Stress Inversion and Fault Instability in the Source Region of the 2021 (M_S 5.0) Yingjiang Earthquake

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Abstract

On 12 June 2021, an earthquake with M_S 5.0 occurred in Yingjiang, adjacent to eastern Myanmar, where seismic activity is frequent due to plate collision. To explore the mechanism of this earthquake, the regional stress field of the Yingjiang zone was inverted using the focal mechanisms of 187 historical earthquakes in this area. Furthermore, based on the obtained orientation of the principal stress axes and the stress shape ratio, the fault slip tendency (Ts) was also estimated to evaluate fault instability in the study area. The stress variation results show that the diffusion and migration of the aftershocks suggested strike–slip-type stress accumulation in Yingjiang with a principal compressive stress axis direction-oriented NNE–SSW. Fault slip tendency results show that the seismogenic faults feature strikes within the ranges of 40~80° and 110~150° and dips of 60~90° and exhibit enhanced stress coupling. The distribution of the aftershock sequence is conjectured to have a high correlation with local fluid migration and was likely controlled by the hydrated rock-induced ruptures of the stressed fault systems near the source region. This study provides insights into potential earthquake risks in this region.

1. Introduction

Yingjiang County is located in the furthest west of Yunnan, China, and in the southeast of the Namcha Barwa syntaxis [1], where seismic activity is frequent (Figure 1). Located in the eastern portion of the boundary between Eurasia and India, Yingjiang has been subjected to plate collision and southward crustal extrusion (Figure 1), which has formed N–S- and NNE-oriented faults (e.g., [2,3,4]). Various lengths of faults are involved in different complex active structures. Near eastern Yingjiang, the Tengchong volcanic region (Figure 1) exhibits strong rock metamorphism and enhanced geothermal activity [5,6].

On 12 June 2021, an earthquake with M_S 5.0 occurred in Yingjiang adjacent to eastern Myanmar [7]. Since 2012, there have been 687 seismic events with magnitudes larger than M 2 near the epicenter of this earthquake, including two M ≥ 5.0 seismic events and an M ≥ 6.0 earthquake (Figure 1). The largest seismic event was the M 6.1 earthquake on 30 May 2014 (NEDC, https://data.earthquake.cn, accessed on 15 December 2022). The source region of the 2021 M_S 5.0 earthquake was delimitated by the Nabang fault to the west (Figure 1, Nabang F.), the Sudian fault to the east (Figure 1, Sudian F.), and the Dayingjiang fault to the south (Figure 1, Dayingjiang F.). A series of secondary faults were present within 50 km of the source region, including the Xima–Panlongshan fault (Figure 1, Xi.-Pan. F.), which was close (4 km) to the epicenter.

The Nabang fault, adjacent to the western margin of the Tengchong block (Figure 1), is an important component of eastern Myanmar’s Mogok metamorphic belt [8,9,10]. Since the Cenozoic, both the Nabang strike–slip fault and the Gaoligong strike–slip fault (Figure 1, Gaoligong F.) have recorded a strong right-lateral strike–slip motion, and they were once the western boundary of the extruded Indo-Chinese block (e.g., [8,9,10]).

The Sudian fault extends northward to Myanmar and southward to Yingjiang with a total length of nearly 100 km. The overall fault strikes ~N–S. The fault has been dominated by dextral strike–slip kinematics since the late Quaternary [11]. The southern tip of the fault was subjected to intense extension, and it coacted with the Dayingjiang fault to form a triangular fault depression area (e.g., [4]).

The Xima–Panlongshan fault is approximately 30 km long, NE–SW-striking, and dips 85° toward SE. It is an early-middle Pleistocene fault [2] and can be considered an extension of the Sudian fault [2].

Figure 1. (a) Seismotectonic map of the Yingjiang area describing the tectonic setting and seismic activity distribution. Black lines are the main faults, and red lines are secondary faults (e.g., [11,12,13,14]). Yellow circles indicate the distribution of historical earthquakes in Yingjiang (data obtained from NEDC, https://data.earthquake.cn, accessed on 15 December 2022). Blue arrows represent the GPS velocity vectors [15]. Red squares indicate the distribution of 24 hot springs in the study area [16,17]. The thick white dashed line illustrates the tectonic boundary of the Tengchong block [18]. Inset is the location map. Black arrows in the inset show GPS velocity vectors of crustal motion relative to Eurasia [19]. (b) Distribution of focal depths of historical earthquakes in Yingjiang.

