Mechanism of rock bursts in a large inclined extra-thick coal seam under the condition of upper slice mining: a case study

Abstract With the increase of mining depth and mining intensity of the large inclined extra-thick coal seam (LIETCS), the number of rock bursts increases significantly. In this paper, based on a rock burst during upper roadway driving of the LIETCS, the distribution characteristics and fracture mechanism of microseismic (MS) events are analyzed using data statistics and seismic moment tensor inversion. By means of mechanical analysis and numerical simulation, this paper studies the static load distribution characteristics and dynamic load action effect of the LIETCS under upper slice mining condition. Based on the theory of dynamic and static combined load, the mechanism of rock bursts is proposed. The results show that the coal and rock mass of the LIETCS are mainly destroyed by shearing. The “shear-clamping” stress produced by the joint action of the roof and floor is the static load source. In upper slice mining, the shear stress and energy of the coal pillar behind the slice and the coal body below the coal pillar always remain high. Under the superposition of floor dynamic load, the regional stress environment changes, which induces rock bursts. This study provides a theoretical basis for rock burst prevention under similar conditions. Key Policy Highlights By means of data statistics and seismic moment tensor inversion, the paper analyzes the distribution characteristics and focal mechanism of MS events in the rock burst area of the LIETCS, and indicates that the coal and rock mass around the rock burst area and the floor rock stratum are mainly destroyed by shearing. By means of theoretical formula and numerical simulation, this paper explores the static load evolution law and dynamic load effect of the LIETCS during the upper slice mining. The results show that the joint action of the roof and floor produces “shear-clamping” stress and dynamic load is the force source of rock bursts. Based on the theory of dynamic and static combined load, the paper reveals the process of rock bursts induced by the high-stress area of the coal and rock mass below during the upper slicing mining of the LIETCS and the dynamic load of the floor.


