Seismic b-Value for Foreshock AE Events Preceding Repeated Stick-Slips of Pre-Cut Faults in Granite

Xinglin Lei,Shinian Li,Liqiang Liu

doi:10.3390/app8122361

Abstract

In this study, the b-values for acoustic emission (AE) events during stick-slip cycles of pre-cut faults in granite (as an analogue of unfavorably oriented immature faults) under triaxial compression (confining pressure: 40 MPa) are investigated. Using a multi-channel AE waveform recording system and two peak detectors, we recorded AE waveforms at 16 bits and at a sampling rate of 25 MHz, as well as the maximum amplitude of AE events with a dynamic range of 55 dB. For stick-slip events, the b-value decreases from 1.2 to 1.5 to approximately 0.6 as the shear stress increases, and then quickly jumps back to 1.0 to 1.3 immediately prior to the dynamic stress drop. The minimum b-value coincides with the maximum event rate and a stress level of 70 to 95% of the shear strength. It is also observed that the AE activity during each cycle was linked with the pre-failure fault slip, which accounts for 30% of the dynamic slip. Our results on b-value evaluation preceding repeated stick-slips can be used as an indicator of the degree of fault maturity and shear stress acting on the fault, which is important in seismic hazard assessment and earthquake prediction, especially for the injection-induced seismicity for fields in which reactivated shear rupture of unfavorable and immature faults or tensile fractures is important.

Highlights

Reactivation of pre-existing immature and/or unfavorably oriented faults is an important issue in earthquake seismology, in cases of seismicity induced by fluid injection or extraction in applications such as enhanced geothermal systems, shale gas fracking, nonconventional oil/gas production, and sequestration of CO2 and other fluid wastes into deep formations [1,2]
The results of the present study indicate that seismic b-values could be an indicator of fault slip history and shear stress acted on the fault
The authors investigated the seismic b-value for acoustic emission (AE) events during repeated stick-slip cycles of pre-cut faults in granite under triaxial compression conditions

Summary

Introduction

Reactivation of pre-existing immature and/or unfavorably oriented faults is an important issue in earthquake seismology, in cases of seismicity induced by fluid injection or extraction in applications such as enhanced geothermal systems, shale gas fracking, nonconventional oil/gas production, and sequestration of CO2 and other fluid wastes into deep formations [1,2]. Earthquakes that occurred on unfavorably oriented faults are considered to be evidence of over-pressurized fluids and have attracted increasing attention (e.g., [3,4,5,6]). Immature and/or unfavorably oriented faults, especially those distributed in sedimentary formations, are important because such faults are locked and show very low levels of background seismicity, but might be reactivated due to man-made local pore pressure and stress changes [2]. In addition to rock blocks, the roughness of the fault surface, geometric

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