Deep tectonics and seismogenic mechanisms of the seismic source zone of the Jishishan M_(s)6.2 earthquake on December 18,2023,at the northeast margin of the Tibetan Plateau

On December 18,2023,an M_(s)6.2 earthquake occurred in Jishishan,Gansu Province,China.This earthquake happened in the eastern region of the Qilian Orogenic Belt,which is situated at the forefront of the NE margin of the Tibetan Plateau(i.e.,Qinghai-Tibet Plateau),encompassing a rhombic-shaped area that intersects the Qilian-Qaidam Basin,Alxa Block,Ordos Block,and South China Block.In this study,we analyzed the deep tectonic pattern of the Jishishan earthquake by incorporating data on the crustal thickness,velocity structure,global navigation satellite system(GNSS)strain field,and anisotropy.We discovered that the location of the earthquake was related to changes in the crustal structure.The results showed that the Jishishan M_(s)6.2 earthquake occurred in a unique position,with rapid changes in the crustal thickness,Vp/Vs,phase velocity,and S-wave velocity.The epicenter of the earthquake was situated at the transition zone between high and low velocities and was in proximity to a low-velocity region.Additionally,the source area is flanked by two high-velocity anomalies from the east and west.The principal compressive strain orientation near the Lajishan Fault is primarily in the NNE and NE directions,which align with the principal compressive stress direction in this region.In some areas of the Lajishan Fault,the principal compressive strain orientations show the NNW direction,consistent with the direction of the upper crustal fast-wave polarization from local earthquakes and the phase velocity azimuthal anisotropy.These features underscore the relationship between the occurrence of the Jishishan M_(s)6.2 earthquake and the deep inhomogeneous structure and deep tectonic characteristics.The NE margin of the Tibetan Plateau was thickened by crustal extension in the process of northeastward expansion,and the middle and lower crustal materials underwent structural deformation and may have been filled with salt-containing fluids during the extension process.The presence of this weak layer makes it easier for strong earthquakes to occur through the release of overlying rigid crustal stresses.However,it is unlikely that an earthquake of comparable or larger magnitude would occur in the short term(e.g.,in one year)at the Jishishan east margin fault.

Preliminary report of coseismic surface rupture(part)of Türkiye's M_(W)7.8 earthquake by remote sensing interpretation

Both M_(W) 7.8 and M_(W) 7.5 earthquakes occurred in southeastern Türkiye on February 6,2023,resulting in numerous buildings collapsing and serious casualties.Understanding the distribution of coseismic surface ruptures and secondary disasters surrounding the epicentral area is important for post-earthquake emergency and disaster assessments.High-resolution Maxar and GF-2 satellite data were used after the events to extract the location of the rupture surrounding the first epicentral area.The results show that the length of the interpreted surface rupture zone(part of)is approximately 75 km,with a coseismic sinistral dislocation of 2-3 m near the epicenter;however,this reduced to zero at the tip of the southwest section of the East Anatolia Fault Zone.Moreover,dense soil liquefaction pits were triggered along the rupture trace.These events are in the western region of the Eurasian Seismic Belt and result from the subduction and collision of the Arabian and African Plates toward the Eurasian Plate.The western region of the Chinese mainland and its adjacent areas are in the eastern section of the Eurasian Seismic Belt,where seismic activity is controlled by the collision of the Indian and Eurasian Plates.Both China and Türkiye have independent tectonic histories.