Introduction
The large inclined coal seam generally refers to the coal seam with a dip angle of 35 $55 , which is extremely difficult to mine.Widely distributed in Sichuan, Chongqing, Yunnan, Guizhou, Xinjiang, Gansu, Ningxia and other regions in China, the large inclined coal seam accounts for 10%$20% of the national reserves, thus being very important for regional economic development (Wu et al. 2020).With the increase of mining depth and mining intensity of large inclined coal seams, the number of rock bursts increase significantly, and the degree of coal and rock dynamic appearance increases.Rock bursts have become one of the serious dynamic disasters in the mining of large inclined coal seams (Zhang and Wang 2014, Feng et al. 2018, Ju et al. 2019).
Scholars have made fruitful achievements in the study of rock burst theory.The most classical theories on rock bursts include strength theory, stiffness theory, energy theory, and impact liability theory (Jiang et al. 2014).Among them, the strength theory was first proposed by Cook, which held that the failure of coal and rock mass is determined by the ratio of coal and rock strength (Cook 1965).Based on the above theories, scholars have successively put forward the theory of deformation system instability, shear-slip theory, three criteria theory, "three-factors" theory, strength weakening and shock reduction theory, etc. (Qi et al. 2019).With the development of research, the energy balance equation and the influence of dynamic load have received extensive attention.Khademian and Ugur (2018), Khademian and Ozbay (2019) developed a computational framework for studying rock burst events based on energy analysis, and quantitatively studied violent failures in potentially complex geological conditions and excavation layouts.He et al. (2017) discovered that the stress wave disturbance of mine tremors would change the stress state, and the coal structure was easy to lose its stability and form a rock bursttype of dynamic disaster.Li et al. (2022) maintained that the total kinetic energy in a rock burst event is generally composed of three portions: one portion from the strain energy stored in the burst rock, a second from the strain energy released from the surrounding rock mass, and a third from the seismic energy.The above research results fully demonstrate that the static load and energy of coal rock mass, as well as the dynamic load disturbance, are the key factors to induce rock bursts.The combination of dynamic and static loads is more likely to induce rock bursts.
Scholars have made some achievements in studying surrounding rock instability and rock burst in large (steeply) inclined coal seams.In terms of large inclined coal seams, Wu et al. (2010) proposed that the inclined masonry structure is formed after the roof of long-wall stope of large inclined coal seam is broken.On this basis, they pointed out that the unbalanced movement of this structure is the dominant influencing factor leading to the instability of the "roof-support-floor" system.Wang et al. (2015aWang et al. ( , 2015b) ) established a thin plate mechanical model of the basic roof of the LIETCS.As is revealed by this model, the spatial order of the first breaking of the basic roof of the LIETCS is "middle upper part !middle lower part !upper part !lower part", while that of the periodic breaking is "middle lower part !middle upper part !upper part !lower part".Xue et al. (2022) found that the roof energy of large inclined coal seam accumulates in the lower end and middle-upper area of the working face, while the floor energy accumulates in the lower end area of the working face; on this basis, the rock burst mechanism is expounded.
In terms of steeply inclined coal seams, Lv et al. (2019) found through mechanical analysis that the roof failure location moves toward the upper area of the working face with the increase of inclination angle and working face length in steeply inclined coal seams.Wang et al. (2022) further pointed out that the roof breaking is the main dynamic load source.Based on the theory of dynamic and static combined load, He et al. (2020) constructed the damage models and damage process of rock bursts in horizontal section mining of steeply inclined coal seam.In the above research results, the scholars mainly discussed the breaking law of the roof in large inclined and steeply inclined coal seams and analyzed the influence of the dynamic load of the roof.Based on energy theory and the theory of dynamic and static combined load, the mechanism of rock bursts is preliminarily expounded.
Many studies have shown that numerical simulation and MS monitoring are powerful tools to study rock mechanics and rock bursts (Starfield and Cundall 1988, Ma et al. 2015, Cai et al. 2018, Cao et al. 2018, Wei et al. 2021).In terms of numerical simulation, Yang et al. (2018) used FLAC 3D to study and found that the solid-coal side of the gobside entry forms an L-shaped stress concentration zone at a dip angle of 30 .With ANSYS software, Ye et al. (2017) obtained that with the increase of the inclination mined-out range of large inclined coal seam, the stress distribution of overlying strata and coal seams at both ends of goaf becomes increasingly dense.Yang et al. (2020) used numerical calculation and other methods to find that the deformation and instability of roadway floor side in horizontal section mining of steeply inclined coal seam begins with shearing slip in the bottom corner.In terms of MS monitoring, Wang et al. (2020) compared the monitoring and warning indices for steeply inclined coal seams and flat seams.The results show that the three indices for dynamic loads and seismic tomography results in steeply inclined coal seams exhibited remarkable precursory information.He et al. (2019) analyzed the temporal and spatial evolution laws of the monitoring parameters of MS and acoustic emissions and the relationship between precursory information of both the acoustic emissions and MS monitoring systems.The main factors of the rock burst in a steeply inclined coal seam are obtained on this basis.The above research demonstrates the importance of numerical simulation and MS monitoring in rock bursts and large (steeply) inclined coal seams.
In the above research, the scholars studied the roof breaking form of large inclined coal seams and the effect of the roof dynamic load on rock bursts.In addition, the distribution characteristics of stress and energy and the evolution laws of MS events in large inclined coal seams are studied.However, the above results do not fully and clearly describe the static and dynamic load source of rock bursts in large inclined coal seams.There needs to be a specific analysis of the dynamic load effect caused by the fracture of the floor and the fracture mechanism of MS events in large inclined coal seams.There is also no targeted research on the rock burst mechanism of the lower heading face during the upper slice mining of the LIETCS.
With a coal mine in China as the engineering background, this paper obtained the dynamic and static load sources by analyzing the spatial distribution characteristics of MS events and the focal mechanism during the upper slice mining of LIETCSs.Combined with the static load distribution characteristics of the coal body in the lower working face during the upper slice working face mining and the dynamic load effect of the floor, the mechanism of rock bursts is revealed.This study provides a theoretical basis for rock burst prevention under similar geological mining conditions.

