The tectonic motions in the Tibetan plateau are related to the Himalayan orogeny where exists mountain-perpendicular compressive stresses [1]. The northeastward motion of India is partitioned by strike– slip and trust faults in the plateau. Furthermore, a significant portion of the lateral slip on these faults is transferred to thrust faults in the margins of the plateau [2]. Large faults lie almost near the NW or NWW direction in the Tibetan plateau [3,4]. The large fault motion, whereas revealed various configurations on the eastern margin of the Tibet Plateau as shown in Figure 1 [5]. Many faults turn the strike direction to near N-S or NE-SW direction. The Jinshajiang fault, Nujiang fault and the Anning river fault, these strike-slip faults all extend along the N-S direction [6,7]. The strike-slip type Xianshuihe fault lies along the NWSE direction [8]. The Jinpingshan-Yulongxueshan fault extends along in the NE-SW direction. The Longmenshan fault also extends along the NE-SW direction, bending somewhat along the northeastern edge of the Sichuan basin. Such a special strike direction of the fault may imply that the Sichuan basin obstructs the southeastward movement of the Tibetan plateau. Regional blocks cut by the above large faults also reveal tectonically complexity as illustrated in Figure 1. The eastern margin of the Tibetan plateau is on the western border of the South China block. The southern segment of the famous North-South Seismic Belt in China (NSSB) likely coincides with the boundary between the eastern margin of the Tibetan plateau and the South China block [9]. The events in the southern segment of NSSB occurred almost in the eastern margin of the Tibetan plateau and Yunnan region indeed. A few events occurred in the Sichuan basin in the South China block. The serious damages, 2008 M8 Wenchuan event and 2013 Lushan M7 event occurred in the Longmenshang fault in the study margin.

Figure 1: Tectonic outline in the eastern margin of the Tibetan planet and its surroundings. I) Longmenshan block. II) East of the Sichuan basin. III) Sichuan-Yunnan rhombic block. IV) Dianzhong block. V) Tenchong-Baoshan block. VI) East of Yunnan block. Red triangle: Tengchong volcano. Blue thick line: main fault.

The tectonic deformation features of the India-Eurasia continental collision zone reveal with thrust compression, lateral extrusion and clockwise rotation. However, in the middle southern plateau, there is subregion with a series of NS striking normal faults [10,11]. Some interested results of seismotectonic motion have been reported [12-15]. Many normal faulting type events concentrated on the high altitude region of the Tibetan plateau, i.e., in the region from the Gangdise mountains in the south to the Kunlun mountains in the north [16-18]. However, the thrust-faulting type event predominated the focus mechanism of earthquake occurrence in low altitude regions surrounding the Tibetan plateau [19]. The seismotectonic motions also show different regional characteristics in the eastern margin of the Tibetan plateau and its surroundings [20,21]. Most of events revealed the strike-slip faulting type there. Few thrust-faulting type events were reported in the eastern margin before the 2008 Wenchuan M8 event (31.1°N, 103.3°E). The serious disaster Wenchuan M8 event and the aftershocks formed a new series of events with thrust-faulting regime in the Longmenshan fault. The new events provided substantial amount of reliable data to investigate the tectonic stress field in the eastern margin of the Tibetan Plateau further [22].

In general the seismotectonic motions are closely related to the tectonic stress. The tectonic stress field in the eastern margin of the Tibetan plateau is related to the collision between the Eurasian Plateau and Indian plates [23,24]. The mountain-perpendicular compressive stresses exists along the Himalayas [1]. The eastward escape hypothesis for the lithosphere usually was reported as the tectonically dynamic causative mechanism of earthquake in the eastern margin of the plateau [25,26]. The seismogenic stress fields were quite complicated in the eastern margin of the Tibetan plateau and the surroundings indeed. Some P-axes aligned NW-SE direction in the eastern margin of the Tibetan plateau although the tectonic forces caused a wide distribution of P-axes near NE-SW direction in most regions of the Tibetan plateau [16]. The temporal variations of seismicity in the eastern margin of the Tibetan plateau were not synchronous with that of either vicinity seismic zone, i.e., the Tibetan plateau or the Southern China block (Yangtze block). It was difficult to find the temporal relativity of seismicity between the southern segment of NSSB and either vicinity seismic zone based on statistic analyses of the seismic temporal series variation [27].