Regional stress field in Yunnan revealed by the focal mechanisms of moderate and small earthquakes

We determined focal mechanism solutions of 627 earthquakes of magnitude M ≥ 3.0 in Yunnan from January 2008 to May 2018 by using broadband waveforms recorded by 287 permanent and temporary regional stations. The results clearly revealed predominantly strike-slip faulting characteristics for earthquakes in Yunnan, with focal depths concentrated in the top 10 km of the crust. The earthquake mechanisms obtained were combined with the global centroid moment tensor solutions of 80 additional earthquakes from 1976 to 2016 to invert for the regional variations of stress field orientation by using a damped regional-scale stress inversion scheme.Results of the stress field inversion confirmed that the Yunnan region is under a strike–slip stress regime, with both maximum and minimum stress axes being nearly horizontal. The maximum compressional axes are primarily oriented in a northwest-southeast direction, and they experience a clockwise rotation from north to south, whereas the maximum extensional axes are oriented largely northeast-southwest. The maximum compressional axes are in line with the global positioning system–inferred horizontal velocity field and the southeastward escape of the Sichuan–Yunnan Rhombic Block, whereas the maximum extensional axes are consistent with anisotropy derived from SKS splitting. Against the strike–slip background, normal faulting stress regimes can be seen in the Tengchong volcanic area as well as in other areas with complex crisscrossing fault zones.

Gravity pattern in southeast margin of Tibetan Plateau and its implications to tectonics and large earthquakes

There are many active faults in the southeast margin of Tibetan Plateau,where three large active faults zones,the Longmenshan,Xianshuihe and Anninghe,merge to form a"Y"shape.Strong crustal deformation and a complicated fault distribution accompany strong earthquake activity in this zone.In this paper,we investigate a multi-scale gravity anomaly in the southeastern margin of the Tibetan Plateau using the wavelet transform;we find that the pattern of the gravity field is closely related to the fault system in the study area.Analyzing the characteristics of this Bouguer gravity anomaly at different orders indicates that the eastern Himalayan syntaxis has produced a strong eastward push during its northward movement,resulting in a shortening of the crust from west to east and a rapid uplift of the Tibetan Plateau.The Songpan–Garzêand Sichuan–Yunnan blocks have been forced to slip and extrude southward and eastward laterally.The distributions of seven large earthquakes from 1970 to 2018 reflects the relationship between large earthquakes and characteristics of the gravity anomaly.Comparing the tectonic backgrounds of several earthquakes reveals that the large earthquakes occur usually in the high gravity anomaly gradient zone,which corresponds in general to the boundary zones of the blocks.We infer that large earthquakes occur primarily in high Bouguer gravity anomaly zones in the upper crust,while low Bouguer gravity anomalies encompass the lower crust and the uppermost mantle.

The Spatial Distribution and Attribute Parameter Statistics of Landslides Triggered by the May 12th,2008,MW7.9 Wenchuan Earthquake

A complete landslide inventory and attribute database is the importantly fundamental for the study of the earthquake-induced landslide.Substantial landslides were triggered by the MW7.9 Wenchuan earthquake on May 12th,2008.Google Earth images of preand post-earthquakes show that 52194 co-seismic landslides were recognized and mapped,with a total landslides area of 1021 km2.Based on the statistics,we assigned all landslide parameters and established the co-seismic landslides database,which includes area,length,and width of landslides,elevation of the scarp top and foot edge,and the top and bottom elevations of each located slope.Finally,the spatial distribution and the above attribute parameters of landslides were analyzed.The results show that the spatial distribution of the co-seismic landslides is extremely uneven.The landslides that mainly occur in a rectangular area(a width of 30 km of the hanging wall of the Yingxiu-Beichuan fault and a length of 120 km between Yingxiu and Beichuan)are obviously controlled by surface rupture,terrain,and peak ground acceleration.Meanwhile,a large number of small landslides(individual landslide area less than 10000 m2)contribute less to the total landslides area.The number of landslides larger than 10000 m2 accounts for 38.7%of the total number of co-seismic landslides,while the area of those landslides account for 88%of the total landslides area.The 52194 co-seismic landslides are caused by bedrock collapse that usually consists of three parts:source area,transport area,and accumulation area.However,based on the area-volume power-law relationship,the resulting regional landslide volume may be much larger than the true landslide volume if the landslide volume is calculated using the influenced area from each landslide.

