Seismic Impact of the M w 9.0 Tohoku Earthquake in Eastern China

Bulletin of the Seismological Society of America, Vol. 14, No. 3, pp. 18 167, June 14, doi: 1.178/11374 Seismic Impact of the M w 9. Tohoku Earthquake in Eastern China by Jia Cheng, Mian Liu, * Weijun Gan, Xiwei Xu, Fuqiong Huang, and Jie Liu E Abstract The 11 Tohoku earthquake (M w 9.) in Japan caused significant coseismic displacement in much of eastern China, where seismic activity has been intense and damaging earthquakes numerous. Did the Tohoku earthquake change the stress field and seismic activity in eastern China? Here, we used a viscoelastic deformation model of layered spherical earth to calculate the coseismic and postseismic displacements and compared the results with Global Positioning System data for eastern China. Using the best-fitting model, we then calculated the coseismic and postseismic changes on major active faults in eastern China. Our results show that the changes caused by the Tohoku earthquake on these faults are less than. MPa, so were unlikely to significantly alter the stress field or trigger earthquakes in eastern China. Online Material: Tables of coseismic displacement at 67 stations and timedependent displacements before and after the M 9. Tohoku earthquake. Introduction On 11 March 11, the M w 9. Tohoku earthquake ruptured a 3 km long and km wide plate interface between the subducting Pacific plate and the overriding Eurasian plate near the east coast of Honshu Island, Japan (Nettles et al., 11; Lengliné et al., 1). The earthquake caused more than 1 m slip on the fault plane (Hayes, 11; Loveless and Meade, 11). The coseismic eastward motion in the overriding Eurasian plate was observed from South Korea to Russia, reaching places more than 3 km from the epicenter (Hwang et al., 1; Shestakov et al., 1). The earthquake temporally altered the stress field (Hirose et al., 11; Kato et al., 11) and triggered earthquakes in the overriding plate near the epicenter (Hirose et al., 11; Lay et al., 11; Toda et al., 11; Fig. 1). Concerns were raised about the seismic impact of the Tohoku earthquake in eastern China, where a series of damaging earthquakes have occurred in the past century, including the M w 7.8 Tangshan earthquake in 1976 that killed more than 4, people (Chen et al., 1988; Fig. 1). The Chinese national Global Positioning System (GPS) network recorded clear coseismic motion in much of eastern China when the Tohoku earthquake struck (Wang, Li, Wang, et al., 11; Yang et al., 11; Chen et al., 1; Zhao et al., 1). The concerns were intensified by a sequence of small earthquakes that rattled the Tangshan region in the spring of 1, including an M s 4.8 event on 8 May 1 (Liu and Wang, 1). Some thought these earthquakes were the aftershocks of the *Also at Key Laboratory of Computational Geodynamics, Chinese Academy of Sciences, Beijing 149, China. 1976 Great Tangshan earthquake; others wondered if these earthquakes were triggered or somehow related to the Tohoku earthquake. The debate was reported by the popular Chinese newspaper Southern Weekend (see Data and Resources). Could the Tohoku earthquake have enhanced earthquake risks in eastern China? We attempt to address this question here by analyzing the coseismic and postseismic deformation in eastern China and calculating the associated stress changes. We also examined the changes of seismicity rates and water level changes. Our results indicate that the seismic impact of the Tohoku earthquake on eastern China is probably insignificant. Geologic Setting and Earthquakes in Eastern China The basement of eastern China was amalgamated during the early Mesozoic by the collision between the North China block and the South China block (Li, 1994). The cratonic root of the North China block was removed during the late Mesozoic, which led to significant lithospheric thinning. This thin lithosphere is partially responsible for the relatively high seismicity in North China (Liu and Yang, ). Most of the seismicity is associated with two systems of active faults, one trending north-northeast and the other west-northwest (Fig. 1). The north-northeast-trending faults include the Tanlu fault, a transform fault developed during the North China South China collision (Yin and Nie, 1993). Although the Tanlu fault shows minimal slip during the Cenozoic, it produced the M 8. Tancheng earthquake in 1668, the largest historical earthquake in China. The Yilanyitong fault is the northern extension of the Tanlu fault; it lies north of the 18

