Abstract
On 21 May 2021, a Mw 6.1 earthquake struck Yangbi County, Yunnan Province, China. In this study, InSAR data from Sentinel-1 SAR images were processed to image the coseismic deformation fields of the Yangbi earthquake. Then, the optimal slip model was obtained by applying the particle swarm optimization method. The interferometry results revealed that the earthquake triggered obvious surface deformation near the epicenter, while the earthquake did not produce an obvious surface rupture zone from field investigation. The optimal slip model suggests that the strike of the seismogenic fault responsible for this event is 139°, the dip angle is 81°, and the average rake angle is -170°. Additionally, the slip was concentrated mainly at depths of 2–8 km, the maximum dip-slip amount was 0.5 m, and the cumulative seismic moment reached 1.43 × 1018 N⋅m, equivalent to a Mw 6.1 earthquake. The geodetic and geophysical inversion results demonstrate that the Yangbi earthquake was dominated by a steeply dipping dextral strike-slip rupture. The rupture fault generally strikes NNW-SSE, which is consistent with that of the Weixi-Qiaohou fault, and may be a relatively new fault formed by an E-W-oriented extension of the western boundary of the Sichuan-Yunnan block. Finally, based on the InSAR results in combination with the spatial distribution characteristics of ground fissures and the strong historical earthquakes, we analyzed the tectonic background preceding the Yangbi earthquake and analyzed the relationship between the Yangbi earthquake and strong historical earthquakes in the region, thereby providing empirical evidence for analyzing seismic risk and fault rupture parameters, interpreting seismic deformation characteristics, and better understanding the seismogenic background of the western boundary of the Sichuan-Yunnan block.
Highlights
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 spatial distribution of crack locations illustrates that the fissured area is roughly distributed in a NW–SE strip that is completely consistent with the fault determined from interferometric synthetic aperture radar (InSAR) (Figure 10)
The NW–SE-trending seismogenic faults in the vicinity of the Yangbi earthquake epicenter are characterized by dextral strike-slip, which is consistent with the InSAR inversion results (139°/81°/-170°), the focal mechanism parameters derived from seismological data (Long Y. et al, 2021; Wang Y. et al, 2021b), and the fault motion characteristics implied by the positive Coulomb stress changes, which inferred that the northwestern RRF is an area requiring special attention
Summary
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). 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 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). 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 FIGURE 8 | Optimal slip distribution of the Yangbi earthquake. There are no notable residual fringes except for some contributions from the atmospheric disturbance in the far field (Figures 9C,F), which suggest that the inversion results are stable and reliable
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