Source rupture characteristics of the September 5,2022 Luding M_(S)6.8 earthquake at the Xianshuihe fault zone in southwest China

On September 5,2022,at Beijing time 12:52 p.m.,an M_(S)6.8 earthquake struck Luding County,GarzêTibetan Autonomous Prefecture,Sichuan Province.The epicenter of the earthquake was at the intersection of the Sichuan-Yunnan,Bayankala,and South China blocks.The tectonic background is extremely complex,and strong earthquakes occur frequently.Based on a predetermined focal location and focal mechanism solution for the earthquake,we reversed the focal depth and rupture process of the earthquake by fitting the teleseismic P and SH waves recorded by the global seismic network.The results show that the focal depth is 16 km,with the main rupture having a length of about 45 km near the epicenter,with a maximum displacement of 1.02 m.Although the rupture mainly propagates from the north–northwest(NNW)to the south–southeast(SSE)along the fault strike,there is a small-scale rupture slip zone at shallow depths in the north–northeast(NNE)direction along the epicenter of the seismogenic fault.This rupture image corresponds to the cluster distribution of aftershocks in the NNW and SSE directions starting from the epicenter,corresponding to the distribution of recorded landslides.The earthquake occurred on the Moxi fault,located in the southeastern section of the Xianshuihe fault.The major tectonic feature in this area is the southeastward movement of the Chuandian block relative to the Bayanhar block.

The Volume Calculation Method of Rock Collapses and Loess Landslides Triggered by the 1556 AD Huaxian M81/2 Earthquake in Shaanxi Province, China

Accurate volume calculation of each individual landslide triggered by strong historical earthquakes can help understand the characteristics of the typical earthquake-induced landslides,thus providing significant information for the modification of the focal parameters of historical earthquakes.In this study,we select one rock fall and three loess landslides triggered by the 1556 AD Huaxian M8⅟􀄟earthquake,compute their volumes using the low-altitude high-precision Unmanned Aerial Vehicle(UAV)photogrammetry and landslide profile restoration methods.The results show that:①the whole influencing area of the Huangjiagou Rock Fall is approximately 3.03×105 m2 and the area of the collapsed rock accumulated at the slope foot is 3.33×104 m2,accounting for approximately 10%of the entire influencing range.However,the estimated volume of the collapsed rock is only 0.699×106 m3,indicating a rock fall with large influencing range but limited collapsed rock;②the geological form of thethree loess landslides are preserved intactly,with volumes of 0.283×108 m3,0.074×108 m3,and 0.377×108 m3.These important geological hazard relics reflect the strong vibrations and severe casualties in the meizoseismal area;③loess landslides are the key reason of the serious death toll in the hilly-gully loess area.Our new method can be used to estimate the influencing area and the actual volume of each individual landslide,and rationally evaluate the role of earthquake landslides in the disaster.In addition,quantitative research on secondary disasters triggered by strong historical earthquakes is beneficial for understanding the surface process and focal parameters of the earthquakes.

Coseismic surface rupture characteristics and earthquake damage analysis of the eastern end of the 2021 M_(S)7.4 Madoi(Qinghai)earthquake

At 02:04 on May 22,2021,an M_(S)7.4 earthquake occurred in Madoi County in Qinghai Province,China.This earthquake is the largest seismic event in China since the 2008M_(S) 8.0 Wenchuan earthquake.Thus,it is critical to investigate surface deformation and damage in time to accurately understand the seismogenic structure of the Madoi earthquake and the seismogenic capacity of the blocks in this region.This study focuses on the Xuema Village,located at the eastern end of the coseismic surface ruptures produced by the event,and assesses the deformation and seismic damage in this area based on field surveys,UAV photogrammetry,and ground penetrating radar(GPR).The results indicate that the rupture scale is substantially smaller at the eastern end of the rupture zone compared to other segments.En echelon type shear tensile fractures are concentrated in a width range of 50–100 m,and the width of single fractures ranges from 20 to 30 cm.In contrast,the degree of seismic damage significantly increases at this site.All of the brick and timber houses are damaged or collapsed,while the steel frame structures and the color steel houses are slightly damaged.More than 80%of the bridge decks on the Changma River Bridge collapse,similar to the terraces along the Youerqu and Changma Rivers and the cut slopes of Provincial Highway S205.We infer that the seismogenic fault of the Madoi earthquake exerts a tail effect in this segment.The tension zone has led to a reduction at the eastern end of the rupture zone,causing shaking damage.Local topography and buildings without earthquake-resistant construction along the strike of the rupture zone have undergone different levels of seismic damage.

