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Article

The 2024 Mw 7.1 Wushi Earthquake: A Thrust and Strike-Slip Event Unveiling the Seismic Mechanisms of the South Tian Shan’s Thick-Skin Tectonics

by
Jiangtao Qiu
1,2,
Jianbao Sun
1,* and
Lingyun Ji
2
1
Institute of Geology, China Earthquake Administration, Beijing 100029, China
2
The Second Monitoring and Application Center, China Earthquake Administration, Xi’an 710054, China
*
Author to whom correspondence should be addressed.
Remote Sens. 2024, 16(16), 2937; https://doi.org/10.3390/rs16162937
Submission received: 25 June 2024 / Revised: 29 July 2024 / Accepted: 7 August 2024 / Published: 10 August 2024
(This article belongs to the Special Issue Monitoring Geohazard from Synthetic Aperture Radar Interferometry)
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Abstract

:
The southern margin of the South Tian Shan has drawn attention due to the intense compressional deformation and seismic activity associated with its thrust structures. However, the deformation and seismic activity in the thick-skinned thrust sheets of the root zones are minimal. The Mw 7.1 Wushi earthquake on 23 January 2024 serves as a window to reveal these unknown aspects of the seismic mechanisms in this structural setting. Using the Leveraging Interferometric Synthetic Aperture Radar (InSAR) technique, we unlock critical insights into the coseismic deformation fields. The seismogenic fault is an unmapped segment within the Maidan Fault system, exhibiting a strike ranging from 241° to 222°. It is characterized by a shallow dip angle of 62° and a deeper dip angle of 56°. Remarkably, the seismic rupture did not propagate to the Earth’s surface. The majority of slip distribution is concentrated within a range of 4 to 26 km along the strike, indicating that this earthquake was a thrust event on a blind fault within the thick-skinned tectonics of the South Tian Shan. Coulomb stress changes indicate that aftershocks primarily occur in the stress-loading region. Interestingly, some aftershocks are very shallow, causing clear surface deformation. Inversion results show that the fault planes of two aftershocks are located above the main shock fault plane at extremely shallow depths (<6 km). Combining geophysical profile data, we infer that ruptures in the deep-seated thick-skinned structures during the main shock triggered ruptures in the shallow thrust structures. This triggering relationship highlights the potential for combined ruptures of the main shocks and aftershocks in the deep-seated thick-skinned structures beneath the South Tian Shan to result in larger disasters than typical seismic events.

1. Introduction

The Pamir–South Tian Shan–Tarim collision zone is acknowledged as the most dynamically active tectonic area within the Tian Shan orogeny, exhibiting significant north–south compression deformation and frequent seismic activity. These factors have contributed to the development of a complex network of faults with varied orientations and kinematic characteristics in this region (Figure 1). These faults play a crucial role in the continuous deformation and heightened seismicity evident in the Tian Shan region [1,2,3]. Along the southern margin of the South Tian Shan, there exists a predominantly thin-skinned thrust system, with the frontal portion of the thrust system being the most concentrated area of deformation and seismic activity. In contrast, the root zone of the thrust system belongs to a thick-skinned tectonic regime, where deformation is minimal and seismic activity is infrequent, leading to a limited understanding of the seismic mechanisms and structural characteristics in this particular tectonic setting.
The Mw 7.1 earthquake that occurred in Wushi County, Xinjiang, on 23 January 2024 was situated at the southern front of the South Tian Shan, at the root zone of the Kepingtag thrust system, within the Wushi Depression, showcasing an extremely complex regional tectonic setting, where the strike of the South Tian Shan orogenic belt undergoes a sudden transition. Some researchers consider this area as a transitional zone between the east–west structures of the South Tian Shan orogeny [4]. Therefore, this Mw7.1 earthquake serves as a window for a deeper understanding of the seismic mechanisms within the thick-skinned tectonics of the southern margin of the South Tian Shan. Following the earthquake, various agencies have provided focal mechanism solutions that confirm it as primarily a thrust event (refer to Table 1 and Figure 1c). These solutions reveal the kinematic characteristics of the faulting structure. However, due to significant differences in the hypocenter locations, determining the precise location and geometric parameters of the faulting structure remains challenging. In particular, post-earthquake investigations have identified approximately a 2 km long surface rupture attributed to aftershocks [5]. The following questions arise: why would aftershocks produce surface ruptures while the main shock did not, and what is the relationship between the two? Addressing these questions is essential.
Interferometric Synthetic Aperture Radar (InSAR) technology has emerged as a crucial tool for capturing coseismic deformation [6,7,8]. In this study, we utilized Sentinel-1A satellite ascending and descending Synthetic Aperture Radar (SAR) data provided by the European Space Agency to obtain the InSAR coseismic deformation fields of the Wushi Mw7.1 earthquake and aftershocks. Using these deformation constraints, we conducted an inversion analysis to determine the geometric parameters and slip distribution of the responsible fault. Furthermore, combining static Coulomb stress analysis with geophysical reflection profile data, we discuss the triggering relationship between the main shock and aftershocks, as well as their significance in indicating the seismic mechanisms within the thick-skinned tectonics of the southern margin of the South Tian Shan.
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Figure 1. Major Cenozoic faults in the Tianshan Zone and the tectonic setting of the 2024 Wushi earthquake. (a) The blue arrow indicates the GPS horizontal velocity field from [9]; the red arrow indicates the GPS horizontal velocity field from [10]; the thin black lines represent faults; the empty circles represent the locations of earthquakes with a magnitude of 6 or higher since 1900. (b) The red circular region delineates the area shown in (a); the gray circles represent the locations of strong earthquakes with a magnitude of 7 or higher since 1900 (data are from https://earthquake.usgs.gov/earthquakes/, accessed on 23 February 2024). (c) shows different organizations’ determined focal mechanism solutions and locations. (d) North–south structural diagram cross-section of the southern margin of the South Tian Shan. The light-blue line delineates the detachment fault. The red lines indicate the thrust faults of the southern margin of the South Tian Shan. Modified from [11,12]. The lemon chiffon area delineates thin skinned structure, medium purple area delineates thick skinned structure. Fault abbreviation: SNBF, the South Naryn Basin Fault; NNBF, the North Naryn Basin Fault; SIKF, the South Issyk-Kul Fault; PFF, the Pamir Frontal Thrust Fault; TFF, Talas-Fergana Fault; MDF, Maidan Fault; TSF, Toshgan Fault; KKSF, Kokesale Fault; NWSF the North Wensu Fault; KTF, Kepingtag Fault.
Figure 1. Major Cenozoic faults in the Tianshan Zone and the tectonic setting of the 2024 Wushi earthquake. (a) The blue arrow indicates the GPS horizontal velocity field from [9]; the red arrow indicates the GPS horizontal velocity field from [10]; the thin black lines represent faults; the empty circles represent the locations of earthquakes with a magnitude of 6 or higher since 1900. (b) The red circular region delineates the area shown in (a); the gray circles represent the locations of strong earthquakes with a magnitude of 7 or higher since 1900 (data are from https://earthquake.usgs.gov/earthquakes/, accessed on 23 February 2024). (c) shows different organizations’ determined focal mechanism solutions and locations. (d) North–south structural diagram cross-section of the southern margin of the South Tian Shan. The light-blue line delineates the detachment fault. The red lines indicate the thrust faults of the southern margin of the South Tian Shan. Modified from [11,12]. The lemon chiffon area delineates thin skinned structure, medium purple area delineates thick skinned structure. Fault abbreviation: SNBF, the South Naryn Basin Fault; NNBF, the North Naryn Basin Fault; SIKF, the South Issyk-Kul Fault; PFF, the Pamir Frontal Thrust Fault; TFF, Talas-Fergana Fault; MDF, Maidan Fault; TSF, Toshgan Fault; KKSF, Kokesale Fault; NWSF the North Wensu Fault; KTF, Kepingtag Fault.
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2. InSAR Data and Processing

