As the main energy in China, the available coal reserves account for about 11.67% of the world reserves. The storage of coal resources has obvious geographic characteristics, and a considerable proportion of coal resources is located within high tectonic stress zones (HTSZs) (Wu 2018; Zhang et al. 2023). In engineering practice, mining activities in HTSZs can easily cause energy release of surrounding rock and even rockburst. With the increase of mining range and depth, the dynamic manifestation degree of coal and rock mass increases as well, which causes huge loss to coal mine.

The research on the rockburst mechanism has been quite mature, leading to some important conclusions. In the early stage, there are strength theory, stiffness theory, energy theory and impact tendency theory (Jiang et al. 2014). Subsequent theories include shear-slip theory, the theory of deformation system instability, fractal theory, catastrophe theory, damage theory, three-criterion theory, “three-factor” theory, dynamic and static combined load theory, etc. (Khademian and Ugur 2018; Khademian and Ozbay 2019; Qi et al. 2019; Tan et al. 2023). According to the dynamic and static combined load theory, once the superposition of static load and dynamic load exceeds the bearing limit of coal and rock mass, it would induce rockburst (Hou et al. 2015). With the further development of the research, more attention has been paid to the influence of dynamic load. He et al. (2016) studied the dynamic loading effect of roof vibration on roadway surrounding rocks. The results show that hard-thick roof will result in high stress concentration on mining surrounding rocks. Through simulation experiments, Wang et al. (2020) found that the damage degree was aggravated under the compound influence of increasing dynamic loading intensity and decreasing distance from the dynamic loading source to the stope. Wang et al. (2019) further pointed out that failure effect of dynamic source is more sensitive to source location, followed closely by source energy, and then the rock property. Wang et al. (2022c) found that dynamic load in steeply inclined roof could increase the stress of coal body and the probability of rockburst occurrence. Li et al. (2022) showed that part of the energy in rockburst events was seismic energy. The above research results show that the effect of dynamic load on rockburst cannot be ignored. As the basic theory of rockburst, the dynamic and static combined load theory has been widely used. Abundant studies have been conducted to explore the effect of dynamic load in the roof.

Scholars also studied the characteristics of stress distribution in HTSZs and its influence on rockburst (Cao et al. 2018a). Lee et al. (2004) found that there was high residual tectonic stress in the fold zones. During mining, high residual tectonic stress is released and rockburst occurs. By combining theory with experiments, Chen (2009) analyzed the critical maximum principal stress of rockburst. On this basis, he further proposed the mechanism of floor rockburst induced by maximum horizontal stress in HTSZs. Through the measurement of ground stress, Kang et al. (2012) further pointed out that the horizontal stress of the synclinal axis in HTSZs increased sharply, and the increase was much greater than the vertical stress. The above research results show that the horizontal stress of the strata in HTSZs is large due to the squeezing action. The area is prone to rockburst.

Many studies have shown that microseismic (MS) monitoring and numerical simulation are powerful tools to study rock mechanics and rockburst in HTSZs (Starfield and Cundall 1988; Cai et al. 2018; Cao et al. 2018b; Wang et al. 2022b ;Liu et al. 2023; Sainoki et al. 2023). In terms of MS monitoring, He et al. (2011) studied the spatiotemporal evolution characteristics of MS events during the mining process of the working face in HTSZs. They found that high energy MS events occurred frequently in the synclinal axis zones. Based on acoustic emission and field MS data, Wang (2021) analyzed the early warning effect of vibration field and energy field indexes in HTSZs. In terms of numerical simulation, Guo et al. (2022) used Flac^3D numerical simulation to find that the influence of the composite fault-fold structure on the roadway group rockburst was mainly caused by high tectonic stress. Xue et al. (2023) studied the stress evolution law of mining face in upright fold zones with high tectonic stress. He found that the occurrence of rockburst disasters in the fold area displayed obvious regional characteristics. Using numerical simulation, Cao et al. (2021) further showed that when the working face was mined to different areas of the fold structure, the distribution of vertical and horizontal stress fields in the fold structure area was significantly different. The above research shows that numerical simulation and MS monitoring are of great significance in the study of stress distribution and rockburst in fold zones with high tectonic stress.