Seismic Impact of the M w 9. Tohoku Earthquake in Eastern China 19 N E 11 E 1 E 1 E 1 E 13 E 13 E 14 E 14 E E N Russia N E 11 E 1 E 1 E 1 E 13 E 13 E 14 E N N Euroasia Plate N YS 4 N 4 N East China TL QX XY M8(133) WDLC HLD M7.8 AS (1976) WFD DD 4 N 4 N Zhangjiakou Fault 3 N M8.(1668) 3 N Shanxi rift Fault Tangshan Fault East China CH Pacific Plate 3 N 3 N Tanlu Fault M7(48) Yilanyitong fault South Korea Japan Honshu Island Japan North America Plate Tohuku Mw9. 11-3-11 > 8 mm/yr N 4 N 4 N 3 N AA BB JIXN BJFS TAIN HLAR LNSY SDYT JLCB N 4 N 4 N 3 N ZHNZ N Philipine Plate N E 11 E 1 E 1 E 1 E 13 E 13 E 14 E 14 E E 3 N WUHN 3 N Figure 1. Tectonic map of the Tohoku earthquake source region and eastern China. The black lines are the plate and national boundaries. The arrow shows the motion of the Pacific plate with respect to the North American plate. The star and bar are the epicenter and rupture length of the Tohoku earthquake, respectively. Gray circles are the earthquake (M s :) epicenters on the Japan Islands after the Tohoku earthquake (Lengliné et al., 1). The gray lines are the faults in eastern China (Deng et al., 3); black circles are M > 7: historic earthquakes since 133 in eastern China (Shen et al., 4); and gray dots are M s 4: earthquakes in eastern China after the Tohoku earthquake. Triangles are the ground water-level stations: standard triangles indicate coseismic rise of the water level, and inverted triangles indicate drop of the water level. The color version of this figure is available only in the electronic edition. tectonically active North China block and has no historical earthquakes larger than magnitude 6. Another north-northeast-trending fault system is the buried Tangshan fault, which was recognized only after the M s 7. Xingtai earthquake in 1966 and the M w 7.8 Tangshan earthquake in 1976 (Zhang et al., 1998). Further to the west, the north-northeast-trending Shanxi rift fault system has produced many large earthquakes in the past millennium (Xu and Deng, 1996), including the M 8 Hongdong earthquake in 133 that killed more than 47, people (Liu et al., 11). However, it has been relatively quiet in the past 3 years (Liu et al., 7). The west-northwest-trending faults include the Zhangjiakou seismic zone, a system of dozens of west-northwesttrending fault strands with sinistral slip (Wang, Liu, Cao, et al., 11). This fault zone hosted a number of devastating earthquakes, including the 1679 Sanhe Pinggu earthquake (M 8.). Many of the large earthquakes occurred where the west-northwest-trending faults intersect the eastnortheast-trending faults. Coseismic Deformation in Eastern China The GPS sites in eastern China recorded clear coseismic motion during the Tohuku earthquake. Figure shows two N Continuous GPS observation 1 mm Observed, Zhao,1 1 mm Observed, Chen, 1 1 mm/yr Background slip rate from GPS E 11 E 1 E 1 E 1 E 13 E 13 E 14 E N Figure. Coseismic displacements observed at Global Positioning System (GPS) stations. Gray arrows are background GPS site velocities, relative to stable Eurasia in the ITRF framework. Solid lines show the faults and national boundaries. The boxes show the GPS sites for calculating coseismic site displacements relative to the Tanlu or the Yilanyitong faults (using the simplified profiles shown in the thick dashed lines) in Figure 3. The color version of this figure is available only in the electronic edition. sets of GPS data. One contains data from 14 continuous GPS sites in eastern China and International GNSS Service (IGS) sites in the surrounding regions (Zhao et al., 1), processed with the GAMIT/GLOBK software; the coseismic offsets were estimated by least-square linear fitting to time series of the daily coordinates for each