SCIENCE CHINA Earth Sciences. Preseismic deformation in the seismogenic zone of the Lushan M S 7.0 earthquake detected by GPS observations

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1 SCIENCE CHINA Earth Sciences RESEARCH PAPER September 2015 Vol.58 No.9: doi: /s Preseismic deformation in the seismogenic zone of the Lushan M S 7.0 earthquake detected by GPS observations LIU XiaoXia 1,2, WU YanQiang 3*, JIANG ZaiSen 2, ZHAN Wei 3, LI Qiang 2, WEI WenXin 2 & ZOU ZhenYu 1,2 1 Institute of Geology, China Earthquake Administration, Beijing , China; 2 Key Laboratory of Earthquake Prediction, China Earthquake Administration, Beijing , China; 3 First Crust Monitoring and Application Center, China Earthquake Administration, Tianjin , China Received December 16, 2014; accepted March 27, 2015; published online June 25, 2015 A continuous GPS array across the southern segment of the Longmenshan fault zone recorded the deformation during the process of the Lushan M S 7.0 earthquake that occurred on April 20, Such data can provide meaningful information regarding the dynamic evolution of crustal deformation in the seismogenic zone. Our studies have shown that the occurrence of the Wenchuan earthquake led to the loading of compressive and sinistral shearing strain on the southern segment of the Maoxian-Wenchuan fault, whereby the extrusion strain accumulated at a greater rate than before the Wenchuan earthquake. The strain time series in the seismogenic zone revealed that the principal compression strain rates decreased from west to east in the direction of N30 45 W. Furthermore, the area to the east of Beichuan-Yingxiu fault behaved as a zone of compressive deformation with obvious sinistral shearing deformation. The surface strain and the first shearing strain time series decreased with time, while the area to the west of the Beichuan-Yingxiu fault behaved as a zone of dextral shear deformation that increased with time. Furthermore, the regional deformation field before the Lushan earthquake showed that the rate of extrusion strain accumulation in the southern segment of the Longmenshan fault zone was obviously larger than before the Wenchuan earthquake. Moreover, the sinistral shearing strain accumulated in the area of the southern segment of the Maoxian-Wenchuan fault. Based on the above analysis, we consider that the eastward movement of the Bayan Har block increased considerably following the Wenchuan earthquake, which enhanced the accumulation of compression strain in the southern segment of the Longmenshan fault zone. Lushan M S 7.0 earthquake, GPS observations, GPS baseline time series, strain time series Citation: Liu X X, Wu Y Q, Jiang Z S, Zhan W, Li Q, Wei W X, Zou Z Y Preseismic deformation in the seismogenic zone of the Lushan M S 7.0 earthquake detected by GPS observations. Science China: Earth Sciences, 58: , doi: /s *Corresponding author ( chdqyw@126.com) A M S 7.0 thrust earthquake shocked the city of Ya an in Lushan County, Sichuan Province on April 20, 2013 ( the seismogenic fault of which belongs to the southern segment of the Longmenshan fault zone. The seismogenic structure and coseismic displacement of the Lushan earthquake were identified by studying the source rupture process, distribution of aftershock positions, coseismic displacement distribution, and by a geological investigation of the fault dislocations following the earthquake (Fang et al., 2013; Wu et al., 2013; Xu et al., 2013; Zhang et al., 2013). Furthermore, the difference in the features of the distribution in different segments of the coseismic displacement (Wu et al., 2013) and coseismic fault slip distribution (Wang et al., 2008; Shen et al., 2009), and the results of a geological investigation (Xu et al., 2008) and inversion of digital seismology (Zhang et al., 2008) of the Science China Press and Springer-Verlag Berlin Heidelberg 2015 earth.scichina.com link.springer.com

