Rupture Dynamics and Ground Motion from Potential Earthquakes Around Taiyuan, China by Zhenguo Zhang, Wei Zhang, Xiaofei Chen, Ping’En Li, and Changhua Fu

Bulletin of the Seismological Society of America, Vol. 107, No. 3, pp. 1201–1212, June 2017, doi: 10.1785/0120160239

Rupture Dynamics and Ground Motion from Potential Earthquakes around Taiyuan, China by Zhenguo Zhang, Wei Zhang, Xiaofei Chen, Ping’en Li, and Changhua Fu

Abstract Using the curved grid finite-difference method, we develop dynamic spontaneous rupture models of earthquakes on the Jiaocheng fault (JF) near Taiyuan, the capital and largest city of Shanxi Province in north China. We then model the wave propagation and strong ground motion generated by these scenario earthquakes. A map of the seismic-hazard distribution for a potential M 7.5 earthquake is created based on dynamic rupture and true 3D modeling. The tectonic initial stress fields derived from the inversion of focal mechanisms of historical earthquakes, a nonplanar fault, and a rough surface are considered in the dynamic rupture simulation. Based on the geological structure of the Taiyuan basin, normal faulting with a dipping angle of 60° is implemented for the scenario earthquake simulations. The largest uncertainty of a potential earthquake in the JF zone is the hypocenter. Four cases are used to nucleate the earthquake at different locations. Using these dynamic rupture sources for the JF, we further simulate and analyze both the seismic wave generated by the scenario earthquake and the strong ground motion. It is found that the low-velocity media of the Taiyuan basin redistribute the ground motion well. The effects of the regional stress fields on the dynamic rupture and hazard distribution are investigated and discussed further. Moreover, a scenario earthquake, which can cause great damage to the city of Taiyuan, is modeled and analyzed.

Introduction The Shanxi rift is an intraplate extension zone in north least three earthquake events since the early Holocene China (Xu and Ma, 1992). Although there is debate about its at 3:74 0:060 ∼ 3:06 0:26 ka, 8:35 0:090 ∼ origin, the extension of the Shanxi rift has been confirmed by 3:74 0:060 ka, and 10:66 0:85 ∼ 8:35 0:09 ka seismic data (Wesnousky et al., 1984), geological surveys ago. Minimum vertical slips of 3.0, 2.5, and 3.2 m, respec- (Zhang et al., 1998), and Global Positioning System (GPS) tively, were observed for these three events, which suggest observations (Shen et al., 2000; He et al., 2003; Qu et al., that their magnitudes were all larger than 7. The fact that 2014). The continuous extension makes the Shanxi rift a sig- historic earthquakes with magnitudes larger than 7 have oc- nificant seismic belt in north China. An earthquake with a curred indicates that it is possible for a destructive earth- magnitude of 8 shook Hongtong, which is located in the Lin- quake to occur on the JF in the future. The JF mainly fen basin of the Shanxi rift, on 17 September 1303 (Deng strikes in the southwest–northeast (SW–NE) direction and et al., 2004). In addition to this large earthquake occurring approaches the city of Taiyuan from the west within a small in the active intracontinental basin, a few earthquakes with distance. Thus, if an earthquake occurs on the JF, a seismic magnitudes larger than 6 have been recorded (Li, 1989). wave with large seismic energy can quickly hit the city of Taiyuan is the capital and largest city of Shanxi Province Taiyuan and can cause extensive damage. Therefore, we in north China. The seismic responses of the city of Taiyuan need to investigate the potential seismic hazards caused by to destructive earthquakes, especially those occurring in the earthquakes occurring on the JF. Taiyuan basin, are important data for engineering seismology Because no analytical solution exists for rupture dynam- analysis to design proper standards for the seismic protection ics and wave propagation in 3D complex media, a numerical of buildings. There are two main faults in the Taiyuan basin: simulation is the tool of choice for obtaining responses to the Jiaocheng fault (JF) and the Taigu fault, which lie on the scenario earthquakes under the complex conditions that north and south of the Taiyuan basin, respectively. No de- approximate the reality. The rupture process and ground- 7 structive earthquake with Ms > has been recorded in the motion distribution of an earthquake are impacted by a few Taiyuan basin. However, a recent geological survey of the JF factors such as the fault geometry, regional stress fields, rock (Xu, personal comm., 2016) indicates that there have been at properties, surface of the Earth, and site effects. A powerful

