The Distance Dependence of Regional Phase Discriminants
Scientific rept. no. 2, 30 Sep 1991-31 Oct 1992
AUSTRALIAN NATIONAL UNIV CANBERRA
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Amplitude ratios of P and S phases recorded at regional distances have been suggested as potential discriminants for the character of seismic sources, because of the differences expected in the radiation patterns of earthquakes and well-contained explosions. The most useful reference phase for P waves beyond 200 km from the source is Pn and for S possible choices are the mantle phase Sn and the crustally guided wave Lg. Each of these phases has a different interaction with the seismic structure of the crust and mantle and such structural effects will impose their own patterns on the radiation characteristics from the source. The range dependent component in LgPn and Sn Pn ratios have been investigated for a dense set of three-component records covering the distance range out to 700 km, for an explosive shot in water in southern Sweden which formed part of the 1979 Fennolora experiment. A stable measure of amplitudes and amplitude ratios is provided by using the vector resultant of ground motion the square root of the total energy. However, even for a common source there are significant variations in the amplitude ratios for LgPn and SnPn of a factor of three or more as a function of range. Three component record sections allow a detailed examination of the way in which individual records relate to the whole wavefields. The role of multiple reflections in building the Pg and Lg phases can be clearly seen as well as the complex substructure of the Pn and Sn phases returned from the uppermost mantle.