In the above studies, the scholars explored the importance of dynamic load for inducing rockburst, and the degree to which dynamic load in the roof affects rockburst. The distribution characteristics of static load and the spatiotemporal evolution of MS events during mining in HTSZs have been analyzed. However, the above-mentioned studies do not fully explore the dynamic load in HTSZs. In particular, there is no specific study on the fracture mechanism of dynamic load source in the floor strata of the HTSZs and its influence on rockburst. In addition, in previous large-scale models, the parameters considered in the simulation of dynamic load are not comprehensive.

Based on the engineering background of a working face in HTSZs, this study obtained the mechanical cause and rupture mechanism of dynamic load source in the floor through MS events analysis and seismic moment tensor inversion. The numerical simulation is carried out according to the focal parameters and the propagation direction of the stress wave. The distribution characteristics of static load and dynamic load effect are studied, and then the mechanism of rockburst is revealed. This study contributed to the exploration of rockburst mechanism in HTSZs. Meanwhile, it also provided theoretical basis for rockburst prevention and control under such geological mining conditions.

2.1 Geological and mining conditions

This study selected a LW face of a mine as the engineering background. The mine is located in the geological fold zones, whose overall tectonic stress is large. According to the in-situ stress test results, the horizontal tectonic stress is dominant in the mine. The ratio of horizontal stress to vertical stress is at least 1.2 and at most 1.6. The LW face is the upper slice working face, mainly mining 5# coal seam, the total thickness of the coal seam is 38 m. The length of the LW face is 200 m, the thickness of the exposed coal seam is 5.6 m to 19.4 m, and the average mining thickness is 13 m. The average mining depth of the LW face is 426 m, and the depth of the rockburst area has reached 500 m. On the west side of the LW face is the goaf of this mine, and on the south side is the large-scale goaf of the adjacent mine. The length and average mining thickness of all goafs in both mines are basically the same as that of the LW face. There are protection coal pillars of the main roadway between the two mines. Among them, the return air lane, the belt lane, the auxiliary transportation lane and the fast track are arranged. The layout of the LW face is shown in Fig. 1.

The stratigraphic column of the rockburst area is shown in Fig. 2. As can be seen, the roof of the 5# coal seam is mainly argillaceous siltstone with large thickness. The thickness of the main roof (argillaceous siltstone) is 51.93 m. The floor is medium-coarse sandstone with high hardness, whose thickness is 24.19 m. According to the bursting liability identification results, the 5# coal seam, roof and floor all exhibit bursting tendency for rockburst.

Layout of working faces

Stratigraphic column in the rockburst area

2.2 A rockburst case

The SOS MS monitoring system was installed in the mine, which recorded the spatial location and energy of MS events. According to the monitoring system, a large-energy MS event occurred in the LW face during the mining period. The energy level of the MS event is 2.47 × 10⁵ J. Subsequently, a MS event with energy of 1.61 × 10³ J occurred in the protection coal pillar area of the main roadway. It caused the roof subsidence and floor heave in the area near the intersection of goaf side roadway and fast track, inflicting severe damage. At this time, the working face was 770 m away from the rockburst area.

The 2.47 × 10⁵ J MS event on the plane was located near the roadway on the solid coal side, 180 m in front of the LW face. The horizontal distance of the MS event from the rockburst area was 600 m. In the profile, it was located in the floor strata, and the vertical distance from the roadway floor was 52 m. Given that the MS event was far away from the rockburst area, it would be an MS event that induces the rockburst. Located near the rockburst area, the 1.61 × 10³ J MS event was generated by the rockburst, with the positioning error taken into account. The location of rockburst and two large-energy MS events are shown in Fig. 3.

Location of rockburst and the MS event that induces the rockburst

3.1 Distribution characteristics of regional microseismic events

In mining engineering, MS events larger than 10⁴J are usually referred to as large-energy MS events. MS events reflect the failure of coal and rock mass. The frequency and energy of MS events are positively correlated with the damage degree of coal and rock mass, and the energy released.

The spatial distribution of large-energy MS events in the region where the 2.47 × 10⁵ J MS event occurred was analyzed. The plan of MS event distribution and the profile along the solid coal roadway were drawn, as shown in Fig. 4. As can be seen, the 2.47 × 10⁵ J MS event occurred in a region far from the anticlinal axis. The fluctuation of coal seam is relatively gentle. There is an accumulation of large-energy MS events in this area. In total, there are 14 large-energy MS events in the accumulation area, 9 of which occur in the coal and rock mass below the roadway floor, 3 in the coal seam of the roadway roof, and 2 in the roof rock strata, accounting for 64.3%, 21.4% and 14.3% respectively. This indicates that the stress environment of the floor coal rock mass in this area is abnormal. In the process of mining, it is easy to break and release energy. In addition, during the data statistics period, there are no large-energy MS events in the protection coal pillar area of the main roadway. This indicates that the protection coal pillar area is less affected by mining. The destruction of the coal body did not release significant energy. However, due to the influence of goaf on both sides, the static load of the protection coal pillar cannot be ignored.