1201 1202 Z. Zhang, W. Zhang, X. Chen, P. Li, and C. Fu

(a) 111˚ 112˚ 113˚ 114˚

Xinzhou C´ D´ (b) Yangqu 38˚Shouyang 38˚ Gujiao Taiyuan Yangquan Qingxu Jinzhong B Jiaocheng B´ Lvliang Wenshui Taigu A Qi Xian A´ Fenyang Pingyao Xiaoyi 37˚Jiexiu 37˚ Jiaokou Lingshi

C D Changzhi 36˚Linfen 36˚

111˚ 112˚ 113˚ 114˚ m 0 1000 2000 3000

Figure 1. (a) Map view of the Jiaocheng fault, which is indicated by the red line. The blue and red arrows illustrate the directions and relative values of the maximum and minimum horizontal compressive stresses. The four black lines AA′,BB′,CC′, and DD′ represent four profiles to illustrate the velocity structure. The velocity profiles are shown in Figure 3. (b) 3D view of fault geometry with roughness. The white stars depict hypocenters used for different models. numerical tool is required to include these complexities in the trace, which is indicated by the red line in Figure 1, is modi- simulation and derive reliable solutions. We use the curved fied from the geological survey (Xu, personal comm., 2016). grid finite-difference method (CG-FDM) to model the spon- The whole JF can be categorized into three main segments taneous dynamic rupture of the scenario earthquake and the (southern, middle, and northern) that have near planar geom- induced seismic-wave propagation in 3D heterogeneous me- etries and are connected by corners located near Wenshui and dia. The CG-FDM is first proposed to simulate seismic wave Qingxu counties. These two corners affect the rupture of the propagating in the presence of irregular topography (Zhang fault as barriers. The complexity of the surface trace of the and Chen, 2006; Zhang et al., 2012) and has been introduced fault infers an inhomogeneous rupture pattern and ground- into modeling the dynamic rupture of irregular fault planes motion distribution. (Zhang et al., 2014b). The geological survey provides the surface trace of the In this work, we investigate the dynamic rupture and fault. However, the 3D geometry under the surface of the seismic-wave propagation of scenario earthquakes occurring Earth needs to be identified by other methods. GPS obser- on the JF. The 3D geometry of the JF used to model the vations show that there is a horizontal extension at the dynamic rupture is constructed with the surface trace of the Taiyuan basin (Shen et al., 2000; He et al., 2003; Qu et al., JF, derived from the geological survey, and then extended in 2014), which indicates the existence of normal faults. In this depth as a dipping fault with an angle of 60°. Scenario earth- work, a 60° dipping angle is considered to construct the 3D quakes triggered at different locations are simulated and dis- geometrical model of the JF (Fig. 1b). cussed in this work. Furthermore, a specific earthquake that Investigation of the fault geometry indicated that the could cause great damage to the city of Taiyuan is modeled fault plane is not ideally planar at the interface of the two and investigated. fault sides but instead shows roughness with different char- acteristic lengths (Power and Tullis, 1991; Sagy et al., 2007). Method and Fault Model Numerical experiments show that this roughness introduces high-frequency seismic-wave radiation (Shi and Day, 2013) We use CG-FDM (Zhang et al., 2014b, 2016) to model or even supershear transition (Bruhat et al., 2016). To model the spontaneous dynamic rupture of the JF. This method has scenario earthquakes under realistic conditions, the surface been validated by recent benchmark models (Harris et al., roughness of the fault plane is incorporated within our 2009). Figure 1 illustrates the surface trace of the JF, which simulations. Because the first order of the fault geometry is extends ∼110 km in the strike direction. The nonplanar fault depicted by the geological survey (red line in Fig. 1a), which Rupture Dynamics and Ground Motion from Potential Earthquakes around Taiyuan, China 1203

(a) Stress (MPa) (b) Dc (m) (c) C0 (MPa) 0 100 200 300 400 0.0 0.4 0.8 1.2 0.0 0.4 0.8 1.2 1.6 0 0 0 σ v σ H σ 5 h 5 5