Figure 1. (a) Seismotectonic map of the Yingjiang area describing the tectonic setting and seismic activity distribution. Black lines are the main faults, and red lines are secondary faults (e.g., [11,12,13,14]). Yellow circles indicate the distribution of historical earthquakes in Yingjiang (data obtained from NEDC, https://data.earthquake.cn, accessed on 15 December 2022). Blue arrows represent the GPS velocity vectors [15]. Red squares indicate the distribution of 24 hot springs in the study area [16,17]. The thick white dashed line illustrates the tectonic boundary of the Tengchong block [18]. Inset is the location map. Black arrows in the inset show GPS velocity vectors of crustal motion relative to Eurasia [19]. (b) Distribution of focal depths of historical earthquakes in Yingjiang.

Previous studies have analyzed the tectonics and seismic activity in the Yingjiang area and obtained meaningful outcomes. Lei et al. [20] presented the results of three-dimensional tomography in Yingjiang, revealing a low-velocity anomaly near the epicenter of the 2011 Yingjiang earthquake, and deduced that the earthquake might be volcano-related and fluid-driven. According to Zhao et al. [21], the 2011 M_S 5.8 Yingjiang earthquake was caused by the horizontal movement of the Baoshan–Tengchong block under the dual effect of the northeastward compression between the Indian plate and the Eurasian plate and the northeastward subduction of the Myanmar arc, which led to left-lateral a strike–slip motion in the Dayingjiang fault zone. Peng et al. [22] studied the changes in b values before and after the Tengchong M_W 5.1 and 5.0 earthquakes in 2011. Based on this work and evidence from the Schuster test and the ETAS model, they concluded that earthquakes could be triggered by tides and that geothermal fluid played an important role in this mechanism. Huang et al. [2] analyzed the seismogenic structures of the Yingjiang M_S 5.6 and M_S 6.1 earthquakes in May 2014 and proposed that the two groups of earthquakes were generated by the clockwise rotation of an NE-trending secondary fault block held by the N–S-trending fault under shear deformation, which resulted in secondary fault activity and a nearly N–S-trending main faulting direction. These studies suggested that the seismic activity in the Yingjiang area is not only controlled by the tectonic stress field, but it is also closely related to the local special geological environment, such as tidal stress-induced triggers and the movement of geothermal fluid. In addition, the geometry and stress field characteristics of faults were also analyzed to assess the seismogenic mechanism of the faults by means of a relocated seismic catalog, focal mechanism solutions, and fault–slip data (e.g., [23,24,25,26,27]). The studies including the regional seismogenic structures, fault–slip tendency, and rock rheological properties [28,29,30] provided further information for our investigation.

In this study, the stress variation in Yingjiang based on inversion of the focal mechanism solutions of historical earthquakes was studied by using synthetic methods to obtain the patterns of seismic migration and regional fault instability in the area and further this provides what types of faults have a high slip-tendency and are prone to instability and earthquake in the future.

2. Data and Methods

2.1. Data Selection and Processing

The moment tensor solution catalog of earthquakes in Yunnan, ranging in time from 2000 to 2014 [31], is used to select events near the epicenter of the M_S 5 Yingjiang earthquake (between 24.5–25.5° N and 97.5–98.5° E). The moment tensor data used in this paper are based on waveforms with good signal-to-noise ratios and derived from stations with good distance and azimuthal coverage by Xu et al. [31]. Based on these criteria, they were successful in estimating moment tensors for 1833 of the 4428 events that they examined.

Figure 2 shows that the seismic activity in this area shows the characteristics of a cluster. To obtain better inversion results, the event closer to the main earthquake was selected. A total of 187 focal mechanisms were selected and used for the regional stress field inversion (Figure 2).