site using data from three days before and after the Tohoku earthquake. The second data set is from 67 continuous GPS stations in northeastern China (Chen et al., 1; E Table S1 in the electronic supplement to this article), processed with the GIPSY-Oasis software; the coseismic displacements were determined using data from five days before and after the Tohoku earthquake. Although there are some discrepancies between these two data sets, both show significant coseismic displacements in eastern China, up to 1 mm within the North China block and mm in northeastern China. The results also generally agree with those of Shestakov et al. (1) at the same GPS stations. The coseismic displacements deviate from the background crustal motion in eastern China (Fig. ), showing the impact of the Tohoku earthquake. Because the Tanlu and the Yilanyitong faults are roughly normal to the directions of coseismic motion, we plotted the coseismic motions across the Tanlu fault within the North China block and across the Yilanyitong fault in northeastern China to see if there are

16 J. Cheng, M. Liu, W. Gan, X. Xu, F. Huang, and J. Liu (a) Coseismic displacement (mm) 1 1 8 6 4 4 6 8 AA Coseismic displacement parallel to the Tanlu fault Coseismic displacement vertical to the Tanlu fault Tanlu fault (b) Coseismic displacement (mm) 1 1 BB Fault-parallel displacement Fault-normal displacement Yilanyitong fault 1 1 16 1 8 4 4 8 Distance from GPS station to the Tanlu fault (km) 3 1 8 6 4 4 Distance from the Yilanyitong fault (km) Figure 3. Coseismic displacement of GPS sites parallel and normal to (a) the Tanlu fault (strike 1 ) (AA profile in Fig. ) and (b) the Yilanyitong fault (strike 4 ) (BB profile in Fig. ). In the plots, dashed lines are linear fitting between coseismic displacement and distance on the right side of the fault. The westward and northward displacements are positive. clear changes across these faults (Fig. 3). Across the Yilanyitong fault, the coseismic motion shows a clear change of the displacement, both normal and parallel to the fault. Similar but less clear changes show across the Tanlu fault. These changes may indicate deep cutting of these faults that allowed differential crustal motion on the two sides of the fault zone; they also imply normal and shear strains induced by the Tohoku earthquake on these faults. The GPS data from Chen et al. (1) also include data from nine continuous GPS stations from the Chinese GPS network (Fig. ; E Tables S S1 in the electronic supplement). We have examined the time-series data from these stations; the clearest coseismic motions are found in four stations (Fig. 4), all near the Tanlu or the Yilanyitong faults and closer to the epicenter of the Tohoku earthquake than the other stations (Fig. ). Modeling the Coseismic and Postseismic Deformation in Eastern China We have calculated the coseismic and postseismic crustal motion in eastern China in response to the Tohoku earthquake, using the model of quasistatic deformation on a layered spherical Earth (Pollitz, 1996, 1997). The model represents the solution to the equations of static equilibrium as the summation of spheroidal and toroidal components, of which both depend on spherical harmonic degree and the moment tensor. Our solutions are the summation of spherical harmonics from degree 1 to 1667. The inclusion of Earth s curvature and rheological layering in the model is essential for this study, which considers a large area. Our model includes a km thick elastic crust on top of a viscous lower crust and upper mantle with variable viscosity (Table 1). The calculated coseismic displacement is consistent with the GPS data (Fig. ). The model results depend on the source model. We have tested four published source models for the Tohoku earthquake: two models based on inversion of teleseismic data (Hayes, 11; Shao et al., 11), one constrained by the geodetic data (Pollitz et al., 11), and a joint inversion of teleseismic, tsunami, strong-motion, and geodetic data (Yokota et al., 11). These rupture models differ in the size of the ruptured areas with different dip angles of the plate