10 10 10 Depth (km) Depth (km) Depth (km) 15 15 15

20 20 20

Figure 2. Schematics to illustrate the distributions of (a) stress fields (σ), (b) critical distance (Dc), and (c) frictional cohesion (C0) varying with depth. is nonplanar with complex geometry in the strike direction, a rock is brittle. Beyond the depth of brittle rock, the fracture of rough surface with H 0 (Andrews and Barall, 2011)is fault changes from brittle to ductile, and a few earthquake rup- considered. Figure 1b shows the 3D geometry of the fault tures occur (Scholz, 1988). In our simulations, we set the brit- which was used in the scenario earthquake simulations. tle fault extending from the Earth’s surface to a 14-km depth Off-fault plasticity is usually implemented to discuss the and then gradually change it to ductile status with σv σH dynamic rupture of a rough fault (Dunham et al., 2011; Shi σh at a 19-km depth, followed by a hydrostatic equilibrium at and Day, 2013). However, as we have no further information deeper depths. The critical depth is chosen by trial and error to about the plasticity of the Taiyuan basin, in this work, we achieve an M ∼ 7:5 scenario earthquake. The variations in the only consider the elastic response of off-fault media. If a re- regional stress fields with depth are schematically shown in liable off-fault plastic model is available, we can use it to Figure 2a. The real values of σ are calculated by integrating calculate the plastic response of dynamic rupture and strong from the Earth’s surface to the depth according to local rock ground motion in future work. densities. For clarity, the stress variations in Figure 2a show Tectonic stress is another major effect that controls rup- those in the homogeneous medium with ρ 2670 kg=m3. ture and seismic-wave radiation behaviors. By investigating The simple slip-weakening law (Ida, 1972) with static the earthquake focal mechanism of the Shanxi rift system in and dynamic friction coefficients of μs 0:42 and north China, Li et al. (2015) derived a stress orientation map μd 0:26, respectively, was used in our dynamic simulations. for the Shanxi rift, including the Taiyuan basin discussed here. The frictional cohesion C0 is also considered in the rupture Their results indicate a stress field with an azimuth of 330° for dynamics simulations. Similar to the variations of the regional the minimum horizontal compressive stress σh and a ratio of stress fields with depth, the frictional cohesion C0 and the slip- R 0:8 defined by R σH − σh= σv − σh,inwhichσH weakening distance Dc are also dependent on the depth, as and σv are the maximum horizontal and vertical compressive illustrated in Figure 2b,c. As seen in Figure 2, a constant Dc stresses. We adopt this stress field inverted from earthquake of 0.4 m is applied for depths between 4 and 14 km. The fric- focal mechanism analysis and assume the following: tional cohesion is 1.6 MPa at the free surface, linearly de- creases to 0.4 MPa at a 4 km depth, and then remains constant. σ ρ − ρ EQ-TARGET;temp:intralink-;df1;55;266 v w gh; 1 The fault plane is embedded within heterogeneous media, which will be introduced in the following paragraphs. The sce- σ 0 9σ nario earthquakes are assumed to spontaneously rupture on the EQ-TARGET;temp:intralink-;df2;55;220 : ; 2 H v fault plane indicated by Figure 1b and then stopped by artificial boundaries specified with high enough strength at the σ 0 5σ EQ-TARGET;temp:intralink-;df3;55;205 h : v; 3 bottom of and at two ends of the fault. When the rupture front reaches this hard fault boundary it generates a stopping phase, in which ρ and ρw are the densities of the rock and water, g is which can be detected by observations (Savage, 1965; Brüstle the gravitational acceleration, and h is the depth. The direc- and Müller, 1987). Another choice is setting some soft boun- tions of two horizontal compressive stresses σH and σh are daries that gradually stop the rupture before it reaches the marked by blue and red arrows in Figure 1a,respectively, boundaries. Because no confirmed information about the fault and that of the vertical compressive stress σv is not shown boundary condition of the JF is available, we chose the hard for clarity. This stress field is resolved on the complex 3D fault boundary condition in this research. geometry to approximate the initial shear and normal stresses As the scenario earthquake is located at the boundary of of the fault planes. It is assumed that earthquakes are caused the Taiyuan basin, which is characterized by solidified or by the rupture of faults within the seismogenic zone, where the unsolidified sedimentary rock and low-velocity shear waves, 1204 Z. Zhang, W. Zhang, X. Chen, P. Li, and C. Fu