According to the standard classification of focal mechanism solutions in a related study [32], among the 187 earthquakes collected for the study area, 8 were classified as purely normal, 2 as reverse, 100 as strike–slip, and 77 as oblique (“other” in Figure 3). The triangle diagram (Figure 3) of focal mechanisms can directly show the bias of seismic data collected in the Yingjiang area. The results show that the Yingjiang area mainly features pure strike–slip earthquakes and earthquakes with strike–slip components (Figure 3). He et al. [33] considered that Yingjiang is located in the southwestern Yunnan block, and the inner part of the Yingjiang area is dominated by NE–SW compression, which results in local faults being prone to slip along strike.

2.2. Stress Field Inversion Method

The orientation of the fault plane and the direction of fault slip are determined by a set of seismic focal mechanisms, which can be used to calculate the most suitable principal stress direction and relative stress magnitude R ($R=\left({\sigma }_2-{\sigma }_1\right)/\left({\sigma }_3-{\sigma }_1\right)$) under the assumption of uniform stress distribution in the source area (e.g., [34]). In this study, the improved method proposed by Gephart [34,35,36,37,38] is applied to invert the regional stress tensor using focal mechanisms. First, the fault slip direction is assumed to be consistent with the shear direction, and the stress model is searched by a grid. Then, the corresponding regional stress tensor can be inferred through the common constraints of multiple focal mechanism solutions.

Stress is a symmetric second-order tensor, so it can be characterized by independent parameters. In this method, four parameters (${\sigma }_1$, ${\sigma }_2$, ${\sigma }_3$, R) can be calculated to represent the stress tensor. First, two sets of Cartesian coordinate systems are considered: one is a non-primed set fixed by the principal directions of stress with ${\sigma }_1$, ${\sigma }_2$, and ${\sigma }_3$ representing the three principal stress axes, and the other is a primed set fixed in the observed geometry of the fault plane, including $x_1^{\prime }$ normal to the fault plane, $x_2^{\prime }$ on the fault plane and parallel to it while normal to the direction of fault slip, and $x_3^{\prime }$ parallel to the slip direction (e.g., [34]). The fault plane and auxiliary plane are assumed. Based on the above assumption, the slip of the fault is in the shear stress direction, which can be expressed as:

$${\sigma }_{12}^{\prime }=0={\sigma }_1{\beta }_{11}{\beta }_{12}+{\sigma }_2{\beta }_{12}{\beta }_{22}+{\sigma }_3{\beta }_{13}{\beta }_{23},$$

(1)

where ${\sigma }_{12}^{\prime }$ is the shear stress in the normal direction of the fault plane ($x_1^{\prime }$), which is normal to the slip direction ($x_2^{\prime }$) and ${\beta }_{ij}$ is the cosine angle between the two sets of coordinate axes. Using the orthogonal expression for $x_1^{\prime }$ and $x_2^{\prime }$:

$$0={\beta }_{11}{\beta }_{12}+{\beta }_{12}{\beta }_{22}+{\beta }_{13}{\beta }_{23},$$

(2)

Combined with Equation (1), the following can be obtained:

$$\left({\sigma }_2-{\sigma }_1\right)/\left({\sigma }_3-{\sigma }_1\right)=-\left({\beta }_{13}{\beta }_{23}\right)/\left({\beta }_{12}{\beta }_{22}\right)=R,$$

(3)

Based on the observed fault plane and assuming that the direction of the principal stress is known, Equation (3) describes the principal stress in the $x_3^{\prime }$ direction consistent with the slip direction, where ${\sigma }_1$ ≥ ${\sigma }_2$ ≥ ${\sigma }_3$, and the value of R is limited by 0 ≤ R ≤ 1 [34]. R is also known as the stress shape factor, and its relationship with the stress shape ratio $\phi$ is $\phi =1-R$. When R = 0, the tensile stress axis remains relatively stable, the compressive stress axis and the intermediate stress axis rotate freely in the plane normal to the tensile stress axis, and both of them are in the state of extrusion. When R = 1, the compressive stress axis remains relatively stable. When R = 0.5, the compressive, tensile, and intermediate stress axes are relatively stable (e.g., [39]).