Distribution of MS events before the rockburst. a Plan. b Profile

3.2 Rupture mechanism of the microseismic event that induces rockburst

The seismic moment tensor inversion is widely used to calculate the rupture type, strike angle, dip angle and rake angle of rupture surface at the source, so as to reveal the rupture mechanism of the seismic source (Khandelwal and Singh 2006). The seismic moment tensors can be represented by the equivalent dipole forces \({M_{ij}}\) acting at the seismic source. The expression is 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. The expression is as follows (Jost and Herrmann 1989; Kan et al. 2022):

where \({G_{ki}}\) denotes Green’s function; \({M_{ij}}\) is moment tensor components of the force couples acting along the \({x_i}\) axis with an arm on the \({x_j}\) axis; and the \(*\) means convolution.

In order to determine the rupture type of the source, it is necessary to decompose the source moment tensor. The approach proposed by Knopoff and Randall (1970) is widely accepted. In this method, the source moment tensor is decomposed into three parts: isotropic (ISO), compensated linear vector dipole (CLVD) and double couple (DC), with + ISO describing explosion, -ISO describing implosion, +CLVD describing tension crack, -CLVD describing compression crack, and DC describing shear mechanism.

According to the proportion of each component in the moment tensor, the source rupture type of MS events can be judged. Ohtsu (1995) proposed that the proportion of DC in the moment tensor can be used to identify the rupture type of coal and rock mass. On this basis, Yang et al. (2023) proposed a more detailed rupture criterion, which is expressed as follows:

where \({P_\text{DC}}\) denotes the proportion of the DC component; and \(P_\text{ISO}\) denotes the proportion of the ISO component.

The source moment tensor of the 2.47 × 10⁵J MS event is decomposed, and the proportion of each component, strike angle, dip angle and rake angle are calculated. Based on the moment tensor inversion results, the characteristic map of the beach ball and the source rupture surface containing all this information are drawn to describe the rupture mechanism of the MS event, as shown in Fig. 5.

Rupture mechanism of the MS event that induces rockburst (dip direction)

It can be seen from the figure that\({P_\text{ISO}}=-33.32\%\), \({P_\text{CLVD}}=-66.66\%\) and \({P_\text{DC}}=0.05\%\) in the moment tensor inversion results. Among them, the percentage of DC component in the moment tensor is only 0.05%, and P_ISO < 0. It indicates that the source is a pure compression failure. The strike angle of the source rupture surface is 173°, which is approximately parallel to the strike of the LW face. The dip angle of the rupture surface is 46° and the rake angle is -90°. It shows that the rupture surface approximately slides along the dip direction of the LW face and toward the roadway on the side of the goaf.

Combined with the MS events location, it can be seen that the 2.47 × 10⁵J MS event is located in the limb of the fold. The fold zones in coal mine are formed under long-term horizontal compression, and the horizontal tectonic stress is the main force. According to the formation mechanism of the fold, the stress state of each part is divided into five zones (Wang et al. 2012): zone I (V) is tensile stress in the vertical direction and compressive stress in the horizontal direction; zone II (IV) is compressive stress in the vertical direction and tensile stress in the horizontal direction; zone III (the limb of the fold) is compressive stress in both horizontal and vertical directions, as shown in Fig. 6. This indicates that before the 2.47 × 10⁵J MS event occurs, the rock mass at the source (52 m below the roadway floor) is under three-way compression. A large amount of elastic energy is accumulated in the rock mass. Under the squeezing action of higher horizontal tectonic stress, the volume of rock mass shrank and implosion occurs. In turn, the large-energy MS event is generated.

