Dynamically triggered seismicity on a tectonic scale:A review

Earthquakes triggered by dynamic disturbances have been confirmed by numerous observations and experiments.In the past several decades,earthquake triggering has attracted increasing attention of scholars in relation to exploring the mechanism of earthquake triggering,earthquake prediction,and the desire to use the mechanism of earthquake triggering to reduce,prevent,or trigger earthquakes.Natural earthquakes and large‐scale explosions are the most common sources of dynamic disturbances that trigger earthquakes.In the past several decades,some models have been developed,including static,dynamic,quasi‐static,and other models.Some reviews have been published,but explosiontriggered seismicity was not included.In recent years,some new results on earthquake triggering have emerged.Therefore,this paper presents a new review to reflect the new results and include the content of explosion‐triggered earthquakes for the reference of scholars in this area.Instead of a complete review of the relevant literature,this paper primarily focuses on the main aspects of dynamic earthquake triggering on a tectonic scale and makes some suggestions on issues that need to be resolved in this area in the future.

Assessment of the Elastic-Wave Well Treatment in Oil-Bearing Clastic and Carbonate Reservoirs

A set of techniques for well treatment aimed to enhance oil recovery are considered in the present study.These are based on the application of elastic waves of various types(dilation-wave,vibro-wave,or other acoustically induced effects).In such a context,a new technique is proposed to predict the effectiveness of the elastic-wave well treatment using the rank distribution according to Zipf’s law.It is revealed that,when the results of elastic wave well treatments are analyzed,groups of wells exploiting various geological deposits can differ in terms of their slope coefficients and free members.As the slope coefficient increases,the average increase in the well oil production rate(after the well treatment)becomes larger.An equation is obtained accordingly for estimating the slope coefficient in the Zipf’s equation from the frequency of the elastic wave.The obtained results demonstrate the applicability of the Zipf’s law in the analysis of the technological efficiency of elastic-wave well treatment methods.

Dynamic strength of rocks and physical nature of rock strength

Time-dependence of rock deformation and fracturing is often ignored.However,the consideration of the time-dependence is essential to the study of the deformation and fracturing processes of materials,especially for those subject to strong dynamic loadings.In this paper,we investigate the deformation and fracturing of rocks,its physical origin at the microscopic scale,as well as the mechanisms of the time-dependence of rock strength.Using the thermo-activated and macro-viscous mechanisms,we explained the sensitivity of rock strength to strain rate.These mechanisms dominate the rock strength in different ranges of strain rates.It is also shown that a strain-rate dependent Mohr-Coulomb-type constitutive relationship can be used to describe the influence of strain rate on dynamic rock fragmentation.A relationship between the particle sizes of fractured rocks and the strain rate is also proposed.Several time-dependent fracture criteria are discussed,and their intrinsic relations are discussed.Finally,the application of dynamic strength theories is discussed.

A roughness parameter considering joint material properties and peak shear strength model for rock joints

This study aims at proposing a reasonable roughness parameter that can reflect the peak shear strength(PSS)of rock joints.Firstly,the contribution of the asperities with different apparent dip angles to shear strength is studied.Then the shear strength of the entire joint asperities is derived.The results showed that the PSS of the entire joint asperities is proportional to a key parameter hs,which is related to the geometric character of the joint surface and the joint material properties.The parameter hsis taken as the new roughness parameter,and it is reasonable to associate the PSS with the geometric characteristics of the joint surface.Based on the new roughness parameter and shear test results of 20 sets of joint specimens,a new PSS model for rock joints is proposed.The new model is validated with the artificial joints in this paper and real rock joints in published studies.Results showed that it is suitable for different types of rock joints except for gneiss joints.The new model has the form of the Mohr-Coulomb model,which can directly reflect the relationship between the 3 D roughness parameters and the peak dilation angle.

Potential triggers for large earthquakes in open-pit mines:A case study from Kuzbass,Siberia

The extraction and movement of rock during mining operations is considered a possible trigger for slip along the fault.In this study,possible anthropogenic causes were analyzed for triggering the earthquake with the local magnitude ofML=6.1(at a depth of 4km just beneath the quarry),in the vicinity of the large coal open-pit mine in Russia.This event was the largest earthquake associated with a quarry(an open pit).A sufficiently deep occurrence of the source testified that seismic vibrations were produced in a dynamic slip along a preexisting and prestressed tectonic fault.Analytical calculations were conducted of increments of normal and shear stresses at fault planes with several dip angles at depths corresponding to the probable location of the hypocenter of the Bachat earthquake.As the results show,long-term rock excavations bring a prestressed thrust fault closer to the ultimate Coulomb strength,and stress variations at those depths may suffice to initiate a movement along the fault.By measuring seismic vibrations at different quarries and mines in Russia,the dynamic effect at the supposed depth of the source occurrence could be reliably estimated.As is shown,the ultimate anticipated dynamic deformations are noticeably lower than corresponding values produced by seismic waves of distant earthquakes in the case when the effects of dynamic triggering were observed.Accordingly,the seismic effect of explosions cannot trigger a large earthquake with deep source occurrence.Operations in open-pit mines can only bring forward the moment of an earthquake at a potentially seismogenic fault.At the same time,the numerical calculations reveal that a branching network of underground tunnels located at several horizons can noticeably reduce the effective shear modulus of the host rock.This effect can even provoke a dynamic movement at a previously aseismic fault.