Geological and mining conditions
This paper takes the working faces between 940 $ 875 level of the coal mine as the engineering background.To be specific, 940 $ 875 refers to the level elevation, and the corresponding ground elevation is 1431 $ 1523.The upper, middle and lower slice mining working faces are arranged between the levels, and the mining sequence is from top to bottom.The LW37121-1 in the upper slice is used to mine 6-1 coal seam, with a total mining height of 8 m; the LW37221-1 in the middle slice is used to mine 6-2 coal seam, with a total mining height of 11 m; the LW37221-2 in the lower slice is used to mine 6-2 coal seam, with a total mining height of 11 m.Fully mechanized caving coal mining technology is adopted for each slice working face, with a mining and caving ratio of 1:3.The dip angle of the coal seam is 23 $42 , with an average of 32.5 .The dip angle gradually increases from south to north.
The upper level of 940 $ 875 level is 35119-1, 35219-1 and 35219-2 goaf, between which are coal pillars with vertical height of about 10 m.The mining depth of LW37121-1 in the upper slice is 556-648 m, with the strike length being 1021 m and the inclination length being 89 $ 153 m.The roadway is driven by trapezoidal section.The layout plan of the working faces is shown in Figure 1(a), and the A-A profile is shown in Figure 1(b).
The stratigraphic column of the rock burst area is shown in Figure 2. It can be seen from the figure that the roof of 6-1 coal seam is mainly composed of sandstone and mudstone; the interlayer between 6-1 and 6-2 (upper) coal seams is mainly composed of medium-fine sandstone.There are multiple layers of mudstone and carbonaceous mudstone bands in 6-2 $ 6-5 coal seams, and the floor of 6-5 coal seam is mainly composed of medium-coarse sandstone.According to the bursting liability identification results, the 6-2 coal seam and floor possess strong bursting liability, while that of the roof is rather weak.In all, the roof, coal and bottom all have the internal conditions to induce rock bursts (Cai et al. 2016).

A rock burst
The SOS MS monitoring system was installed at the mine in January 2019 to monitor rock bursts.The layout of the monitoring sensors is shown in Figure 3.According to the error analysis of the network, the monitoring accuracy of the system in both vertical and horizontal direction is high, and the error can be managed within 20 m.MS events represent the fracture of coal and rock mass.The greater the energy of MS events is, the more energy the fracture of coal and rock mass would release.
According to the records of the MS monitoring system, on April 16, 2020, a rock burst occurred at the 37221-1 upper roadway driving face, with a seismic source energy of 1.18 Â 10 4 J, resulting in floor heave at a position of 210 $ 360 m behind the head.At that time, the LW37121-1 in the upper slice has been mined for 750 m, and the nearest horizontal distance from the rock burst area is about 200 m. Figure 1 shows the exact location of the rock burst.

Distribution characteristics of microseismic events
MS events with energy !10 3 J are usually defined as large-energy MS events in rock burst monitoring.The spatial distribution of large-energy MS events within 15 days   before the rock burst occurred on April 16 was counted; and the distribution plan and section map of MS events were drawn and shown in Figure 4. Select the regions of the floor and floor rock roadway in the profile and the side of the coal pillar and upper goaf in the plan.The frequency of MS events with energy !10 3 J and MS events with energy !10 4 J in each area was counted.Their percentages in the total frequency of MS events with energy !10 3 J and MS events with energy !10 4 J were calculated, respectively, as shown in Table 1.
As can be seen from Table 1, the frequency of MS events with energy !10 3 J in coal seam floor is high, accounting for 82.8%; and that of MS events with energy !10 4 J accounts for 100%.The frequency of MS events with energy !10 3 J in floor rock roadway area accounts for 55.0%, and that of MS events with energy !10 4 J accounts for 100%.The frequency of MS events with energy !10 3 J in coal pillar and upper-level goaf accounts for 21.5%, and that with energy !10 4 J accounts for 15.8%.The analysis shows that the MS events in the above regions are concentrated to a certain extent, and the coal and rock mass fracture releases energy frequently.Among them, 82.8% of the large-energy MS events occur in the floor, indicating that the rock burst of the LIETCS is mainly affected by the floor strata under the condition of upper slice mining.In addition, the floor rock roadway, section coal pillar and the upper-level goaf further aggravate the stress concentration degree of coal and rock mass and the failure range of floor rock stratum.