2 Liu X X, et al. Sci China Earth Sci Wenchuan earthquake, showed that the southern segment of Longmenshan fault zone did not rupture during the Wenchuan earthquake. Other studies have indicated that the MS8.0 Wenchuan earthquake, which occurred on May , accelerated the preparation process of the Lushan earthquake (Wang et al., 2013; Wu et al., 2013; Xu et al., 2013). Based on the above researches, a study of the evolving features of crustal deformation before the Lushan earthquake would increase the understanding of tectonic dynamical problems such as the preparation process and coseismic rupture of the Lushan earthquake, as well as the post-seismic effects of the Wenchuan earthquake. Following the Wenchuan earthquake, a GPS network of 10 stations, which has been recording data since September 2008, was established across the southern segment of the Longmenshan fault zone by the Institute of Earthquake Science, China Earthquake Administration. Five years continuous GPS data have recorded the deformation related to the post-seismic effects of the Wenchuan earthquake and the preparation process of the Lushan earthquake, which offers the opportunity to study the evolutionary features of crustal deformation before the Lushan earthquake. In this paper, we first analyze the continuous GPS data by calculating the baseline and strain parameter time series prior to the Lushan earthquake. Considering the obvious effects of the Wenchuan earthquake, we then analyze the regional GPS deformation field before the Lushan earthquake. Finally, we analyze the features of crustal deformation across the southern segment of the Longmenshan fault zone by combining the continuous and campaign GPS September (2015) Vol.58 No measurements. 1 GPS time series across the southern segment of the Longmenshan fault zone The continuous GPS measurements from the 10 stations were processed using GAMIT/GLOBK (Herring et al., 2010a, 2010b) and QOCA (Dong et al., 1998) software. Data of 30 Crustal Movement Observation Network of China (CMONOC) and 90 IGS stations which are distributed uniformly on the earth were involved in the GAMIT processing. To obtain the single-day solution, the parameters were set as follows: 30 s of sampling intervals, relax mode for the satellite orbit, the earth gravity field, solid tide and pole tide followed the IERS2003 model, and the global ocean tide followed the FES2004 model. During the process, the ionosphere-free linear combination phase observable (LC) and the GMF for troposphere mapping functions were used. Furthermore, a parameter of zenith delay was estimated every 2 h. Overall, 37 stable stations were set to frame stations during the overall adjustment and the coordinate time series of each station combined with ITRF2005 (Zhan et al., 2011). The location continuous GPS stations and some of their original coordinate time series were shown in Figure Time series of GPS cross-fault baselines The coordinate time series of the GPS stations are periodic Figure 1 Tectonic setting of the continuous GPS stations across the southern segment of the Longmenshan fault zone. (a) Tectonic setting of GPS stations; (b) original GPS coordinate time series.

3 1594 Liu X X, et al. Sci China Earth Sci September (2015) Vol.58 No.9 to some extent because of seasonal effects (Dong et al., 2002; Zhang et al., 2002; Wang et al., 2005). To avoid these effects, the analyze_tseri package of the QOCA software was used to eliminate the annual and semi-annual periodicities from the 3D coordinate time series. In the GPS data processing, the reference framework systemically affects the positioning results, the time series of the baselines between the GPS stations and the strain parameters calculated from several stations discussed in this paper, can effectively reduce these systemic effects and highlight the crustal deformation. The GPS baseline time series based on the annual and semi-annual information removed are shown in Figures 2 and 3. Figure 2 shows that all the baseline time series decrease quickly except LS04_LS10, and the shortening rate is about 8 12 mm/a with an average shortening rate in unit length of /yr, which is clearly faster than the rate of /yr before the Wenchuan earthquake (Jiang et al., 2009; Li et al., 2009). Station LS10 is located at the junction between the Xianshuihe and Longmenshan fault zones and its eastward movement is obviously smaller than those stations in the interior of the Bayan Har block, e.g. LS09, LS03 and LS08. Consequently, the shortening rate per unit length of baseline LS04_LS10 is significantly smaller than that of the other three baselines. This feature might indicate that the eastward movement of the Bayan Har block was prevented by the hard crust of the Sichuan Basin in the southern segment of the Longmenshan fault