The Differential InSAR (DInSAR) technique utilizes differential interferograms, derived from radar images captured at different times, to quantify the surface deformation of the Earth [13], which has been extensively and maturely applied in numerous earthquake case studies. This study also employs this technique, utilizing multi-temporal Sentinel-1 SAR data, to acquire the deformation fields of the Mw7.1 mainshock and its aftershocks.

2.1. InSAR Deformation of the Mw7.1 Earthquake

Within two to three days following the Wushi Mw7.1 earthquake, the European Space Agency’s Sentinel-1 satellite conducted ascending and descending orbit observations of the seismic zone, respectively. We promptly downloaded the multiperiod images from both orbits, which covered the epicentral area (detailed information is provided in Table 2). These images were then processed using the GAMMA software (version 2018) platform [14,15] for two-pass DInSAR processing [16]. The primary processing steps included the following: the removal of the topographic phase using the 30 m resolution Shuttle Radar Topography Mission (SRTM) digital elevation model (DEM) obtained from the Consultative Group on International Agricultural Research Consortium for Spatial Information (CGIAR-CSI, http://srtm.csi.cgiar.org, accessed on 1 January 2020); range and azimuth multi-looking with a configuration of 10 × 2, utilizing the weighted power spectral filtering method with a filter window size of 64 × 64 to generate interferograms [17]; and phase unwrapping using the robust minimum-cost flow method. To ensure the reliability of the deformation results, pixels with a coherence lower than 0.2 were masked during the unwrapping process. A second-degree polynomial fitting method was employed to remove the residual phase caused by orbital effects in the interferograms. Since the epicenter is located at the intersection of a basin and mountains, an atmospheric phase delay model was established based on the available digital elevation model to correct for the phase delay caused by atmospheric water vapor stratification. During the analysis of the interferograms from both ascending and descending orbits, significant long-wavelength errors were observed, which were corrected by a flattening correction. Finally, the deformation maps were geocoded in radar coordinates to obtain the line-of-sight (LOS) coseismic deformation fields in geographic coordinates (refer to Figure 2, and the wrapped interferogram is shown in Supplementary Material Figure S1).
The deformation fields of the Wushi Mw7.1 earthquake and aftershock, as depicted in Figure 2, clearly demonstrate the capability of Sentinel-1 SAR data to effectively monitor the coseismic deformation associated with this event, with deformation features being prominently visible. Regardless of the ascending or descending orbits, a nearly elliptical uplift zone is observed on the northwest side (with a major axis of approximately 45 km), exhibiting significant LOS displacement. The maximum LOS displacement values are approximately 72.5 cm for ascending track T56 and 48.8 cm for descending track T34. On the southeast side, a subsidence zone is observed (with a major axis of approximately 24 km), with maximum values of approximately 10.8 cm for the ascending track and 13.8 cm for the descending track. The deformation patterns observed on both sides of the fault in the ascending and descending interferograms exhibit substantial consistency, indicating that the primary deformation induced by the earthquake is vertical uplift. We utilized the deformation fields from both ascending and descending orbits to separate the vertical and east–west components of deformation (detailed in the Supporting Information Text S1 and Figure S2). Our findings reveal that the Mw 7.1 earthquake triggered a peak uplift of 60.7 cm in the northwest block, accompanied by a maximum westward displacement of 52.5 cm. In contrast, the southeast block experienced a significant subsidence of 12.8 cm and an eastward displacement of 9.5 cm.