Figure 3. Distributions of VP along four profiles across the Taiyuan basin. The surface projections of (a) AA′, (b) BB′, (c) CC′, and (d) DD′ are illustrated in Figure 1. the 3D heterogeneous media properties must be included. We An important uncertainty regarding the potential earth- use the 3D heterogeneous media model derived from a local quake is the hypocenter. Earthquakes that are triggered at survey (Wang, personal comm., 2016), which provides the different locations may experience quite different rupture sediment depths of the Quaternary and Tertiary periods and histories and seismic hazards, especially those that occur on those of the media layers. The media model contains seven faults with complex geometries. In this work, we set up four layers in total, including two sediment layers and five other models with hypocenters located at different points in the layers of basement rock. Table 1 lists the P-andS-wave veloc- strike direction. The hypocenters of the second and fourth ities for each layer. The depths for the layered media are models are located in two corners, that is, Wenshui and irregular. The maximum depth of the latest sediment Qingxu counties. The scenario earthquakes of the first and (Quaternary period) is ∼600 m, covering the surface of the third models are triggered by the hypocenters located in the southern and middle segments. Moreover, the nucleation Taiyuan basin with VP 2000 m=sandVS 1000 m=s. The sediment of the Tertiary period that fills the Taiyuan basin patches in the four models are of the same size (with a radius of 2 km) and depths, as illustrated by the white stars in has VP 3200 m=sandVS 1600 m=s. The maximum depth of the Tertiary sediment is 2200 m. To illustrate more Figure 1b. Within the nucleation patch, a relatively high ’ details about the heterogeneous media, we select four vertical stress larger (0.1%) than the fault s strength is assumed to profiles (AA′,BB′,CC′, and DD′). The surface projections of initialize the dynamic modeling. After that, the rupture spon- the four profiles are presented by the four named black lines in taneously propagates outside the unbroken area. Figure 1.Figure3 shows the distributions of VP along these four profiles. Because the rock density is not included in the Results ρ 0 31 0:25 media model, we approximate it by : VP (Gardner et al.,1974). The grid interval determines the accuracy of numerical simulations. Generally, a smaller grid interval results in solutions with higher accuracy. Compared with wave propaga- Table 1 tion simulation, dynamic rupture modeling usually requires a Media Parameters Used in the Simulations finer grid, due to that the accuracy of dynamic rupture mod-

Layers VP (m=s ) VS (m=s ) eling is controlled by the spatial resolution of cohesive zone Quaternary 2000 1000 (Day et al., 2005; Cruz-Atienza et al., 2007; Tago et al., Tertiary 3200 1600 2012). However, for wave propagation simulation, the accu- 1 4500 2300 racy is controlled by the spatial resolution of wavelength. 2 5500 3000 Moreover, computational limitations also require the use of 3 6000 3400 coarser grids to perform wave propagation simulation in a 4 7000 4000 5 8000 4600 larger volume compared to the rupture dynamic modeling, which only concerns a smaller area near the fault plane. Rupture Dynamics and Ground Motion from Potential Earthquakes around Taiyuan, China 1205

Figure 4. Rupture time contour every 1 s for earthquakes triggered by different hypocenters, which are represented by the white stars.

The simulation run for each scenario earthquake involves the ruptures spontaneously stop at the bottom boundary of the two steps. The first is the spontaneous dynamic rupture proc- fault plane. As an example, Figure 5 illustrates snapshots of ess on the nonplanar fault. The CG-FDM that we use to solve the slipping rate (dipping component) for the first model. the rupture dynamics specifies grids along the irregular fault The maps of the final slips for the four cases are illus- plane, with a grid interval of 100 m, as well as the parameters trated in Figure 6. It is clear that the rupture can propagate the and conditions discussed in the Method and Fault Model sec- entire fault plane except for the northern part. The moment tion. The computational size for the rupture dynamics mod- magnitude of each simulation is marked in Figure 6 to better eling is 120, 30, and 25.2 km in the three dimensions, understand the earthquakes. The moment magnitudes of the discretized into 1200 × 300 × 252 grids. Subsequently, the earthquakes triggered at different locations do not differ very rupture process of the first simulation is added as source time significantly and are all close to Mw 7.55. However, the functions to calculate the seismic-wave propagation and patterns of the slip distribution of the four models are quite strong ground motion, with a grid interval of 250 m. Each different, which can radiate distinct ground motions, causing ground-motion simulation is performed within a volume ex- different seismic hazards. tending 300 km along both longitudinal and latitudinal directions and 60 km in depth. The perfectly matched layer scheme (Zhang and Shen, 2010; Zhang et al., 2014a) is used Strong Ground Motions in both dynamic rupture and ground-motion simulations. With the rupture processes on the fault plane, we further simulate the strong ground motion to investigate the seismic hazards caused by earthquakes triggered at different loca- Dynamic Rupture tions. Figure 7 illustrates maps of synthetic Chinese seismic Figure 4 illustrates the time contour of the fault plane intensity calculated from the synthetic horizontal peak every 1 s for the scenario earthquakes nucleated at different ground velocity (PGVh). The intensity maps indicate that locations. The rupture time for each point is recorded when seismic-hazard distributions depend strongly on the hypo- the slipping velocity first exceeds 1 mm=s. The ruptures for center location. Because of seismic directivity, the ground all four models cannot propagate in the northernmost segment, motion in the front of the rupture direction is larger than that where the strike direction changes markedly, and the resolved in the opposite direction. This effect is most notable when the initial shear stress here is too small to continue the original hypocenter is located close to the end of the fault plane. For rupture. Another common feature of these simulations is that example, for the first model (Fig. 7a), the rupture is nucleated 1206 Z. Zhang, W. Zhang, X. Chen, P. Li, and C. Fu

