Local stress and strain fields near a fault as indicators of movement on its surface: Mechanics and seismicity

Abstract

Experimental and theoretical studies show that displacements along fault planes cause regular distortions in the initial stress field near the fault. One would naturally expect these distortions to influence source mechanisms of small earthquakes and the number of seismic events. We attempt to reveal this effect and to show its possible use for recognizing displacements along large faults. Some variants of a three-dimensional stress field conditioned by the displacement along the fault planes were obtained on the basis of a theoretical two-dimensional stress field near the fault and the third superimposed principal stress. The variants were used to plot theoretical prognostic schemes for secondary shear fractures in the active fault zone. The schemes enable the prediction of location, dimensions, and configuration of areas with shear fractures of different types (thrusts, strike-slip and normal faults), orientation of their planes and zones of high shear stress. The areal distribution of seismotectonic deformation types and the number of seismic events for two 3-year periods were studied in the source zone of the 1978 Alai earthquake. The results were compared with the theoretical prognostic patterns of secondary shear fractures in the active fault zone. This investigation discovered that the deformations might have resulted from aseismic shear movement (creep) along one of the two regional large Pamir-Alai faults. Thus, it is shown to be possible to reveal the active part of the fault, as well as its length and the direction of the displacement along the fault, by data on weak seismicity alone.