zone. Therefore, the eastward movement of those stations located near the southeast boundary was smaller than recorded for those stations in the interior of the Bayan Har block. The time series of baselines LS01_LS03, LS04_LS08 and LS04_LS10 clearly decrease nonlinearly with time, and their average annual shortening rates decrease year by year. The shortening rate of baseline LS01_LS03 decreases from 16 mm/yr in to 7 mm/yr in , that of baseline LS04_LS08 decreases from 12 mm/yr in to 8 mm/yr in , and that of baseline LS04_LS10 decreased from 7 mm/yr in to 1.7 mm/yr in However, the decrease of baseline LS04_LS09 behaves linearly, which indicates steady eastward movement of the Bayan Har block, because the station LS09 is located in the interior of the Bayan Har block. The time series of baselines across a single fault are showed in Figure 3. The average shortening rate of baseline LS05_LS06 across the Jiangyou-Guanxian fault and of LS06_LS07 across the Beichuan-Yingxiu and Maoxian- Wenchuan faults are both about mm/yr. The average crustal shortening rate per unit length is about /yr, and it has decreased year by year since The time series of baseline LS04_LS05 decreases nonlinearly with an annual shortening rate diminishing from 3.4 to 1.5 mm/yr, and an average crustal shortening rate per unit length reducing from to /yr. The shortening Figure 2 Time series of GPS baselines across the southern segment of the Longmenshan fault zone. The ordinates are the variation of baseline length, the gray circles represent the original observations, red lines represent the filtering results of the Least Squares Collocation method, thick blue lines represent the secular trend with the wavelet filter, and the number in lower left corner of each picture is the length of each baseline.

4 Liu X X, et al. Sci China Earth Sci September (2015) Vol.58 No Figure 3 Time series of GPS baselines across one single fault. The ordinates are the variation of baseline length, the gray circles represent the original observations, red lines represent the filtering results of the Least Squares Collocation method, thick blue lines represent the secular trend with the wavelet filter, and the number in lower left corner of each picture is the length of each baseline. rates of the baselines in Figure 3 are obviously smaller than in Figure 2. According to statistical results (Jiang et al., 2000), if the fault is not totally locked, the variation of cross-fault baselines should be larger than those that are not cross-fault, and the strain rate of small-scale baselines should be larger than large-scale baselines. However, the statistical results for the baselines across the southern segment of the Longmenshan fault zone reveal the converse. Because the movement of station LS09 is relatively faster than the others, the time series of baselines LS04_LS09 and LS07_LS09 decrease rapidly with annual shortening rates of and 8 12 mm/yr respectively. This might indicate that the eastward movement of the east Bayan Har block was obviously faster than that before the Wenchuan earthquake, and that it did not slow down until the occurrence of the Lushan earthquake. 1.2 ure 4 to determine the zone of intensive strain accumulation. Figure 4 shows that the principal compressive strain Time series of strain parameters Although cross-fault baselines can reflect relative movement between two sides of a fault, the stability of a baseline time series at a certain level does not mean the absence of relative motion; because the baseline involves only two points, it could indicate shearing motion instead. Based on high precision data, local strain allocation can be investigated by calculating the strain in triangles (Shen et al., 1996). The average principal strain rates in triangles, obtained from the time series of GPS points are shown in Fig- Figure 4 Principal strain rate in the southern segment of the Longmenshan fault zone. Triangles enclosed by the red dashes are calculating units of the strain time series and different cover colors represent different shearing features. Annotation: As there are large differences between different triangle strains, two scales are used in Figure 4, indicated by red and black arrows. Arrows pointing toward each other indicate compressive deformation; arrows pointing away from each other indicate extensional deformation.