interface. The source model from the joint inversion (Yokota et al., 11) gives a better fit between the predicted and observed coseismic displacements in eastern China than the other source models (Fig. ). Therefore, we use this source model for our calculations of postseismic stain and stress perturbations. The continued GPS data from stations in eastern China can be used to constrain the lithospheric rheology, which is needed for calculating postseismic strain and stress evolution. Using the same viscoelastic model that produced the results in Figure, we calculated the time series of postseismic displacements and compared the results with selected continuous GPS sites in eastern China (Fig. 6). The results are generally good but depend on the viscosity structures of the model lithosphere. We tested a range of viscosity values for the lower crust and upper mantle; the optimal values (Table 1) are 1 1 19 Pa s for the lower crust and 1 1 1 Pa s for the mantle, which are within the typical ranges for the lower crust and upper mantle (e.g., Mitrovica and Forte, 4). The detailed variations in the time series, such as the step change of the northward motion in station SDYT (Fig. 6), may be related to local perturbations such as small earthquakes. Stress Perturbations in Eastern China by the Tohoku Earthquake We have shown that the 11 Tohoku earthquake caused clear coseismic and postseismic displacements in eastern

( Seismic Impact of the M w 9. Tohoku Earthquake in Eastern China 161 Dispalcement (mm) Northward GPS station: JIXN 4 GPS station: JIXN 1 1 4 11-3-11 11-3-11 1.4 1.8 11. 11.6 1. 1.4 1.8 11. 11.6 1. E astward Dispalcement mm) 3 1 1 3 (mm) Upward Dispalcement GPS station: JIXN 4 3 1 1 3 4 11-3-11 1.4 1.8 11. 11.6 1. Northward Dispalcement (mm) 1 1 GPS station: JLCB 11-3-11 E astward Dispa lcement (mm ) 6 4 4 6 GPS station: JLCB 11-3-11 1.6 1.8 11. 11. 11.4 11.6 11.8 1. 1.6 1.8 11. 11. 11.4 11.6 11.8 1. 1.6 1.8 11. 11. 11.4 11.6 11.8 1. Upward Dispalcement (mm) GPS station: JLCB 4 3 1 1 3 4 11-3-11 Dispalcement (mm) Northward 3 GPS station: LNSY 1 1 (mm) Dispalcement Eastward 4 GPS station: LNSY 3 1 1 3 3 4 11-3-11 11-3-11 4 11-3-11 3 1.4 1.6 1.8 11. 11. 11.4 11.6 11.8 1. 1.4 1.6 1.8 11. 11. 11.4 11.6 11.8 1. 1.4 1.6 1.8 11. 11. 11.4 11.6 11.8 1. Upward Dispalcement (mm) GPS station: LNSY 4 3 1 1 Dispalcement (mm) Northward 1 1 GPS station: SDYT 11-3-11 1.4 1.8 11. 11.6 1. E astward D isp a lcement (mm ) 4 3 1 1 3 4 GPS station: SDYT 11-3-11 1.4 1.8 11. 11.6 1. U pwa rd D ispalcement (mm ) GPS station: SDYT 4 3 1 1 3 4 11-3-11 1.4 1.8 11. 11.6 1. Figure 4. Time series of the GPS stations in eastern China before and after the Tohoku earthquake. Data are from Chen et al. (1). Dashed lines mark the time of the Tohoku earthquake. Solid lines are the average slip rates of each GPS station before the earthquake. Error bars are 1σ. Station locations are shown in Figure.

16 J. Cheng, M. Liu, W. Gan, X. Xu, F. Huang, and J. Liu E 11 E 1 E 1 E 1 E 13 E 13 E 14 E N N and two-year postseismic period following the Tohuku earthquake. The change, ΔCFF, is given by N N ΔCFF Δτ μ Δσ n ; 1 4 N 4 N 4 N 4 N 3 N 3 N Tanlu fault Yilanyitong fault 3 N 3 N 1 mm Modeled N 1 mm Observed, Zhao et al (1) N 1 mm Observed, Chen et al (1) E 11 E 1 E 1 E 1 E 13 E 13 E 14 E Figure. Comparison of the calculated and observed coseismic displacements in eastern China and the surrounding regions. The source model from the joint inversion by Yokota et al. (11) was used in the calculation. The color version of this figure is available only in the electronic edition. China. To what degree are stress perturbations associated with these crustal motions? Using the best-fitting kinematic model, we calculated the coseismic and postseismic stress changes on four main active faults in eastern China (Fig. 7): the northeast-trending Yilanyitong fault, the east-northeasttrending Tanlu fault, the east-northeast-trending Shanxi rift fault zone, and the west-northwest-trending Zhangjiakou seismic zone (Fig. ). Table shows the slip behaviors of these faults, based on large historic earthquakes on these faults (Shen et al., 