In the present analysis, the complicated seismotectonic motion and earthquake generating stress field in the eastern margin of the Tibetan plateau were further investigated on view of the tectonics on the boundary between the Tibetan plateau and the South China block employing seismicity and focal mechanism solution parameters. Specially, the extensional seismotectonic motions in the eastern margin and the dynamics are discussed in detail.

Focal mechanism solutions of 239 earthquakes with magnitude equal to or greater than M5 on the eastern margin of the Tibetan plateau and its vicinities during the period between 1933 and 2008 were analyzed in this study. 85 fault plane solutions of events were determined form the distribution of the polarities of initial P-wave motion on the focal hemisphere by the authors referring to data from the Bulletin of the International Seismological Center (ISC). Another 77 solutions determined by P-wave first motions were selected from the other reports referring to data from the Chinese Seismic Net and the Bulletin of the International Seismological Center (ISC) using P-wave first motions [28-30]. All the solutions used in the analysis were well selected based on the contradictory rate of the plus to minus signs for the first polarities of P-wave motions. The rest of 77 CMT solutions were (determined by the Centroid Moment tensor solutions) selected from the Harvard University and USGS. The CMT solutions for events with magnitude equal to or greater than 5.0 from 1976 to 2003 were also used in this study [31,32]. Both of mechanisms published and derived from authors were employed to investigate the seismotectonic motion. The events used in this analysis occurred in the crust. The depths of most events are less than 70 km in the analysis. The earthquake catalogue for seismicity is drawn from the seismic networks of the Seismic Net Center of the China Earthquake Administration.

T?he faulting motion regime of the focal mechanism solution shows the regional characteristics of the seismotectonic motion. In order to investigate the seismotectonic motions, Figure 2 shows the focal mechanism solutions of the equal area projection of events in the eastern margin region of the Tibetan plateau and its surroundings. The occurrence regimes of events revealed the distinguished regional characteristics although the mechanism diagrams of strike-slip faulting type events widely appeared in many areas in Figure 2. T?he events were almost strike-slip type ones in the northwestern segment of the Xianshuihe fault, whereas some events of strike-slip faulting with normal faulting component occurred in the southeastern segment of the Xianshuihe fault besides strike-slip ones as shown in Figure 2 e.g., the largest one along the Xianshuihe fault, the 1955 M7 Kangding event was the event of strike-slip faulting with normal faulting component occurring in the southeastern end of the Xianshuihe fault in Figure 2.

Figure 2: Distribution of the fault plane solution diagrams projected on the equal area in low hemisphere in the eastern margin of the Tibetan plateau and its surroundings. Green dash circle: N-S extension normal faulting type event region southwest of the Xianshuihe fault (SWXSH-NSNF), ranging about 98º-102ºE, 29º-32ºN; Green dash ellipse: E-W extension normal faulting type event region east of the lower reach of the Jinshajiang fault (ELRJSJ-EWNF), ranging about 98º-102ºE, 26º-29ºN. Diagrams in the legend in the lower right corner, N: normal faulting type event; T: thrust faulting type event; S: strike-slip faulting type event.

While many normal faulting type events occurred in the region south of the Xianshuihe fault, extending southward for hundreds of kilometers along the eastern side of the Jinshajiang river fault in Figure 2. These normal faulting type events might be taken as two groups based on the faulting dislocation regimes indeed. The first, the normal faulting type events concentrated on the region southwest of the Xianshuihe fault and east of the upper reach of the Jinshajiang river ranging about 99°E-102°E and 29°N-31°N, marked with the green dash circle in Figure 2, for hundreds of kilometers in the longitudinal and latitudinaldirections. Both of the strike directions of the fault planes and the fault auxiliary planes of focal mechanism solutions of the normal faulting type events lay likely in the E-W and near E-W directions. It showed that the dislocations of these normal faulting type events extended along or near the N-S direction in the region southwest of the Xianshuihe fault. T??he north-south components of fault slips were all great. This region is taken as the N-S extensively normal faulting type event region southwest of the Xianshuihe fault (SWXSH- NSNF) marked with the green dash circle in Figure 2. The fifteen events occurring in the SWXSH-NSNF were all normal faulting type ones during the study period. Three events among these fifteen were equal to or greater than M6.0. The greatest one was the April 15, 1998 Litang, Sichuan Province M6.4 event (29.98°N, 99.24°E). The normal fault dislocating extensively along or near N-S direction in the normal faulting type event region southwest of the Xianshuihe fault as mentioned above, is a newly notable property on the seismotectonic motion. The property implies that the earthquake rupture regimes are predominated by the normal faulting type events with the N-S extensive dislocation in SWXSH -NSNF in the eastern margin of the Tibetan plateau. It is probably different from the traditionally eastward escape movement in the eastern Tibetan plateau.