5 1596 Liu X X, et al. Sci China Earth Sci dominates most triangles except for the two formed by LS02, LS03, LS08 and LS09, in which the principal extensional strain rate is larger. Figure 4 also shows that the southern segment of the Longmenshan fault zone was mainly dominated by compressive deformation. The principal compressive strain was higher in the area to the west of the Maoxian-Wenchuan fault than to the east. This might be the result of the strong lateral heterogeneity of the crust in this region (Lou et al., 2010). The principal strain rate was relatively low in the focal region of the Lushan earthquake, and the principal compressive strain increased gradually from east to west in the direction of the principal compressive strain (i.e., about N30 45 W). Sinistral shearing occurred on the eastern side of the Beichuan-Yingxiu fault, setting N45 E as the direction of the main fault zone (hereinafter the same), dextral shearing occurred at the northern and southern ends of the western side, and the area formed by LS02, LS08, LS09 and LS07 also underwent sinistral shearing deformation. The principal compressive strain rate increased gradually from /yr on the eastern side to /yr on the west. To analyze the evolutionary characteristics of crustal deformation in different regions further, the time series results of the first shear strain (Jiang et al., 2003) and surface strain September (2015) Vol.58 No.9 which typifies the compressive and expansive characteristics are shown in Figure 5. The first shear strain can reflect shear deformation on faults with strikes of N45 W or N45 E according to its formula R1 e n / 2.0. Thus, this strain parameter can effectively reflect the shear deformation features of the southern segment of the Longmenshan fault zone as its fault strike is close to N45 E. The regions of sinistral shearing shown in Figure 5(a) and (b) lie on the east of the southern segment of the Longmenshan fault zone containing the Jiangyou-Guanxian fault. The time series results show that the cumulative first shear strains reach and , respectively, and that the sinistral shear deformation in the southern side of the focal region of the Lushan earthquake (Figure 5(a)), is slower than that in the focal region (Figure 5(b)). The time series of surface strain shown in Figure 5(a) decreases rapidly, while that illustrated in Figure 5(b) decreases gradually. Figure 5(c) and (d) shows dextral shear deformation occurred with gradual acceleration in regions located to the northwest of the Beichuan-Yingxiu fault. The cumulative first shear strains reach and and the average strain rates are significantly higher than those shown in Figure 5(a) and (b). The surface strain results presented Figure 5 Time series of first shear strain and area strain around the southern section of the Longmenshan fault zone. (a) Strain time series in the area formed by LS01, LS04, LS05, LS06 and LS02; (b) strain time series in the area formed by LS04, LS05, LS06 and LS10; (c) strain time series in the area formed by LS02, LS03, LS09 and LS08; (d) strain time series in the area formed by LS06, LS07, LS09, and LS10. The plus value of the first shear strain typifies sinistral shear, while the minus value typifies dextral shear. Thick black lines represent the secular trend with the wavelet filter.

6 Liu X X, et al. Sci China Earth Sci in Figure 5(c) show that no obvious compressive or expansive deformation occurred in the region formed by LS02, LS03, LS09 and LS08, while the first shear strain indicates gradual trend of acceleration in dextral shear in the northeast direction. This is manifested by the rapid contraction in the area shown in Figure 5(d), and the acceleration of dextral shear strain and compressive strain during The continuous GPS observations show that the southeast extrusion deformation oriented the deforming mode of the southern segment of the Longmenshan fault zone, and a regional sinistral shear deformation mode on the eastern side of the fault zone, which agrees with the inversion results of the coseismic slip distribution by Jiang et al. (2014). It also shows dextral shear deformation mode on the western side of the fault zone. The results shown in Figures 4 and 5 indicate that the extrusion strain accumulation in the focal region of the Lushan earthquake might be higher than on the western side of the fault zone, which lead to the gradual decrease from west to east of the principal compressive strain rate (maintained almost vertical to the fault) in recent years. The statistical results of Jiang et al. (2000) showed significant correlation between the strain rate in areas of non-uniform deformation and the calculated scale, as for larger computing scale lower values of strain would be obtained. Contrary to this law, the calculating scale used in Figure 5(d) is larger than in Figure 5(b), but the average rate of shear strain and surface strain are also slightly larger. Furthermore, Figure 5 also shows a gradual decrease of regional strain rate from the area in the west side toward the near-field of the Longmenshan fault zone, which to some extent indicates that the level of strain accumulation in the