The Calculation Method for the Volume of Landslides and Dammed Lake Sediments Triggered by a Strong Historical Earthquake in the Loess Plateau:A Case Study of Qiuzigou, Gansu Province, Northwest China

The quantitative calculation of the volume of large earthquake-triggered landslides and related dammed lake sediments is of great significance in the study of secondary disasters and focal parameters of strong historical earthquakes.In this study,the dammed lake induced by Qishan M7 earthquake(Lingtai County,Gansu Province,Northwest China)is selected as the research object.Based on the information collected from the 4 boreholes in the dammed lake area,we further take advantage of the lowlevel Unmanned Aerial Vehicle(UAV)photogrammetry and the morphology recovery method,to calculate the volume of the dammed lake and landslides,respectively.Finally,major conclusions are obtained as follows:①the AMS-14C age at the bottom of the Qiuzigou Dammed Lake sediments is 2890±30 BP,which coincides with the 780 BC Qishan earthquake;furthermore,the Qiuzigou Landslides seem to have been triggered by the earthquake,forming an enclosed dammed lake deposition environment after the upstream sediments accumulate;②the Qiuzigou landslides are opposite-sliding landslides that have blocked the river valley;in detail,landslide volumes at the right and left banks are 235×104 m3 and 229×104 m3,respectively.The length of the dammed lake is 2.6 km,with a thickness of approximately 43 m near the landslides,and the total sedimentary volume is 573×104 m3;③the erosion rate of Qiuzigou Landslide Dammed Lake is 0.44 mm/a,the accumulation rate is 15.05 mm/a,and the soil erosion modulus is 593 t/(km2/a),characterized as slight erosion.Quantitative research on the formation of landslides and dammed lakes from strong historical earthquakes is vital for increasing our understanding of the vibrational characteristics and surface action processes of these types of earthquakes.

Relationship between the Landslides Triggered by the Tongwei M71/2 Earthquake in 1718 AD and the Disappearance of Yongning Ancient Town

Although China’s historical earthquake documentation is relatively rich,it is not all based on scientific records.Therefore,the verification of the seismic information in historical records can effectively avoid exaggerating or underestimating the damage they produced.In this paper,we analyze the detailed information of the 1718 AD Tongwei M7⅟􀄟earthquake through field surveys,document sorting,and manual visual interpretation of UAV images.Major conclusions are listed as follows:①The low-level terraces of Weihe River between Gangu and Wushan are fully developed with flatted surface,and the residents here are mostly killed by house collapses.In addition,the disappearance of Yongning Ancient Town is not directly related to the earthquakeinduced landslides;②In fact,“Yongning Town is entirely buried by the earthquake”in the historical records describes the phenomenon that loess dust has pervaded the entire Weihe Valley.These dust grains are produced by the sliding of earthquakeinduced landslides.Thus,there is no possibility that large-scale landslides have slipped over Weihe bedrock and buried Yongning Town;③After the earthquake,survivors abandoned the ruins and selected a new site to live.They built a new town named“Pan’an(means always peaceful in the future)”.earthquake-induced landslides may be the reason of burying the residential areas on valley-side slopes,while those locations inside the valley are associated with the amplification effect of ground vibration.On the basis of compiling historical seismic data,scientific methods are used to explore the real meaning of these documents,for the purpose of providing basic data for the risk evaluation of strong historical earthquakes.

