A major challenge in the inversion of subsurface parameters is the ill-posedness issue caused by the inherent subsurface complexities and the generally spatially sparse data. Appropriate simplifications of inversion models are thus necessary to make the inversion process tractable and meanwhile preserve the predictive ability of the inversion results. In the present study, we investigate the effect of model complexity on the inversion of fracture aperture distribution as well as the prediction of long-term thermal performance in a field-scale single-fracture EGS model. Principal component analysis (PCA) was used to map the original cell-based aperture field to a low-dimensional latent space. The complexity of the inversion model was quantitatively represented by the percentage of total variance in the original aperture fields preserved by the latent space. Tracer, pressure and flow rate data were used to invert for fracture aperture through an ensemble-based inversion method, and the inferred aperture field was then used to predict thermal performance. We found that an over-simplified aperture model could not reproduce the inversion data and the predicted thermal response was biased. A complex aperture model could reproduce the data but the thermal prediction showed significant uncertainty. A model with moderate complexity, although not resolving many fine features in the “true” aperture field, successfully matched the data and predicted the long-term thermal behavior. The results provide important insights into the selection of model complexity for effective subsurface reservoir inversion and prediction.

The Xianshuihe (XSH) fault in eastern Tibet is one of the most active faults in China, with the next large earthquake most likely to occur along its SE part, where the fault splits into three parallel branches: Yalahe, Selaha and Zheduotang. Precisely quantifying their slip rates at various timescales is essential to evaluate regional earthquake hazard. Here, we expand our previous work on the Selaha fault, to the nearby Zheduotang and Moxi faults, and add observations on the Yalahe fault and on the newly discovered Mugecuo South fault zone. Using tectonic-geomorphology approaches with 10Be dating, we had determined average late Quaternary slip rates of 9.75±0.15 and 4.4±0.5 mm/yr along the NW and SE Selaha fault, respectively. Using the same methods here, we determine a slip rate of 3.7-5.4 mm/yr on the Zheduotang fault and of 10.4-14.8 mm/yr on the Moxi fault. This is consistent with the southeastward slip rate increase we had proposed along the XSH fault system from 6-8 mm/yr (Ganzi fault) to ~10 mm/yr (Selaha fault), and >10.4 mm/yr (Moxi fault). We eventually propose a new model for the SE Xianshuihe fault, where the large-scale Mugecuo pull-apart basin lies within an even larger scale compressive uplift zone in the XSH fault’s restraining bend, where the highest peak in eastern Tibet is located (Gongga Shan, 7556 m). Our slip rates determination allows to estimate a relatively high regional earthquake hazard of Mw~7 at present in the SE Xianshuihe fault.

The Xianshuihe fault in eastern Tibet is one of the most active faults in China, with the next large earthquake most likely to occur along its SE part near Kangding. Quantifying its slip rate along the three parallel branches (Yalahe, Selaha and Zheduotang) as well as along the Moxi fault is essential to evaluate regional earthquake hazard, necessary to the construction of the Chengdu-Lhasa railroad. Here, we expand our previous work on the Selaha fault to the Zheduotang and Moxi faults, with observations on the Yalahe fault and the newly discovered Mugecuo South fault zone. Using tectonic-geomorphology approaches (LiDAR, UAV and 10Be dating), we had determined late Quaternary slip rates of 9.75±0.15 and 4.4±0.5 mm/yr along the NW and SE Selaha fault, respectively, hence had inferred a ~5 mm/yr rate along the parallel Zheduotang fault. Here, using the same methods, we confirm such rate (4.5[+0.9/-0.8] mm/yr, ZDT moraine site) thus suggest a total slip rate of >8.9±1.4 mm/yr in the SE Xianshuihe fault. Our rate along the Moxi fault (12.5[+2.3/-2.1] mm/yr, MX moraine site) is higher than those along the Ganzi (6-8 mm/yr) and Xianshuihe (~10 mm/yr) faults farther NW, which reinforces our earlier finding of a southeastward slip rate increase, in agreement with the eastward decrease of GPS vector values (with respect to Eurasia) located north of the fault. Our study reveals a high regional earthquake hazard (Mw6.5 to 7.3) in the near future, which adds to the challenge of building the new railroad in such mountainous area.