Abstract

Regional seismic networks are occasionally useful for purposes other than earthquake studies. Three such applications which involve surface and atmospheric phenomena are described in this report: (1) the detection and location of a missile silo explosion in Arkansas; (2) analysis of an aircraft sonic boom; and (3) attempts to determine the time and location of ground impacts of aircraft and a meteorite. When the source of the disturbance is in the air, seismic instruments respond to the air-coupled surface waves generated in the wake of the atmospheric shock front advancing with the speed of sound in air. Thus, the disturbance's point and time of origin can be determined using a standard iterative hypocenter program with a “crustal” velocity model appropriate to the prevailing atmospheric temperature and pressure conditions. Simplified theory predicts that air-coupling is significant only when the speed of sound in air is close to the shear wave velocity of the surface crustal layer. This is normally the case only when unconsolidated material is present at the surface. The observations discussed here support this prediction: seismic network instruments sited on soft ground recorded signals from atmospheric disturbances much more clearly than did higher gain instruments on hard rock.

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