Interpretation of the Spatial Distribution Characteristics of the Co-seismic Landslides Induced by the 1920 Haiyuan M81/2 Earthquake Using Remote Sensing Images

Analyzing the spatial distribution characteristics of earthquake-induced secondary disasters based on advanced techniques is significantly important,especially in understanding the process of strong earthquakes in the Loess Pateau.Using ArcGIS,this study interprets multi-temporal high-resolution satellite images,field investigation data,and historical seismic records.Major conclusions are obtained as follows:①Landslides induced by the Haiyuan earthquake are mainly distributed in the intersection area of the end of the Haiyuan fault and Liupanshan fault,as indicated by multiple dense distribution centers;②The landslide distribution of the Haiyuan Earthquake is determined by the distance to the fault,topographic relief,slope,lithology,and other factors.In detail,the closer the distance to the fault,the greater the density of the landslide.The greater the slope and relief of the terrain,the greater the density and the smaller the average area of a landslide.Compared with tertiary strata,Quaternary strata has a larger average area,and the density of the landslides is smaller;③The density curve of the death toll in the Haiyuan earthquake can be used as a reference for the distribution of co-seismic landslides.Several Haiyuan co-seismic landslides are distributed in the Tongwei landslide area;however,the major landslides here are induced by the 1718 Tongwei earthquake rather than the 1920 Haiyuan earthquake;④The co-seismic landslides of the Haiyuan earthquake exhibits the“slope effect”in the south-west plate of Haiyuan fault,presenting the dominant sliding direction towards the fault and epicenter;however,the“slope effect”is not evident in the northeast plate of the fault.

Abnormal Phenomena of Volume Strain before Large Earthquakes

This paper studies the imminent anomalies observed by the Sacks volume strainmeter in Erzhangying station and Tiantanghe station before 80 earthquakes with Ms≥ 7. 0 which took place from January 2011 to April 2014 all over the world. Then, preconditions for anomaly identification are put forward for complex earthquake cases. Statistical results show that volume strain observation has a better earthquake reflecting ability for earthquakes with magnitudes larger than 7. 0 and epicentral distance within 8000kin. In addition, these results also reflect that the volume strain observation can better reflect precursory anomalies of such earthquakes. Based on categorization and description of those anomalies, we divide the anomalies into three types, that is, earth tide distortion type, abrupt change type and slow earthquake type. Furthermore, the paper makes a statistical analysis of these types and preliminarily discusses their mechanical properties as well. According to research, volume strain anomaly has an indicative significance to future strong earthquakes in the world.

Coseismic displacements and inospheric changes of the 2013 Ms7. 0 Lushan earthquake from GPS measurements

By inverting GPS data recorded at stations of the Crustal Movement Observation Network of China （CMONOC） near the 2013 Lushan Ms7.0 earthquake, we found a horizontal displacement of 22 mm at a site about 32 kin SW of the epicenter and vertical displacements of as much as 12.4 mm at several sites. The vertical displacements were generally uplift on the west side of the nearby Longmenshan fault zone and subsidence on the east side. We also found coseismic ionospheric disturbances about 0.5 to 0.9 TECU in amplitude that lasted for about one hour.

A case-based reasoning method of recognizing liquefaction pits induced by 2021 M_(W) 7.3 Madoi earthquake

Earthquake-triggered liquefaction deformation could lead to severe infrastructure damage and associated casualties and property damage.At present,there are few studies on the rapid extraction of liquefaction pits based on high-resolution satellite images.Therefore,we provide a framework for extracting liquefaction pits based on a case-based reasoning method.Furthermore,five covariates selection methods were used to filter the 11 covariates that were generated from high-resolution satellite images and digital elevation models(DEM).The proposed method was trained with 450 typical samples which were collected based on visual interpretation,then used the trained case-based reasoning method to identify the liquefaction pits in the whole study area.The performance of the proposed methods was evaluated from three aspects,the prediction accuracies of liquefaction pits based on the validation samples by kappa index,the comparison between the pre-and post-earthquake images,the rationality of spatial distribution of liquefaction pits.The final result shows the importance of covariates ranked by different methods could be different.However,the most important of covariates is consistent.When selecting five most important covariates,the value of kappa index could be about 96%.There also exist clear differences between the pre-and post-earthquake areas that were identified as liquefaction pits.The predicted spatial distribution of liquefaction is also consistent with the formation principle of liquefaction.

