Influence of crustal rheology and heterogeneity on tectonic stress accumulation characteristics of North China constrained by GNSS observations

Highlights

• North China is characterized by lithosphere heterogeneity and dense faults.

• Established 3-D viscoelastic FEM based on NSGL, faults, and GNSS velocities.

• Analyzed stress accumulation across NSGL and relationship with seismicity.

• High maximum shear stress corresponds to the focal depth.

• Crustal thickness across NSGL is a major factor dominating earthquake depth.

Abstract

North China is characterized by significant lithosphere heterogeneity and numerous faults, with the occurrence of many historical and ongoing devastating earthquakes. To extend our understanding of the mechanisms of seismicity initiation and fault activity due to lithospheric rheology and lateral difference, we first established a three-dimensional viscoelastic finite element model based on the lithospheric lateral heterogeneity of physical properties across the north–south gravitational lineament (NSGL), spatial distribution of faults, and interseismic global navigation satellite system (GNSS) velocities (1999–2018). We then analyzed the stress accumulation characteristics across the NSGL and its relationship with seismicity. Finally, we explored the temporal and spatial variations of stress along major faults and further analyzed potential relationships between the stress components and rupture mechanisms of typical faults. The results showed that high maximum shear stress, mainly distributed in eastern North China (ENC) and western North China (WNC), corresponding to the focal depth, which suggests that the different brittle crustal thicknesses across the NSGL may be one of the major factors that dominates earthquake depth in North China. Significant maximum shear stress is mainly accommodated on faults around the Ordos Block in WNC and northern faults in ENC, which is consistent with frequent seismic activity in these regions. The relationship between the calculated stress components and rupture mechanisms of typical faults imply that the differential tectonic loading from neighboring blocks may be one of the major dynamic factors for seismogenic processes in North China.

Introduction

North China is located in the eastern Asian continent (central China), which is adjacent to the Pacific Plate to the east, Philippine Plate to the southeast, Tibetan Plateau to the west, Indian Plate to the southwest, Amurian Plate to the north, Eurasian Plate to the northwest, and Yangtze Craton to the south (Chen et al., 2014, Kusky et al., 2007) (Fig. 1a). The collision between the Indian and Asian plates and continuous northward push, combined with the subduction of the Pacific Plate have produced complex structures, numerous faults, and caused intense intraplate seismicity in North China (Deng et al., 2003, Ye et al., 2015). For example, the 1556 Mw 8.0 Huaxian earthquake (Du et al., 2017), 1668 Ms 8.5 Tancheng earthquake (Li, 1986), 1976 Ms 7.8 Tangshan earthquake (Nábělek et al., 1982), and 1998 Mw 5.8 Zhangbei earthquake (Li et al., 2008). These earthquakes are concentrated in the periphery of the Ordos Block and in the northern North China Basin (Fig. 1b). Frequent seismicity and unique structure have rendered North China an ideal natural laboratory to understand tectonic activity relationships among crustal movement, fault activity, and earthquake initiation.

Previous research on crustal deformation and seismicity in North China can be categorized into three aspects. First, seismic tomography and artificial seismic source have been adopted to investigate the influence of crustal and mantle heterogeneity on seismogenic mechanisms. Previous studies on this aspect have suggested that the strain and stress concentration in seismic regions can be attributed to partial Moho upheaval, low velocity zone anomaly, and upwelling of mantle materials (Bi and Jiang, 2019, Dong et al., 2018, Wang et al., 2017). Second, interseismic GNSS observations have been applied to investigate the relative movement of faults, which implied that the sinistral motion of the NNE-trending faults was potentially determined by differential motion among their adjacent blocks, such as the shear couple caused by the Amurian Plate and South China Block (Zhang et al., 2018, Zhao et al., 2017). Crustal strain fields and fault locking were further examined based on kinematic models, such as the least square collocations (Qu et al., 2017), multiscale spherical wavelets (Hao et al., 2019, Meng et al., 2019), and linear elastic coupling estimator (Wang et al., 2011). Third, dynamic simulation models have been used to investigate crustal deformation and stress characteristics under different displacements or stress boundary conditions (Sun et al., 2015), as well as to discuss the relationship between the seismogenic mechanism and lithospheric structure (Liu et al., 2016a, Zhu et al., 2010) and the impact of adjacent large earthquakes on seismicity in North China (Feng et al., 2016, Qu et al., 2019a).

Geodetic observations have revealed present low deformation and fault activity in North China (Liu et al., 2011). However, at least 94 earthquakes greater than M 6 (including 24 earthquakes greater than M 7) have occurred in North China. The contradiction between low crustal movement and intense seismicity implies that deep tectonic activity may contribute to stress accumulation and fault rupture (Liu et al., 2011, Wang et al., 2017). Therefore, further considerations must be placed on stress characteristics influenced by deep lithospheric tectonic loading. The finite element method (FEM) is an effective technique to examine the coupling between crustal deformation and deep tectonic activity, which has been widely used in simulating stress characteristics, seismic cycling, and interaction. High-precision GNSS observations can provide reliable boundary conditions for FEM simulations (Luo and Liu, 2009, Luo and Liu, 2010, Parson, 2002). Previous studies are of great importance to better understand the geodynamics of North China. However, there are a number of key issues that should be resolved in current studies, such as the previous research was usually based on elastic model, which does not account for stress relaxation due to lithospheric rheology (Feng et al., 2016, Qu et al., 2019a); boundary conditions were usually treated as constants instead of referring to actual monitoring data (Liu et al., 2016a), which may be inconsistent with actual crustal motion. In addition, there is a north–south gravitational lineament (NSGL) throughout North China, with significant differences in topography and Moho depth on both sides of the NSGL (Fig. 2). Seismic tomography, artificial seismic source, and receiver function inversion all indicate that North China was separated by the NSGL into two topographically and tectonically heterogeneous domains, i.e., eastern North China (ENC) and western North China (WNC) (Xu, 2007, Xu et al., 2004). Previous studies have not yet investigated the impact of the stress accumulation and seismicity of different lithospheric thicknesses and physical properties that characterize the two sides of the NSGL. More importantly, numerous faults encompassing the Ordos Block and distributed throughout ENC, have been the focal region of historical and ongoing earthquakes. The different stress states of these faults during tectonic evolution and their relationship with historical seismic activity, as well as possible future fault mechanisms, also require further exploration.

Therefore, to further solve these issues and investigate the influence of lithospheric heterogeneity and rheology on fault activity and the mechanisms of seismic initiation in North China, we first analyzed crustal deformation based on long-term and recent GNSS velocities from 1999 to 2018. Then, a three-dimensional (3-D) dynamical viscoelastic FEM was established with reference to the actual lithospheric layers, spatial distribution of faults, and lateral heterogeneity of physical properties on the two sides of the NSGL, followed by the generation of the optimal model through adjustments to the boundary conditions and viscosity. Afterward, stress accumulation on both sides of the NSGL at different time scales were obtained, along with an analysis of the temporal and spatial differences in stress accumulation in the main fault zone. We also discuss the influence of the difference in viscosity coefficient on the stress structure loading and seismicity in deep lithosphere. Finally, we explored the temporal and spatial characteristics of the stress components on typical faults and their potential relationships with the fracture rupture mechanism. The results allow a better understanding of the differences in the stress distribution and segmentation along the main faults, as well as providing a reference for an in-depth understanding of the stress accumulation and seismic activity under the influence of the lateral and longitudinal heterogeneity of the lithosphere in North China.