4). Although it has no historic or instrumental records of earthquakes larger than magnitude 6, the Yilanyitong fault has paleoseismic evidence for large events with dominantly dextral slip (Xu and Deng, 1996). Thus, we assumed pure dextral slip on the Yilanyitong fault. We calculated the normal, shear, and changes on the fault planes of these faults for the coseismic in which Δτ and Δσ n are the shear and normal stress changes on the fault plane; μ is the effective friction coefficient, taken to be.4, a typical value for most cases (King et al., 1994). As shown in Figure 7a, the coseismic normal stress change on the Yilanyitong fault is positive, because the Tohoku earthquake causes extension across the fault (Fig. ). The coseismic shear stress decreases slightly, especially on the northern part of the fault, due to the sinistral slip induced by the Tohoku earthquake. The change, which is dominated by the positive normal stress, is positive on the entire Yilanyitong fault, with the maximum value around.1 MPa. Postseismic viscous relaxation increases the negative shear stress on the Yilanyitong fault except near the northern end, where shear stress becomes positive. On the other hand, the extensional normal stress continued to increase on the whole fault plane, leading to an overall increase of the on much of the Yilanyitong fault. However, the maximum change is. MPa, less than the.1 MPa threshold that is deemed to be effective in triggering earthquakes (Stein, 1999; Yu et al., 6). On the Tanlu fault, the coseismic shear stress change is positive on the southern part but negative on the northern part. The coseismic normal stress change is positive along the entire fault plane, giving an overall positive coseismic change (Fig. 7b). Two years after the Tohoku earthquake, due to the increased negative shear stress change on the entire Tanlu fault, the change becomes slightly (< :1 MPa) positive on the southern part and negative on the northern part. The Shanxi rift fault has experienced positive coseismic, which was increased by the postseismic process (Fig. 7c). The maximum change is about.1 MPa. On the north-northwest-trending Zhangjiakou fault, the coseismic shear stress change (Fig. ) is negative, whereas the normal stress change is slightly positive, so the coseismic change is negative (Fig. 7d). With time elapsing, the normal stress change on the fault plane decreases, whereas the shear stress changes on the fault increases; hence, the change increased negatively over time, but the peak value is less than.1 MPa. Table 1 Parameters of the Lithosphere Layers Layer Thickness (km) Density (g=cm 3 ) Shear Modulus (1 1 1 Pa) Bulk Modulus (1 1 1 Pa) Viscosity ( Pa s) Upper crust.9 3.99 6.99 Elastic Lower crust 13.9 4.11 7.81 1 1 19 Upper mantle 3.1 6.81 1.969 1 1 1

Seismic Impact of the M w 9. Tohoku Earthquake in Eastern China 163 Northward Displacement (mm) JIXN 3 8 6 4 1 1 4 6 8 1 1 14 11-3-11 1 11-3-11 1 3 11-3-11 3 11. 11.4 11.6 11.8 1. 1. 11. 11.4 11.6 11.8 1. 1. 11. 11.4 11.6 11.8 1. 1. Eastward Displacement (mm) Upward Displacement (mm) Northward Displacement (mm) JLCB 4 1 4 1 3 3 1 1 3 11-3-11 11-3-11 11-3-11 3 4 11. 11.4 11.6 11.8 11. 11.4 11.6 11.8 11. 11.4 11.6 11.8 Eastward Displacement (mm) Upward Displacement (mm) 1. Northward Displacement (mm) Northward Displacement (mm) 6 4 4 6 8 1 1 14 16 11.1 11. 11.3 11.4 11. 11.6 11.7 11.8 11.9 14 1 1 4 6 8 1 LNSY SDYT 8 6 4 11-3-11 Eastward Displacement (mm) 4 3 1 1 11-3-11 11-3-11 3 11.1 11. 11.3 11.4 11. 11.6 11.7 11.8 11.9 Eastward Displacement (mm) 3 3 1 Upward Displacement (mm) 3 3 1 11.1 11. 11.3 11.4 11. 11.6 11.7 11.8 11.9 11-3-11 11-3-11 11-3-11 6 11. 11.4 11.6 11.8 1. 1. 11. 11.4 11.6 11.8 1. 1. 11. 11.4 11.6 11.8 1. 1. Upward Displacement (mm) 3 1 1 3 4 Figure 6. Postseismic time series of the selected continuous GPS sites (locations shown in Fig. ). The dashed lines mark the time of the Tohoku earthquake. Circles with error bars are the GPS data from Chen et al. (1). The solid lines show the calculated postseismic displacements. The color version of this figure is available only in the electronic edition.