Figure 5. Snapshots of slipping rate (dipping component) on the fault surface for the first model. The white star represents the hypocenter for each snapshot. in the southwestern segment of the JF and propagates toward surface traps seismic waves, causing long-lasting vibration the NE. The seismic-intensity map for this case shows great in the basin. The hanging-wall effect of the normal fault damage in the NE of the JF, including in Taiyuan, Xinzhou, aggravates the vibration in the basin. and Gujiao. The ground-motion pattern for the fourth case The intensity map for each scenario earthquake (Fig. 7) (Fig. 7d), in which the rupture is nucleated in the clearly shows a swallow-tailed pattern in the southwestern northeastern segment of the JF and propagates toward the part of the JF but none in the northeastern segment. This SW, performs oppositely. Although the moment magnitudes swallow-tailed pattern in the ground-motion distribution is of the four scenario earthquakes with different hypocenters caused by the stop phase of a rupture when stopped by the are nearly the same, the strong ground motions derived from artificial boundary with infinitely high strength. Because this the numerical simulations are quite different. abruptly changing rupture speed radiates strong seismic All four scenario earthquakes introduce great damage to waves (Madariaga, 1983), the stop of the rupture at the ar- the Taiyuan basin. An important reason for this is the sedi- tificial boundary in the southwestern end of the JF generates ment structure with low velocity. The reflection between the strong seismic-wave radiation. This is followed by the swal- rock basement with a relatively high velocity and the free low-tailed pattern in the intensity map. However, the rupture Rupture Dynamics and Ground Motion from Potential Earthquakes around Taiyuan, China 1207

Figure 6. Final slip patterns and moment magnitudes (Mw) for earthquakes nucleated at different locations. The nucleation patch of each model is represented as a white star. process on the other end of the JF is quite different. The its center. It is clear that the ground motion in Figure 8b has a original faulting gradually heals before it reaches the artifi- long time duration and a large amplitude. In the basin, multi- cial boundary, without the stop phase associated with the reflection of the seismic wave between the free surface and abrupt changes of the rupture speed. It is then reasonable that the base rock in the basin contributes to the long duration and the seismic intensity at the northeastern end of the JF rapidly large amplitude vibration. attenuates with increasing distance. Another phenomenon of the intensity map is the wide Discussion extension of the area with high intensity at the Qingxu corner. This feature is caused by the changing rupture direction. Variations in Stress The JF turns left to strike north within a short distance from Tectonic stress is the driving force of faulting that causes the original SW–NE direction. When the normal rupture seismic-wave radiation followed by seismic hazards. Varia- associated with large energy changes its direction of propa- tions in the background stress fields may result in gation, a strong seismic wave is radiated. This extension is different rupture patterns on the target fault and in different weaker in the fourth scenario than in the other three scenar- strong ground motion distributions for the scenario earth- ios, in which the rupture is initialized further from the quakes. Moreover, there is great uncertainty when measuring Qingxu corner. At sufficient distances to accumulate energy, tectonic stress fields. In this section, we rotate the orientation – the rupture in the three models (Fig. 7a c) arrives at the of the stress fields as shown in Figure 1 counterclockwise corner with large amplitude. Under this circumstance, the (CCW) and clockwise (CW) by 10° to investigate the effects change of the rupture direction radiates strong seismic waves of the regional stress fields on rupture dynamics and ground and causes wide-spread high intensity in the Qingxu corner. motion. Except for the azimuth of regional stress fields, all In the last scenario earthquake (Fig. 7d), the energy of the other parameters are the same as those described in the rupture is small when it reaches the Qingxu corner because Method and Fault Model section. the propagation distance from the hypocenter is short. Figure 9 illustrates the slip patterns of the earthquakes Figure 8 presents the seismogram for the second model triggered by different nucleation patches and rotated by at two stations with more details. One is located in the city of CCW 10° regional stress fields. Compared with those derived Taiyuan, and the other is located at the intersection point of from the original stress fields (with an azimuth of 330°; profiles BB′ and DD′. The first receiver (Fig. 8a) is located at Fig. 6), the rupture slips for the 10° CCW rotation are rela- the edge of the Taiyuan basin and the other one (Fig. 8b)in tively larger, meaning that the CCW-rotated regional stress 1208 Z. Zhang, W. Zhang, X. Chen, P. Li, and C. Fu

