Abstract

It is generally accepted that the frictional strength of pre-existing faults represents a lower limit on rock shear strength in the seismogenic layer. The paper proposes an alternative concept based on a recently identified shear rupture mechanism (fan-mechanism). The fan-mechanism creates paradoxical effect providing the formation of new faults in intact rock mass at shear stress levels significantly below the frictional strength. This indicates that the lower limit of lithospheric strength is not restricted by the frictional strength. The varying efficiency of this mechanism with depth provides a specific depth distribution of lithospheric strength.

1 Introduction

Earthquakes are normally related to pre-existing faults,which implies an essential role of them in earthquake activity. It is assumed that the Earth's crust is riddled with faults. Modern understanding of the nature of earthquakes is that the primary mechanism comprises stick-slip instability on pre-existing faults, along which the frictional strength represents a lower limit on rock shear strength in the lithosphere (Scholz 2002). Depth distribution of the lithospheric shear strength within the seismogenic layer is determined by the Byerlee's law (Byerlee 1978) in consideration of enhanced temperature at great depths (Brace & Kohlstedt 1980).

The paper proposes an alternative concept based on a recently identified self-unbalancing ‘fan-head’ shear rupture mechanism (Tarasov 2008, 2010, Tarasov & Randolph 2011, Tarasov & Guzev 2013). The fanmechanism is activated at stress conditions corresponding to the seismogenic layer and can provide the formation of new faults in intact rock mass at shear stress levels significantly below the frictional strength. This indicates that the lower limit of lithospheric strength is not restricted by the frictional strength. An important feature is that in nature the fanmechanism nucleates in the vicinity of pre-existing discontinuities (boundaries between tectonic plates, faults, deep mines, etc.) which serve as local stress concentrators necessary for the initial formation of the fan-structure. The proximity of the pre-existing faults to the area of instability caused by the fan-mechanism creates the illusion of stick-slip instability on the preexisting faults, thus concealing the real situation. The new concept proposes a new mode of instability in the Earth's crust in addition to the generally accepted frictional stick-slip instability. The varying efficiency of this mechanism with depth provides a specific depth distribution of lithospheric strength.

This content is only available via PDF.
You can access this article if you purchase or spend a download.