The stress change of the major activity faults in the Weihe Graben after the Wenchuan earthquake

Sheng Dong ,Wein Yun,＊ ,Fuqing Shi

a School of Geological Engineering and Geomatics,Chang'an University,Xi'an,710064,China

b Shaanxi Earthquake Agency,Xi'an,710068,China

Keywords:Coulomb stress Seismic activity The Weihe Graben

ABSTRACT The seismicity of small earthquakes in the Weihe Graben has changed after the Wenchuan earthquake.In detail,the seismicity around the Qishan-Mazhao fault in the western Weihe Graben decreased,while the seismicity in Gaoling and Jingyang Counties in the middle portion of the Weihe Graben and that in the area between Hancheng and Yuncheng Cities in eastern Weihe Graben increased.In this paper,the stress loading on the major activity faults in the Weihe Graben induced by the Wenchuan earthquake is discussed based on the Coulomb stress theory.The results show that the Wenchuan earthquake has exerted an unloading effect in the western Weihe Graben and a loading effect in the middle and eastern Weihe Graben.The spatially varied Coulomb stress is consistent with the seismicity distribution,indicating that the seismicity change is closely associated with the stress loading caused by the Wenchuan earthquake.

1.Introduction

The Weihe Graben lies between the southern margin of the Ordos Block and the northern part of the Qinling Orogen.Its western boundary is Baoji City,Shaanxi Province and its eastern part reaches Yuncheng City,Shanxi Province,Lingbao and Sanmenxia Cities,Henan Province.The Weihe Graben is characterized by a roughly west-east trending with a total length of approximately 400 km.The geological structure of the Weihe Graben is complicated by a list of well-developed active faults in the basin,such as the Qishan-Mazhao fault,Weihe fault,Qinling fault,Kouzhen-Guanshan fault,Huashan fault,and Hancheng fault(Han et al.,2002).According to historical records,destructive earthquakes,such as the QishanM7.0 earthquake in 780 BC and HuaxianM8¼earthquake in 1556,have occurred in this area,resulting in huge casualties and economic losses(Ma et al.,2016).

The seismicity in the Weihe Graben is not high and mainly dominated by earthquakes that have a magnitude smaller thanML3.0 since 1970.The strongest historical seismic event is the JingyangML5.2 earthquake in November 5th,2009.After the WenchuanMS8.0 earthquake,the frequency of earthquakes in Shaanxi province has increased obviously.However,the overall magnitude of these events are not high and the strongest is the GaolingML4.8 earthquake (Shi,2008).Based on the minimum completeness magnitude (ML1.6) in and around the Weihe Graben(Wang et al.,2014),we find that the seismicity in the vicinity of the Qishan-Mazhao fault (Region A in Fig.1) in the western Weihe Graben decreased(Figs.1 and 2(a)),while those in Gaoling and Jingyang counties in the middle portion of the Graben(Region B in Fig.1)(Figs.1 and 2(b))and those in the area between Hancheng and Yuncheng Cities(Region C in Fig.1) (Figs.1 and 2(c))increased.

Previous studies have demonstrated that regional seismicity is controlled by the lithospheric stress state(e.g.Jia et al.,2018;Shi et al.,2020).Based on the Coulomb stress theory and ETAS model,Jia et al.(2018) shows that the decrease of regional seismicity prior to the Jiuzhaigou earthquake is controlled by the stress unloading associated with the Wenchuan earthquake.In order to discuss the changes of the seismicity of small to moderate earthquakes in the Weihe Graben before and after the Wenchuan earthquake,the spatial distribution and the possible mechanism of small to moderate earthquakes are studied according to the Coulomb stress theory.

Fig.1.Tectonic settings and seismic activities around the Weihe Graben.Black dots and red dots are the ML＞1.5 earthquakes before and after the Wenchuan earthquake,respectively.

Fig.2.Seismic activities in the selected regions in Fig.1.(a) for region A,(b) for region B and (c) for region C.Red and black lines are the ML ≥3.0 and ML＜3.0 earthquakes,respectively.