2.2. InSAR Deformation of Aftershock

In the aftermath of the Wushi Mw 7.1 earthquake, the China Earthquake Networks Center recorded a total of 1742 seismic events with magnitudes greater than 1.5 in the vicinity of the epicenter between 23 January 2024 and 23 February 2024. The largest event during this period was a magnitude 5.6 earthquake (78.698°E, 41.108°N) on 30 January.
We obtained SAR images from descending track T34 and ascending track T56 on 6 February (14 days after the Wushi earthquake) and 7 February (15 days after the Wushi earthquake), respectively. These images underwent interferometric processing using the most recent post-earthquake SAR image, following the same processing strategy as employed for the InSAR deformation field analysis of the Wushi Mw 7.1 earthquake. The resulting ascending and descending InSAR deformation fields (Figure 2e,f, the wrapped interferogram is shown in Supplementary Material Figure S3) indicate that the aftershock-induced deformation is primarily localized within an approximate area measuring 12 km in length and 8 km in width. In ascending track T56, the maximum observed deformation is 38.7 cm, while the minimum is −13.6 cm. For descending track T34, the maximum deformation value is 48.4 cm, with a minimum of −8.9 cm.

3. Fault Geometry and Slip Exploration

The Okada model [18,19] defines the physical link between surface deformation and fault plane slip. In this section, we apply the Okada model to conduct a further inversion and ascertain the geometric parameters and distribution of coseismic slip within the source structure. To obtain a reasonably accurate representation of the fault geometry and slip distribution, we conducted multiple testing iterations. Additionally, to analyze the relationship between the fault distributions of the mainshock and aftershocks, we also performed an inversion on the fault planes responsible for the aftershock deformation.

3.1. Fault Parameters Setting of the Mw7.1 Earthquake

Prior to conducting slip distribution inversion, it is essential to determine the geometric parameters of the causative fault. The geometric parameters include the reference location (longitude and latitude), length, width, strike, and dip angle. The kinematic parameters encompass the amount of strike-slip displacement and dip-slip displacement. The inversion of geometric parameters is based on an elastic half-space model [19], assuming uniform slip on the fault plane and seeking the best fit between modeled deformation values and actual observations [20]. Equation (1) represents the relationship between the observed data, slip model vector, Green’s function, and observation error:
d = G(m) + ε
Here, d represents the observed data, m represents the slip model vector, G is the Green’s function that relates the model and observed data, and ε represents the observation error.
Bagnardi and Hooper (2018) introduced a Bayesian approach for inverting multiple geodetic datasets, enabling the rapid characterization of posterior probability density functions (PDFs) of source model parameters [21]. In this study, we utilize this method and perform the inversion of fault geometric parameters using InSAR deformation fields. Initially, we establish the search range for model parameters. By delineating the boundaries of uplift and subsidence revealed by LOS deformation, we preliminarily determine the spatial distribution of the seismogenic fault. The northwest side corresponds to the hanging wall, and the surface trace of the fault is expected to extend southwestward (the white line segment shown in Figure 1a,b). Therefore, we set the fault strike range as 220°–240°. Multiple seismogenic mechanism solutions from various institutions (Table 1) indicate a relatively concentrated range for fault dip values (38°–50°). Valuable geophysical profile data [4,22], located near the epicenter, serve as valuable prior information for fault slip inversion. Two profiles, one on the east and the other on the west side of the epicenter (Figure S4), both indicate fault dip angles ranging from approximately 56° to 64° in the frontal mountain position. Hence, we set the search range for the fault dip as 38°–70°. To enhance computational efficiency, we downsample the original high-resolution InSAR observations using a quadtree partitioning method, resulting in a final dataset of 3319 ascending observations and 1995 descending observations. This downsampling preserves deformation details while reducing data redundancy. The optimization process reveals an optimal dip angle of 60.369° and an optimal strike of 229.98° for the seismogenic fault (Table 3 and Figure 3).

