Experimental study of AE from rock under cyclic loading - in relation to the effect of oceanic tide to the earthquake swarm

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In the case of the 1980 Izu-hanto-toho-oki earthquake swarm, a very clear correlation between the seismic activity and the oceanic tide was found (Mogi, 1983). To obtain a clue to clarify this tide effect, laboratory measurements of AE from rock specimens under cyclic loadings were carried out. Since it is deduced that the swarm-type earthquakes in the Izu occurred in an extension stress field, cyclic loadings of a tensile type were applied to a rectangular parallelepipeds rock sample by a bending method. The amplitude of cyclic loadings were increased at successive intervals.According to this experiments, AE events occur both in the upward and the downward processes of loadings. It is concluded that AE events under cyclic loadings occur by the following two different mechanisms, (a) development of micro-fractures and (b) stick-slip along micro-crack surfaces.The experimental results show that there are some common character istics between AE and natural earthquakes under repeated loadings. They are: (1) The peak of activity dose not always coincide with the maximum point of loadings; (2) Both AE and seismic events occur at specific stress levels (the appearance of AE and earthquake families); (3) The activity in the earthquake swarm is high in the upward process of the oceanic tide, and the AE activity is also high in the upward process of loadings for the case of small amplitudes of cyclic loadings. These results may give a clue to make clear the effect of tidal stress to the earthquake occurrence.