Figure ). September (2015) Vol.58 No near-field of the southern segment of the Longmenshan fault zone was fairly high. 2 Regional crustal deformation fields before the Lushan earthquake As the continuous GPS stations above distribute in small to medium spatial scale across the fault zone, they could reflect the evolution of near-field deformation a few years prior to the occurrence of earthquakes. The CMONOC regional stations began observing in 1999, and four stages were undertaken up until Campaigns involving an extension of the CMONOC network have been conducted in 2009 and 2011, and a network comprising 1000 newly built regional sites and 233 continuous GPS stations has been established (Li et al., 2012). The characteristics of large spatial scale deformation can be reflected by the regional GPS deformation field. Then, by analyzing the GPS deformation in different periods, the evolutionary characteristics of crustal deformation before the Lushan earthquake can be well comprehended. In this paper, we present the processed velocity results of regional GPS data observed during and in and around the Longmenshan fault zone using the GAMIT/GLBOK (Herring et al., 2010a, 2010b) and QOCA (Dong et al., 1998) software packages. The specific data processing methods and processes can be found in the literature (Niu et al., 2005; Jiang et al., 2009; Wu et al., 2013). Figure 6 shows that no obvious crustal deformation occurred in the near-field area of the Longmenshan fault zone GPS velocity field before the Lushan earthquake referring to the Huanan block. (a) Results in ; (b) results in (Wu et al.,

7 1598 Liu X X, et al. Sci China Earth Sci September (2015) Vol.58 No.9 before the Wenchuan earthquake. However significant different movements occurred among the stations located inside the Bayan Har block, away from the fault zone before the Wenchuan earthquake. This feature is consistent with the findings of studies on the characteristics of strain accumulation before the Wenchuan earthquake (Zhang et al., 2008; Jiang et al., 2009; Li et al., 2009). Therefore, this might suggest that the Longmenshan fault zone was locked in the near-field, while experiencing far-field loading before the Wenchuan earthquake. Figure 6(b) shows significant tensile strain release during in the Bayan Har block, which is the hanging wall of the northern section of the Longmenshan fault zone. This was manifested as faster movement in the SEE direction of the measuring stations near the fault than those inside the block, and the significant non-continuous differential movement between near-field sites. What significant differences is that the southern segment of the Longmenshan fault zone exhibited characteristics of extrusion strain accumulation, indicating the continuation of the locked state (Wu et al., 2013). The features of the extrusion and shear strain accumulation in the focal region of the Lushan earthquake were studied through the profile results presented in Figure 7. Because the GPS velocity field in period was observed after the Wenchuan earthquake, the region near the epicenter was affected significantly by post-seismic adjustment. Furthermore, the number of GPS sites increased significantly since 2009 involving 1000 regional sites of CMONOC extension and the 10 newly built continuous stations around the southern segment of the Longmenshan fault zone. Therefore, the range of the profile selected for was slightly smaller than that for the to avoid the areas significantly affected by the Wenchuan earthquake and to ensure that it reflects the deformation state of the southern section of the Longmenshan fault zone. In addition, the responses in the southern segment of the Longmenshan fault zone, associated with the coseismic processes of the Wenchuan earthquake, were analyzed through the coseismic displacement profile shown in Figure 7 (Wei et al., 2014), in which the direction of the parallel component is the N45 E strike of the Longmenshan fault zone and the vertical component is S45 E. Figure 7 shows the dextral shear and extrusion strain accumulation in the southern segment of the Longmenshan fault zone prior to the Wenchuan earthquake. It can be seen that the rate of dextral strain is higher than that of the extrusion strain, which is consistent with the research results of Jiang et al. (2009) and Li et al. (2009). Figure 7(a) shows that the crustal shortening rate was 5.0 mm/yr (strain rate was about /yr) within a range of 350 km to the west of the Maoxian-Wenchuan fault. During the Wenchuan earthquake, the SE displacement in the northwestern plate of the southern segment of the Maoxian-Wenchuan fault was about 80 mm with no decrease over the range of 300 km. The SE displacement in the southeastern plate of the southern segment of the Maoxiano-Wenchuan fault was significantly smaller than in the northwestern plate with no response features of fault dislocation and strain relief movement. Furthermore, the amount of crustal shortening in the fault zone and over a range of 90 km to the east reached at least 100 mm, behaving as continuous deformation, and indicating the significant load of extrusion. The parallel component