3.2. Slip Distribution Inversion of the Mw7.1 Earthquake

The geometric parameters obtained in the previous step represent an initial approximation of the fault shape. However, in theory, the slip distribution on the fault plane is not uniform. This is evident from the InSAR deformation field, which shows that the surface trace of the fault exhibits bends rather than a straight line (Figure S1 and Figure 2a). Additionally, two geophysical profiles suggest the possibility of variation in the fault’s dip angle (Figure S4). Therefore, in this step, it is crucial to continuously refine and test the geometric parameters of the fault for inversion in order to achieve a reasonable slip distribution on the fault plane. The SDM (steepest descent method) utilizes the InSAR deformation field as a constraint and employs the Okada elastic half-space dislocation model to invert for the distribution of coseismic slip [23]. This method has been widely applied to various earthquakes [24,25,26].
Through multiple iterations using the SDM code and considering various parameter combinations, we generated several coseismic slip models. We configured the fault with a length of 66 km and a width of 40 km, discretized into 33 × 20 patches, each measuring 2 km × 2 km. The results of these models are presented in Table S2 and Figure S5 in the Supporting Information. All models indicate thrust and left-lateral slip during seismic rupture, with slip primarily concentrated at depths ranging from 4 to 26 km along the dip. The distribution of slip along the strike, the magnitude of maximum slip, and its location differ depending on the specific model used. Among these models, parameters with a strike ranging from 241° to 222° from east to west, a shallow dip of 62°, and a deep dip of 56° exhibit the highest correlation (0.9955) between the model and the observation. This model produces a slip distribution that aligns well with the deformation observed on both the hanging wall and footwall of the fault (Figure 4). The slip rupture in this model extends along the strike for 50 km, with a maximum slip displacement of 3.2 m (a dip-slip of −2.2589 m and a strike-slip of 3.1334 m), located at a depth of 11.08 km (78.60°E, 41.20°N). The slip near the Earth’s surface is relatively minor. The corresponding moment magnitude (Mw) of the inverted slip model is estimated to be between 7.01 and 7.02, which aligns with the values provided by various organizations, as listed in Table 1.

3.3. Seismic Structures and Slip Distribution Inversion of the Aftershocks

The InSAR data in Figure 2e,f illustrate that the aftershock deformation extent is relatively limited. However, the deformation characteristics are notably intricate, displaying two distinct zones of deformation polarity transition: one trending NEE and the other oriented approximately north–south (Figure S3). To understand why aftershocks result in surface ruptures and to elucidate their seismogenic structural patterns, we further conducted a structural inversion based on the aftershock deformation fields.
The deformation fields associated with the ascending and descending tracks of aftershocks from this earthquake (Figure 2e,f and Figure S3) exhibit a complex pattern, from which we discerned two distinct rupture traces. Consequently, we developed a set of simulation models comprising two faults—one trending NEE and the other oriented approximately north–south. After multiple tests, we ultimately determined a set of plausible fault models: Fault 1 (f1) exhibits a shovel-shaped geometry with a strike of 61°, a shallow dip angle of 58°, and a deep dip angle of 30°. Fault 2 (f2) had a strike of 191° and a dip angle of 76°, with the maximum slip distribution (refer to Figure 5) indicating that the rupture primarily occurred on Fault 1, with a maximum slip of 1.15 m located at a depth of 1.43 km (78.635°E, 41.158°N). The predicted deformation field can replicate the main observed deformation characteristics; however, significant residuals are present near f1. The simulated seismic moment is Mw 5.8, larger than the M5.6 magnitude, confirming that this deformation field encompasses not only the deformation associated with the M5.6 event but also a distinct aftershock event with a nearly east–west striking causative fault.

4. Discussion

4.1. Seismogenic Structure of the Mw7.1 Wushi Earthquake

The analysis of ascending and descending track InSAR deformation fields provides valuable insights into the deformation characteristics on both sides of the seismogenic fault. The northwest side of the fault exhibits uplift in LOS measurements, while the southeast side shows subsidence. This consistent pattern of deformation in both ascending and descending tracks, predominantly with uplift, indicates that the surface deformation resulting from this earthquake is primarily associated with the reverse slip motion. Furthermore, the decomposition of the deformation fields reveals left-lateral slip displacement. The northwest side of the fault corresponds to the hanging wall, while the southeast side corresponds to the footwall. Based on these observations, we delineated the surface trace of the seismogenic fault as a polyline composed of two segments (Figure S1 and Figure 2). The eastern segment trends at 241°, approximating the northern branch of the Maidan Fault, but then turns to 222° and extends southwest towards the vicinity of the southern branch of the Maidan Fault (Figure S1). This suggests that the seismogenic fault associated with this earthquake is a previously unmapped fault.
To determine a reasonable slip distribution for the fault model, we conducted inversion tests with various combinations of parameters (Supplementary Material Table S2 and Figure S5). The results showed the highest correlation coefficient achieved for the fault model, with a strike ranging from 241° to 222° from east to west, a shallow dip of 62°, and a deep dip of 56°. This fault model exhibits strong agreement with the InSAR observation field, providing a robust fit to the data. The geophysical profile data further elucidate the predominant structural deformation in this region, characterized by thick-skinned thrusting and associated folding [22,27,28]. The fault architecture demonstrates an upper steep segment and a lower gentle segment, with dip angles that progressively decrease with increasing distance from the mountains. Consistent with these geophysical observations, our geometry and kinematics models of the coseismic fault inversion depict a slightly listric fault geometry, with a dip angle ranging from 62° at the top to 56° at the bottom.
The distribution of reverse and strike-slip components on the fault plane aligns with the Maidan Fault’s geometry and kinematics. However, the co-seismic rupture primarily occurred at depths between 4 and 26 km along the dip, without reaching the surface. This suggests that the seismogenic fault associated with this earthquake may be a concealed secondary reverse fault within the Maidan Fault zone. This earthquake event falls within a blind thrust fault in the root of the South Tian Shan’s thick-skinned tectonics. We hypothesize that the earthquake rupture initiated with an oblique slip at depth. At the location of the fault bend, the rupture transitioned to primarily a strike-slip motion, resulting in maximum slip displacements of −2.2589 m along the dip and 3.1334 m along the strike. Subsequently, the rupture propagated upward in a reverse slip manner, reaching shallower depths. This earthquake demonstrates the reverse and strike-slip characteristics of the Maidan Fault, underlining the importance of root faults in accommodating compression and providing valuable insights into the mechanics of the Tianshan region.

