Paradoxes of strength and brittleness of rocks at seismic depths

Abstract

The present models of deep-focus earthquakes and rock bursts rest upon the modern views of brittle rock fracture under conditions of non-equicomponent triaxial compression at seismic depths. Spontaneous macroscopic fracture can only develop beyond the ultimate stress limit. Thus, reliable information on post-limiting fracture mechanism and properties is critical for the appropriate understanding of dynamic phenomena. It is thought today that post-limiting fracture at any lateral pressure takes place in the stress range between the limit strength and residual strength governed by friction (static or dynamic) between fracture surfaces, and the residual strength is considered as the minimum strength of rocks. These basic conditions are included in all current models of dynamic processes. Yet this article demonstrates that strong rocks at seismic depths under conditions of high triaxial compression and at the time of post-limiting fracture exhibit anomalously low strength which can be low than residual strength by an order of magnitude. The strength reduction is accompanied by the anomalous increase in brittleness which can be tens times higher than brittleness under uniaxial compression. The combination of these properties makes the process of fracturing extremely violent and unpreventable, thus preventing disclosure of the true strength and brittleness characteristics of rocks in the post-limiting domain so far. The discovery of anomalous properties of high-strength rocks unveils new opportunities for getting a deeper insight into the real causes of deep-focus earthquakes and rock bursts.