In the actual inversion situation, there is an infinite number of fault plane structures corresponding to a given stress model. To obtain the best fitting model, Gephart [34] chose the one-norm measurement to minimize the misfit and used a grid search to find the best fitting model and its confidence limit. At the same time, uncertainty in the fault plane has a great impact on the inversion results. The method proposed by Gephart [34] can objectively identify the likelihood of the two possible fault planes from each focal mechanism and thus avoid the problem of the ambiguity of nodal planes. In addition, based on the linear inversion method proposed by Michael [40], Hardebeck and Michael [41] introduced a set of damping parameters to remove the influence of artificial grid division, which can simultaneously invert the stress tensor of all of the grid regions and minimize the difference in the stress tensor between adjacent grids. Thus, the smooth transition of adjacent grids can be realized. In this study, an appropriate damping factor is also added in the process of the study to carry out the constraints of inversion.

2.3. Slip Tendency Analysis Method

The reactivation of frictional faults is also critical to earthquake generation, as most earthquakes are caused by frictional instability on existing faults [42,43,44,45,46,47]. The analysis of fault stability based on the existing regional stress field is helpful to analyze the development mechanism of seismic activity and identify the evolution trend of seismic activity. According to Amonton’s law:

$$\tau ={\mu }_s{\sigma }_n^{\prime }={\mu }_s\left(\sigma -P_f\right),$$

(4)

where $\tau$ and ${\sigma }_n^{\prime }$ are the shear stress and effective normal stress acting on the fault plane, respectively, $\sigma$ is the normal stress (positive compression), $P_f$ is the pore pressure, and ${\mu }_s$ is the sliding friction coefficient. According to this relationship, the fault slip tendency [48] $T_s$ is an intuitive variable to evaluate the stability of a fault plane, and it corresponds to the ratio of shear stress to normal stress acting on the fault plane [48]:

$$T_s=\tau /{\sigma }_n^{\prime },$$

(5)

When $T_s>{\mu }_s$, the fault plane is unstable, and the fault easily slips.

2.4. Pore-Pressure Diffusion Method

We employed the method described by Shapiro et al. [49] to further analyze the diffusion properties. The estimation of hydraulic diffusivity uses the concept of the so-called ‘triggering front’. Assuming that in an infinitely uniform and isotropic fluid-saturated medium, the outermost diffusion distance of the pore pressure triggered by the point source satisfies:

$$r=\sqrt{4\pi Dt},$$

(6)

where r defines the approximate outermost envelope of the distances between seismic event locations and the injection point, t is the time when the diffusion starts and D is the hydraulic diffusivity.

3. Results

Based on the above principles and methods, we analyzed the tectonic stress field of the 2021 M_S 5.0 Yingjiang earthquake in Yunnan Province. Then, we investigated the fault slip tendency of the area. Finally, the tectonic factors related to earthquake generation were obtained from these two aspects.

3.1. Inversion of the Tectonic Stress Field

When the damping parameter is small, the data misfit has a relatively larger effect on the inversion results, while the model length has a smaller effect. The selection of an appropriate damping parameter can reduce the complexity of the stress model and make the model smoother [41]. Based on the trade-off curve of data misfit and model length, this study determined the appropriate damping parameter as 1.4, the confidence region of the stress model was set to 95%, and stress field inversion with 1000 bootstrap resamplings was conducted. The results are shown in Figure 4.

Figure 4a shows the probability density function of the stress shape ratio $\mathsf{\phi }=\left({\sigma }_2-{\sigma }_3\right)/\left({\sigma }_1-{\sigma }_3\right)$ of the three principal stress axes, and the stress shape ratio equals 0.62 ± 0.05. Figure 4b shows the azimuth distribution results of the three principal stress axes. The azimuth of the maximum principal stress ${\sigma }_1$ is 97.2 ± 86.9° in the NNE direction, the azimuth of the intermediate principal stress ${\sigma }_2$ is 276.7 ± 59.6°, and the azimuth of the minimum principal stress ${\sigma }_3$ is 124.8 ± 22.7°. Figure 4c–e are the dip distribution functions of the three principal stresses. The maximum principal stress ${\sigma }_1$ is approximately horizontal, and the dip angle is 3.1 ± 5.0°. The intermediate principal stress is nearly vertical, and the dip angle is 84.5 ± 2.1°. The minimum principal stress is nearly horizontal, and the dip angle is 4.5 ± 2.3°. The maximum and minimum principal stresses are close to horizontal, while the intermediate principal stress is close to vertical, indicating that the Yingjiang area has a strike–slip stress system, which is basically consistent with previous research results (e.g., [50]).

