According to the China Seismic Network, on 21 May 2021, a Mw 6.1 earthquake struck Yangbi County, Yunnan Province, at (25.67°E, 99.87°N), the depth of hypocenters was 8 km (Li C. et al., 2021; Hu et al., 2021). Different research institutions have provided focal mechanism solutions for this earthquake using the global or regional network data (Table 1). This earthquake is generally characterized as a dextral strike-slip event (Liu et al., 2021). Based on the earthquake epicenter, this event occurred along the western boundary of the Sichuan–Yunnan block, which is consistent with the movement properties and trends of the Red River and Weixi–Qiaohou fault zones (Figure 1), the latter of which has been characterized by dextral strike-slip movement since the late Quaternary (Chang et al., 2016a; Zhang K. et al., 2021). There are no obvious surface ruptures that have been found in field investigations. Consequently, the identity and geometric characteristics of the seismogenic fault responsible for the Yangbi earthquake need to be further clarified. In particular, intensive research must be conducted to ascertain whether the Weixi–Qiaohou fault zone, which represents the closest fault to the epicenter of the Yangbi event or an unknown hidden fault, was responsible for this earthquake, and the characteristics of such a hidden fault must be thoroughly investigated.

In recent years, earth observation technologies such as interferometric synthetic aperture radar (InSAR) have been employed to remotely monitor the Earth’s surface to quantify global surface microdeformation globally with high precision (Li et al., 2020; Li K. et al., 2021; Li B. et al., 2020). In this paper, the InSAR coseismic deformation fields of the Yangbi earthquake were mapped using Sentinel-1 images acquired in terrain observation with progressive scans (TOPS) mode, and the focal mechanism and fault slip distribution were calculated and discussed. Analyzing the focal mechanism of the Yangbi Mw6.1 earthquake has essential theoretical and practical significance for fundamentally understanding the seismotectonic background, the structural deformation mechanism, and the activities along the block boundary in NW Yunnan. In particular, studying the seismotectonic activity in this region is crucial to understanding the tectonic background of the southeastern Qinghai–Tibet Plateau.

The Yangbi earthquake occurred along the southwestern boundary of the rhombic Sichuan–Yunnan block (Figure 1), which has been forming since the early Cenozoic as a result of the eastward extrusion of the Qinghai–Tibet Plateau. During this time, the southeastward motion of the Sichuan–Yunnan block resulted in the development of a series of active right-lateral, oblique NW-trending faults in this region, obstructed by the South China block. The Sichuan–Yunnan block shifted toward the SE and rotated clockwise around the East Himalayan tectonic junction. Consequently, the Sichuan–Yunnan block is the most representative active block (featuring the strongest lateral extrusion) along the eastern margin of the Qinghai–Tibet Plateau (Long et al., 2021) and, being a research hotspot for studying the movement and tectonic deformation of rigid blocks, is the focus of many investigations on active tectonics and earthquake monitoring and prediction (Chang et al., 2016b). The northeastern and eastern boundaries of the Sichuan-Yunnan block are controlled by the Ganzi-Yushu, Xianshuihe-Anninghe-Zemuhe, and Xiaojiang fault segments and other fault zones, all of which are characterized by sinistral strike-slip and high slip rates. In particular, the eastern boundary of this block has a clear structure and frequently experiences earthquakes (Ren et al., 2007).

The Yangbi earthquake occurred in an active fault zone traversing the western boundary of the Sichuan-Yunnan block and is where the Weixi-Qiaohou-Weishan fault connects with the Red River fault, both of which are NW-trending dextral strike-slip faults. The main well-known active segments are situated near the epicenter of the 2021 Yangbi event. As major faults, the Weixi-Qiaohou-Weishan fault and Red River fault control the crustal deformation along the southwestern boundary of the Sichuan-Yunnan block (Wang S. et al., 2021). Using the cut-and-paste (CAP) waveform inversion method (Wang Y. et al., 2021a), the epicenter of the Yangbi earthquake sequence was shown to be approximately 3–10 km SW along the Weixi–Qiaohou fault and the long axis of the aftershock area was reported to trend NW–SE.

The Weixi–Qiaohou fault is a large-scale boundary fault that starts in northwestern Weixi County and connects with the Red River fault in southern Weishan, spanning approximately 280 km with a general strike of NNW–SSE (Figure 1). The fault shows strong late Quaternary activity, which has manifested in new active landforms such as straight fault troughs, clear fault triangles, and fault scarps (Ren et al., 2007; Duan et al., 2021). The fault is dominated by dextral strike-slip with a minor normal slip component, and ridges and water systems show apparent synchronous dextral dislocations along the fault (Li D. et al., 2021). Because the historical records of earthquakes in this region are not noteworthy, this area has received little attention for many years. However, many moderately strong earthquakes have recently occurred, such as the 2017 Yangbi Ms5.1 earthquake (25 km from the epicenter of the 2021 Yangbi Mw6.1 earthquake), the 2016 Yunlong Ms5.0 earthquake (59 km from the epicenter), and the 2013 Eryuan Ms5.5 earthquake (31 km away from the epicenter) (Figure 1). These phenomena suggest that the tectonic activity in this region is gradually increasing.