Another normal faulting type event region marked by a green dash ellipse in Figure 2 was located in the region east of the lower reach of the Jinshajiang fault ranging about 99°E-101°E and 26°N-28°N, for hundreds of kilometers in the longitudinal and latitudinal direction. Both of the strike directions of the fault planes and their fault auxiliary planes of the normal faulting type events were almost along or near the N-S direction. It implied that the dislocations of the normal faulting type events extended along or near the E-W direction in the region east of the lower reach of the Jinshajiang fault. The E-W components of fault slips were all great. The earthquake rupture regime was predominated by the normal faulting type event that the dislocation extended along the E-W direction although the large tectonic faults with N-S strike direction lay on the western and eastern side of the normal faulting type event region. This region is taken as the E-W extensional normal faulting type event region east of the lower reach of the Jinshajiang fault (ELRJSJ-EWNF) marked by a green dash ellipse in Figure 2. The ELRJSJ-EWNF is particularly located near joint region between the Jinshajiang and Jinpingshan-Yulongxueshan faults in Figures 1 and 2. The seventeen normal faulting type events occurred in the region east of the lower reach of the Jinshajiang. Five of them were equal to or greater than M6.0 and the greatest one was the February 3, 1998 Lijiang, Yunnan Province M7.0 event (27.3°N, 100.21°E) in ELRJSJEWNF. The regimes of normal faulting dislocations along the E-W direction in the ELRJSJ- EWNF are quite different from those along the N-S direction in the SWXSH-NSNF although the ELRJSJ-EWNF is on the southern border of SWXSH- NSNF in Figure 2.

The Longmenshan fault with NE-SW strike direction crosses to the Xianshuihe fault with NW-SE strike direction in the eastern margin of the Tibetan plateau north of 28°N in Figure 1. The earthquake faulting regimes are obviously different each other in Figure 2. The majority of events occurring in the Longmenshan fault were the thrust faulting type ones although few thrust-faulting type events were reported in the eastern margin before the 2008 Wenchuan M8 event. The Wenchuan M8 event in the Longmenshan fault and its strong aftershocks were thrust faulting events except only a strike-slip faulting one [33]. The 2013 Lushan M7 event was also a thrust one there. Moreover some thrust faulting type events occurred in the Longmenshan block in Figure 1, the northwestern neighborhood of the Longmenshan fault in Figure 2. The thrust faulting dislocations played an important role for the regimes of earthquake occurrence in the Longmenshan fault and its northwestern neighborhood.

As mentioned above, the thrust faulting type events occurred almost in the northeastern region in the eastern margin of the Tibetan plateau. The normal faulting type events occurred almost in the western region of the eastern margin in Figure 2. Except the two normal faulting zones and one thrust faulting zone listed above, the events almost revealed strike-slip faulting type in other zones in Figure 2, i.e., the wide Yunnan region and the southwestern edge of the Sichuan basin.

The occurrence regimes of events showed evidently regional properties in the eastern margin of the Tibetan plateau as mentioned above. The regime of earthquake occurrence or seismotectonic motion property is likely related to the regional tectonic stress field. The eastern margin of the Tibetan plateau is located in the boundary between the Tibetan plateau and the South China block. The seismogenic stress field in and around the boundary was usually investigated in view of the plate tectonics. Figures 3 and 4 showed the distribution of horizontal projections for the principal compressive stress P-axes and extensional stress T-axes in the study region.