Intermediate to Long-term Estimation of Strong Earthquake Risk Areas in the Chinese Mainland Based on Geodesic Measurements

Based on previous research results,present-day crustal deformation and gravity fields in the Chinese mainland are analyzed using the GPS data,leveling,gravity and cross-fault deformations. We analyzed strain accumulation of the major faults,and identified locked or high strain accumulation segments. Combining the effects of large earthquakes in the study area,the long-term (decade) probability of large earthquakes in the Chinese mainland is estimated.

Analysis and Study on the Change of the Observed Geo-electric Field Data at Lhasa Geomagnetic Station before and after the Nepal MS8.1 Earthquake

In this paper,the main purpose is to analyze and research the characteristics of the geoelectric field observed data with a long time span and large amplitude abnormal change,at the Lhasa geomagnetic station( hereafter referred to as "Lhasa station "),before and after the Nepal M_S8. 1 strong earthquake,which occurred on April 25,2015. Based on the observation conditions,the observation system,and the observed data of Lhasa station preliminary discussed,the main characteristics of the abnormal change and evolution process are analyzed and studied,using the following two methods; the "synthesis energy accumulation"and the "power as MSA spectrum"analysis,from the two aspects of the"Time Domain"and"Frequency Domain. "The results show that the abnormal change of the geo-electric field observation of Lhasa station experienced a development stage following the process of "trend change- disturbance change- earthquake period-recovery period",and an evolution process of "low frequency change- high frequency change- smooth change- high frequency change ",before and after the Nepal M_S8. 1strong earthquake. Comprehensive analysis shows that the variation characteristics and evolution process of the geo-electric field at Lhasa station are basically consistent with the results of the relevant mechanism and phenomenon research. So far,this is valuable information with certain objectivity,which is typical and representative to reflect the whole process of the gestation, occurrence and complete development of such strongearthquakes.

Improving image accuracy of ambient noise data by temporary seismic arrays at different observation periods

When using ambient noise data to invert velocity and anisotropic structures,the two-station inter-correlation method requires synchronous stations.If there are multiple temporary seismic arrays with different observation periods in the study area,the seismic arrays are usually used selectively.This paper takes the Sanjiang lateral collision zone as an example,and utilizes the ambient noise data of multiple temporary seismic arrays at different observation periods to improve the accuracy of regional velocity structure and anisotropy by anchoring permanent seismic stations.In this paper,notable enhancements in S-wave velocity and azimuthal anisotropy imaging accuracy are achieved by integrating data from three temporary seismic arrays(SJ-Array,SL-Array,and Chin Array-I)with the permanent seismic network.The imaging resolutions for the S-wave velocity and azimuthal anisotropy above 40 km are 0.4°×0.4°and 0.5°×0.5°,respectively.In the region of the most concentrated array coverage,the imaging resolution of S-wave velocity can reach 0.33°×0.33°at depths of less than 30 km.These findings underscore the significant improvement in deep structure imaging accuracy by the synergistic integration of ambient noise data from multiple temporary seismic arrays.

