Dynamic Deformation And Fault Locking of The Xianshuihe Fault Zone, Southeastern Tibetan Plateau: Implications For Seismic Hazards

The Xianshuihe Fault Zone is one of the most historically seismically active regions in mainland China. However, the seismicity along this fault zone has been quiescent for the past 40 years, since the Daofu M6.9 earthquake in 1981. Understanding its current deformation patterns and fault coupling characteristics is of great significance to estimate the potential risk of strong earthquakes. In this study, we analyzed the dynamic deformation and fault coupling characteristics along the Xianshuihe Fault Zone using Global Positioning System (GPS) data for 1999–2007 and 2016–2020. The results show that the deformation pattern of the Xianshuihe fault zone underwent a dynamic adjustment after the Wenchuan and Lushan earthquakes, i.e., the maximum shear strain accumulation rates of the Luhuo and Daofu sections significantly decreased from 6.0×10-8/a to 3.2×10-8/a, while that of the southeastern segment (i.e., Kangding and Moxi sections) increased from 4.5×10-8/a to 6.2×10-8/a. Additionally, the slip rate and deformation width of the Xianshuihe Fault Zone also changed during these two periods. Combined with the near-field cross-fault observation data, we suggest that the surrounding strong earthquakes 2008 Wenchuan Mw7.9 and 2013 Lushan Mw6.6 had evident differential impacts on the deformation pattern of the Xianshuihe Fault Zone. The fault-coupling inversion results show that the locking degree of the Xianshuihe Fault Zone continued to increase after the Mw7.9 Wenchuan and Mw6.6 Lushan earthquakes, especially the Qianning and Moxi sections increased significantly, with an average coupling coefficient of greater than 0.9 and left-lateral slip-rate deficits of ~5 mm/a and ~8 mm/a, respectively. In contrast, the locking degree of the Kangding section decreased with almost no slip-rate deficit, which may be due to the partial energy release caused by the Mw5.9 and Mw5.6 Kangding earthquakes in 2014. The analysis of the recent rupture history and strain accumulation characteristics of the Xianshuihe Fault Zone indicates that both the Qianning and Moxi sections have a high seismic potential for the next strong earthquake in the Xianshuihe Fault Zone.

Multisegment Rupture Hazard Modeling along the Xianshuihe Fault Zone, Southeastern Tibetan Plateau

The Xianshuihe fault is a remarkable strike-slip fault characterized by high slip rate (∼10 mm/yr) and frequent strong historical earthquakes. The potential for future large earthquakes on this fault is enhanced by the 2008 Mw 7.9 Wenchuan earthquake. Previous works gave little attention to the probabilities of multisegment ruptures on the Xianshuihe fault. In this study, we build five possible multisegment rupture combination models for the Xianshuihe fault. The fault slip rates and historical earthquakes are used as input constraints to model the future seismicity on the fault segments and test whether the rupture combination models are appropriate. The segment combination model, based essentially on historical ruptures, has produced the seismicity rates most consistent with the historical records, although the model with ruptures on both the entire northern section and southern section should also be considered. The peak ground acceleration values with a return period of 475 yr calculated using the modeled rates on the Xianshuihe fault for both two models are on average larger than the values of the China Seismic Ground Motion Parameters Zonation Map.

Hydrogeochemical Characteristics and Conceptual Model of the Geothermal Waters in the Xianshuihe Fault Zone, Southwestern China

Abundant geothermal waters have been reported in the Yalabamei, Zhonggu, Erdaoqiao, and Yulingong geothermal areas of the Xianshuihe Fault Zone of western Sichuan, southwestern China. This study focused on the hydrogeochemical evolution, reservoir temperature, and recharge origin of geothermal waters using hydrochemical and deuterium-oxygen (D-O) isotopic studies. Shallow geothermal waters represented by geothermal springs and shallow drilled water wells are divided into two hydrochemical groups: (1) the Ca–Na–HCO3 type in the Erdaoqiao area, and (2) Na–HCO3 in other areas. Deep geothermal waters represented by deep drilled wells are characterized by the Na–Cl–HCO3 type. The major ionic compositions of geothermal water are primarily determined by the water–rock interaction with silicate and carbonate minerals. The reservoir temperatures estimated by multi-geothermometries have a range of 63–150 °C for shallow geothermal water and of 190–210 °C for deep geothermal water, respectively. The δ18O and δD compositions indicated geothermal waters are recharged by meteoric water from the elevation of 2923–5162 m. Based on the aforementioned analyses above, a conceptual model was constructed for the geothermal system in the Xianshuihe fault zone.

Chemical and isotopic compositions of boron in the geothermal waters in the Xianshuihe Fault Zone, Western Sichuan Province, China

In this study, boron contents and boron isotopic compositions of geothermal waters are first reported in the Xianshuihe Fault Zone (XSHFZ), Western Sichuan. The results show that boron contents of geothermal water are 0.03-10.50 mg/L, and the δ11B values range from -6.75 to 4.01‰, indicating the non-marine origin. The δ11B values and Cl/B molar ratios reveal that boron in geothermal water is mainly leached from reservoir rocks, such as carbonate and igneous rocks. Comparing it with other geothermal systems in the Yunnan-Tibet Geothermal Belt, we found that the samples from XSHF have the largest variations of Cl/B molar ratio and the intermediate δ11B values.