4.2. Deformation and Potential Seismic Structures of Aftershock

Following the occurrence of the Wushi earthquake, immediate field investigations were conducted by geologists. On 30 January 2024, after the largest aftershock (M5.6) of the Wushi earthquake, an unexpected surface rupture was discovered in the Chalemati River Valley (78°36′, 41°10′). The surface rupture zone exhibited an overall trend of N60°E, spanning approximately 2 km in length, with a maximum vertical displacement of about 1 m [5]. The surface rupture is governed by a reverse fault dipping southeastward, contrary to the dip of the mainshock causative fault. Our wrapped InSAR interferograms from ascending and descending tracks reveal areas of maximum deformation exhibiting surface traces of incoherence, indicating aftershock rupture reaching the surface based on empirical evidence.
The InSAR data in Figure 2e,f illustrate that the aftershock deformation extent is relatively limited, showcasing maximum LOS displacements of 38.7 cm (ascending track) and 48.4 cm (descending track). However, the deformation characteristics are notably intricate, displaying two distinct zones of deformation polarity transition: one trending NEE and the other oriented approximately north–south. To understand why aftershocks result in surface ruptures and to elucidate their seismogenic structural patterns, we further conducted a structural inversion based on the aftershock deformation fields.
We developed a set of simulation models comprising two faults—one trending NEE and the other oriented approximately north–south. After multiple tests, we ultimately determined a set of plausible fault models: Fault 1 (f1) exhibits a shovel-shaped geometry with a strike of 61°, a shallow dip angle of 58°, and a deep dip angle of 30°. Fault 2 (f2) has a strike of 191° and a dip angle of 76°, with the maximum slip distribution (refer to Figure 5) indicating that the rupture primarily occurred on Fault 1, with a maximum slip of 1.15 m located at a depth of 1.43 km (78.635°E, 41.158°N). The predicted deformation field can replicate the main observed deformation characteristics; however, significant residuals are present near f1. The simulated seismic moment is Mw 5.8, larger than the M5.6 magnitude, confirming that this deformation field encompasses not only the deformation associated with the M5.6 event but also a distinct aftershock event with a nearly east–west striking causative fault. This outcome reveals that the hypocenter of the aftershock is very shallow, leading to rupture at extremely shallow depths and eventually propagating to the surface. The geometric relationship of the seismogenic faults for the main shock and aftershocks is shown in the Supporting Material Figure S6.

4.3. The Triggering Relationship of Main–Aftershocks

The triggering relationship between the main shock and aftershocks of the Wush earthquake is of particular interest as the main shock did not rupture to the surface while the aftershocks did. This raises questions about the structural and triggering mechanisms between the aftershocks and the main shock. The occurrence of earthquakes is closely related to the regional tectonic stress field, and stress changes on fault planes before and after an earthquake can trigger aftershocks. In recent years, Coulomb stress [29] has been widely used to study stress triggering and stress perturbation on major active faults [30,31].
When calculating the stress changes caused by an earthquake, it is necessary to determine the geometric morphology of the fault rupture surface and the amount of fault slip [32,33]. Considering that the regional stress field is continuous in space and not limited to any specific fault plane, here, we adopt the approach proposed by [29] to calculate the Coulomb stress changes on faults oriented optimally for failure. This approach, assuming a diversity of fault orientations, assesses stress-triggered earthquake potential, including aftershocks, by focusing on faults most susceptible to stress perturbations. We employed the detailed fault slip distribution model obtained in Section 3.2 to calculate the static Coulomb stress changes produced by the Wushi Mw7.1 earthquake at depths of 5 km, 10 km, and 20 km (refer to Figure S7). The results indicate that the coseismic Coulomb stress changes induced by the Wushi earthquake primarily accumulate in localized areas, with the stress rapidly attenuating as the distance from the causative fault increases. Aftershocks mainly occur in regions where Coulomb stress levels have increased at varying depths.
We specifically calculated the influence of Coulomb stress changes on two aftershock fault planes originating from the main shock fault plane (illustrated in Figure 6a). Positive Coulomb stress changes (depicted in red) on the fault plane indicate an approaching rupture state and the presence of sliding hazards [34]. By integrating geophysical profile data, we established the structural relationship between the main shock and aftershocks (as shown in Figure 6b) and inferred that the entire process involved a rupture of the shallow thrust fault in response to the main shock rupture occurring in the deep thick-skinned structure. This triggering relationship underscores the possibility that the combined rupture of main and aftershock events within the deep thick-skinned structures at the base of the South Tian Shan may lead to catastrophes greater than those seen in typical seismic events.

