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Radiation from tensile fractures. Mansinha, Lalatendu


The geographical distribution of the sense of the first motion of P waves (and to a very limited extent, S waves) has been studied by seismologists to provide information on the focal mechanism of earthquakes. In this thesis we investigate the inverse problem; knowing the type and form of displacement at the focus at the focal instant, we study the azimuthal distribution of the sense of first P and S motion, using model seismic technique. The source of elastic energy is a thermally induced tensile fracture in a glass plate. Two types of fractures have been studied: Initial (Bilaterally propagating) Fractures and Extended (Unilaterally propagating) Fractures. The azimuthal distribution of the P and S wave amplitudes is indicated. The experiments reported in this thesis constitute a partial test of a recent theory by Knopoff and Gilbert (1960) on first motions from seismic sources. The type of fracture studied corresponds to Case 3 of Knopoff and Gilbert. Our results show significant discrepancies with the theory. The sense of the measured first S motion is opposite to that predicted by the theory, for both Initial and Extended Fractures. The ratios Pө /P₉₀ and Sө/Pө differ in magnitude from the theory in many azimuths. It is suggested that the discrepancies are possibly due to the neglect in the theory of non-linear elastic effects near the tip of the fracture.

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