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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 σ3 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 σ3 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.

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