Fault Geometry and Mechanism of the Mw 5.7 Nakchu Earthquake in Tibet Inferred from InSAR Observations and Stress Measurements

Yujiang Li,Xingping Hu,Haoqing Liu,Yongsheng Li

doi:10.3390/rs13245142

Abstract

Different types of focal mechanism solutions for the 19 March 2021 Mw 5.7 Nakchu earthquake, Tibet, limit our understanding of this earthquake’s seismogenic mechanism and geodynamic process. In this study, the coseismic deformation field was determined and the geometric parameters of the seismogenic fault were inverted via Interferometric Synthetic Aperture Radar (InSAR) processing of Sentinel-1 data. The inversion results show that the focal mechanism solutions of the Nakchu earthquake are 237°/69°/−70° (strike/dip/rake), indicating that the seismogenic fault is a NEE-trending, NW-dipping fault dominated by the normal faulting with minor sinistral strike-slip components. The regional tectonic stress field derived from the in-situ stress measurements shows that the orientation of maximum principal compressive stress around the epicenter of the Nakchu earthquake is NNE, subparallel to the fault strike, which controlled the dominant normal faulting. The occurrence of seven M ≥ 7.0 historical earthquakes since the M 7.0 Shenza earthquake in 1934 caused a stress increase of 1.16 × 105 Pa at the hypocenter, which significantly advanced the occurrence of the Nakchu earthquake. Based on a comprehensive analysis of stress fields and focal mechanisms of the Nakchu earthquake, we propose that the dominated normal faulting occurs to accommodate the NE-trending compression of the Indian Plate to the Eurasian Plate and the strong historical earthquakes hastened the process. These results provide a theoretical basis for understanding the geometry and mechanics of the seismogenic fault that produced the Nakchu earthquake.

Highlights

On 19 March 2021, the Mw 5.7 Nakchu earthquake (92.74◦ E, 31.94◦ N) occurred in the eastern part of the Qiangtang Block in the interior of the Tibetan Plateau (Figure 1).After the earthquake, based on seismic data from the local seismic network or teleseismic data from the global seismic network, seven groups reported focal mechanism solutions for the earthquake (Table 1)
The results show that the solutions provided by IEF and GeoAu suggest the dominant sinistral strike-slip motion, while the others indicate that this event is dominated by normal faulting with minor sinistral strike-slip components, in which the strike is predominant NEE-trending, the dip angle ranges
As can be seen from the comparison of the ascending and descending interferograms, the Interferometric Synthetic Aperture Radar (InSAR) deformation fields have the same spatial distributions, and they all have a significant subsidence deformation region, with the maximum deformation occurring in the line of sight (LOS) direction in the meizoseismal areas of the descending and ascending data, i.e., approximately 20 mm (Figure 3a) and 25 mm (Figure 3d), respectively

Summary

Introduction

After the earthquake, based on seismic data from the local seismic network or teleseismic data from the global seismic network, seven groups reported focal mechanism solutions (strike/dip/rake) for the earthquake (Table 1). The results show that the solutions provided by IEF and GeoAu suggest the dominant sinistral strike-slip motion, while the others indicate that this event is dominated by normal faulting with minor sinistral strike-slip components, in which the strike is predominant NEE-trending, the dip angle ranges. 55–64◦ , and the rake angle is larger than 47◦. These different focal mechanisms severely limit our understanding of the seismogenic mechanism and geodynamic processes of the earthquake. The coseismic displacement fields provided by such data are widely used to constrain fault geometry parameters and for the inversion of rupture models [5,6,7,8,9,10,11]

Methods

Results

Discussion

Conclusion