Dramatic Weakening and Embrittlement of Intact Hard Rocks in the Earth’s Crust at Seismic Depths as a Cause of Shallow Earthquakes

Abstract

Frictional stick-slip instability on pre-existing faults is well studied experimentally and considered as the general mechanism for shallow earthquakes. At the same time, post-peak properties of intact hard rocks under high confining stresses σ 3 corresponding to seismic depths of shallow earthquakes are still unexplored experimentally due to uncontrollable and violent failure of rock specimens even on modern stiff and servocontrolled testing machines. The lack of knowledge about post-peak properties of the majority of the earthquake host rocks prevents us from understanding and quantifying the contribution of these rocks to shallow earthquakes. This paper discusses a recently identified shear rupture mechanism operating in hard rocks under high σ 3 which causes dramatic rock weakening and embrittlement (by tenths of times) during the post-peak failure. The unknown before 'abnormal' properties of hard rocks imply the fundamentally different general mechanisms for shallow earthquakes. It is shown that in the earth's crust, the new mechanism acts in the vicinity of pre-existing faults only and provides the formation of new dynamic faults in intact rocks at very low shear stresses (significantly less than the frictional strength). The fault propagation is characterised by extremely low rupture energy and small stress drop. These 'abnormal' properties make hard rocks the main and more dangerous source of shallow earthquakes in comparison with pre-existing faults.

Fan‐hinged shear instead of frictional stick–slip as the main and most dangerous mechanism of natural, induced, and volcanic earthquakes in the earth's crust

“…This means that the specimen strength at these stages of failure is equal to τ fan , which is significantly lower than the frictional strength τ f . This stage of the postpeak rupture is classified as class III (Tarasov, 2019). When the fan crosses the specimen, the strength of the specimen increases and is determined by the residual frictional strength τ f .…”

“…It was found that at a lower unloading rate, the stress-time curves take the form of an AEC curve, and for spontaneous rupture without servo-controlled unloading, they take the form of an AF curve. A more detailed consideration of the duality of the strength properties of hard rocks in the rupture process controlled by the fan mechanism is presented in the findings of Tarasov (2019).…”

“…Since extraordinary features of the fan mechanism and the properties of rocks determined by it are contradictory to the modern ideas, the fan‐hinged approach is viewed with great skepticism and distrust. The problem is that in the brief papers published so far, it was impossible to present this extremely complex and extensive topic with clear arguments, reliable evidence, and detailed discussion (Tarasov, 2008, 2010, 2011, 2013, 2014, 2016, 2017, 2019; Tarasov & Guzev, 2013; Tarasov & Ortlepp, 2007; Tarasov & Potvin, 2013; Tarasov & Randolph, 2008, 2011, 2016; Tarasov & Sadovskii, 2016; Tarasov et al, 2016, 2017).…”

New physics of supersonic ruptures

“…Accepting this fact, the models were simplified by presenting the future fault as consisting of "predetermined" slabs. A detailed description of the physical and mathematical models is presented in previous papers (Tarasov, 2010(Tarasov, , 2014(Tarasov, , 2017(Tarasov, , 2019Tarasov & Guzev, 2013;Tarasov & Randolph, 2011Tarasov et al, 2017). This paper uses The structure of the future fault consists of a row of "predetermined" slabs (tiles) inclined at an angle α 0 to the rupture plane.…”

“…Since the extraordinary features of the fan mechanism and the properties of rocks determined by it contradict the classic theories, the fan‐hinged approach was perceived with great difficulty and distrust. The problem is that in the brief papers published so far, it was impossible to present this extremely complex and extensive topic with clear arguments, reliable evidence, and detailed discussion (Tarasov, 2008, 2010, 2011, 2013, 2014, 2016, 2017, 2019; Tarasov & Guzev, 2013; Tarasov & Ortlepp, 2007; Tarasov & Potvin, 2013; Tarasov & Randolph, 2008, 2011, 2016; Tarasov & Sadovskii, 2016; Tarasov et al, 2016, 2017).…”