Characterization of Elastic Waves Induced by Growth of Cracks in the Earth's Crust and its Appliation to Evaluation of Crack Properties

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Characterics of elastic waves (AE:acoustic emission) induced by growth of a crack in the deep earth's crust are studied in the intermediate region where the distance between the crack (the wave source) and an observation station is several ten times the crack size, and, furthermore, methods are discussed for the evaluation of crack properties, such as the size location, orientation and deformation mode. The results obtained in the present work are summarized as follows:(1) Spectrum of the circumferential displacement due to the elastic waves has a peak at wl/c_2=1.5, where w is angular frequency, l a representative length of the crack and c_2 the phase velocity of the shear wave. Therefore, the crack size can be estimated as l 1.5c_2/(lw_p), were w_p is the angular frequency of the peak.(2) The orientation of the crack can be determined by the displacement vector, the radiation pattern and the vector representing the direction from the observation point to the wave source.(3) The energy of the elastic waves due to tensile mode crack growth is very small compared with that due to shear mode crack growth. this means that mixed mode crack growth it apt to be misunderstood as if it is pure shear mode crack growth, since the contribution of tensile mode is very faint and is easily buried in the elastic waves due to shear mode.(4) The displacement vector just after longitudinal wave arrivals points the portion where the crack growth takes place. In shear mode crack growth, there exist the cases that the displacement victor can hardly be distinguished. In such case, the direction of the wave source can be determined by the displacement victor just after shear wave arrivals.