The stress state of each area of the fold

The rock mass rupture at the source transmits two kinds of waves, P wave and S wave. The S wave is divided into SV wave and SH wave. The vibration direction of P wave is parallel to the propagation direction, which mainly affects the outburst of coal and rock mass. The vibration direction of S wave is perpendicular to the propagation direction, which mainly affects the shear slip of coal and rock mass (Bai et al. 2023). In an isotropic, unbounded, and uniform elastic medium, the displacement generated by force source f (r,t) in the far-field displacement field is expressed as follows (Cao and Dou 2008):

where, u^P, u^SV, u^SH, and u^S denote the far-field displacement fields of P wave, SV wave, SH wave and S wave; ρ is density; r represents the distance between the source and the monitoring point;, \(v_\text{p}\) is the P wave velocity; \(v_\text{s}\) is the S wave velocity; r^P, r^SV and r^SH stand for the far-field radiation patterns corresponding to P wave, SV wave and SH wave; ϒ, Θ and Φ are unit displacement vectors, namely:

where \(\theta\) is the dip angle of the rupture surface, and \(\varphi\) is the strike angle of the rupture surface.

Based on Eqs. (4) and (5), displacement field radiation pattern of the source that induces the rockburst is obtained, as shown in Fig. 7. As can be seen, the influence of seismic wave (compression wave) transmitting outward from the source on coal and rock mass displays obvious directionality. The maximum displacement amplitude of P wave is perpendicular to the rupture surface. The angle between the maximum displacement amplitude of the S-wave and the rupture surface is ± 45°. The coal and rock masses in this direction are most disturbed by the dynamic load.

Feld radiation pattern of the source that induces the rockburst. a P wave b S wave

4.1 Numerical modeling

The numerical model is based on the geology and mining of the mine. Given that the scale of the simulation is the whole mine and the fluctuation of coal seam in the rockburst area is gentle, the Flac^3D numerical model is established after appropriate simplification, as shown in Fig. 8. The model length, width and height is 6520 m × 3500 m × 1000 m (x, y, and z), respectively. In the numerical model, six working faces in the upper slice and one working face in the lower slice of adjacent mine are set as goaf. Meanwhile, four working faces on the side of the LW face are set as goaf as well. Both the upper and lower slice mining thickness are designed to be 13 m. The model is used to simulate the stress distribution characteristics in protection coal pillar area of the main roadway when rockburst occurs.

FLAC^3D model diagram

In static load simulation, the top surface of the model is the ground surface. The bottom surface of the model is set as a fixed boundary, and the other surfaces as rolling support boundary. According to the in-situ stress test results of the mine, 30 MPa and 40 MPa initial stress are applied to the x and y directions at the bottom of the model respectively. To accelerate the balance of the model, a gravitational acceleration of 9.8 m/s² is applied. This simulation adopts Mohr-Coulomb model. According to the rock mechanics test results of the mine and adjacent mines, parameters are adjusted, and the final physical and mechanical parameters of coal and rock are obtained as shown in Table 1 (Wang et al. 2003; Zhang and Einstein 2004).

4.2 Stress and energy distribution characteristics in protection coal pillar area

The protection coal pillar of the main roadway in HTSZs is greatly affected by both horizontal stress and vertical stress. In numerical simulation, it is not accurate to reflect the stress state of protection coal pillar by horizontal stress or vertical stress. Therefore, the maximum principal stress is selected to reflect the degree of stress concentration in this area, and the energy density is used to reflect the degree of energy accumulation in the coal and rock mass. This will provide a basis for revealing the mechanism of rockburst.

In order to study the stress state of the protection coal pillar area before the occurrence of the large-energy MS event, the maximum principal stress cloud map is intercepted at the roof and floor of the main roadway in the upper slice, respectively, as shown in Fig. 9. As can be seen from the figure, the overall stress level of the protection coal pillar is relatively high due to the influence of large-scale goaf on both sides. Among them, the maximum principal stress at the roof and floor of the main roadway in the rockburst area is 40.3 MPa and 40.8 MPa, respectively, which are in the high stress area.

The monitoring line L1 is used to further extract the maximum principal stress at the rockburst area in Fig. 9, and the curve is drawn. The position of the monitoring line L1 is shown in Fig. 9(b), and the position of the monitoring line in Fig. 9(a) is the same as that in Fig. 9(b). The curves drawn are shown in Figs. 10(a) and 10(c). It can be seen from the figure that the maximum principal stress of coal body at the roof and floor of the main roadway 35 m away from the rockburst area (along the direction of the adjacent mine goaf) is 44.5 MPa and 43.5 MPa. The maximum principal stress of coal body 20 m away from the rockburst area is 41.6 MPa. With the approach to the LW face, the stress decreases first and then increases slightly. In other words, the stress of the fast track in the roadway group is high due to the influence of the adjacent mine goaf. The stress of coal body between the fast track and the adjacent mine goaf is also high. The stress of coal body around return air roadway, belt roadway and auxiliary transportation roadway decreases successively. In addition, there are roadway crossings in the fast track area (rockburst area). This will further intensify the degree of stress concentration and make the coal prone to failure (Zhou et al. 2022).