(a) (b) 111˚ 112˚ 113˚ 114˚ 111˚ 112˚ 113˚ 114˚

Xinzhou Xinzhou

VIII 38˚V Yangqu 38˚ 38˚V Yangqu 38˚ Shouyang VIII Shouyang Gujiao Gujiao Taiyuan Yangquan Taiyuan Yangquan VII Qingxu Jinzhong Qingxu Jinzhong Jiaocheng X VII Jiaocheng X Lvliang Wenshui Lvliang Wenshui VII Taigu Taigu VII Qi Xian Qi Xian Fenyang Fenyang VIII IX Pingyao IV Pingyao Xiaoyi Xiaoyi V 37˚Jiexiu 37˚ 37˚Jiexiu 37˚ Jiaokou Jiaokou Lingshi Lingshi

VIII VII

Changzhi Changzhi 36˚Linfen 36˚ 36˚Linfen 36˚

111˚ 112˚ 113˚ 114˚ 111˚ 112˚ 113˚ 114˚ m/s m/s 0.0 0.4 0.8 1.2 0.0 0.4 0.8 1.2

Xinzhou Xinzhou

38˚V Yangqu 38˚ 38˚Yangqu 38˚ VII Shouyang VI Shouyang Gujiao Gujiao Taiyuan Yangquan Taiyuan Yangquan VI VII Qingxu IX Jinzhong Qingxu Jinzhong Jiaocheng Jiaocheng IX VI Lvliang Wenshui Lvliang Wenshui Taigu VI Taigu Qi Xian Qi Xian Fenyang Fenyang X IX Pingyao IX Pingyao Xiaoyi V Xiaoyi V 37˚Jiexiu 37˚ 37˚Jiexiu 37˚ Jiaokou Jiaokou Lingshi Lingshi

VIII VIII

Changzhi Changzhi 36˚Linfen 36˚ 36˚Linfen 36˚

111˚ 112˚ 113˚ 114˚ 111˚ 112˚ 113˚ 114˚ m/s m/s 0.0 0.4 0.8 1.2 0.0 0.4 0.8 1.2

Figure 7. Maps of Chinese seismic intensity calculated by horizontal peak ground velocity (PGVh) for earthquakes triggered at different locations. The value of PGVh is presented by the color. We present the intensity scales by black contour lines. For clarity, some scales are also printed at the appropriate places in blue Roman numerals with blue boxes. The surface projection of the hypocenter for each model is represented by the white star. The white line indicates the fault plane at the free surface. The light gray lines are the administrative boundaries. fields make the JF more likely to rupture. Based on a quan- are stopped by the nonplanar segments instead. As we know, titative comparison, the moment magnitude of each scenario the entire JF can be divided into three segments (southern, earthquake in the CCW 10° case is larger than that of the middle, and northern) due to the strike changes in Wenshui corresponding simulation for the unrotated stress fields. and Qingxu counties (Fig. 1). The corners bar the rupture More importantly, the ruptures of earthquakes with different propagation when it passes through them. The final slip dis- hypocenter locations can all propagate in the northernmost tributions show that the entire JF is unfavorable to rupture segment, which is close to the city of Taiyuan. Much stronger under the CW 10° rotation of the regional stress fields. The seismic hazards can be inferred at the city of Taiyuan under barrier effect of the Qingxu corner is the same for all four CCW-rotated regional stress fields. models and prevents the rupture from further propagating A CW 10° rotation of the regional stress fields results in in the northern segment. However, these barrier effects are very different slip patterns (Fig. 10) compared to the unro- not uniform for all ruptures on the fault plane. When the tated (Fig. 6) and the CCW 10° rotation (Fig. 9) cases. All the hypocenter is located near the Wenshui corner (Fig. 10a, ruptures cannot propagate through the entire fault plane and c), the ruptures reach the barrier before accumulating enough Rupture Dynamics and Ground Motion from Potential Earthquakes around Taiyuan, China 1209

