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The Effect of Topographic Shape and Shot Depth on Seismic Waveform Generation at Local and Regional Distances

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Technical Report,12 Sep 2017,11 Sep 2018

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Leidos San Diego United States

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We use three-dimensional nonlinear calculations of explosions at the North Korean test site, together with the representation theorem, to determine the effect of topography and depth on local and regional phases, and on long period surface waves and far-field body waves. We completed 5 three-dimensional nonlinear finite difference calculations using the topography of the North Korean test site at the estimated location and depth of the September 2017 nuclear test. The new calculations have yields of 12.5, 20 and 180 kilotons, plus two additional calculations at 180 kilotons that include tectonic stresses. We calculate large displacements of 2-4 meters on the mountain surfaces near the explosion, but very little displacement on the surface directly above the explosion which is very close to the mountain peak. This is most likely due to the topography with gravity increasing displacement on downhill slopes. We find that the topography increases the amplitude of the surface reflected pP phase for all events at this test site, however the effect is reduced for the 2017 explosion because of strong nonlinear interaction with the free surface. The North Korean explosions appear to be anomalous in generating much larger surface waves than are expected based on surface waves observed from buried explosions in other areas, however surface waves derived from the 3D calculations are a very good match to the observed surface waves. Calculations show that explosions at the base of a mountain have amplified surface waves, which explains part of the anomaly. The mountain reduces horizontal stresses, which has an effect similar to tensile tectonic stresses, so added tensile stresses cause only a small additional amplification. The remaining difference is attributed to low Ms bias in the global data set due to free surface interaction and compressive tectonic release.

Subject Categories:

  • Nuclear Weapons

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