4.4. Seismicity of the Maidan Fault

The geological evolution of the Tian Shan has shaped a multifaceted geological framework, characterized by folding, thrusting, and crustal shortening deformation in the foreland basins flanking the Tian Shan [13,35,36]. Geological investigations and GPS observations have revealed significant activity in the thin-skinned thrust structures on both sides of the Tian Shan, particularly in tectonic belts consisting of reverse faults and folds, which have accommodated a substantial portion of the shortening deformation in the region through horizontal crustal shortening and vertical structural uplift [37,38]. Strong earthquakes are predominantly concentrated along active folds and thrust faults [3]. The Keping thrust nappe, located in the southern margin of the South Tian Shan, represents a seismically active zone, whereas the Maidan Fault, positioned between the Keping thrust nappe and the South Tian Shan, exhibits lower activity and relatively slower slip rates. From a macroscopic tectonic perspective, this difference can be attributed to their respective positions within the region. The Kepingtag thrust nappe is situated at the forefront of the thin-skinned tectonic zone, while the Maidan Fault is located further back. As a result, the structural stress on the Maidan Fault is significantly lower than the intense compressional thrusting experienced by the frontal segment of the thrust nappe. However, seismic activity has intensified at the eastern end of the Maidan Fault. The Wushi earthquake provides an opportunity for us to reassess the seismic activity along the Maidan Fault.
The epicenter of the Wushi earthquake is located at the eastern end of the Maidan Fault, with its northwestern segment corresponding to the Kokesale northeast-trending tectonic zone and its southern segment representing the Wushi thrust-fold zone. Within this specific region, the strike of the Tian Shan orogen abruptly transitions from an almost east–west orientation to a northeast–southwest direction. Fu (2010) considers this area as a transitional junction between the east–west and northeast–southwest structural patterns that characterize the Tian Shan orogeny [4]. Geophysical profiling data indicate that the shallow layers of the Wushi thrust-fold zone exhibit a forward-vergent thrust nappe structure, while the deeper layers are characterized by thick-skinned structures involving the basement (Figure 1d) [22,27]. Geological and geodetic data have revealed that the structural deformation along the Maidan Fault is segmented [11,39,40]. The western segment of the Maidan Fault, located west of Aheqi, primarily exhibits reverse movement with a minor component of left-lateral strike-slip. However, to the east of Aheqi, the slip rate on the Maidan Fault decreases, and it diverges into several branch faults. The Wushi earthquake occurred at the eastern terminus of the Maidan Fault, presenting a horse-tail-shaped distribution that includes root fault, central fault, and leading fault segments [41]. The root fault is located in front of the Kuokesale Mountain and controls the northern boundary of the Wushi Depression [4,28,42]. It is worth noting that as it extends into the Wushi Depression, the orientation of the Maidan Fault shifts from nearly east–west to southwest, coinciding with an increase in seismic activity [22]. An analysis of GNSS profile data reveals a decrease in the shortening rate of the Tianshan Mountains from approximately 10 mm/y at 75°E to around 5 mm/y at 85°E [9]. Therefore, this region may play a crucial role in understanding the mechanical transition mechanism between the Tarim Basin and the South Tianshan Mountains.
Notably, unlike the Kashgar Depression and Kepingtag thrust sheet in the western segment of the Maidan Fault, the Wushi Depression lacks well-established folds, implying that stress concentration is more likely to occur along fault structures. We can speculate that the occurrence of the Wushi earthquake can be attributed to its location at the junction of the South Tianshan Mountains and the Wushi Depression, a region characterized by a sudden change in the orientation of the South Tianshan structural belt.

5. Conclusions

The occurrence of the Wushi earthquake on the southern Tianshan root fault provides a unique opportunity to reassess the role of activity within the root structure of the southern Tianshan region in modulating regional movements. This study utilized ascending and descending orbit Sentinel-1 SAR data provided by the European Space Agency and employed InSAR techniques to retrieve the coseismic deformation field. By utilizing this information as a constraint, the coseismic slip distribution of the earthquake was inverted, yielding valuable insights into the complex structures associated with this event. The research findings are as follows:
(1) The Wushi earthquake represents a reverse and slip rupture event within the thick-skinned structure located at the southern margin of the South Tian Shan. The seismic event occurred along the eastern segment of the Maidan Fault, striking a direction from 241° to 222° from east to west, characterized by a shallow dip of 62° and a deep dip of 56°.
(2) The main slip distribution resulting from the Mw7.1 mainshock primarily occurred at depths ranging from 4 to 26 km. Notably, the maximum slip, approximately 3.2 m, was observed at a depth of 11.08 km. Subsequent to the mainshock, the aftershock on 30 January 2024 (M5.6) ruptured along an extremely shallow fault plane trending N61°E. This aftershock exhibited a maximum slip of 1.15 m at a depth of 1.43 km, extending all the way to the surface. This triggering relationship serves as a reminder that the combined rupture of mainshocks and aftershocks within the thick-skinned structure at the base of the South Tian Shan may result in greater disaster potential than typical seismic events.
In conclusion, the study of the Wushi earthquake and its associated deformation and seismic structures provides valuable insights into the dynamics of the southern Tianshan region and contributes to our understanding of the mechanical behavior and seismic hazard potential in this area.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/rs16162937/s1, Figure S1: Wrapped coseismic descending and ascending interferograms of the 2024 Wushi Mw7.1 earthquake; Figure S2: The vertical and east–west components of deformation fields; Figure S3: Wrapped coseismic descending and ascending interferograms of the 2024 Wushi aftershocks; Figure S4: The fault model derived from the InSAR deformation field and geophysical profile data; Figure S5: The different coseismic slip results; Figure S6: The combined slip model of the mainshock and aftershock seismogenic structures; Figure S7: Coulomb stress changes varying depths caused by the Wushi earthquake; Text S1: Using the InSAR results of ascending and descending tracks to decompose the east–west and upward deformation components; Table S1: Fault models for the Wushi event from various inversions with different fault configurations.