Figure 3: Distribution of the P-axis horizontal projections of events in the eastern margin of the Tibetan plateau and its surroundings. Solid segments: P-axis horizontal projections. Longitudinal black dash line: boundary of the P-axes’ directions between the western and eastern regions in the study region. E1, E2 and E3: the stress subregions in the eastern region. W1 and W2: the stress subregions in the western region.

Figure 4: Spatial distribution of the T-axis horizontal projections of events in the eastern margin of the Tibetan plateau and its surroundings.

Regional stress field related to earthquake faulting type

With respect to the tectonic stresses for the normal faulting type event region southwest of the Xianshuihe fault ranging about 99°E-102°E and 29°N-31°N , the green circle region (SWXSH-NSNF), the P-axes of normal faulting events aligned almost in the ENE-WSW or near E-W direction in Figure 3. The major T-axes of normal faulting events aligned almost conformably in the NNW-SSE or near N-S direction with great horizontal component projection in Figure 4. In order to investigate completely the spatial orientation of stress axes of the normal faulting type events Figure 5a, 5b showed the distributions of the projections of P- and T- axes on the vertical profile AB which was the diameter of the green dash circle along the N-S orientation from the north (A) to south (B) in SWXSH- NSNF in Figure 2, respectively. It could be seen that most P-axes aligned almost along the vertical direction with great vertical components. Most T-axes lay horizontally on the profile along N-S direction with great horizontal components. These characteristics of stress field showed the seismogenic mechanism of the normal faulting type events dislocating extensively along the N-S direction in the region southwest of the Xianshuihe fault in Figure 2.

Figure 5: Distribution of the projections of P- (a) and T- axes (b) on the profile AB in the N-S extensional normal faulting type event region southwest of the Xianshuihe fault (SWXSH-NSNF) marked with the green dash circle region in Figure 2. AB is the diameter of the green dash circle in Figure 2 from the north to south. A: northern end of the diameter. B: southern end.

In the normal faulting type event region east of the lower reach of the Jinshajiang river (ELRJSJ-EWNF) ranging about 99°E-101°E and 26°N-28°N marked with the green dash ellipse in Figure 2, the horizontal component of P-axes of normal faulting events were very little in Figure 3. The horizontal projections of the T-axes of normal faulting type events were generally great as shown in Figure 4. Major T-axes orientated near NNE-SSW direction. Figure 6 showed the distributions of the projections of P- and T- axes on the vertical profile CD. CD is the axis of the green dash ellipse along the E-W orientation from the west (C) to east (D) in ELRJSJ-EWNF in Figure 2. The P-axes aligned almost near the vertical direction with great vertical components. Most T-axes lay horizontally on the profile along E-W direction with great horizontal components. The characteristics of the stress field identified the dynamic mechanism of the extensional dislocation motion along the E-W direction of normal faulting type events in the region east of the lower reach of the Jinshajiang river.

Figure 6: Distribution of the projections of P- (a) and T- axes (b) on the profile CD in the E-W extensional normal faulting type event region east of the lower reach of the Jinshajiang fault (ELRJSJ-EWNF) marked with the green dash ellipse region in Figure 2. CD is the axis of the green dash ellipse in Figure 2 from west to east. C: western end of the axis. D: eastern end.

The orientation of the P-axes in a general ENE-WSW direction in the northwestern segment of the Xianshuihe fault was different from those in a general E-W or WNW-ESE direction in the southeastern segment in Figure 3. The T-axes turn the general NW-SE or NNWSSE direction in the northwestern segment into the near NNE-SSW direction in the southeastern segment of the Xianshuihe fault in Figure 4. The horizontal components of P- and T- axes, however were great. This was the typical mechanism of stress field of strike-slip faulting type event. In the thrust faulting type event region, the Longmenshan fault and its northwestern neighborhood, the horizontal components of P-axes of thrust faulting events were great and the P-axes aligned almost in the WNW-ESE or NW-SE direction in Figure 3. The T-axes of thrust faulting events aligned almost in the NNE-SSW or NE-SW direction with little horizontal component in Figure 4. Such stress characteristics showed the dynamic mechanism of the dislocation of thrust faulting type events. The azimuth of P-axis of the Wenchuan event ranged 290° to 302°, dip angle ranged about 6° to 8°, little [19,30]. The little dip angle of P-axis showed the stress field characteristic of thrust faulting type event occurrences in the Longmenshan fault and its northwestern neighborhood as shown in Figure 2 [34].