2.Methods and models

Coulomb stress change can be obtained from the incremental form of the Coulomb criterion for rock failure,as expressed in Equa.(1).It is usedto represent the stress increment in the sliding direction on the fault plane.Coulomb stress change consists of normal stress change(Δσn)and shear stress change(Δτ).The effective friction coefficient in Equa.(1)is relative to the mechanical property,type,slipping rate of fault,as well as liquid injection(Ali et al.,2008;Parsons and Dreger,2000).It is an uncertain parameter variable and has little influence on the calculated Coulomb stress change (Shi et al.,2020;Wan and Shen,2010).In this paper,the value is set to 0.4 considering the fault type of the Weihe Graben.The PSGRN/PSCMP proposed by Wang et al.(2006) is used to calculate Coulomb stress change.

where μ′is the effective friction coefficient.

2.1.Lithospheric structure model

The lithospheric structure from the Longmenshan fault which triggers the Wenchuan earthquake to the Weihe Graben exhibits a significant lateral difference (Xu et al.,2014).In order to analyze the effect of lithospheric structure,two construction models(A and B)are established based on the Ordos Block (Li et al.,2015) and the Longmenshan fault zone (Xu et al.,2017).The two models are listed in Table 1 and are compared to discuss the stress loading effect of the Wenchuan earthquake on the major faults in the Weihe Graben.

Table 1 Mechanical properties of the lithosphere structure.

Table 2 Parameters of the main major faults in Weihe Graben.

2.2.Receiving fault model

Coulomb stress change is the projection of the regional stress increment in the specific sliding direction on the fault plane.The positive Coulomb stress change indicates that stress loading on the fault in the sliding direction may promote the fault activity,and vice versa.Consequently,the geometric parameters (such as strike,dip,and dipping angle) of the receiving fault should be clarified in the simulation of Coulomb stress change.This study assumes that the stress loading of the Wenchuan earthquake is on the Weihe Graben.The geometric parameters of the major faults in the Weihe Graben,which are summarized from previous investigations,are presented in Table 2.The Coulomb stress change indicates a depth of 10 km based on the regional focal depth(Yuan et al.,1998).

2.3.Source model of the Wenchuan earthquake

The WenchuanMS8.0 earthquake was triggered by the Longmenshan fault in the border of the Tibet Plateau and the Sichuan Basin on May 12th,2008.The coseismic rupture model of the Wenchuan earthquake is proposed by Wang et al.(2008) using the teleseismic waveforms and local coseismic displacement.Their results show that the Wenchuan earthquake is mainly associated with a thrusting and dextral strike slip rupture event along the Longmenshan fault.Two high slip areas with slip amount of up to 12–15 m have uplifted in Yingxiu and Beichuan Counties successively,resulting in the most serious local damage.The source model of the Wenchuan earthquake in this study is established based on their works.

3.Simulation results

The effect of the Wenchuan earthquake on the crustal movement of the Weihe Graben is simulated using the source model and lithospheric structure model A established in Section 2.Overall,the influence of the Wenchuan earthquake on the crustal movement of the Weihe Graben is not highly intense and the coseismic displacement is also in millimeter magnitude and consistent with the observed coseismic displacement obtained by Wang and Shen (2020).The coseismic displacement of the western area is larger than that of the eastern area.The coseismic displacement pattern in Fig.3a shows that the compression in the direction perpendicular to the Qishan-Mazhao fault has enhanced,which is contrary to the activity of the strike-slip Qishan-Mazhao fault.The dextral shearing action of the normal faults in the central Weihe Graben(Kouzhen-Guanshan fault and Weihe fault)and the right lateral strike slip normal faulting in the eastern Weihe Graben (Hancheng fault,Zhongtiaoshan fault and Shuangquan-Linyi faults) increased,possibly promoting the stress loading on these faults.