The stress field inversion results show that the maximum principal stress ${\sigma }_1$ in the Yingjiang M_S 5.0 earthquake area in 2021 has an NNE orientation, which is consistent with the geological structure of the Yingjiang area and the velocity field direction based on GPS measurements [15] (Figure 1). Related studies have proposed that the Nabang fault and Sudian fault on the east and west sides are both dextral strike–slip faults, and their inner regions easily form conjugate shear planes with NE and NW orientations under the action of force coupling [4,8,51,52]. Coupled with the east–west extensional action of the Dayingjiang fault at the southern tip, the overall stress field in the Yingjiang area is oriented NNE. In addition, the epicenter of the studied earthquake was close to the intersection of the NE–SW-striking Xima–Panlongshan fault and the N–S-striking Sudian fault. Researchers [11,53] believe that under the influence of the principal stress structure in the NNE direction, tectonic stress is easily concentrated at the intersection, and the earthquake might have been triggered by the right-lateral strike–slip movement of the Sudian fault and the pushing effect of the Xima–Panlongshan fault.

The lower the value of the stress shape factor R ($R=1-\phi$), the closer the intermediate stress axis is to the principal compressive stress axis, and the compressive stress regime becomes more dominant; when the R value is near 0.5, it indicates that the maximum, intermediate, and minimum principal compressive stress axes remain relatively stable (e.g., [54]). The R value calculated in this study is 0.38 (close to 0.5), indicating that the compressive and extensional stress axes in the study area are relatively balanced, but the horizontal extensional stress is the main stress, which is also consistent with the active geothermal hot spring activities in this area [8,55]. The results of stress inversion preliminarily indicate that the occurrence of the 2021 M_S 5.0 Yingjiang earthquake was primarily controlled by the tensile stress field.

3.2. Analysis of Fault Slip Tendency in the Yingjiang Area

Based on the stress inversion results, the slip tendency of faults in the Yingjiang area was further analyzed (Figure 5 and Figure 6). Figure 5 shows the Mohr circle under the normalized normal-shear stress coordinates, and the frictional coefficient is prescribed as 0.6 [56]. To analyze the relationship between different fault kinematics and slip tendency and the relationships among faults and strike, dip, and rake angles, we discussed these aspects separately according to the classification method in Section 2.1. Figure 6 further describes the distribution characteristics of the slip angle and normalized slip tendency in the strike-dip coordinate system and the distribution of historical earthquakes in the Yingjiang area.

As shown in Figure 6, the faults with a considerable slip tendency strike ~40° to ~80° or ~110° to ~150° and dip ~60° to ~90°. In Figure 6a, there are only two historical compressive earthquakes, and it can be seen that the slip tendencies of the two earthquakes are low. Figure 6c includes the distribution of strike–slip earthquakes, and it is notable that these strike–slip earthquakes have high slip tendencies. Figure 6e shows the distribution of 7 historical normal fault earthquakes, two of which have relatively high slip tendencies. Figure 6g describes oblique earthquakes, most of which have high slip tendencies according to the distribution of focal spheres. In summary, most of the seismogenic faults of historical earthquakes in the Yingjiang area have a high slip tendency; that is, they have a high degree of instability. Among them, strike–slip faults and oblique faults are more prone to slip, and NE-trending faults have a relatively higher slip tendency.