of the profile in Figure 7(d) shows different response characteristics of crustal deformation during the Wenchuan earthquake. The largest dextral shear movement was in the area within 20 km of the fault (amount of displacement reached 79.8 mm), and it behaved similarly to strain release on the hanging wall of the fault. Apart from this, it displayed continuous sinistral shear strain in the area to the east of the fault with 25 mm of sinistral strain within the range of 90 km. The Maoxian-Wenchuan fault marks the main boundary of the different deformational response of the vertical and parallel deformation components to the fault in the southern section of the Longmenshan fault zone during the Wenchuan earthquake. The direction of the principal tectonic stress was approximately perpendicular to the main fault zone, therefore, the continuous extrusion deformation in the direction vertical to the fault (Figure 7(c)), mainly reflects the significant load of the extrusion strain accumulation in this segment during the rupture process of the Wenchuan earthquake, whereas the deformation responses of other faults to the east of the fault zone were relatively weak. The profile results from , shown in Figure 7(e) and (f), include data from the 10 continuous GPS stations established by Institute of Earthquake Science, China Earthquake Administration. Data from the 10 GPS stations have high reliability because of the continuous observation mode used, and some stations have been marked in Figure 7(e). The results presented in Figure 7(e) and (c) show extrusion strain accumulation in the southern section of the Longmenshan fault zone, especially from the results of the continuous stations. And the crustal shortening rate of 5 mm/a within the range of 90 km was larger than that before Wenchuan earthquake. The results shown in Figure 7(f) and (d) show obvious dextral shearing deformation along the southern segment of the Maoxian-Wenchuan fault at 7.0 mm/yr. However, there was sinistral strain accumulation in the area to the east of the fault with a deformation amount of about 4 mm within the range of 110 km. According to the profile results presented in Figure 7, the rate of extrusion strain accumulation in the southern segment of the Longmenshan fault zone was about before the Wenchuan earthquake. In the focal region of the Lushan earthquake, the extrusion and sinistral strain caused by the Wenchuan earthquake were about and about respectively, while the rates of extrusion and sinistral strain were about and /yr, respectively. The above results indicate that the Wenchuan earthquake strengthened the extrusion strain in the southern

8 Liu X X, et al. Sci China Earth Sci September (2015) Vol.58 No Figure 7 GPS displacement profile results in the southern segment of the Longmenshan fault zone. Conventions: Negative slope indicates extrusion or dextral shear, and Figure 7(c) to (f) quote from Wei et al. (2014). section of Maoxian-Wenchuan fault and its eastern region with the extrusion strain rate being larger than that before the Wenchuan earthquake. In addition, the Wenchuan earthquake caused sinistral shear strain in the eastern region of the southern segment of the Maoxian-Wenchuan fault, which was maintained during Discussion The time series of the baselines across the entire Longmenshan fault zone, i.e., LS01_LS03, LS04_LS08 and LS04_LS10, and those across one single fault, i.e., LS04_LS05, LS05_LS06 and LS06_LS07, showed nonlinear shortening deformation. The time series of baselines LS04_LS09 and LS07_LS09 showed rapid shortening deformation as LS09 is located in the interior of the Bayan Har block. The time series of surface strain in the focal region of the Lushan earthquake showed nonlinear shortening characteristics which indicated that the Bayan Har block continued to move into the Longmenshan fault zone at a greater rate than before the Wenchuan earthquake. The time series, which showed a gradual decrease might indicate that the strain accumulation increased gradually in the focal region of the Lushan earthquake, which indicates approach of a seismic hazard. Otherwise, inevitably, the continuous GPS data were affected by the post-seismic effects of the Wenchuan earthquake as they were observed after the earthquake event. According to the current research results, the physical mechanism of post-seismic deformation modes

9 1600 Liu X X, et al. Sci China Earth Sci September (2015) Vol.58 No.9 caused by large earthquakes are mainly constituted by afterslip, viscoelastic medium relaxation in the lower crust and upper mantle and block movements. Afterslip means crustal deformation caused by fault slip over the scale of months after the main shock, which generally exhibits logarithmic decrement (Marone et al., 1991). The continuous GPS data in this study were observed four months after the Wenchuan earthquake, and the main deformation characteristics identified in this paper did not occur within a one year time scale, therefore, the effects of afterslip should not affect the recognition of the characteristics of GPS continuous deformation in the southern section of the Longmenshan fault zone. The effect of