Stress field difference between the eastern and western regions

The P-axes’ azimuths in the eastern region looked widely divergent from those in the western regions on the whole in Figure 3. Most of the compressive stress P-axes aligned in a general NW-SE direction in the eastern region. The majority of P-axes in the western region, however, aligned along a general NE-SW direction in Figure 3. The boundary of the P-axis orientation between the eastern and western regions lay roughly along longitude 100°E in the region north of the about latitude 28°N and about longitude 101°E in the region south of the about latitude 28°N, displayed by a longitudinal black dashed line in Figure 6. In order to study the regional stress characteristics further, the study region was zoned into the five subregions in Figure 3. Figure 7 showed statistic rose diagrams of the azimuths of P- and T-axes of events in the eastern subregions Figure 6. The predominant direction of P-axes was about in the NNW-SSE direction in the southern subregion E3, and WNW-ESE direction in the northern subregion E1 in Figures 3 and 7, respectively. It looks as though the P-axes turn gradually the orientations from the NNW-SSE direction in the southern region E3 to the WNW-ESE direction in the northern region E1 through region E2. Many P-axes aligned near the E-W direction in subregion E2 in Figure 3 east of the normal faulting type event regions of SWXSH-NSNF in Figure 2. Figure 8 showed statistic diagrams of the azimuths of Pand T-axes of events in the western subregions of the study region. Similarly, P-axes turn gradually the orientations from the NNE-SSW in the southern region W2 to the ENE-WSW in the northern region W1. i.e., the N-S components of P-axis horizontal projections decreased and the E-W components increased from the south to the north in Figure 3. The horizontalprojections of P-axes got great E-W components in the region north of the 28°N and great N-S components in the Yungui plateau south of the 28°N. The predominant orientations of P-axes in the eastern region and western regions seem to cross into an obtuse angle in the region north of about 28°N and cross into a sharp angle south of about 28°N as shown in Figures 3 and 7. The regional characteristics of the T-axes’ orientations in Figure 4 were investigated similarly referencing the stress longitudinal boundary between the eastern and western regions in Figure 3. In the western region of Figure 4, the T-axes’ orientations aligned about in the WNW-ESE direction in the southern part (south of 28ºN) and NNW-SSE direction in the northern part (north of 28ºN). In the eastern region, the T-axes’ orientations aligned about in the ENE-WSW direction in the southern part and NNE-SSW direction in the northern part in Figure 4.

Figure 7: Statistical rose diagrams of the azimuths of P- axes in subregion E1, E2 and E3 in the eastern region in the eastern margin of the plateau in Figure 3 and the corresponding T-axes of the above subregions in Figure 4 respectively.

In another view, the majority of T-axes likely had great horizontal component along near N-S direction in the region north of about 28°N and great horizontal component along near E-W direction in the region south of 28°N. Referencing results in Figure 3, the belt region along about latitude 28°N looks a latitudinal boundary for the orientation changes of P- and T-axes between the north and south parts of the study region, which is also near the southeastern boundary of the Tibetan plateau as shown in Figure 1.

The findings of seismotectonic motions in the eastern margin of the Tibetan plateau in the present analysis showed that the normal faulting type event region existed southwest of the Xianshuihe fault (SWXSHNSNF) where the normal faulting type events dislocated extensively along the N-S direction. The above observed normal faulting type event region indicate that there are extensional motions along the N-S direction besides the eastward tectonic escape in the eastern margin of the Tibetan plateau [35]. The another normal faulting type event region, east of the lower reach of the Jinshajiang river (ELRJSJ-EWNF) where the dislocations of normal faulting type events extended along the E-W direction. The ELRJSJ-EWNF is on the southern border of the SWXSH- NSNF. The two normal faulting type event regions, are approximately located in the western study region. The events are almost thrust faulting type ones along the Longmenshan fault, in the eastern study region. The strike-slip faulting events occurred along the Xianshuihe fault and other wide regions. The faulting regimes of events show more complex seismo-tectonic motions in the eastern margin of the Tibetan plateau which is boundary between the south China block and the Tibetan plateau.

