2-D AXISYMMETRICAL CALCULATIONS OF SURFACE-WAVES GENERATED BY AN EXPLOSION IN AN ISLAND, MOUNTAIN AND SEDIMENTARY BASIN

The generation of long period (20-50 s) surface waves by an explosion on an island, inside a mountain, or near a material discontinuity is s tudied using 2-D axisymmetrical finite difference calculations. The ge neration of surface waves can be reduced substantially if the explosio n is close to a vertical boundary such as a material velocity reductio n, mountain slope, or island/ocean boundary. This occurs because surfa ce waves from a shallow explosion are generated primarily by the horiz ontal stress components which are reduced at the boundary. Three sets of finite difference calculations were performed for an 'island' surro unded by an 'ocean' of air, water, and a low-velocity solid. The surfa ce waves were measured on 'land' on the other side of the 'ocean'. A ' solid' calculation with a uniform structure was also performed for com parison. Calculations include 'ocean' depths of 3 and 6 km, and the 'i sland' varies in diameter from 6 to 48 km. For a 48 km island, the mat erial boundary has little effect on the surface waves. For an island w idth equal to the ocean depth, however, the surface wave generation is reduced by an order of magnitude for air and water, and reduced by mo re than a factor of 2 for the low-velocity solid. Comparisons with the geography of the Amchitka and Mururoa island test sites show that thi s effect is small for explosions at those sites, however, the effect a ppears to be important for Novaya Zemlya and other sites where explosi ons are detonated inside mountains, and at sites with strong material variations. To estimate the effect of a high-velocity boundary surroun ding a low-velocity source region, we performed a calculation for an e xplosion in a sedimentary basin modelled after Yucca Valley at the Nev ada Test Site. In order to include finer details of the source region, a finely gridded finite difference calculation was performed in the s ource region, and surface waves exterior to the calculation were calcu lated using the representation theorem. We find that surface waves are amplified by about a factor of 2 by the high-velocity interface surro unding the low-velocity basin.