Maximum principal stress cloud map of upper slice. a Roadway floor b Roadway roof

Fish language is used to edit the energy density of the unit body in the FLAC^3D numerical model, and the calculation formula is as follows (Wang et al. 2023):

where \({\sigma _1}\) is the maximum principal stress of rock mass unit (MPa); \({\sigma _2}\) is the intermediate principal stress of rock mass unit (MPa); \({\sigma _3}\) is the minimum principal stress of rock mass unit (MPa); µ denotes Poisson’s ratio; and E is the elastic modulus (MPa).

The energy density values corresponding to the maximum principal stress points in Figs. 10(a) and 10(c) are extracted. The curves are drawn and shown in Figs. 10(b) and 10(d). As can be seen, the energy density at the roof and floor of the main roadway in the rockburst area is 4.2 × 10⁵ J and 4.1 × 10⁵ J, respectively. The peak energy density of at the roof and floor between the fast track and the adjacent mine goaf reaches 4.7 × 10⁵ J and 4.9 × 10⁵ J. The energy accumulated in these regions is substantial. Once the coal body is destroyed, a large amount of energy is to be released, and in turn rockburst occurs.

Maximum principal stress and energy density curve of upper slice. a Maximum principal stress of roadway floor b Energy density of roadway floor c Maximum principal stress of roadway roof d Energy density of roadway roof

4.3 Effect of the microseismic event in the floor

4.3.1 Application of dynamic load in the floor

Based on the static load numerical model, the dynamic module is used to simulate the dynamic load. The MS event that induces rockburst is located 52 m below the floor of the roadway, and the energy density is 2.47 × 10⁵J. According to the relevant statistical results, when the MS event of 10⁵J level occurs, the maximum recorded peak vibration velocity can reach over 3.0 m/s (Li 2016). In this simulation, the peak vibration velocity at the source radius is taken as 3.0 m/s. The equivalent stress at the source can be expressed as (J and A 1994):

where \({\sigma _\text{dp}}\) and \({\sigma _\text{ds}}\) represent the equivalent stresses generated by P wave and S wave (Pa); \(\rho\) denotes rock density (kg/m³); \(C_\text{P}\) and \(C_\text{s}\) are the propagation velocities of P and S waves in the medium (m/s); \({v_\text{pp}}\) and \({v_\text{sp}}\) are the peak vibration velocities caused by P wave and S wave (m/s).

With,

the equivalent stress generated by P wave and S wave is 27 MPa and 16 MPa, respectively.

Since the simulated wave is the excitation pulse at the source, there is almost no time difference between P wave and S wave. Therefore, the mixed waveform of P wave and S wave is applied at the source radius (Cao 2009). In addition, the slip direction of rupture surface at the source and the displacement amplitude of seismic wave should be considered in the numerical simulation of dynamic load (Wang 2016). Section 3.2 determines the sliding direction of the fracture surface and the maximum displacement amplitude direction of seismic waves. To be specific, P wave is perpendicular to the direction of the rupture surface, and S wave is ± 45° with the rupture surface. On this basis, the dynamic load is applied.

As a source mechanical parameter to characterize the disturbance scale, the source radius needs to be reflected in the numerical simulation. Assuming that the rupture surface is a circular rupture surface, the rupture radius of the source exhibits the following relationship:

where \(f_\text{c}\) represents the corner frequency of the source; k denotes the model coefficient, and for the Brune model, k = 2.34.

Based on Brune model, the source radius distribution of MS events in this mining area was calculated, as shown in Fig. 11 (Chen 2019). The results indicate that the source radius of MS events related to rockburst is between 35 and 80 m. Within this range, the source radius of 40 m accounts for the highest proportion, up to 28%. Since the seismic wave energy is constant in the sphere, the energy decays from the surface of the sphere (Liu et al. 2022). Therefore, in numerical simulation, the same equivalent stress is applied in the range of 40 m around the source.

