Located in the eastern boundary of the Qinghai-Tibetan plateau, the Xianshuihe fault zone is one of the most active left-lateral strike-slip faults in Chinese mainland. As the southern boundary of the Bayanhar block, the Xianshuihe Fault accommodates the southeastward transport of material toward southeastern Asia. Earthquakes have occurred frequently along this fault, especially in the northwestern segment. More than 20 earthquakes with M_W>6.0 have ruptured since 1700. The most recent M_W>7 earthquake was the Luhuo earthquake in 1973, and the most recent M_W>6 earthquake was the M_W6.6 Luding earthquake in 2022. As one of the most active faults in mainland China, the present slip pattern of the Xianshuihe Fault, especially the shallow creep characteristics along its northwestern segment, has attracted much attention.

The primary goal of determining slip rates of active faults using geodetic data is to quantify the seismic potential of the faults. Illuminating the long-term slip rate and shallow creep distribution of faults is the basis for calculating the seismic moment rate and evaluating the seismic potential. Due to the backwardness of early measurement methods, the seismic deformation along the Xianshuihe Fault was previously based on geologic, cross-fault short baseline and leveling surveys. With the application of GPS in tectonic geodesy, more and more GPS stations are installed near active faults, which provide accurate constraints on the long-term slip rates of the fault. Subsequently, the appearance of InSAR technology has brought a beneficial supplement to GPS, providing high spatial resolution surface velocity maps, which have been widely used to measure deep and shallow creep along active faults. It is the key to accurately characterize the fault slip behavior and evaluate the seismic potential.

In this study, 119 Sentinel-1 satellite descent data from December 2014 to December 2021 were processed to obtain the average line-of-sight(LOS)velocity field of the northwestern segment of the Xianshuihe Fault based on the small baseline InSAR method. Then the elastic screw dislocation model was used to fit the fault normal InSAR LOS velocity profiles to estimate the long-term slip rates and shallow creep rates. Combined with the viscoelastic earthquake cycle model, the effects of the earthquake recurrence period, and rheology of the lower crust and upper mantle on slip rate estimation in Luhuo segment are analyzed. The main results are as follows:

(1)The average InSAR LOS velocity field is in the northwestern segment of the Xianshuihe Fault during 2014—2021 has been obtained with a large range and high spatial resolution. The velocity field results show an obvious velocity gradient across the surface trace of the Xianshuihe Fault, which is consistent with the left-lateral strike-slip characteristics of the Xianshuihe Fault.

(2)To investigate the slip rate variation along the northwestern segment of the Xianshuihe Fault, we used the two-dimensional elastic screw dislocation model to fit the 14 fault-normal velocity profiles selected along the northwestern segment of the Xianshuihe Fault and estimated the long-term slip rates and shallow creep rates using the Markov Chain Monte Carlo(MCMC)method. The results show that the overall slip rates of the NW segment of the Xianshuihe Fault range from 7.2mm/a to 11.0mm/a, and gradually decrease from west to east. The shallow creep rate ranges from 0.3mm/a to 3.1mm/a. The high creep rate appears mainly at Xialatuo and the segment from Daowu to Songlinkou. The shallow creep rates in other places are close to zero, implying that the fault is completely locked.

(3)According to historical earthquake records, the recurrence interval of the Luhuo segment is set to be 150 years, 200 years, and 400 years, and the viscosity of the lower crust and upper mantle is set to be 5.0×10¹⁸Pa·s, 1.0×10¹⁹Pa·s, and 5.0×10¹⁹Pa·s. The slip rate of the Luhuo segment is estimated to be (7.91±0.3)~(9.85±0.4)mm/a using the MCMC method, which is slightly lower than the (10.14±0.5)mm/a obtained by the pure elastic model. In addition, when the earthquake recurrence interval is 150 years and the viscosity of the lower crust and upper mantle is 5.0×10¹⁹Pa·s, we simulate the fault-normal velocity at 5 years, 20 years, 75 years, and 125 years after the 1973 Luhuo earthquake, and find that in any period of the seismic cycle, the estimation of fault slip rate will be biased to some extent if the viscoelastic contribution of the lower crust and upper mantle is ignored.

