Two-Dimensional Axisymmetric Calculations of Surface Waves Generated by anExplosion in an Island, Mountain and Sedimentary Basin

摘要

The generation of long period (20-50 second) surface waves by an explosion on anisland, inside a mountain, or near a material discontinuity is studied using two-dimensional axisymmetric finite difference calculations. The generation of surface waves can be reduced substantially if the explosion is close to a vertical boundary such as a material velocity reduction, mountain slope, or island/ocean boundary. This occurs because the horizontal components of the static stress field are reduced at the boundary. Three sets of finite difference calculations were performed for an 'island' surrounded by an 'ocean' of air, water and a low velocity solid. Calculations include 'ocean' depths of 3 km and 6 km, and the 'island' varies in diameter from 6 km to 48 km. For a 48 km island material boundary has little effect on the surface waves. For an island width equal to the ocean depth, however, the surface wave generation is reduced by an order of magnitude for air and water, and reduced by more than a factor of 2 for the low velocity solid. Comparisons with the geography of the Amchitka and Mururoa island test sites show that this effect is small for explosions at those sites, however the effect appears to be important for Novaya Zemlya and other sites where explosions are detonated inside mountains, and at sites with strong material variations. A similar numerical experiment for an explosion in a sedimentary basin shows that surface waves can be amplified by the presence of a high velocity non-source boundary.