164 J. Cheng, M. Liu, W. Gan, X. Xu, F. Huang, and J. Liu (a) Yilanyitong fault (Coseismic) NE Yilanyitong fault ( years later) NE (c) km 13 km (Pa) Shanxi rift fault (Coseismic) NE Shanxi rift fault ( years later) NE km 8 km 1 1 (Pa) (b) (d) Tanlu fault (Coseismic) NNE NNE Tanlu fault ( years later) km 1 km Zhangjiakou fault (Coseismic) NWW Zhangjiakou fault ( years later) NWW km 7 km 1 1 (Pa) (Pa) Figure 7. Calculated coseismic and postseismic shear, normal, and the changes on the fault planes of selected faults: (a) the Yilanyitong fault, (b) the Tanlu fault, (c) the Shanxi rift, and (d) the Zhangjiakou fault. The color version of this figure is available only in the electronic edition. Discussion and Conclusions Intensive studies in the past decades have shown that earthquake-induced changes of static can trigger earthquakes and raise seismicity rates. With postseismic viscous relaxation, this change of static can last for years and affect seismicity, especially aftershocks, in the nearby regions. Over long distances from the rupture plane, the triggering effects are mainly associated with the passage of seismic waves. However, one may wonder if the 11 Tohoku earthquake is different: it was an M w 9. event, and, being produced on a megathrust fault, the overriding Eurasian plate has shown widespread coseismic motion toward the trench. In eastern China, the Tohoku Table Fault Parameters from the Largest Historical Earthquakes (Shen et al., 4) Fault Name Largest Earthquake Magnitude (year) Strike ( ) Dip ( ) Rake ( ) Shanxi rift fault M 8 (133) 19 7 3 Tanlu fault M 8. (1668) 8 Zhangjiakou seismic M 7 (48) 9 8.1 zone Yilanyitong fault 4 8 18 earthquake caused 1 mm coseismic displacement. Consequently, it raises the question of whether the Tohoku earthquake has affected seismicity rates and may even trigger earthquakes in eastern China. We found the coseismic and postseismic stress perturbations in eastern China imparted by the Tohoku earthquake are insignificant. Although the details of the calculated would vary with the choice of the frictional coefficient, as it will change the relative proportions of normal and shear stress changes (equation 1), the magnitude of the static changes are within the range of : MPa. This is comparable to tidal (around.1 MPa; Stein, 4) but much lower than the empirical threshold of.1 MPa that is thought to cause significant changes of seismicity rate and trigger sizable earthquakes. Our results of stress changes are consistent with seismicity rates around the major faults in eastern China, which do not show significant increases following the Tohoku earthquake (Fig. 8). On 8 May 1, an M s 4.8 event occurred within the source region of the 1976 Tangshan earthquake (M w 7.8). There were speculations about whether this and a sequence of recent small earthquakes in the Tangshan region were triggered by the Tohoku earthquake. Our calculations show that the change is about : MPa to :9 MPa in the Tangshan region, depending on the rupture planes of these small events. Such small stress perturbation is unlikely to have triggered the recent seismicity in Tangshan, which alternatively has been

Seismic Impact of the M w 9. Tohoku Earthquake in Eastern China 16 (a) N (b) 13 1 Zhangjiakou fault Tanlu fault Yilanyitong fault Shanxi fault 4 N 4 N 3 N Shanxi fault Zhangjiakou Fault Tanlu fault Yilanyitong Fault Number of Earthquake/month 9 7 6 4 3 N N 11 E 1 E 1 E 1 E 13 E 13 E 4 16 1 8 4 4 8 1 16 4 Time before and after the Tohuku earthquake (month) Figure 8. (a) Seismicity in eastern China (M L 1:) between 11 March 9 and 11 March 13. Boxes are the regions for counting the seismicity rates for each fault zone. (b) Seismicity rates before and after the Tohoku earthquake. Earthquake catalog data are from China Earthquake Network Center (see Data and Resources). The color version of this figure is available only in the electronic edition. explained as aftershocks of the 1976 Tangshan earthquake (Liu and Wang, 1). We have also examined the change of water table in eastern China, as measured by the Groundwater Monitoring Network of China. Four stations that show clear coseismic response (Fig. 9) are near the Yilanyitong fault or the Tanlu fault (Fig. 1). If the changes of water table are due to coseismic normal stress change, they should show a coseismic drop, because the normal stresses on both faults are predicted to be extensional (Fig. 7). The data, however, show mixed