The focal mechanisms and waveform inversion results reported by previous studies show that the seismogenic fault that produced the Yangbi earthquake exhibited dextral strike-slip motion (Ye et al., 2021; Yang Z. et al., 2021). However, the overall deformation amplitude is small from field investigation, and there are almost no signs of rupture on the surface. To image the coseismic deformation field generated by this earthquake, the Sentinel-1 satellite equipped with C-band SAR launched by the European Space Agency was employed in this paper. Sentinel-1 descending track 135 and ascending track 99 data with interferometric wide swath (IW) mode were obtained (Figure 1). Using an automatic seismic deformation monitoring system for Sentinel-1 SAR data (Li Y. et al., 2021), the ascending and descending InSAR coseismic deformation fields were obtained, as shown in Figure 2. The Advanced Land Observing Satellite Global Digital Surface Model (ALOS World 3D) with a 30-m resolution was used as external digital elevation model (DEM) data to eliminate the phase contribution of undulating terrain (Tadono et al., 2014). Additionally, an adaptive Goldstein filtering algorithm was applied to filter the original interferograms (Goldstein et al., 1988), and the minimum cost flow algorithm was used for phase unwrapping (Chen and Zebker, 2000). The atmospheric delay error was preliminarily corrected by the Generic Atmospheric Correction Online Service for InSAR (GACOS) method (Yu et al., 2018), and the residual orbit was fitted by linear fitting.

The resulting coseismic deformation field based on Sentinel-1 satellite data can clearly describe the spatial distribution and magnitude of deformation caused by the Yangbi earthquake (Figure 2). The results predominantly indicate that the strike of the fault is NNW–SSE, which is consistent with recent aftershock location findings (Su et al., 2021). After correcting the atmospheric contribution, the coseismic interferogram of the descending track in Figure 2A reflects a clear deformation pattern. The long axis is distributed in the NW–SE direction. There are obvious deformation signals distributed on both sides of the inferred fault (the surface to the SW of the fault is moving away from the satellite, while that to the NE is moving toward the satellite). Cross sections perpendicular to the inferred fault were selected to conduct a profile analysis (Figure 3A), revealing that the maximum deformation in the line-of-sight (LOS) direction on the NE side reached approximately 8 cm, while that on the SW side was approximately 5 cm. The ascending coseismic interferogram (Figure 2B) shows an obvious deformation pattern moving toward the satellite in the epicenter area, reaching 5 cm in the LOS direction (Figure 3B). However, the deformation field cannot accurately describe the coseismic deformation characteristics of this strike-slip earthquake, mainly because the azimuth of the ascending track is nearly consistent with the fault strike. Therefore, the SAR sensor is not sensitive enough to capture the deformation signal parallel to the surface movement direction.

After unwrapping the InSAR interferograms, the data were downsampled using the quadtree method constrained by the data resolution (Lohman and Simons 2005; Feng et al., 2018; 2019). The large deformation gradients in the downsampled interferograms are mainly in the areas with large surface deformation, while the deformation gradients are low in the far field (Figure 4). This downsampled measurement ensures the inversion accuracy and dramatically reduces the computational cost of the inversion. Then, the downsampled deformation fields were employed as inversion constraints, and the geometric parameters and slip distribution of the fault were estimated by a two-step inversion method (Feng et al., 2013). The fault parameters and slip mechanism of the Yangbi earthquake were studied as described below.

In this study, the coseismic deformation field produced by the Yangbi earthquake that occurred on 21 May 2021, was derived from Sentinel-1 ascending and descending track data. Taking the deformation fields as constraints, a two-step inversion strategy was applied to estimate the geometric structure and slip distribution of the rupture plane. The PSOKINV method was employed to find the best solution that minimizes the fitting function in the whole parameter domain. The inferred optimal slip model suggests that the rupture surface of the earthquake trends NNW–SSE and is predominantly a steeply dipping dextral strike-slip fault. This reveals that the coseismic slip distribution was controlled by secondary faults west of the Weixi–Qiaohou fault. The earthquake nucleated at a shallow depth on the rupture plane, and the major seismogenic fault was a dextral strike-slip fault with a strike of ∼139°, a dip of ∼81° to the southwest, and an average rake angle of -170°. A maximum slip of ∼0.5 m was achieved at a depth of 6 km. The cumulative seismic moment reached up to 1.43 × 10¹⁸ N⋅m, equivalent to a magnitude of Mw 6.1. According to the resulting Coulomb stress changes, the strong historical earthquakes produced an obvious stress increase at the epicenter of the Yangbi earthquake and promoted the occurrence of the Yangbi earthquake. The efficient and accurate analysis of the focal mechanism of the Yangbi earthquake is of great significance for interpreting coseismic deformation characteristics and facilitating the rapid deployment of earthquake emergency rescue personnel. Moreover, this work is expected to benefit further research on the geological structure and kinematic mechanism of the Weixi–Qiaohou fault and the active prediction of geological disasters.