Rupture mechanism of microseismic events
Seismic moment tensor inversion, as a powerful tool to study the rupture characteristics of seismic source, is widely applied to calculate the focal mechanism of MS events (Khandelwal and Singh 2006).The seismic moment tensors can be represented by the equivalent dipole forces M ij acting at the seismic source, and it could be expressed as follows: The far-field displacement caused by a seismic source can be described as a convolution of the moment tensor and Green's function, and the expression is as follows (Jost andHerrmann 1989, Kan et al. 2022): where G ki is Green's function; M ij denotes moment tensor components of the force couples acting along the x i axis with an arm on the x j axis; and the Ã refers to convolution.
To determine the rupture type of the seismic source, the seismic source moment tensor needs to be decomposed into isotropic (ISO), compensated linear vector dipole (CLVD) and double couple (DC) parts (Knopoff and Randall 1970).ISO mainly characterizes explosion and implosion; CLVD mainly characterizes compressive crack and tensile crack; and DC mainly describes shear mechanism.Figure 5 reflects the corresponding relationship between seismic source type and moment tensor component, where the DC component is represented by colour intensity.The top and bottom of the diamond correspond to explosion and implosion, the edges to compressive crack and tensile crack, and the origin to pure shear mechanism.On this basis, the seismic source rupture type can be determined quantitatively by calculating the proportion of the DC component (Stec 2012), and the expression is as follows: where, PDC denotes the proportion of the DC component.MS events in the rock burst area and floor strata are selected to decompose the seismic source moment tensor and calculate the proportion of each component, as shown in Figure 5.Among the 20 MS events in the rock burst area, the MS events with PDC !60% account for 75%, and the MS events with 40%!PDC !60% account for 25%.Among the 10 MS events in floor rock strata, the proportion of PDC !60% MS events is 80%.This indicates that the failure of coal and rock mass around the rock burst area and floor rock strata is mainly shear failure (Song et al. 2022).
In addition, the beach ball represents the correspondence between MS events and focal mechanism solutions, as well as the strike angle, dip angle and rake angle, as shown in Figure 6.

Mechanical model establishment
The load distribution characteristics of LIETCS in dip direction are different from those in near horizontal seam.The load analysis mechanical model of LIETCS along the dip direction is established to analyze the load distribution characteristics of LIETCS under the condition of upper-level goaf, as shown in Figure 7.
In this figure, r y denotes the y-direction stress in the limit equilibrium zone (MPa); r x refers to the x-direction stress in the limit equilibrium zone (MPa); s is the shear stress at the interface between roof, coal seam and floor (MPa); b denotes the distance between the peak value of coal body and goaf in limit equilibrium zone (m); r b is the peak stress of coal body in the limit equilibrium zone (MPa); m stands for the thickness of coal seam (m); c refers to the average volume force of coal body (MPa); and a denotes the dip angle of coal seam ( ).
In this paper, the coordinate system is established with the roof as the object.Assume that the downward direction along the inclined direction of the coal seam is the positive direction of the x-axis, and the upward direction perpendicular to the coal seam is the positive direction of the y-axis.When discussing the load distribution characteristics of the coal body in the limit equilibrium zone with width b, the influence of shear stress should be considered.

Mechanical model solution
Based on the basic assumptions of elasticity and the Mohr-Coulomb criterion, the equilibrium differential equation in the limit equilibrium zone is established as:

@r
where, c x and c y are the average volume forces of coal in the x and y directions in the limit equilibrium zone respectively (MPa); c denotes the cohesion at the interface between roof, coal seam and floor (MPa); u refers to the friction angle at the interface between roof, coal seam and floor ( ).
According to relevant research results (Xie et al. 2006), the vertical stress of LIETCS can be expressed as: where, C 1 , C 2 , C 21 and C 22 are constants to be solved.According to Eq. ( 5), the expressions of r y and s can be obtained as long as the four constants C 1 , C 2 , C 21 and C 22 are obtained.
According to limit equilibrium conditions: the x-direction equilibrium equation of the limit equilibrium zone could be established as follows: The solution is as follows: where C 3 denotes the constant to be solved.When x ¼ b and y ¼ m/2, (5) and ( 8) can be obtained as follows.
From the simultaneous equations ( 7) and ( 8), the following can be obtained: From the simultaneous equations ( 9) and ( 10), the following can be obtained: Therefore, the vertical stress at any point in the limit equilibrium zone is: The shear stress at any point in the limit equilibrium zone is: The lateral pressure coefficient b in elastic area of LIETCS is: where K denotes the coefficient of lateral pressure, and b denotes the lateral pressure coefficient of large inclined plane.The horizontal stress is generally larger in the LIETCS.According to Eq. ( 14), with the increase of dip angle, the effect of horizontal crustal stress component on coal seam becomes more obvious.By means of the stress solution method in limit equilibrium zone, the expression of stress variation in elastic zone of coal-rock interface can be obtained as follows: r v ¼ Ne À 2 tan uðK sin aþ cos aÞ mðK cos aÀ sin aÞ x À 2c À mc sin a 2 tan u s v ¼ À Ne À 2 tan uðK sin aþ cos aÞ mðK cos aÀ sin aÞ where r v denotes the vertical stress in the elastic zone (MPa); s v is the shear stress in the elastic zone (MPa); and N refers to the initial vertical stress in the elastic zone (MPa).With LW37221-1 as an example, combined with the actual mine conditions, a ¼ 32.5 , m ¼ 11 m, c ¼ 2.5 Â 103kg/m 3 , c ¼ 2.8mpa, u ¼ 28 , y ¼ m/2 and other parameters are substituted into Eqs.(12) ( 13) and ( 15).The variation law of vertical stress and shear stress of the LIETCS along the inclined direction is shown in Figure 8.
It can be seen from Figure 8 that the LIETCS bears the vertical stress exerted by the roof and floor.The vertical stress changes exponentially with the increase of the distance from the coal wall, and the overall effect is "clamping".Under the action of "clamping", the shear stress exists at the interface of coal and rock mass and inside the coal body, and the shear stress changes exponentially with the increase of the distance from the coal wall.With the mining of LW37121-1 in the upper slice, the stress at the roof side decreases, and the load is further transferred to the floor, which intensifies the stress concentration and energy accumulation degree of coal and rock mass on the floor.This provides a stress environment for the induction of the rock burst at 37221-1 driving face.

Numerical simulation study on mining of the LIETCS
The results of theoretical analysis reveal the load distribution characteristics of LIETCS after mining, and reflect the changing trend of vertical stress and shear stress of coal seam.In the part of FLAC 3D numerical simulation, the dynamic and static load sources of the rock burst and the evolution laws of stress and energy would be further studied.

Numerical model establishment
Based on the geology and mining of the coal mine, a 3D numerical model was established after appropriate simplification, as shown in Figure 9.The length, width and height of the model body in the figure are 507 m Â 850m Â 300m (x, y, z).In the numerical model, 35119-1, 35219-1 and 35219-2 are all set as goafs, and the mining and caving heights are designed to be 8 m, 11 m and 11 m, respectively.To be specific, with 10 m as the unit, the LW37121-1 is excavated in the upper slice between 940 $ 875 levels.According to the actual conditions of the rock burst area, the model dip angle is 38 .
In the static load simulation, 400 m above the coal seam is not set with rock strata, so 10 MPa equivalent uniform load is applied on the upper surface of the model.The bottom surface of the model is set as a fixed boundary, and the other surfaces are set as rolling support boundary.In the numerical simulation of dynamic load, in order to prevent the reflection and refraction of dynamic stress wave at the boundary, the surface of the model is adjusted as a viscous boundary.According to the ground stress test results of the coal mine, the initial stresses of 17.5mpa and 21 MPa were applied in x and y directions of the model, respectively.To accelerate the balance speed of the model, a gravity acceleration of 9.8 m/s 2 was applied.Based on Mohr Coulomb strength criterion, the parameters were corrected according to the rock mechanics test results of the coal mine and adjacent mines (Wang et al. 2003, Zhang  and Einstein 2004).The final physical and mechanical parameters of coal and rock are shown in Table 2.