The extensional motion along the N-S direction due to the normal faulting type event took place southwest of the Xianshuihe fault. It is a new finding of the seismo-teconic motion in the present analysis. The dynamic cause of the N-S extensional motion probably is attributable to the regional tectonic motions and the stress fields of the subblocks around the N-S extensional normal faulting type event region southwest of the Xianshuihe fault (SWXSH-NSNF) in Figure 2. The Longmenshan block, region I in Figure 1 north of the Xianshuihe fault and northeast of the SWXSH-NSNF moved southeastward very slowly because of encountering strong obstructs from the Sichuan basin [36,37]. The channel flow of the deep crust may be inhibited by the rigid Sichuan Basin [38]. The compressive stress P-axes are also along WNW-SES direction here as shown in Figures 3 and 7. The horizontal projection of fault slip vector of the Wenchuan M8 earthquake in the Longmenshan fault is also southeastward. The strike direction, dip angle and rake angle of the M8 event fault were 230°, 30° and 128° respectively. The annual ratio of the surface displacement between Tibetan plateau and the south China block in near ESE-WNW direction was about 3 mm/y along the Longmenshan fault, very little based on GPS observations [39]. While the large faults lie along the N-S direction in the Sichuan- Yunnan rhombic block (region III, south of the Xianshuihe fault) as shown in Figure 1. The rhombic block moved southward with greater annual ratio than that in the Longmenshan block [39]. So the SWXSHNSNF being located in the north end of the rhombic block might undergo extensional status southward. Furthermore the azimuths of many P-axes are in or near the E-W direction (greater than thirty percent between 270°and 280°) in the region E2 east of SWXSH- NSNF as shown in Figures 3 and 7. It implied that the SWXSH-NSNF was undergone strong compressive action in near E-W direction also. The T-axes aligning along N-S direction are greater than seventy percent in region E2 as shown in Figures 4 and 8. Such dynamic mechanism is evidently benefited to the extensional motion of normal fault in N-S direction.

Figure 8: Statistical rose diagrams of the azimuths for P- axes in subregion W1 and W2 in the western region in the eastern margin of the plateau in Figure 3 and the corresponding T-axes of the above subregions in Figure 4, respectively.

The E-W extension normal faulting type event region east of the lower reach of the Jinshajiang fault (ELRJSJ-EWNF) is south of the SWXSH-NSNF in Figure 2. The northern end of the ELRJSJ-EWNF might be compressed by the southward extensional motion from the SWXSH-NSNF as motioned above. The southern end of ELRJSJEWNF is located northeast of the southern end of the Jinshajiang fault in Figures 1 and 2. The Jinshajiang fault along the N-S strike direction in the northern segment is turned into the NW-ES direction in the southern end of ELRJSJ-EWNF. This is an interesting tectonic change for the normal faulting type events. It might imply that the ELRJSJ-EWNF in the southern end of the rhombic block (region III₁ in Figure 1) may encounter the tectonic obstruction from its southern neighborhoods. The stress field also shows that the ELRJSJ-EWNF is compressed by its southern vicinities. The P-axes aligned in the NNE or near the N-S direction in the stress subregion W3 southwest of the ELRJSJ-EWNF and in the NNW or near the N-S direction in the stress subregion E3 southeast of the ELRJSJ-EWNF as illustrated in Figures 3,7 and 8. It shows that the strong compressive stress distributed in the southwestern (W2 stress subregion) and southeastern neighborhood (E3 subregion) of the ELRJSJ-EWNF. Simultaneously, a strong extensional stress field existed in the eastern neighborhoods of ELRJSJEWNF as shown in Figure 4. Such surroundings for both of tectonic motions and stress fields imply the southern end of ELRJSJ-EWNF is undergoing the compression near the N-S direction and the extension near the E-W direction. The tectonic motion and the stress field benefits evidently the normal faulting extensional motion in E-W direction in ELRJSJ-EWNF.