Crustal thicknesses and Poisson's ratios beneath the Chuxiong-Simao Basin in the Southeast Margin of the Tibetan Plateau

In the Southeast Margin of the Tibetan Plateau, low-velocity sedimentary layers that would significantly affect the accuracy of the H-κ stacking of receiver functions are widely distributed.In this study, we use teleseismic waveform data of 475 events from 97 temporary broadband seismometers deployed by ChinArray Phase I to obtain crustal thicknesses and Poisson's ratios within the Chuxiong-Simao Basin and adjacent area, employing an improved method in which the receiver functions are processed through a resonance-removal filter, and the H-κ stacking is time-corrected.Results show that the crustal thickness ranges from 30 to 55 km in the study area, reaching its thickest value in the northwest and thinning toward southwest, southeast and northeast.The apparent variation of crustal thickness around the Red River Fault supports the view of southeastern escape of the Tibetan Plateau.Relatively thin crustal thickness in the zone between Chuxiong City and the Red River Fault indicates possible uplift of mantle in this area.The positive correlation between crustal thickness and Poisson's ratio is likely to be related to lower crust thickening.Comparison of results obtained from different methods shows that the improved method used in our study can effectively remove the reverberation effect of sedimentary layers.

Characteristics of Late-Quaternary Activity and Seismic Risk of the Northeastern Section of the Longmenshan Fault Zone

Following the 2008 Wenchuan M8 earthquake,the seismic risk of the northeastern section of the Longmenshan fault zone and the adjacent Hanzhong basin has become an issue that receives much concern.It is facing,however,the problem of a lack of sufficient data because of little previous work in these regions.The northeastern section of the Longmenshan fault zone includes three major faults：the Qingchuan fault,Chaba-Lin＇ansi fault,and Liangshan south margin fault,with the Hanzhong basin at the northern end.This paper presents investigations of the geometry,motion nature,and activity ages of these three faults,and reveals that they are strike slip with normal faulting,with latest activity in the Late Pleistocene.It implies that this section of the Longmenshan fault zone has been in an extensional setting,probably associated with the influence of the Hanzhong basin.Through analysis of the tectonic relationship between the Longmenshan fault zone and the Hanzhong basin,this work verifies that the Qingchuan fault played an important role in the evolution of the Hanzhong basin,and further studies the evolution model of this basin.Finally,with consideration of the tectonic setting of the Longmenshan fault zone and the Hanzhong basin as well as seismicity of surrounding areas,this work suggests that this region has no tectonic conditions for great earthquakes and only potential strong events in the future.

Heterogeneous strain regime in the eastern margin of Tibetan Plateau and its tectonic implications

The eastern margin of Tibetan Plateau is one of the most active zones of tectonic deformation and seismicity in China. To monitor strain buildup and benefit seismic risk assessment, we constructed 14 survey-mode global position system （GPS） stations throughout the northwest of Longmenshan fault. A new GPS field over 1999-2011 is derived from measurements of the newly built and pre-existing stations in this region. Sequentially, two strain rate fields, one preceding and the other following the 2008 MwT.9 Wenchuan earthquake, are obtained using the Gausian weighting approach. Strain field over 1999-2007 shows distinct strain partitioning prior to the 2008 MwT.9 Wenchuan earthquake, with compression spreading over around Longmenshan area. Strain fieldderived from the two measurements in 2009 and 2011 shows that the area around Longmenshan continues to be under striking compression, as the pattern preceding the Wenchuan earthquake, implying a causative factor of the sequent of 2013 Mw6.7 Lushan earthquake. Our GPSderived dilatation shows that both the Wenchuan and Lushan earthquakes occurred within the domain of pro- nounced contraction. The GPS velocities demonstrate that the Longriba fault underwent slight motion with the faultnormal and -parallel rates at 1.0 -4- 2.5 mm and 0.3 4-2.2 mm/a; the Longmenshan fault displayed slow activity, with a fault-normal rate at 0.8 ± 2.5 mm/a, and a fault-parallel rate at 1.8 4- 1.7 mm/a. Longriba fault is on a par with Longmenshan fault in strain partitioning to accommodate the southeastward motion of eastern margin of the Tibetan Plateau. Integrated analysis of principal strain tensors, mean principal stress, and fast directions of mantle anisotropy shows that west of Sichuan is characterized as mechanically strong crust-mantle coupling.