calculations with topography are difficult because the grid must be brought into gravitational equilibrium before the explosion calculation can be started. We perform an initial calculation without a source to accomplish this. The equilibrium calculation can take quite a long time, so we have developed a new procedure that speeds up the equilibrium run substantially.