Shear stress and energy distribution characteristics of coal seam
During the LW37121-1 (upper slice) mining, in order to study the distribution of shear stress in the coal body under the working face, the cloud map of the maximum shear stress along the dip direction was obtained by numerical simulation and shown in Figure 10.According to the location of the rock burst on site, the section 200 m behind the LW37121-1 was selected to draw the cloud map.As can be seen from the figure, the shear stress of section coal pillar is concentrated under the influence of the upper-level goaf and 37121-1 goaf.The shear stress in the region with high stress concentration is more than 20 MPa, and the distribution is "diagonal."The coal body at the upper roadway side of the LW37221-1 below is affected by the shear stress concentration area, and the overall stress level is relatively high before mining.The shear stress distribution characteristics in the coal body show that due to the larger inclination angle, the forces of the roof and floor can lead to a higher concentration of shear stress in the section coal pillar and the coal body below it; and there is a trend of shear failure, which is consistent with the results of MS event analysis.
During the upper slice mining, the change law of shear stress of LW37221-1 was analyzed by stress curve.When the LW37121-1 was mined at 550 m (the rock burst position), 650 m (100 m ahead of the rock burst position) and 750 m (200 m ahead of the rock burst position), the maximum shear stress in the middle line of the LW37221-1 at the upper roadway side in Figure 10 was respectively intercepted, and the curve is drawn and shown in Figure 11.As can be seen from the figure, when the LW37121-1 (upper slice) reaches 100 m in front of the rock burst position, the peak shear stress of coal body in the upper roadway side of the LW37221-1 is 10.81 MPa, an increase of 33%; and the average shear stress is 7.68 MPa, an increase of 22%.When the LW37121-1 reaches 200 m in front of the rock burst position, the peak shear stress is 11.02 MPa, an increase of 36%; and the average shear stress is 7.76 MPa, an increase of 23%.It can be seen from the analysis that the shear stress of coal body on the upper roadway side of the LW37221-1 increases obviously due to the upper slice mining of LIETCSs.With the further mining of the LW37121-1, although the increase of shear stress is not obvious, the shear stress is always at a high level; and the regional coal body is prone to shear failure under the action of such continuous high shear stress.
Fish language was used to edit the energy density of the unit body in the FLAC 3D numerical model, and the calculation formula is as follows (Cao et al. 2021): where r 1 , r 2 and r 3 denote the three principal stresses of the rock mass unit (MPa); l is Poisson's ratio; and E is the elastic modulus (MPa).
The energy density value corresponding to the maximum shear stress point in Figure 11 is selected and the curve is drawn and shown in Figure 12.As can be seen from the figure, the energy accumulation in the shear stress concentration area of LIETCS is large.When LW37121-1 is mined at 550 m, 650 m and 750 m, the peak  values of energy concentration area are 1.46 Â 10 5 J, 1.83 Â 10 5 J, 1.90 Â 10 5 J, respectively.Compared with mining at 550 m, the energy at 650 m and 750 m increases by 25% and 30% respectively.This indicates that the energy accumulation of coal body on the upper roadway side of the LW37221-1 is relatively large due to the upper slice mining of LIETCSs.When the coal body is shear damaged, a large amount of energy will be released, and then rock bursts will occur.

Failure characteristics of floor and effect of dynamic load
By means of numerical simulation, the distribution of plastic zone in inclined direction at 200 m behind the LW37121-1 (upper slice) when mining at 750 m is obtained and shown in Figure 13.Combined with Figure 10, it can be seen that due to the mining of LW37121-1, the coal and rock body of the floor is damaged and a plastic zone is formed consequently; and the failure form is mainly shear failure.The stress level in the plastic zone is low and the distribution is "circular arc".With the increase of depth, the range of low stress zone formed by failure decreases.It can be seen from the analysis that the shear stress is the main force leading to the destruction of the coal and rock mass below during the upper slicing mining of the LIETCS.Compared with rock mass, coal body is more prone to failure, which makes the stress further transfer to the deep floor rock stratum.At the same time, the interleaved roadway in floor strata aggravates the regional stress concentration and provides space for surrounding rock deformation, which leads to regional rock mass reaching shear strength and failure, resulting in large energy MS events.This is consistent with the analysis results of MS events.When the rock burst occurred, the seismic source was located in the floor stratum, and the seismic source energy was 1.18 Â 10 4 J.Based on the simulation results of 750 m upper slice mining, the dynamic load numerical simulation was carried out by using FLAC 3D dynamic module.According to the location and energy of the seismic source, the dynamic load was applied in the form of sine wave in the floor rock stratum of the LIETCS.The specific location is shown in Figure 14(a).After the occurrence of a large energy MS event, the particle vibration velocity reaches the maximum at the seismic source boundary and gradually attenuates as it propagates away from the seismic source.According to the statistical results of MS monitoring data, particle vibration velocity increases with the rise of MS event energy.The normal stress generated by MS events above 10 4 J can generally reach over 32.7 MPa (Li 2016).Accordingly, a dynamic load stress wave of 32.7 MPa could be applied, as shown in Figure 14(b).
In order to study the response characteristics of LW37221-1 coal body to dynamic load of floor strata, the monitoring points are set in the stress concentration area of LW37221-1 upper roadway.The monitoring points are located at the bottom of the LW37221-1 with a vertical distance of 16 m from the upper-level goaf, and the middle of the LW37221-1 with a vertical distance of 16 m from the upper-level goaf.The dynamic change curve of stress with loading time is obtained and shown in Figure 15.As can be seen from Figure 15, when dynamic load is applied, the maximum shear stress at the bottom and middle of LW37221-1 increases and decreases repeatedly with the dynamic load time, displaying obvious periodicity.When the dynamic load is applied for about 0.06s, the maximum shear stress at the bottom and middle of LW37221-1 increases by 9% and 5%, respectively.After about 0.3s of dynamic load, the stress change caused by dynamic load disturbance gradually attenuates, and the shear stress of coal body tends to be stable.The maximum shear stress at the bottom and middle of LW37221-1 decreases by 11% and 4% respectively.
The analysis results show that the dynamic load will cause repeated changes of the shear stress in a short time, and the peak shear stress of coal body at different positions will increase as well.After the dynamic disturbance, the shear stress of coal body at different positions is reduced.Although the dynamic load does not increase the overall stress level of the coal body much, it repeatedly changes the stress environment in the stress concentration area, resulting in the regional coal body prone to damage and release energy.That is, dynamic load can induce rock bursts and increases the possibility of rock bursts.This is consistent with relevant research results (Jiang et al. 2018).In addition, with the increase of distance from the seismic source, the increase degree of peak shear stress and the decrease degree of overall stress are reduced, and the influence of dynamic load is gradually weakened.