results. Except the station WFD, which shows a drop of the water table, the other stations (especially CH and YS) show clear coseismic rise of the water table. These changes of water table level in eastern China are inconsistent with the coseismic stress changes. Instead, they may be better explained by the interactions between the aquifer and seismic waves. Hence, we conclude that, although the 11 Tohoku earthquake caused up to mm coseismic displacement in eastern China, its impact on the stress field and seismicity in eastern China is insignificant. The coseismic and postseismic changes of static on the major active faults in eastern China is less than. MPa, and the recent small earthquakes in the Tangshan region are unlikely triggered by the Tohoku earthquake. Data and Resources The GPS data used in this paper are from two National Key Scientific Projects: Crustal Movement Observation Network of China (CMONOC I) and Tectonic and Environment Observation Network of Mainland China (CMONOC II). Water level observation data were from Groundwater Monitoring Network of China. The newspaper report in Southern Weekend about the Tangshan earthquake is available on the webpage at http://www.infzm.com/content/ 767, last accessed October 13; in chinese. The earthquake catalog data in Figure 8 are from China Earthquake Network Center (www.ceic.ac.cn; last accessed October 13; in Chinese). Acknowledgments The GPS data are from Crustal Movement Observation Network of China (CMONOC). We express our thanks to all the Chinese participants in constructing the network and making the GPS measurements. We thank Fred Pollitz from U.S. Geological Survey (USGS) for providing the computer code and his source model of the Tohoku earthquake that we used in this study, and Yusuke Yokota from University of Tokyo for providing his joint inversion results of the rupture model. Comments by Associate Editor Roland Bürgmann, and two anonymous reviewers improved the paper. The figures are drawn using Generic Mapping Tools (Wessel and Smith, 1998). This research was supported by the National Scholarship of China for the leading author to visit the University of Missouri. M. L. acknowledges

166 J. Cheng, M. Liu, W. Gan, X. Xu, F. Huang, and J. Liu (a) 48 Water level value (m) (b) 46 44 4 11.1 11.3 11. 11.7 11.9 11.11 1.1 Time (year.month) 3 Water level value (m) 1 (c) Water level value (m) (d) 11.1 11.3 11. 11.7 11.9 11.11 1.1 Time (year.month) 3 1 11.1 11.3 11. 11.7 11.9 11.11 1.1 Time (year.month) Water level value (m) 1 11.1 11.3 11. 11.7 11.9 11.11 1.1 Time (year.month) Figure 9. Time series of the water-level changes at four observation sites: (a) AS, (b) YS, (c) WFD, and (d) CH (locations shown in Fig. 1). Data are from the Groundwater Monitoring Network of China Earthquake Network Center. The vertical lines mark the time of the Tohoku earthquake. support from U.S. National Science Foundation: Partnerships for International Research and Education (Grant 734), Chinese Academy of Sciences, National Natural Science Foundation of China (Grant 91141), and the Spark Program of Earthquake Science of China (Grant XH18). References Chen, W., W. Gan, G. Xiao, S. Liang, and C. Sheng (1). The impact of 11 Tohuku-Oki earthquake in Japan on crustal deformation of northeastern region in China, Seismol. Geol. 34, 4 439 (in Chinese with English abstract). Chen, Y., K. L. Tsoi, F. B. Chen, Z. H. Gao, Q. J. Zou, and Z. L. Chen (1988). The Great Tangshan Earthquake of 1976: An Anatomy of Disaster, Pergamon Press, Oxford, New York. Deng, Q., P. Zhang, Y. Ran, X. Yang, W. Min, and Q. Chu (3). Basic characteristics of active tectonics of China, Sci. China Earth Sci. 46, 36 37. Hayes, G. P. (11). Rapid source characterization of the 11 M w 9. Off the Pacific Coast of Tohoku earthquake, Earth Planets Space 63, 9 34. Hirose, F., K. Miyaoka, N. Hayashimoto, T. Yamazaki, and M. Nakamura (11). Outline of the 11 Off the Pacific Coast of Tohoku earthquake (M w 9.) Seismicity: foreshocks, mainshock, aftershocks, and induced activity, Earth Planets Space 63, 13 18. Hwang, J., H. Yun, H. Huang, T. Jung, D. Lee, and K. We (1). The 11 Tohoku-Oki earthquake s influence on Asian plates and Korean geodetic network, Chin. J. Geophys., 1884 1893 (in Chinese with English abstract). Kato, A., S. Sakai, and K. Obara (11). A normal-faulting seismic sequence triggered by the 11 Off the Pacific Coast of Tohoku

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