The urban active fault survey is of great significance to improve the development and utilization of urban underground space, the urban resilience, the regional seismic reference modeling, and the natural hazard prevention. The Beijing-Tianjin metropolitan region with the densest population is one of the most developed and most important urban groups, located at the northeastern North China plain. There are several fault systems crossing and converging in this region, and most of the faults are buried. The tectonic setting of the faults is complex from shallow to deep. There are frequent historical earthquakes in this area, which results in higher earthquake risk and geological hazards. There are two seismicity active belts in this area. One is the NE directed earthquake belt located at the east part of the profile in northern Ninghai near the Tangshan earthquake region. The other is located in the Beijing plain in the northwest of the profile and near the southern end of Yanshan fold belt, where the 1679 M8.0 Sanhe-Pinggu earthquake occurred, the largest historical earthquake of this area. Besides, there are some small earthquake activities related to the Xiadian Fault and the Cangdong Fault at the central part of the profile.
    The seismic refraction experiment is an efficient approach for urban active fault survey, especially in large- and medium-size cities. This method was widely applied to the urban hazard assessment of Los Angeles. We applied a regularized tomography method to modeling the upper crustal velocity structure from the high-resolution seismic refraction profile data which is across the Beijing-Tianjin metropolitan region. This seismic refraction profile, with 185km in length, 18 chemical explosive shots and 500m observation space, is the profile with densest seismic acquisition in the Beijing-Tianjin metropolitan region up to now. We used the trial-error method to optimize the starting velocity model for the first-arrival traveltime inversion. The multiple scale checker board tests were applied to the tomographic result assessment, which is a non-linear method to quantitatively estimate the inversion results. The resolution of the tomographic model is 2km to 4km through the ray-path coverage when the threshold value is 0.5 and is 4km to 7km through the ray-path coverage when the threshold value is 0.7. The tomographic model reveals a very thick sediment cover on the crystalline basement beneath the Beijing-Tianjin metropolitan region. The P wave velocity of near surface is 1.6km/s. The thickest sediment cover area locates in the Huanghua sag and the Wuqing sag with a thickness of 8km, and the thinnest area is located at the Beijing sag with a thickness of 2km. The thickness of the sediment cover is 4km and 5km in the Cangxian uplift and the Dacang sag, respectively. The depth of crystalline basement and the tectonic features of the geological subunits are related to the extension and rift movement since the Cenozoic, which is the dynamics of formation of the giant basins.
    It is difficult to identify a buried fault system, for a tomographic regularization process includes velocity smoothing, and limited by the seismic reflection imaging method, it is more difficult to image the steep fault. Velocity and seismic phase variations usually provide important references that describe the geometry of the faults where there are velocity differences between the two sides of fault. In this paper, we analyzed the structural features of the faults with big velocity difference between the two sides of the fault system using the velocity difference revealed by tomography and the lateral seismic variations in seismograms, and constrained the geometry of the major faults in the study region from near surface to upper crust. Both the Baodi Fault and the Xiadian Fault are very steep with clear velocity difference between their two sides. The seismic refraction phases and the tomographic model indicate that they both cut the crystalline basement and extend to 12km deep. The Baodi Fault is the boundary between the Dachang sag and the Wuqing sag. The Xiadian Fault is a listric fault and a boundary between the Tongxian uplift and the Dachang sag. The tomographic model and the earthquake locations show that the near-vertical Shunyi-Liangxiang Fault, with a certain amount of velocity difference between its two sides, cuts the crystalline basement, and the seismicity on the fault is frequent since Cenozoic. The Shunyi-Liangxiang Fault can be identified deep to 20km according to the seismicity hypocenters.
    The dense acquisition seismic refraction is a good approach to construct velocity model of the upper crust and helpful to identify the buried faults where there are velocity differences between their two sides. Our results show that the seismic refraction survey is a useful implement which provides comprehensive references for imaging the fault geometry in urban active fault survey.