Improved concept of lithospheric strength and earthquake activity at shallow depths based upon the fan-head dynamic shear rupture mechanism

Abstract

The typical depth–frequency distribution of earthquake hypocentres (DFDE) demonstrates that, below an upper cutoff, the earthquake frequency increases with depth up to a maximum value and then decreases and ceases at a lower cutoff. Such regular behaviour of earthquakes implies the existence of some fundamental mechanisms responsible for the distribution. Conventional models of lithospheric strength based upon the assumption that the frictional strength along pre-existing faults represents a lower limit on the rock shear strength do not provide any intrinsic logic for the observed DFDE. The paper shows that these models ignore the specific properties of intact hard rocks which can exhibit extremely low transient strength (significantly lower than the frictional strength) during failure under the high confining stresses corresponding to seismogenic depths. The low transient strength is provided by a recently identified fan-head shear rupture mechanism which can be initiated in intact rocks in the proximity of pre-existing faults. The low transient shear strength of intact rock determines the correspondingly low transient strength of the lithosphere, which favours generation of new earthquake faults in the intact rock mass adjoining pre-existing faults in preference to frictional stick-slip instability along these faults. The efficiency of the fan-mechanism within the seismogenic layer is variable, with maximum efficiency at the middle range between the upper and lower cutoffs, thus providing minimum transient strength of the lithosphere and maximum earthquake frequency at that depth. We believe that this intrinsic property of hard rocks is responsible for the observed DFDE. Importantly, the formation of new faults in intact rock generated by the fan-mechanism can be accompanied by very small stress-drops (similar to, or lower than, stress-drops for frictional stick-slip instability) combined with abnormally high energy release. The paper proposes an improved concept of lithospheric strength and earthquake activity at seismogenic depths.