Mechanism of rock bursts
According to the theory of dynamic and static combined load, as a serious dynamic disaster of coal and rock, rock bursts are induced by the superposition of local high static and dynamic load (Hou et al. 2015).Based on the analysis of MS monitoring data, theoretical formulas and numerical simulation research results, the theory of dynamic and static combined load is used to describe the process of rock bursts in the LIETCS under upper slice mining condition, as shown in Figure 16.
As can be seen from the figure, the upper level of LW37121-1 is the goaf before mining.The LIETCS where the working face is located is subject to shear stress, which changes exponentially with the increase of the distance from the coal wall and is prone to shear failure.With the mining of LW37121-1, in terms of static load, the goaf roof collapses and forms a plastic zone, and the static load on the roof side decreases as a whole.The shear stress concentration degree of the coal pillar behind LW37121-1 and the coal body below the coal pillar increases obviously, and always remains at a high level.At the same time, a large amount of energy is accumulated in the coal body, resulting in a large static load on the upper roadway of LW37221-1 in this area.
In terms of dynamic load, the coal and rock masses in the floor of LW37121-1 undergo shear failure, and the stress is transferred to the depth of the floor.At the same time, the rock roadway in the depth of the floor provides the space for surrounding rock deformation and further intensifies the stress concentration of surrounding rock mass, which causes the rock mass to fracture and triggers large energy MS events.The instantaneous release of dynamic load is superimposed on the upper roadway bottom coal of LW37221-1, making it exceed its bearing limit and thus inducing floor heave.Because of the large superimposed load in this area, the energy release is high when the coal and rock mass are damaged, and the rock burst damage is serious.In this rock burst, shear stress is the main static load source, and dynamic load plays a role of disturbance and induction.

Comparison of study results
Scholars have also studied the mechanism of rock bursts under similar conditions and obtained similar conclusions.Wang et al. (2019) established the mechanical analysis model of the steeply inclined coal seam, analyzed the variation rule of principal stress in dip direction, and concluded that a steeply inclined coal seam is "clamped" by roof and floor.The above conclusion supports the results of this paper.However, the mechanical model established by this scholar does not consider shear stress, and the conclusion is one-sided.The research in this paper shows that the coal seam with a large dip angle is subject to "shear-clamping," which fully reflects the static load source of the coal seam under similar conditions.
Using UDEC simulation software, Cao et al. (2020) found that the static load stress of coal seam is higher under the "clamping" action of roof and floor during the horizontal section mining of steeply inclined coal seams.Strong dynamic loads are thus formed by breakage of the roof and the failure of multiple hinged beam structures.The results of the static stress study above are similar to those in this paper.The research object of the dynamic load source above is the roof, while the dynamic load source of this paper is the floor.They are different.