viscoelastic relaxation after the earthquake has received much attention because its overall impact gradually increases with the elapsed time (Rice and Gu, 1983; Pollitz et al., 2001; Xiong et al., 2010). The software package PSGRN/PSCMP has been used to calculate the effect of viscoelastic relaxation by Wang et al. (2006). However, the calculations depend on a variety of parameters, and the rupture slip distribution of the Wenchuan earthquake calculated by either seismological methods (Wang et al., 2008) or the deformation data inversion method (Shen et al., 2009) in the southern section of the Longmenshan fault zone were relatively small with low precision. Thus, accurate assessment of the viscoelastic relaxation effect cannot be obtained based on current research. In addition, regional GPS data showed no extrusion strain release in the southern section of the Longmenshan fault zone caused by the Wenchuan earthquake. Therefore, the earthquake afterslip, viscoelastic relaxation and eastward movement of the Bayan Har block after the Wenchuan earthquake were conducive to the extrusion strain accumulation in the focal region of the Lushan earthquake. Furthermore the specific impact mode to the southern section of the fault zone requires further study. The coseismic and post-seismic GPS deformation fields in Figure 7 show some dextral shear strain release in the southern section of the Longmenshan fault zone caused by the Wenchuan earthquake. However, the rate of continuous extrusion deformation in the SE direction was larger than before the Wenchuan earthquake. Due to the strong compressive background of the Longmenshan fault zone, the deformation adjustment are more likely occurred along the direction of the relatively weak shear paralleling to the fault, which could result in this characteristic. Thus, the distribution of the observation points revealed that the fault segment remains a zone of continuous deformation. The GPS coseismic displacements mainly reflect the elastic deformation of the upper crust, and therefore, the observed eastward movement of the Bayan Har block, which accelerated overtime, might be affected by deep tectonic deformation. There are many tectonic models of the Longmenshan fault zone (Hubbard and Shaw, 2009; Li et al., 2010; Yao et al., 2012), however, it is difficult to define how the deep tectonics affect crustal deformation clearly due to the current limitations in the accuracy and density of the GPS observations. In subsequent work, we will study the observed crustal deformation in combination with the deep tectonics. 4 Conclusions The following findings were identified by analyzing continuous GPS observations and regional GPS deformation field prior to the Lushan earthquake: (1) The time series of GPS cross-fault baselines showed that the eastward movement of the Bayan Har block accelerated the deformation of the southern segment of the Longmenshan fault zone after the Wenchuan earthquake. However, the internal extrusion rate of the fault was relatively slow, especially in the seismogenic zone of the Lushan earthquake, which revealed a strong strain accumulation feature. All these results indicate the increasing risk of an earthquake. (2) The strain time series showed that extrusion deformation dominated the southern segment of the Longmenshan fault zone with an obviously larger strain rate than before the Wenchuan earthquake, while the near-field strain rate was smaller than that in the area to the west of the Maoxian-Wenchuan fault. The shear deformation modes on both sides of the fault zone were opposite. The shear deformation on the eastern side of the major fault with strike of N45 E exhibited sinistral shear, while dextral shear occurred on the western side, although with local differences. The extrusion rate gradually increased from east to west. (3) The regional GPS deformation field before the Lushan earthquake indicated that the Wenchuan earthquake accelerated the preparation process of the Lushan earthquake. The extrusion component in the SE direction on the southern section of the Maoxian-Wenchuan fault exhibited features of continuous deformation with an increasing extrusion rate, caused by the Wenchuan earthquake. The coseismic displacement of the Wenchuan earthquake showed that the distribution of the extrusion deformation was continuous in the southern segment of the Longmenshan fault zone, whereas features of strain release were apparent in the middle and northern segments, which indicated a lockout state of the southern segment of the Longmenshan fault zone in recent years (Wu et al., 2013). Furthermore, the comparison of different scales in this paper showed that the southern segment of the Longmenshan fault zone was in a state of strong extrusion strain accumulation. This work was supported by the National Natural Science Foundation of China (Grant Nos , ), the Basic Research Project of Institute of Earthquake Science of China Earthquake Administration (Grant No. 2014IES010101). We sincerely thank Professor Wen XueZe for his careful guidance and the peer reviewers for their advanced recommendations.

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