Figure 6 shows the characteristics of faults with a considerable slip tendency, which indicates that the faults may rupture and cause earthquakes in the future. The Xima–Panlongshan fault, which is close to the epicenter of the 2021 Yingjiang earthquake, has a strike of approximately N45°E and a dip angle of 85° [2], which coincides with our estimation. However, the nearby Sudian fault, whose strike is close to N–S and whose dip angle range is 65° ~ 85°, is not within this range. According to the focal mechanism solution of the M_S 5.0 Yingjiang earthquake determined by Guo et al. [7], nodal plane I has a strike of N31°W and a dip angle of 90°, and plane II has a strike of N59°E and a dip angle of 85°. The focal fault indicated by nodal plane II has a near-NE–SW strike and an approximately vertical dip angle, which is similar to the characteristics of the Xima–Panlongshan fault, which has a considerable slip tendency. In addition, a total of 116 relocated aftershock events following the 2021 Yingjiang mainshock until December 31, 2021, were selected for performing temporal and spatial characteristic analyses. Figure 7a displays the spatial distribution of aftershocks, and two sections (AA’ and BB’) were selected along and perpendicular to the direction of the aftershock distribution. The BB’ profile line is also approximately parallel to the strike of the Xima–Panlongshan fault. The sections along the depth are shown in Figure 7b,c, and the color change of the circles shows the distribution of aftershocks over time. Figure 7b shows that the aftershock migrates northwestward (near A) with time. Figure 7c shows that the aftershock also migrates along a direction approximately parallel to the Xima–Panlongshan fault (near B). The results show that the distribution of aftershocks is influenced by the joint action of two faults, and the influence of the Xima–Panlongshan fault seems to be more significant (Figure 7c). Therefore, the results indicate that the Xima–Panlongshan fault has a considerable slip tendency and is likely the seismogenic fault of the studied earthquake.

4. Discussion

Based on the analysis of the tectonic stress field and fault slip tendency of the 2021 M_S 5.0 Yingjiang earthquake, it is notable that the earthquake was mainly controlled by the tensile stress field and that the inferred strike and dip angles are similar to those of the Xima–Panlongshan fault. The active geothermal fluid activity in this area may have certain implications for the generation mechanism and evolution trend of the mainshock and aftershocks.

Relevant studies have proposed that earthquakes caused by the infiltration and diffusion of deep geothermal fluids often show focal migration characteristics, which can be explained by fluid diffusion (e.g., [49,57,58]). This study further analyzed the evolution of subsequent aftershock activities. The results are illustrated in Figure 7 and Figure 8.

Figure 8 shows the distribution of all of the aftershocks in the three-dimensional direction over time (12 June 2021–3 December 2021). Notably, the aftershock sequence has an outward migration trend overall. Figure 7a shows the aftershock evolution into two distinct spatial subsequences, which are circled with gray and blue ellipses and labeled as aftershock sequences S1 and S2. Subsequence S1 includes the aftershock events occurring between the main earthquake and 3 September 2021, represented by small gray circles. Subsequence S2 includes the aftershock events occurring between 4 September 2021, and 31 December 2021, represented by small blue circles. S1 takes the M_S 5.0 main shock as the starting point, and S2 takes the M_S 3.2 aftershock (24.99° N, 97.85° E) on 4 September 2021, as the starting point. There are obvious differences between S1 and S2 in terms of the spatial distribution of aftershock activity. Subsequence S1 closely surrounds the epicenter of the M_S 5.0 mainshock, while subsequence S2 is relatively closer to the Tenglagong hot spring (25° N, 97.77° E).

We utilized the method described by Shapiro et al. [49] to further analyze the diffusion properties of subsequences S1 and S2. Figure 9 shows the diffusion analysis results of the sequence. Figure 9a shows the distribution of the magnitudes of events during the entire earthquake sequence over time. The M_S 3.2 aftershock on 4 September 2021 was the first large aftershock after the main earthquake. Shapiro et al. [49] proposed that the distribution of the aftershock sequence corresponds to the displacement curve of pore pressure diffusion if the appropriate hydraulic diffusivity D is selected. Figure 9b,c depicts the diffusion distributions of the two subsequences with different hydraulic diffusivities. Figure 9b demonstrates that subsequence S1 has good consistency with the diffusion front curve of D = 4.50 ${\mathrm{m}}^2/\mathrm{s}$, while Figure 9c shows that subsequence S2 corresponds well to the diffusion curve of D = 2.80${\mathrm{m}}^2/\mathrm{s}$. These results indicate that the distributions of the two aftershock sequences were affected by the diffusion and migration of geothermal fluids to a certain extent, but the correlation of subsequence S2 is more conspicuous. In addition, the two subsequences include some aftershocks close to D = 10.20${\mathrm{m}}^2/\mathrm{s}$ and D = 5.00${\mathrm{m}}^2/\mathrm{s}$, which suggests the presence of some faults with high diffusivity in the Yingjiang area, which is consistent with previous studies [49].