Distribution of focal radius in mining area

In addition, in the numerical simulation of dynamic load, the model surfaces are set as viscous boundaries to prevent the reflection and refraction of dynamic load stress waves at the boundary. The damping coefficient is set to 0.01. A sinusoidal wave with a frequency of 20 Hz is used to simulate the propagation of seismic waves. The cycle time of the dynamic load is 0.05s. The source location and dynamic load stress waves are shown in Fig. 12.

Application of the dynamic load. a The seismic source location b Dynamic load stress waves

4.3.2 Effect of dynamic load in the floor on rockburst area

In order to study the effect of dynamic load on rockburst area, monitoring points are designed in the model. The monitoring points are arranged on the roof and floor of the main roadway. The monitoring points are set at the rockburst area, 20 m, and 35 m away from the rockburst area (direction of the adjacent mine goaf). The change curve of the maximum principal stress with loading time is obtained and shown in Fig. 13. As can be seen, with the increase of dynamic load action time, the maximum principal stress of coal body at the roof and floor of main roadway decreases and increases repeatedly, displaying obvious periodicity. When the dynamic load is applied about 0.4s, the maximum principal stress of coal body at the roof and floor of the main roadway reaches the peak. The stress increment of coal body at different positions is 0.2% to 0.3%. After the dynamic load is applied about 1.0s, the stress variation caused by dynamic load gradually attenuates. The maximum principal stress of coal body tends to be stable. The stress of coal body at different positions decreases by 0.2% to 0.5%.

The analysis results indicate that dynamic load can cause repeated changes of the maximum principal stress in a short time. During the action of dynamic load, the stress of coal body in different areas will increase instantaneously. After the action of dynamic load, the stress of the coal body decreases, and the strength also decreases accordingly. Since the rockburst area is far from the dynamic load source, dynamic load attenuates sharply. Therefore, the stress level of coal body in this area has little change. Although the single dynamic load disturbance is small, there have been many large-energy MS events in this area before the 2.47 × 10⁵ J MS event. The dynamic loads generated by these MS events repeatedly act on the highly stressed coal body in the rockburst area, which increase the cumulative damage of the coal body and decrease the critical stress of rockburst. This will cause highly stressed coal body to easily reach the critical stress of rockburst. Under the action of the 2.47 × 10⁵ J dynamic load source, the coal body under the critical stress state may fail and release energy in a short time. The above analysis fully shows that the dynamic load mainly plays a role in increasing coal body damage and inducing rockburst.

Stress evolution law under dynamic load disturbance. a Dynamic load stress at floor of roadway b Dynamic load stress at roof of roadway

5.1 Mechanism of the rockburst

According to the above MS events analysis and numerical simulation results, the theory of dynamic and static combined load is used to describe the rockburst process in high stress area of coal pillar induced by the MS event in the floor, as shown in Fig. 14.

Process of the rockburst

As is shown, in terms of static load, the coal and rock mass in HTSZs are in the limb of the fold and are mainly subjected to horizontal compression. The overall horizontal tectonic stress is large. At the same time, the fast track (rockburst area) is located in the roadway group in the coal pillar area. Due to the influence of the adjacent mine goaf and roadway crossings, the stress concentration in the coal body in this area is significant, resulting in a large static load. A large amount of elastic energy is accumulated in the coal body.

In terms of dynamic load, the floor rock is under three-way compression, with a great amount of elastic energy accumulated inside. Under the horizontal stress squeezing force in HTSZs, the volume of the rock mass shrinks and implodes, transmitting energy outward in the form of stress wave. This causes the cumulative damage and stress of coal body in the roof and floor of the fast track increase in a short time. At this time, the coal body in this area, denoted as the green ball in the figure, has been in the critical stress state. Small stress changes can easily lead to coal (the green ball) instability and instant failure. The dynamic and static superimposed loads in this area are large, and high energy is released during the failure of coal and rock mass. This causes the coal body at the roof and floor of the fast track to bulge in the direction of free plane, and then the roof subsidence and floor heave occur, resulting in serious roadway damage. In the case of rockburst, the high static load of the coal body is the main force source. The dynamic load plays a role in increasing coal body damage and inducing rockburst.