Fig.3.(a)Coseismic displacement and(b)current postseismic velocity.The colored contours represent the vector sum of east and north components induced by the Wenchuan earthquake.The quivers mark the direction of (a) horizontal coseismic displacements and (b) postseismic velocities,respectively.

The velocity pattern of regional crustal movement induced by lithospheric rheological relaxation is presented in Fig.3b.The Ordos Block and the northeastern margin of the Qinghai-Tibetan Plateau exhibit northeastward movement and the velocity decreases along the movement direction.Consequently,northeastward compression has been exerted on these areas.The Qinling Orogen exhibits southwestward movement and the velocity increases along the movement direction,and thus southwestward tension has been exerted on this area.These crustal movements promote the tension and dextral shear of the Weihe Graben.

The results of Coulomb stress change are presented in Fig.4.The stress unloading on the Qishan-Mazhao fault in the western Weihe Graben is observed and the maximum unloading stress reaches up to 5000–10 000 Pa.Stress loading is exerted on a list of faults (Kouzhen-Guanshan fault,Qinling fault,Hancheng fault,and Shuanquan-Linyi fault) with loading stress as low as approximately 1000 Pa.The stress loading and unloading areas in Fig.4 are corresponding to the high and low seismic activity areas in Fig.1,respectively.This manifests that the change of the seismic activity in the Weihe Graben should be ascribed to be the great Wenchuan earthquake.The maximum loading stress may be lower than the commonly used trigger threshold(10 000 Pa)(King et al.,1994);however,many previous studies have demonstrated that a large number of small earthquakes,even some big earthquakes,have occurred in areas with stress loading of less than 1000 Pa(Arnad'ottir et al.,2003;Wan et al.,2003;Ziv et al.,2000),substantiating that the results are reasonable.Besides,the difference in lithospheric rheology between model A and model B has affected the value of the Coulomb stress change.However,it could not change its polarity according to the comparative analyses of the results in Fig.4(a)and 4(b).

4.Discussion

Fig.4.Cumulated co-and post-seismic Coulomb stress change on the major faults induced by the Wenchuan earthquake(μ′=0.4).(a)based on rheological model A and (b) based on rheological model B.

Effective friction is an important parameter of fault mechanism.However,it is uncertain due to the difficulty of field observation and may influence the simulation results.In order to test the uncertainty of effective friction,we calculate the cumulated co-and post-seismic Coulomb stress change induced by the Wenchuan earthquake,using different effective friction values based on model B.The results in Figs.4(b) and 5 indicate that the calculated Coulomb stress change patterns are consistent among the three selected regions in Fig.1.Nevertheless,for the West Qinling fault and the Liupanshan fault,the calculated Coulomb stress changes shows an obvious decreasing trend and even become negative,as the effective friction increases.This is mainly due to the relative difference between normal and shear stress changes induced by the great Wenchuan earthquake.Wan and Shen(2010) analyze the Coulomb stress changes on the surrounding faults induced by the great Wenchuan earthquake and obtain similar results.Therefore,our results are able to provide valuable information to interpret the regional seismicity in the three selected regions in Fig.1.

5.Conclusion

We investigate the stress change on the major faults in the Weihe Graben induced by the Wenchuan earthquake based on a rheological model.The results show that the stress unloading induced by the Wenchuan earthquake occurs on the western Weihe Graben where the seismic activity decreased,and stress loading is exerted on the middle and eastern Weihe Graben where the seismic activity increased.The stress loading and unloading areas are respectively corresponding to the high and low seismic activity areas,indicating that the change of seismic activity in the Weihe Graben should be associated with the great Wenchuan earthquake.

Fig.5.Cumulated co-seimic and post-seismic Coulomb stress change on the major faults induced by the Wenchuan earthquake.(a) μ’=0.1;(b) μ’=0.7.

Acknowledgement

This project is sponsored by the Program of Science for Earthquake Resilience,China Earthquake Administration（XH21032）and the Program from Xi 'an Geological Survey Center of China Geological Survey([2018]01–38).