Considering that the Yingjiang area is located in the western Tengchong volcanic area, hot springs are widely distributed, and geothermal activity is strong. The Tenglagong hot spring (25° N, 97.77° E) is closest to the epicenter of this earthquake. Spring water has a temperature of 62 °C and a flow rate of 1.5 L/s [16]. Moreover, the M_S 5.0 Yingjiang earthquake was located at the intersection of two strike–slip faults (Figure 1). Under the extensional action of the two groups of faults, the intersection area has a low strength and is conducive to fluid activity. Zhang and Sanderson [59] asserted that underground fluids are more likely to flow along the tensional area of strike–slip faults. In addition, the 2011 M_S 5.8 Yingjiang earthquake (24.7° N, 97.9° E) occurred in the same area as the epicenter of the 2021 M_S 5.0 Yingjiang earthquake. The tomographic imaging results found that the epicenter of the 2011 Yingjiang earthquake was located adjacent to a low-velocity anomaly caused by the upwelling of an active volcano (e.g., [20]). Abundant geophysical evidence suggests that the Yingjiang earthquake may have been a fluid-driven earthquake associated with volcanism (e.g., [20]). Furthermore, previous studies (e.g., [60,61]) have proposed that there are several key factors affecting fault activation, including fault geometry (orientation, size, and surface features), stress field characteristics, and frictional properties (friction coefficient and cohesion). When a fault is in the state of imminent slip, a small stress change driven by geothermal fluid may enhance fault instability, thus controlling the occurrence of earthquakes. There are anomalous low-velocity areas near the focal points of some earthquakes, and these anomalies are considered as evidence of the presence of fluids. These fluid-filled areas have high pore pressures, which reduces the effective stress of faults and encourages fault slip and destabilization, resulting in earthquakes (e.g., [20,62,63,64]). It is worth noting that previous studies [55,65] have noted a low-velocity anomaly region located in the epicenter zone of the Yingjiang earthquake sequence, and this low-velocity region is proposed to reflect magma activity in a molten or semi-molten state. In addition, the water radon at Longling station (approximately 86 km away from the Yingjiang M_S 5.0 earthquake, east of the epicenter) has been detected with an abnormally low value since late February 2021, and the temperature of water has also been detected with a high value anomaly since early August 2020 [e.g., 66]. After the Yingjiang earthquake, these anomalies gradually changed into normal conditions. A previous study suggested that these fluid anomalies might be related to this 2021 M_S 5.0 Yingjiang earthquake occurrence [66]. Although the seismogenic mechanism of the 2021 M_S 5.0 Yingjiang earthquake was dominated by the tectonic stress field, considering its seismogenic mechanism and the conspicuous migration of the aftershock sequence, we recognize that the diffusion and migration of geothermal fluids influenced the development of subsequent seismic activity. Therefore, the 2021 M_S 5.0 Yingjiang earthquake was dominated by the tectonic stress field and influenced by fluid migration.

5. Conclusions

In this study, focal mechanism data from 2008 to 2014 in Yingjiang, Yunnan, were selected for regional stress field inversion. Through detailed analyses, we obtained the following conclusions.

• The main compressive stress axis of the regional stress in the Yingjiang area has an NNE–SSW distribution, which is consistent with the tectonic setting of the Yingjiang area. The two principal stress axes are nearly horizontal, and the intermediate axis is vertical. The extensional stress field is predominantly a strike–slip type.
• The results of slip tendency analysis show that the faults striking ~40° to ~80° or ~110° to ~150° and dipping ~60° to ~90° in the Yingjiang area have relatively higher slip tendencies. The Xima–Panlongshan fault with a considerable slip tendency is likely to be the seismogenic fault of the 2021 earthquake.
• The calculated corresponding diffusion fronts of the aftershock sequences reach as high as 2.8~4.5 m²/s. The 2021 M_S 5.0 Yingjiang earthquake and aftershocks are likely to have been greatly influenced by the migration of geothermal fluids.