Research on the influence of rock fracture toughness of layered formations on the hydraulic fracture propagation at the initial stage

Deep underground rocks exhibit significant layered heterogeneity due to geological evolution and sedimentation.Rock fracture toughness, as one of the important indicators of hydraulic crack propagation, also exhibits heterogeneous distribution. In order to investigate the influence of non-uniform fracture toughness of layered rockson hydraulic crack propagation, this paper establishes a planar three-dimensional hydraulic crack propagationmodel. The model is numerically solved using the 3D displacement discontinuity method (3D-DDM) and the finitedifference method. The calculation results indicate that when the distribution of the fracture toughness of layeredrocks changes from uniform to non-uniform, the fracture morphology develops from a standard circular crack toan elliptical crack. When the difference of the rock fracture toughness between adjacent rock layers and themiddle rock layer (pay zone) is large enough, the fracture morphology will develop towards a rectangular shape.In addition, when the fracture toughness of rock layers is non-uniformly distributed, the hydraulic crack not onlyrapidly expand in the softening layer (rock layer with lower fracture toughness), but also slowly propagate in thestrong layer (rock layer with higher fracture toughness). However, the propagation speed in the softening layer ismuch faster than that in the strong layer. The results indicate that the heterogeneity of rock fracture toughness hasan important impact on the morphology, propagation speed, and direction of hydraulic fractures.

Internal variable gradient model for active earth pressure of rigid retaining wall moving with translation

The instability of retaining wall is a key factor for many geo-hazards,such as landslides.To estimate the stability of retaining wall,the distribution of earth pressure is necessary.The results of in-situ observations and indoor experiments demonstrate that the distribution of earth pressure behind the retaining wall exhibits remarkable nonlinearity.When the results are analyzed in details,the oscillation and quasi-periodicity of the distribution of earth pressure are observed,which has not been given widely concerns and cannot be described by the existing analytical models.Based on the internal variable gradient theory and operator averaging method,a gradientenhanced softening constitutive model is proposed in this paper to describe the oscillation and quasiperiodicity of the distribution of earth pressure acting on the retaining wall,by introducing the high-order gradient terms of the hydrostatic pressure into Mohr-Coulomb yield condition.In order to check the applicability of the proposed formulation,the predictions from the formulations are compared with the full-scale and laboratory-scale test results as well as the existing formulations.It is noted from the comparisons between predicted and measured values that the results of gradient-dependent softening constitutive model provides the comparable approximations for active earth pressure and describes the oscillation and quasi-periodicity very well.This model may enhance the comprehension of soil mechanics and provide a novel view for the design of the retaining wall.

Preliminary study on the determination of the Weibull modulus of strength distribution in quasi-brittle materials

In this paper,how to determine the Weibull modulus of a fracture strength distribution is discussed with its physical implications for quasi-brittle materials.Based on the Markov chain assumption,it is shown that the lifetime(i.e.,the time taken for formation of a critical defect)in a quasi-brittle material can be described by a gamma probabilistic distribution function.Prior to macroscopic failure,the effective number of energy barriers to be overcome is determined by the slope of the energy barrier spectrum,which is equivalent to the Weibull modulus.Based on a fracture mechanics model,the fracture energy barrier spectral slope and Weibull modulus can be calculated theoretically.Furthermore,such a model can be extended to take into account the crack interactions and defect-induced degradation.The predicted Weibull modulus is good agreement with that derived from available experimental results.

Investigation on the physical mechanism of cavity percentage dependent shear strength for rock joints considering the real contact joint surface

To explain the effect of joint roughness on joint peak shear strength(JPSS)and investigate the effect of different contact states of joint surface on JPSS,we try to clarify the physical mechanism of the effect of joint cavity percentage(JCP)on JPSS from the perspective of the three-dimensional(3D)distribution characteristics of the actual contact joint surface,and propose a JPSS model considering the JCP.Shear tests for red sandstone joints with three different surface morphologies and three different JCPs were performed under constant normal load condition.Based on test fitting results,the reduction effect of the JCP on JPSS is investigated,and a JPSS model for cavity-containing joints is obtained.However,the above model only considers the influence of JCP by fitting test data,and does not reveal the physical mechanism of JCP affecting the JPSS.Based on the peak dilation angle model for consideration of the actual contact joint morphology,and the influence of JCP on the roughness of the actual contact joint surface,a theoretical model of the JPSS considering the JCP is proposed.The derivation process does not depend on the test fitting,but is entirely based on the joint mechanical law,and its physical significance is clear.It is proposed that the essence of the influence of the JCP on JPSS is that the JCP first affects the normal stress of the actual contact joints,further affects the roughness of actual contact joints,and then affects the shear strength.

An Analytical Method for Predicting Direct Tensile Creep Fracture in Brittle Solids Containing Initial Microcracks

The tensile creep fracture behaviors in brittle solids are of great significance for the safety evaluation of brittle solid engineering.However,micromechanics-based tensile creep fracture behavior is rarely studied.In this study,a micromechanics-based method for predicting direct tensile creep fractures is presented.This method is established by combining the suggested expression of the mode-I stress intensity factor,the subcritical crack growth law,and the relationship between wing crack length and axial strain.This suggested mode-I stress intensity factor is formulated by the use of the basic theory of fracture mechanics under different loading modes.The rationality of the proposed tensile creep fracture model is verified by comparing with the experimental results.The correspondences of time-dependent axial strain,strain rate,wing crack length,and crack velocity are plotted under constant stress and stepping stress during tensile creep fracture.The effects of the initial crack size,inclination angle and density on the crack initiation stress,tensile strength,tensile creep fracture time,steady-state strain rate,initial strain,crack coalescence strain,and failure strain are discussed.