5.2 Comparison of study results

Prior to the comparison of study results, the relationship between the rockburst location and the seismic source location needs to be explored first. When the rockburst is close to the source, it is generally believed that the MS event is caused by the rockburst. When the rockburst is within the source radius, the rockburst occurs under the combined action of dynamic load stress waves and the rock mass failure caused by the source. When the rockburst is outside the source radius, it is usually considered that the rockburst occurs under the action of dynamic load stress waves generated by the source (Wang et al. 2022a). In addition, when the seismic source is located on the roof, the instantaneous gravity loading effect of the roof breaking should also be considered. The dynamic load source discussed in this study is the seismic source in the floor, which is far away from the rockburst area. In this condition, the rockburst is mainly induced by dynamic load stress waves.

Scholars have also studied the mechanism of rockburst under the above different conditions, and obtained some similar conclusions. Jiang et al. (2018) used a testing system to test the dynamic behavior of blocky rock masses and analyzed the effect of impact energy on sliding instability. The results show that the sliding instability of blocky rock mass triggered by dynamic load depends on the shear force. To be specific, the shear force is negatively correlated with the external disturbance required. Kong et al. (2019) established a small-scale roadway model with FLAC^3D software to study the rockburst potential of roadway under different dynamic and static loads. The results show that rockburst is more likely to occur in a deep mine roadway with a combination of a high static load and a weak dynamic load. The above research used experimental and numerical simulation methods to study the influence of dynamic load on a small scale and obtained similar conclusions. In this study, the rockburst was analyzed from the macroscopic scale by using the field monitoring data and numerical simulation. The results are more consistent with the engineering site. Using UDEC software, Cao et al. (2020) found that the strong dynamic load was formed by breakage of the steeply inclined roof and the failure of multiple hinged beam structures. The dynamic load source of the above research was in the roof, and the strong dynamic load was the result of the instantaneous gravity loading of the roof breaking and the stress wave. In this paper, the dynamic load source is in the floor strata, and the influence of dynamic load stress waves is mainly studied.

5.3 Suggestions on rockburst prevention

Based on the results of this paper, the high static load in the protection coal pillar area of the main roadway is the main force source to induce rockburst. Therefore, the protection coal pillar should be the main object of rockburst prevention. Combined with the site conditions, it is suggested to use deep-hole blasting or directional hydraulic fracturing for the roof of the fast track and auxiliary transportation roadway. The boreholes in both roadways are oriented towards the adjacent goafs to reduce the coal pillar load. At the same time, large diameter boreholes are used in the walls and floor of the fast track to further reduce the stress concentration of coal body around the roadway.

In this study, MS data analysis was used to obtain the fracture mechanism of the MS event that induces the rockburst in HTSZs and the distribution characteristics of regional MS events. Combining seismic moment tensor inversion and numerical simulation, a rockburst research scheme is proposed to increase the rationality of dynamic load simulation. Based on this, the evolution characteristics of static load and the influence of dynamic load in rock burst area were studied. According to the dynamic and static combined load theory, the rockburst mechanism was proposed. The specific conclusions are as follows:

The analysis results of MS events show that the stress environment of floor strata in HTSZs is abnormal. The squeezing action of high horizontal stress is the force source of the MS event that induces the rockburst. The rupture surface of the seismic source faces the roadway on the side of the goaf and slides along the dip direction of the LW face. The maximum displacement amplitude of P wave generated by the source is perpendicular to the rupture surface. The maximum displacement amplitude of S-wave is ± 45° with the rupture surface.

The static load numerical simulation results show that the stress of coal body in the fast track area where rockburst occurs is high due to the influence of the adjacent mine goaf. In addition, roadway crossings in this area significantly increases the degree of stress concentration and energy accumulation. The numerical simulation results of dynamic load show that, on the one hand, the dynamic load increases the cumulative damage of the coal body and reduces the critical stress of rockburst. On the other hand, the dynamic load will cause the increase of stress in a short time, and then induce rockburst.

The rockburst mechanism is described based on the dynamic and static combined load theory. The volume contraction of floor rock strata causes implosion, and then generates large-energy MS events, which are transmitted outwards in the form of stress waves. This causes the cumulative damage and stress of the coal body in the roof and floor of the fast track increase in a short time. Since the coal body in this area is already in the critical stress state, even the slight stress change can easily lead to the instantaneous failure of the coal body and release a large amount of energy. In the case of rockburst, the high static load of the coal body is the main force source, and the dynamic load plays a role in increasing coal body damage and inducing rockburst.