Due to the ongoing collision between Indian and Eurasian plates, the internal blocks of the Tibet plateau are experiencing eastward extrusion. Resulting from the blocking of the Sichuan Basin along the eastern boundary of the Bayanhar block, the plateau begins to rotate clockwise around the eastern syntaxis, and continues to move toward the IndoChina Peninsula. Such process forms the Hengduan Mountains with thousands of gullies in the Sichuan-Yunnan region, and generates major earthquakes across the entire Red River Fault, where infrastructures and residents are seriously threatened by the frequent earthquakes. InSAR observations feature a high spatial resolution and short intervals, ranging from several days to over a month, depending on the satellite revisit period.
On May 21, 2021, an earthquake struck the Yangbi city. This event provides a rare opportunity to look at the local tectonic and seismic risk in the north of the Red River Fault. We processed the Sentinel-1 SAR data with D-InSAR technology and generated the surface deformation caused by the Yangbi M_S6.4 earthquake occurring on May 21, 2021. Due to the abundant vegetation and moisture in Yunnan, significant atmospheric noise needs to be corrected for the derived InSAR displacement field. The results show a maximum deformation of~0.07m in line-of-sight for ascending track and~0.08m for descending track. The quality of interferogram on the ascending track is low, and only one of the quadrans can be distinguished, the rest of the interferogram is regarded as phase noise. However, the descending interferogram contains two deformation regions, with its long axis roughly along the NW-SE direction. The northeast part of interferogram moves towards the satellite, while the southwest part moves away from the satellite. The InSAR interferograms pattern shows a right-lateral strike-slip movement. Then, we combined coseismic displacement data obtained from the Global Navigation Satellite System(GNSS)and InSAR(both the ascending and descending)to invert the coseismic slip model of the Yangbi earthquake. The inversion test shows that our data cannot give strong constraints for the dip orientations, and the two slip models with opposite dip orientation can explain the observations within the noise level. No matter what the dip orientation is, the slip models show that the coseismic slip concentrated at depth of 2~10km, with a maximum slip of~0.8m, which corresponds to a moment magnitude of M_S6.4, and is consistent with body-wave-based focal mechanism. But the relocated aftershocks in 3 hours immediately after the mainshock reveal a SW-dipping fault plane 10km away to the west of Weixi-Qiaohou-Weishan Fault, we therefore conclude that the Yangbi earthquake ruptured a SW-dipping dextral fault, which is previously unknown. To analyze the effects of the Yangbi earthquake on the seismic risk of the regional dextral faults, we estimated the Coulomb stress change caused by our preferred slip model. The Coulomb stress at 7.5km depth is negative, indicating stress unloading, while the Coulomb stress at 15km depth is positive, indicating slightly loading, but still less than the empirical triggering threshold. The results indicate that Yangbi earthquake partially relieved the strain accumulated on the nearby faults, thus restraining the seismic risk of these faults.

The Talas-Fergana Fault(TFF)with a total length of more than 1 000km is a large dextral strike-slip fault across the West Tianshan Mountains in the northwest direction. The fault plays an important role in accommodating deformation in Central Asia and has attracted much attention by geologists due to the huge controversy in its strike-slip rate and kinematic pattern. Previous studies indicated that its average dextral strike-slip rate is 8~20mm/a since Late Holocene based on offset ephemeral stream valleys and ¹⁴C dating method. Some researchers recently updated the strike-slip to 2.2~6.3mm/a by the application of multiple dating methods(¹⁰Be, ²⁶Al, ³⁶Cl, luminescence, and radiocarbon)and satellite images with higher precision. But the strike-slip rates derived from modern GPS velocity field are only~2mm/a or even as low as 0.8mm/a. Thus, there is a substantial divergence between geological results and geodetic results in the strike-slip rate of the TFF. Some scholars believe that the huge difference between the geological rate and the rate obtained by geodetic measurements is caused by fault locking. In this study, the updated GPS data was used to establish velocity field of the West Tianshan Mountains relative to the stable Eurasian framework and the velocity field without self-rotation. The velocity field shows that the Tianshan Mountains are under intense crustal shortening and deformation. Moreover, for the TFF, as an important boundary fault in the western Tianshan Mountains, whether the far velocity field or the near velocity field, the differential movement of the crust is not obvious. And far-field velocity vectors away from the TFF show that there is minor difference of crustal movement along the fault. The TFF does not have the typical characteristic of locked fault that there is a big difference in velocities of far-field vectors, but a small difference in that of near/mid-field vectors. Thus, the activity of the fault is weak actually.
To further illustrate the overall low slip rate of the TFF, we compare the maximum shear strain rate and its distribution characteristics along the Altyn Fault and the Haiyuan Fault with large slip rates with the results of the TFF. The maximum shear strain rates along the Altyn Fault and the Haiyuan Fault are concentrated along the fault, and are as high as~60nano strain/a and~40nano strain/a, which are much larger than the overall maximum shear along the TFF. This shows that the sliding rate of the TFF is much lower than the strike-slip rate of the Altyn Fault of 9~15mm/a, and even slightly lower than the sliding rate of the Haiyuan Fault of 4~8mm/a. Therefore, we are more certain that the current activity rate of the TFF is far less than 8~20mm/a estimated by some geological methods.
The half-space elastic dislocation model is used to more rigorously re-constrain the current strike-slip rate of the TFF. The results show that the fault is divided into three segments. The TFF dextral strike-slip rate increases from the northwest section to the middle section and decreases from the middle section to the southeast section. And the strike-slip rates of the northwestern, middle and southeastern segments are(2.1±0.7)mm/a, (3.3±0.4)mm/a and(2.4±0.7)mm/a, respectively. The TFF is dominated by strike-slip motion, but there is also a weak dip-slip motion in the middle section of the TFF, with a magnitude of about 1mm/a.
The above results confirm the current low strike-slip rate of the TFF obtained by GPS which is much less than the strike-slip rate of 8~20mm/a estimated by geological methods. And through the GPS results, it is certain that the TFF presently has a low fault activity rather than a locked fault. To reconcile the high geological strike-slip rates and the geodetic results, a new deformation pattern of the West Tianshan Mountains may be needed. And more detailed GPS observations are required to explore whether the TFF has penetrated into the southern foreland basin of the West Tianshan Mountains.