(a) energy to break it and continue propagating. In these two models, the scenario earthquakes are limited to rupture V 0.48306 x within a single segment of the JF, thus causing earthquakes with small magnitudes. For the model in which the hypocen- Vy 0.39575 ter is set at the corner (Fig. 10b), the high artificial initial stress triggers the barrier to rupture, thus causing a large V 0.31829 earthquake propagating through the entire fault plane. An- z other possibility that would force the Wenshui barrier to rupture is indicated in Figure 10d, where the hypocenter is located sufficiently distant from the barrier to allow enough (b) energy accumulation when the rupture front meets it.

Vx 0.79863 Northern Segment of the JF

Vy 0.6017 The northern segment of the JF is close to the city of Taiyuan, a metropolis in north China. The earthquake activity of the JF, especially in the northern part, is very important V 0.60784 z to seismic-hazard distributions and needs to be considered. In this section, we set up a model that allows rupture in the northern segment of the JF to investigate the potential great 0 50 100 seismic hazards introduced to the city of Taiyuan. Time (s) There is evidence showing that the northern segment of the JF has a high possibility of rupture during an earthquake Figure 8. Seismogram of three-component particle velocity for the second model at the two receivers: (a) at the city of Taiyuan and that occurs on the entire JF (Zhang et al., 2011). It is very (b) at the intersection point of profile BB′ and DD′. The first useful for us to calculate the seismic-wave propagation receiver is located at the edge of the basin and the other one in and ground-motion distribution caused by an earthquake its center. The maximum absolute value of each seismogram is rupturing on the northern segment of the JF. However, in marked at the end of the time history. our previous simulations, the rupture cannot propagate in

Figure 9. Final slip patterns and moment magnitudes (Mw) for earthquakes nucleated at different locations with stress fields rotated counterclockwise by 10°. These slip patterns are similar to those shown in Figure 6 with the same nucleation locations, except that the ruptures occur in the northernmost segment in all cases. 1210 Z. Zhang, W. Zhang, X. Chen, P. Li, and C. Fu

Figure 10. Final slip patterns and moment magnitudes (Mw) for earthquakes nucleated at different locations with stress fields rotated clockwise by 10°. All ruptures cannot propagate the entire fault plane under these rotated stress fields. the northern part of the JF. Because of the strike directions of northern segment of the JF, we use 310° as the azimuth of the JF and the tectonic field, the relative stress is low in the σv. Moreover, seismic activity analyses (Zhang et al., 2011) northern segment of the JF and unfavorable to rupture. Under indicated that the northern segment has a higher possibility the assumption of a 330° azimuth of the minimum horizontal of rupture than the southern segment of the JF, indicating that compressive stress, for all four cases with different nuclea- the dynamic properties of the JF are inhomogeneous. We set tion patch locations, both the rupture time (Fig. 4) and the inhomogeneous distributions of the dynamic friction coeffi- final slip (Fig. 6) distributions of the fault indicate that the cient μd for this observation. A larger value of the dynamic rupture cannot propagate in the northern part of the JF. This friction coefficient (μd 0:27 and a smaller stress drop) is situation improves when the tectonic stress is rotated CCW employed for the segment with a strike distance less than by 10° (Fig. 9), although the total slip in the northern part of 80 km. For the rest of the fault, a small value (μd 0:24 and the JF is relatively small compared to that on other segments. a larger stress drop) is used. The uniform distribution of μd The increased possibility of the northern JF segment to rup- on the fault plane suggests that the main seismic energy ture for stress fields rotated CCW by 10° can be easily iden- release is focused in the northern segment of the JF close to tified using the initial stress distributions. In contrast to that the city of Taiyuan. It can be predicted that in such a case, the in the unrotated case, the minimum horizontal compressive scenario earthquake poses intense hazards to the city. stress σh of the CCW 10° rotated case is almost perpendicular With assumptions about the condition and parameters, to the northern segment of the JF. Under reasonable criteria, we rerun the simulations of rupture dynamics and seismic- it is acceptable to rotate the stress fields CCW to force the wave propagation. The results are shown in Figure 11, northern part of the JF to rupture, causing a large earthquake including the rupture time, final slip distributions, and syn- and great damage to the city of Taiyuan. thetic intensity map. Only the third hypocenter (Fig. 1)is The GPS observation (Qu et al., 2014) indicated that used to model the scenario earthquake simulation. An impor- there is an extension movement at the azimuth of 310°, which tant discrepancy of this model is that the rupture propagates is different from the stress fields inverted from the earthquake in the northernmost segment of the fault until it is stopped by mechanisms (Li et al., 2015). There is a gap between the the artificial boundary. Moreover, the northern segment ex- extension movement indicated by the observation and the periences a larger slip than other parts of the fault plane and regional stress fields inferred from the earthquake mecha- releases the main moment energy of this scenario earthquake. nism because the strain does not necessarily occur along The synthetic Chinese seismic intensity calculated based on the stress (Townend and Zoback, 2006; Yang and Hauksson, strong ground motion simulation also suggests that the inten- 2013). To force the scenario earthquake to rupture in the sive motion area is focused in the northern segment of the JF. Rupture Dynamics and Ground Motion from Potential Earthquakes around Taiyuan, China 1211

