Study On Crustal Movement And Gravity Variation In Sichuan-Yunnan Region And Their Relationship With Regional Strong Earthquakes

With the development of modern geodetic observation techniques（Global Navigation Satellite System, GNSS; Interferometric Synthetic Aperture Radar, InSAR; Modern Gravity Measurement）, we can study the regional crustal movement, gravity field and its change features etc. with a higher accuracy, spatial and temporal resolution, which bring much convenience for the research of fault activity and earthquakes. The Sichuan-Yunnan region is located in the eastern of Qinghai-Tibet plateau. In the dynamic tectonic environment that the Indian Plate northward colliding with Eurasia, the north-southward squeezing and eastward extrusion of Qinghai-Tibet plateau, the Sichuan-Yunnan region is characterized with complex geological structure, dramatically contrast of landform, intense tectonic activity, high frequency and strong intensity of earthquakes. So, the study of crustal movement, lithosphere structure, gravity fields and its change features in the Sichuan-Yunnan region, and research on their relationship with regional crustal structure and strong earthquakes are always one of the hotspots in geo-scientific research.Using the global latest data and models, this study focuses on the displacement field, strain field evolution, lithosphere structure, gravity and its change features in the Sichuan-Yunnan region. In addition, we investigate their relationship with regional tectonic structure, seismic and fault activity. The main researches and results are as follow:1. Using the GPS velocities data of the period of 2001 to 2004, 2004 to 2007 and 1999 to 2013, we first study the evolution characters of crustal strains in Sichuan-Yunnan region, then we analysis their relation with the big earthquakes（>Ms6.0） which occurred during corresponding period. The results show that, the distribution of surface strain rate agrees well with the scope of sub-blocks, and the surface strain differences formed corresponding faults block types in the sub-block or its boundaries, which has a certain relationship with the location and focal mechanism of the earthquakes. The high activity faults in Sichuan-Yunnan region is character by high value of maxim strain rate, and rarely big earthquakes occurs on these faults; The lower value of maxim strain rate, the less activity of these faults, and the easier accumulation of the strain energy under the intense tectonic activity setting in Sichuan-Yunnan region, which bring big earthquakes to these faults zone.2. Using the CRUST1.0 model, based on the Airy isostatic theory, we study the state of crustal equalisation in Sichuan-Yunnan region. The crustal isostatic apparently shows that the Sichuan Basin is in isostatic state; On the contrary, the Longmen Mountains Fault Zone represents the most remarkable disequilibrium area in Sichuan-Yunnan region. The distribution of recently strong earthquakes in regional crustal isostatic map has a certain regularity, thrust or normal earthquakes often broke out the fault zone where the isostatic state change severely, while the strike-slip earthquakes often occurred in the region where the change of isostatic is not obvious. The regularity is the same with the distribution of regional faults type.We present a rapid method based on the assumption of lithosphere local isostasy to map the equilibrium lithosphere thickness in Sichuan-Yunnan region. The lithosphere heterogeneity is complicate in Sichuan-Yunnan region, and has rarely mirror-image relation with regional Moho discontinuity. Qinghai-Tibet Plateau, like the Aba sub-block and ZangDong sub-block is characterized with thick lithosphere of 150 kilometer, which maybe correlates with their plate’s collision environment. Sichuan Basin craton has a thick lithosphere root, which is about 150 kilometer. The lithosphere thickness of North-South seismic belt is thinner comparing with its surrounding blocks. The sub-blocks located in the south of 26°N have a lithosphere thickness of 100¹30 km, and is characterized with the north to south thinning trend. There are good agreements between our results and the published seismic lithosphere thickness in Sichuan-Yunnan region, which mean the regional lithosphere maybe in an equilibrium state.3. Using the newly ultra-high order gravity field model EIGEN-6C2, the global DEM model topo₁5.1.img and the crust model CRUST1.0 etc., we calculate the Free-air gravity anomaly, Bouguer anomaly and residue gravity anomaly in Sichuan-Yunnan region, and study the relationship between these static gravity anomalies and regional earthquakes. As a whole, the Free-air gravity anomaly has a clear mirror-image relation with regional topography, which means that the regional topography has a big effect on Free-air gravity anomaly. The Bouguer anomaly in Sichuan-Yunnan region presents the features of lower northwest and higher southeast pattern, which basically reflects the crustal thickness changes in Sichuan-Yunnan region, and be superposed of the intensity of various ranges of local anomalies. The residue gravity anomaly in Sichuan-Yunnan region is characterized with a sharply lateral variation, which means the crustal density anomaly also change with a big lateral variation. The Longmenshan fault is on the border of positive to negative changing area of residual gravity anomaly, and locates near the southeast side of the positive abnormal; Ganzi-Yushu and Xhanshuihe fault zone are in the stable area of residual gravity anomaly; But around the intersection of Xianshuihe fault, Longmenshan fault and An’ninghe fault, the residual gravity anomaly change dramatically. The thrust or normal type earthquakes are likely to occur on the high gradient zone of three kinds of static gravity anomalies, and the strike-slip type earthquakes are likely to break on the smooth and steady area of these static gravity fields, which may be concerned with the character of the faults and their tectonic background.4. Using the GFZ Release 05 gravity field models, we calculate and analysis the features of satellite gravity field change in Sichuan-Yunnan region, and study its relationship with regional earthquakes. The results show that, the satellite gravity fields change dramatically in Sichuan-Yunnan region during the period from the year 2003 to 2012, which reflect its strong regional tectonic movement. From the year 2006, the cumulative gravity change images have form its basic pattern, that is a "four quadrant distribution" pattern around the Sichuan-Yunnan rhombus-shape block, and the only little change happen are the location and magnitude of high and low-value center of cumulative gravity change images. The satellite gravity change features has a close relationship with the gestation and occurrence of regional strong earthquake. There is an “increasing, slowly increasing, reducing” gravity change feature before the Wenchuan earthquake, and the trend of high gradient zone of gravity variation is changed to parallel with the fault just two year before Wenchuan earthquake. From the year 2008 to 2009, the gravity change pattern in Sichuan-Yunnan region formed a "four quadrant distribution" pattern centered around the Longmenshan fault, which may reflect the Wenchuan earthquake’s impact on regional crustal movement. The satellite gravity field in Sichuan-Yunnan region change strongly after the Wenchuan earthquake, and the regional seismicity is enhanced, which shows that Wenchuan earthquake may has triggered effects on the its following strong earthquakes in Sichuan-Yunnan region.5. We calculate the gravity change feature caused by dislocation and disclination movement of a rectangle fault using numerical method, and analysis its relationship with the fault occurrence, like the dip angle and buried depth. The results show that, the gravity change on deformed earth surface and space fixed point both have a close relation with the fault occurrence and the magnitude of the dislocation and disclination. The bigger of the magnitude of the fault’s dislocation and disclination, the lager of the range and magnitude of the gravity change. The magnitude of the gravity change also has an inverse correlation with fault’s buried depth. At macro level, the gravity change caused by the rectangular fault dislocation movement shows an approximate symmetrical pattern. The gravity change caused by the rotation of the fault（disclination） break this symmetrical pattern, and the bigger of the fault rotation angle, the greater of the asymmetry.