Author Contributions

All the authors participated in editing and reviewing the manuscript. Conceptualization, J.S. and L.J.; methodology, J.S. and L.J.; software, L.J. and J.Q.; validation, J.S.; data curation J.S. and J.Q.; writing—original draft preparation, J.Q.; writing—review and editing, J.S., L.J. and J.Q. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Natural Science Foundation of Shaanxi Province, China (No. 2023-JC-QN-0296) and Social Science Planning Fund Project of Xi’an, China (No. 24QL47).

Data Availability Statement

The Sentinel-1 data used in this study are downloaded from the European Space Agency (ESA) through the ASF Data Hub website, https://vertex.daac.asf.alaska.edu/ (accessed on 1 March 2024).

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 2. Unwrapped InSAR deformation fields of the Wushi earthquake and aftershocks. (a,b) LOS deformation (cold colors indicate motion away from the satellite, while warm colors indicate motion towards the satellite). White lines denote of the surface trace of the seismogenic fault that we inferred. (c,d) Deformation profiles along A-A’ and B-B’ in (a,b). (e,f) InSAR deformation fields of aftershocks. The blue circles represent precise locations of aftershocks with depths less than 10 km and magnitudes greater than M4.5 between 24 January 2024 and 7 February 2024. The size of the circles corresponds to the magnitude of the aftershocks. Data source: https://data.earthquake.cn/gxdt/info/2024/334671642.html, (accessed on 12 March 2024). (e) Ascending track 56, time interval 20240125_20240207. (f) Descending track 34, time interval 20240124_20240206.
Figure 2. Unwrapped InSAR deformation fields of the Wushi earthquake and aftershocks. (a,b) LOS deformation (cold colors indicate motion away from the satellite, while warm colors indicate motion towards the satellite). White lines denote of the surface trace of the seismogenic fault that we inferred. (c,d) Deformation profiles along A-A’ and B-B’ in (a,b). (e,f) InSAR deformation fields of aftershocks. The blue circles represent precise locations of aftershocks with depths less than 10 km and magnitudes greater than M4.5 between 24 January 2024 and 7 February 2024. The size of the circles corresponds to the magnitude of the aftershocks. Data source: https://data.earthquake.cn/gxdt/info/2024/334671642.html, (accessed on 12 March 2024). (e) Ascending track 56, time interval 20240125_20240207. (f) Descending track 34, time interval 20240124_20240206.
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Figure 3. Marginal posterior probability distributions for the fault model parameters for the Wushi earthquake. Red lines represent the maximum a posteriori probability solution (cold colors for low frequency, warm colors for high frequency).
Figure 3. Marginal posterior probability distributions for the fault model parameters for the Wushi earthquake. Red lines represent the maximum a posteriori probability solution (cold colors for low frequency, warm colors for high frequency).
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Figure 4. Slip distribution model and deformation and residuals predicted of the Wushi Mw7.1 earthquake. (a) 3D display and (b) 2D display, arrows indicate the slip direction of the hanging wall relative to the footwall. (c,f) represent the observed values of ascending track 56 and descending track 34 after downsampling; (d,g) represent the predicted values; (e,h) represent the residual values. The red solid line indicates the determined trace of seismogenic fault.
Figure 4. Slip distribution model and deformation and residuals predicted of the Wushi Mw7.1 earthquake. (a) 3D display and (b) 2D display, arrows indicate the slip direction of the hanging wall relative to the footwall. (c,f) represent the observed values of ascending track 56 and descending track 34 after downsampling; (d,g) represent the predicted values; (e,h) represent the residual values. The red solid line indicates the determined trace of seismogenic fault.
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Figure 5. Estimated slip distribution and predicted deformation of the aftershocks. (a) A three-dimensional visualization showcasing the slip distribution of two faults. Red circles depict aftershock events occurring during the SAR imagery acquisition period. (b,c) Two-dimensional representations of the same slip distribution for enhanced clarity. (d–f) Observed deformation, simulated deformation, and residual errors derived from ascending track T56, respectively. (g–i) Similarly, the observed deformation, simulated deformation, and residual errors for descending track T34 are presented in sequence. The thin blue lines represent the seismogenic fault of the Wushi Mw 7.1 earthquake. The red solid line indicates the surface trace of f1 and f2 faults.
Figure 5. Estimated slip distribution and predicted deformation of the aftershocks. (a) A three-dimensional visualization showcasing the slip distribution of two faults. Red circles depict aftershock events occurring during the SAR imagery acquisition period. (b,c) Two-dimensional representations of the same slip distribution for enhanced clarity. (d–f) Observed deformation, simulated deformation, and residual errors derived from ascending track T56, respectively. (g–i) Similarly, the observed deformation, simulated deformation, and residual errors for descending track T34 are presented in sequence. The thin blue lines represent the seismogenic fault of the Wushi Mw 7.1 earthquake. The red solid line indicates the surface trace of f1 and f2 faults.