Suggestions on rock bursts prevention
The results of this paper show that the high static load of coal body around the upper roadway of LW37221-1 and the dynamic load of floor rock are the main force sources to induce rock bursts.Therefore, section coal pillar and floor rock stratum are the objects of rock bursts prevention.For the prevention and control of rock bursts, scholars have done detailed research (Mazaira and Konicek 2015, Yan et al. 2015, Wei et al. 2018).Combined with the actual conditions of the site, the coal blasting measures were taken at the coal pillar side of the upper roadway of LW37221-1.In the floor rock roadway, deep-hole blasting measures were taken on the coal seam floor of the upper roadway side to reduce the stress concentration degree of the coal seam and floor.

Conclusions
By means of MS data statistics, seismic moment tensor inversion, theoretical formula and numerical simulation, this paper analyzes the distribution characteristics and focal mechanism of MS events in rock-burst area of the LIETCS, and explores the static load evolution law and dynamic load effect of the LIETCS during the upper slice mining.On this basis, the rock burst mechanism is proposed based on the theory of dynamic and static combined load.The specific conclusions are as follows: Based on the data statistics and seismic moment tensor inversion, this paper analyzes the distribution characteristics and focal mechanism of MS events in the lower rock burst area (driving face of the upper roadway) during the upper slice mining of the LIETCS.The results of analysis indicate that the coal and rock mass around the rock burst area and the floor rock stratum are mainly destroyed by shearing.
The results of theoretical analysis and static load numerical simulation show that the LIETCS is subject to the "shear-clamping" stress caused by the joint action of the roof and floor.During the upper slice mining of the LIETCS, the shear stress and energy of the coal pillar behind the working face and the coal body below the coal pillar always remains at a high level, which is prone to shear failure and energy release.This provides the static load condition for the occurrence of the rock burst.
The research of dynamic load numerical simulation shows that when the floor rock stratum reaches the shear strength and is destroyed, the released dynamic load will lead to repeated changes in the shear stress of the coal body in the high-stress zone above in a short time.Based on the theory of dynamic and static combined load, a rock burst will be induced when the superposition of static load and dynamic load exceeds the bearing limit of the coal body.In the rock burst area, the superimposed load is large, and the rock burst damage is serious.
Although this study has discussed the mechanism of rock bursts in the LIETCS under the condition of upper slice mining, several limitations also apply.Firstly, taking a rock burst in the LIETCS as a case study, this paper focuses on revealing the static and dynamic load sources and the process of the rock burst.The research results provide a reference for the mechanism analysis and prevention of the rock burst under similar conditions.Secondly, the theoretical formula adopted in this paper is based on the basic assumptions of elasticity and the Mohr-Coulomb criterion.
In the following research work, the author will continue to follow up the rock bursts of the LIETCS, record the data before and after the occurrence of rock bursts in time, and analyze the mechanism and countermeasures of different rock burst cases.This will help further expand the application scope of the research results.

Figure 1 .
Figure 1.Layout of working faces (a) Plan (b) A -A profile.

Figure 2 .
Figure 2. Stratigraphic column of the rock burst area.

Figure 3 .
Figure 3. Sketch of the distribution of MS monitoring sensors until 8 May 2020.

Figure 4 .
Figure 4. Distribution of MS events before the rock burst (a) Plan (b) A -A profile.

Figure 6 .
Figure 6.Beach Ball of MS Events in the rock burst area and floor rock strata.

Figure 7 .
Figure 7. Load analysis mechanics model of LIETCS along the dip direction.

Figure 8 .
Figure 8. Stress variation trend of the LIETCS along the inclined direction.

Figure 10 .
Figure 10.Cloud diagram of maximum shear stress along the dip direction at 200 m behind the upper slice.

Figure 11 .
Figure 11.Variation curve of maximum shear stress along the dip direction during upper slice mining.

Figure 12 .
Figure 12.Change curve of energy density in shear stress concentration area.

Figure 13 .
Figure 13.Distribution characteristics of plastic zone of floor.

Figure 15 .
Figure 15.Stress evolution law under dynamic load disturbance (a) Dynamic load stress wave at the bottom of 37221-1 face (b) Dynamic load stress wave in the middle of LW37221-1.

Table 1 .
Frequency statistics of large energy MS events.

Table 2 .
Mechanical parameters of the model.