Far Eastern Federal University, Vladivostok, Russia:

B. G. Tarasov, Professor, Doctor of Engineering Sciences, bgtaras@gmail.com

Spontaneous rock fracture can only occur beyond the ultimate stress limit. The currently known three classes of the post-limit rock behavior (class I, class II and class III) feature different dynamics of spontaneous fracture at the same initial conditions. This article discusses post-limit properties and fracture energy balances in these classes of the post-limit rock behavior. The author focuses on least-studied class III typical of seismic depths. It is shown that class III features the lowest destructive energy and the highest elastic energy release, which creates conditions for the uppermost dynamics of spontaneous fracture. Class III rocks are most instable at seismic depths and are most susceptible to initiation of both natural and induced earthquakes and deep-level rock bursts. Ideas on the spontaneous fracture conditions in strong and brittle rocks were developed concurrently with techniques of studying post-limit properties of rocks. The techniques were improved with sequential discovery of the three classes of the behavior of rocks having post-limit properties illustrated in this article. The bar chart of typical change in the frequency of earthquakes and aftershocks with depth is explained by the action of the fan mechanism. The depth of rocks having minimal fan strength conforms with the depth of the maximum seismic activity. At shallow depths outside the influence zone of the fan mechanism, the stick–slip mechanism is active.
The study was supported by the Ministry of Science and Higher Education of the Russian Federation, Grant No. RFMEFI58418X0034.

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