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Figure 6. (a) Coulomb stress triggering and (b) source structure locations of the main shock–aftershock sequence of the Wushi earthquake. (a) Changes in positive Coulomb stress (depicted in red) on the fault plane indicate proximity to failure and sliding hazard, while negative values (depicted in blue) signify a lack of sliding hazard. (b) The black lines represent the fault locations delineated based on the inversion of fault geometry parameters and geological cross-section base map (adapted from [27]). The red lines depict faults, the black line segments represent the mainshock fault and the aftershock (f1) fault identified in this study. Shallow light yellow areas indicate reverse-thrust overlying strata.
Figure 6. (a) Coulomb stress triggering and (b) source structure locations of the main shock–aftershock sequence of the Wushi earthquake. (a) Changes in positive Coulomb stress (depicted in red) on the fault plane indicate proximity to failure and sliding hazard, while negative values (depicted in blue) signify a lack of sliding hazard. (b) The black lines represent the fault locations delineated based on the inversion of fault geometry parameters and geological cross-section base map (adapted from [27]). The red lines depict faults, the black line segments represent the mainshock fault and the aftershock (f1) fault identified in this study. Shallow light yellow areas indicate reverse-thrust overlying strata.
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Table 1. Focal mechanism solutions of the 2024 Wushi event from different institutions.
Table 1. Focal mechanism solutions of the 2024 Wushi event from different institutions.
AgencyLocationMagnitude
(Mw)		Depth
(km)		Nodal Plane 1
Strike, Dip, Rake		Nodal Plane 2
Strike, Dip, Rake
GCMT *41.19°N 78.57°E7.014.0236°, 48°, 47°110°, 57°, 127°
NEIC41.269°N 78.649°E7.013.0235°, 45°, 42°113°, 62°, 126°
IPGP41.294°N 78.594°E7.122234°, 50°, 51°105°, 53°, 127°
CEA-IGP41.2938°N 78.5937°E7.127.4250°, 42°, 59°109°, 55°, 115°
GFZ41.28°N 78.73°E7.0115251°, 38°, 73°93°, 54°, 103°
* Note: GCMT (Global Centroid Moment Tensor), NEIC (National Earthquake Information Center), IPGP (Institut de Physique du Globe de Paris), CEA-IGP (Institute of Geophysics, China Earthquake Administration), GFZ (GeoForschungsZentrum Potsdam).
Table 2. InSAR parameters for Sentinel-1A images.
Table 2. InSAR parameters for Sentinel-1A images.
DirectionTrack No.Detection TimeTime Interval
(Day)		Spatial
Baseline
(m)		Event
MasterSlave
Asc5614 January 202425 January 202412−35.9Main shock
Des3413 January 202424 January 202412−1.4
Asc5625 January 20247 February 202412106.8Aftershock
Des3424 January 20246 February 202412−92.2
Table 3. Seismogenic fault parameters of the Mw7.1 earthquake.
Table 3. Seismogenic fault parameters of the Mw7.1 earthquake.
Model Param.OptimalMeanMedian2.5%97.5%
Fault Length83,783.783,721.483,783.482,69484,815.7
Fault Width13,17913,195.513,244.112,260.313,916.2
Fault Depth18,221.618,190.118,211.917,733.118,552.4
Fault Dip *−59.8794−59.8732−59.8719−60.4573−59.2678
Fault Strike *49.168849.199349.197148.916749.4954
Fault StrSlip−1.58574−1.58309−1.58101−1.65472−1.51888
Fault DipSlip−1.94319−1.93905−1.93353−2.02938−1.86268
* According to the GBIS code’s specifications, negative slip values represent left-lateral strike-slip motion, while negative dip values indicate a downward inclination. It should be noted that the definition of strike and dip angles used here differ from the conventional right-hand rule commonly employed in geology. Specifically, the strike angle of 49.1688, in accordance with the right-hand rule, would be 229.1688, and the dip angle of −59.8794 should be interpreted as 59.8794.
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Qiu, J.; Sun, J.; Ji, L. The 2024 Mw 7.1 Wushi Earthquake: A Thrust and Strike-Slip Event Unveiling the Seismic Mechanisms of the South Tian Shan’s Thick-Skin Tectonics. Remote Sens. 2024, 16, 2937. https://doi.org/10.3390/rs16162937

AMA Style

Qiu J, Sun J, Ji L. The 2024 Mw 7.1 Wushi Earthquake: A Thrust and Strike-Slip Event Unveiling the Seismic Mechanisms of the South Tian Shan’s Thick-Skin Tectonics. Remote Sensing. 2024; 16(16):2937. https://doi.org/10.3390/rs16162937

Chicago/Turabian Style

Qiu, Jiangtao, Jianbao Sun, and Lingyun Ji. 2024. "The 2024 Mw 7.1 Wushi Earthquake: A Thrust and Strike-Slip Event Unveiling the Seismic Mechanisms of the South Tian Shan’s Thick-Skin Tectonics" Remote Sensing 16, no. 16: 2937. https://doi.org/10.3390/rs16162937

APA Style

Qiu, J., Sun, J., & Ji, L. (2024). The 2024 Mw 7.1 Wushi Earthquake: A Thrust and Strike-Slip Event Unveiling the Seismic Mechanisms of the South Tian Shan’s Thick-Skin Tectonics. Remote Sensing, 16(16), 2937. https://doi.org/10.3390/rs16162937

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