Elastoplastic source model for microseismicity and acoustic emission

Alexander Minakov,Viktoriya Yarushina

doi:10.1093/gji/ggab207

Abstract

SUMMARYThe microseismic events can often be characterized by a complex non-double couple source mechanism. Recent laboratory studies recording the acoustic emission during rock deformation help connecting the components of the seismic moment tensor with the failure process. In this complementary contribution, we offer a mathematical model which can further clarify these connections. We derive the seismic moment tensor based on classical continuum mechanics and plasticity theory. The moment tensor density can be represented by the product of elastic stiffness tensor and the plastic strain tensor. This representation of seismic sources has several useful properties: (i) it accounts for incipient faulting as a microseismicity source mechanism, (ii) it does not require a pre-defined fracture geometry, (iii) it accounts for both shear and volumetric source mechanisms, (iv) it is valid for general heterogeneous and anisotropic rocks and (v) it is consistent with elasto-plastic geomechanical simulators. We illustrate the new approach using 2-D numerical examples of seismicity associated with cylindrical openings, analogous to wellbore, tunnel or fluid-rich conduit and provide a simple analytic expression of the moment density tensor. We compare our simulation results with previously published data from laboratory and field experiments. We consider four special cases corresponding to ‘dry’ elastically homogeneous and elastically heterogeneous isotropic rocks, ‘dry’ transversely isotropic rocks and ‘wet’ isotropic rocks. The model highlights theoretical links between stress state, geomechanical parameters and conventional representations of the moment tensor such as Hudson source type parameters.

Highlights

1.1 Background and definitionsMicroseismicity and acoustic emission research is a part of earthquake science which focuses on weak seismic signals that often lack a clear main shock and form so-called microseismic clouds and earthquake swarms
We derive the seismic moment tensor based on classical continuum mechanics and plasticity theory
This representation of seismic sources has several useful properties: (i) it accounts for incipient faulting as a microseismicity source mechanism, (ii) it does not require a pre-defined fracture geometry, (iii) it accounts for both shear and volumetric source mechanisms, (iv) it is valid for general heterogeneous and anisotropic rocks and (v) it is consistent with elasto-plastic geomechanical simulators

Summary

Introduction

1.1 Background and definitionsMicroseismicity and acoustic emission research is a part of earthquake science which focuses on weak seismic signals that often lack a clear main shock and form so-called microseismic clouds and earthquake swarms. In comparison to normal earthquakes, microseismic events are characterized by smaller magnitudes, higher frequencies, shorter duration and a more complex source mechanism (Foulger et al 2004; Kamei et al 2015; Eaton 2018). Microseismicity is observed both in natural conditions and during geotechnical operations (Vavrycuk 2002; Fischer & Guest 2011). We reserve the word ‘microseismicity’ for small events (M < 0) that occur in geological environment (either naturally or induced by human operations) with a frequency of 101−103 Hz, whereas mainly ultrasound ‘acoustic emissions’ (104−106 Hz) is referred to laboratory deformation experiments on rock specimens

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