(a) (b)

Xinzhou

Yangqu 38˚V Shouyang 38˚ Gujiao Taiyuan VIIYangquan X Qingxu Jinzhong Jiaocheng Lvliang Wenshui VII Taigu Qi Xian Fenyang Pingyao IX Xiaoyi V 37˚Jiexiu 37˚ Jiaokou Lingshi

VII

Changzhi 36˚Linfen 36˚

111˚ 112˚ 113˚ 114˚ m/s 0.0 0.4 0.8 1.2

Figure 11. (a) Rupture time contour every 1 s, (b) final slip distribution, and (c) synthetic intensity map for the scenario earthquake with inhomogeneous dynamic friction coefficients. The white star presents the position of the hypocenter. The value of PGVh is presented by the color. We present the intensity scales by black contour lines. For clarity, some scales are also printed at the appropriated places in blue Roman numerals with blue boxes. The light gray lines are the administrative boundaries.

The earthquake introduces serious damage to the city of motion. The complexity implies that a rupture dynamics Taiyuan, approximately degree XI according to the Chinese simulation for the seismic-hazard assessment of a scenario seismic intensity scale. earthquake is necessary. When the northern part of the JF is allowed to rupture with a large slip distance, this worst- Conclusion case modeling suggests that great motion will be brought into the city of Taiyuan. This study can be applied to seismic en- We numerically simulated the spontaneous dynamic gineering of the local city as well as extended to other places. rupture and the seismic-wave propagation for potential earthquakes on the JF, located in the SW of Taiyuan, China. 3D Data and Resources heterogeneous media, including the 3D Taiyuan basin, and tectonic stress inferred from observations are implemented in The data of fault trace on surface and media are from scenario earthquake simulations. The roughness on fault sur- personal communications. face, depth-depended stress, and friction parameters are considered to approximate the scenario earthquake. The results Acknowledgments indicate that although the moment magnitudes of different We are grateful to Associate Editor Michel Bouchon as well as two earthquakes nucleated at different locations are the same, the anonymous reviewers for their constructive comments. This work is sup- patterns of strong ground motion are quite different. The rup- ported by National Natural Science Foundation of China (Grants ture directivity affects the ground motion in such a way that 41661134014 and 41504040), Central Public-interest Scientific Institution Basal Research Fund of Institute of Geophysics, China Earthquake Admin- shaking is more intensive at the front of the rupture than at its istration (Grants DQJB14C02 and DQJB14C06), and China Postdoctoral back. Moreover, the basin with low-velocity media increases Science Foundation (Grant 2016T90575). The simulations were performed the seismic hazard and lengthens the duration of ground in National Supercomputing Center in Shenzhen (China). 1212 Z. Zhang, W. Zhang, X. Chen, P. Li, and C. Fu

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