Furthermore, the Indian plate subducts from the Myanmar region under the eastern end of the Himalayans in the Eurasian plate along near E-W direction. Based on seismic data this is an eastward subduction zone form the Myanmar region under the southeastern Tibetan plate and Yunnan region, China. The subduction slab extends for about 500 km from the southwest to northeast and the front reaches 150 km depth. The subduction zone ranges between about 20°N-27°N and 92°E-98°E [40]. The Tengchong volcano in Yunnan province, China is located east of the subduction zone in Figure 1. The normal faulting type event regions, SWXSH-NSNF and ELRJSJ-EWNF are about 200 km away from the subduction zone in the northeast and the east, respectively. The occurrences of the normal faulting type events in ELRJSJ-EWNF and SWXSH-NSNF are likely attributable to the extensional motion in the region back the subduction zone.

The directions of the P-axes in nearly NE-SW or NEE-SWW on the western region generally coincide with those on the Tibet Plateau. The seismogenic stress field on the Tibet Plateau and the plateau motion are caused by compression from the northward movement of the Indian plate [41-43]. The P-axes aligning in the NW-SE and WNWESE directions in the eastern regions E1, E2 and E3 are quite different from those in the NE-SW and ENE-WSW directions in the western regions W1 and W2. The orientations of the principle compressive stress P-axes reveal discontinuous change as crossing the longitudinal boundary of the P-axis as shown in Figures 3,7 and 8. It perhaps implies that another independent dynamic source being able to generate the compressive stress shown by the P-axes in the NW–SE and WNW-ESE directions might exist potentially in the eastern region in Figure 3. The principal compressive stress P-axes in a general NW-SE and WNWESE direction in the eastern region in the eastern margin of the Tibetan plateau approximate those in the South China and Taiwan region. The orientation of the P-axes in the eastern region in Figure 3 probably is affected by the stress field in the South China block and the Taiwan region [44]. This dynamic source of stress field in the eastern region in Figure 3 might be attributable to the transmission of the tectonic force resulting from subduction of the Philippine Sea plate along the Ryukyu trench and the collision between the Philippine Sea and Eurasian plates in the Taiwan region [45].

It is noticed that the longitudinal boundary of the P-axis between the eastern and western subregions lies within the Tibetan plateau and even passed through the large Xianshuihe fault. The longitudinal stress boundary in Figure 3 is also unrelated to the tectonic boundary between the Tibetan plateau and the South China block at the surface in Figure 1. It might be relation with lateral differences in the lithospheric structure [46]. Based on the tomography results, a westward extension front of a high velocity zone beneath the South China block has crossed the Longmenshan fault by 20 km westward and arrived in the eastern margin of the plateau in the lower crust and the upper mantle at 50 km depth [13,47]. Aftershocks of the M8 Wenchuan earthquake were almost concentrated on the inner side of the high velocity zone front of the South China block [48]. This also implies that the tectonic stress in the South China block might have affected the earthquake generating stress field for the eastern region of the eastern margin of the plateau in Figure 3.

The results in the present analysis showed that the dislocation of normal faulting type events extended along the N-S direction in the normal faulting type event region southwest of the Xianshuihe fault (SWXSH-NSNF). Another, in normal faulting type event region was east of the lower reach of the Jinshajiang river (ELRJSJ-EWNF) the dislocations of normal faulting type events extended along the E-W direction. The ELRJSJ-EWNF is on the southern border of SWXSHNSNF.

The P-axes aligned in the NE-SW or ENE-WSW directions in the western region and in the NW-SE or WNW-ESE directions in the eastern region. The seismogenic stress field in the eastern region in the eastern margin of the Tibetan plateau is related to the collision between the Eurasian and Philippine Sea plates along the Taiwan region through the South China block and the subduction from the Philippine Sea plate along the Ryukyu trench under the Eurasian plate. The longitudinal boundary of the P-axis of seismogenic stress field between the eastern and western regions lies within the eastern margin of the Tibetan plateau along about the longitude 100°E or 101°E. It does not coincide with the boundary between the Tibetan plateau and the South China block.

This study was supported partly by the Natural Science Foundation of China (Grant No. 41374052).