Microfractures, aftershocks, and seismicity

Laboratory investigation of microfracturing in brittle rock has revealed that microfracturing events can be detected after brittle fracture of rock in compression, provided the specimen remains intact. If the sample is isolated after fracture, microfracturing activity decays hyperbolically in a manner similar to typical earthquake aftershock sequences. If reloaded, the sequence is disturbed and a cumulative aftershock pattern develops which is similar to that described by Benioff as strain release due to shear creep. These two types of sequences are discussed with respect to a Markovian model of time dependent fracture in an inhomogeneous brittle medium. This model is then expanded to apply to earthquake aftershock sequences. According to this theory aftershocks are produced by creep rupture due to stress corrosion in the regions of stress concentration following the main shock. The conclusions from laboratory investigations of microfracturing are summarized with respect to the implications regarding the sequence of earthquakes