Deformation Mechanism Of Seismogenic Faults Derived From Surface Dynamic Geodetic Data

Spatial and temporal characters of crustal movement and surface deformation are the most direct expressions of strain accumulation and release during the earthquake preparation, occurrence and the post-seismic adjustment, which reflect the state of strain accumulation of seismogenic faults. As a common geodetic data, GPS observation time period is only a time window compared with the entire earthquake recurrence.How to use GPS data which observation time is short to study strain accumulation of seismogenic faults and further analyzing seismic hazard, which is a significance reserach. Tectonic earthquakes often occur on the seismogenic fault. Since tectonic earthquakes occur at the active faults (Usually less than 10 km deep). The relation of stress-strain of surface and depth of crust on seismogenic faults and Dynamic changes of crustal deformation is studied to reveal seismogenic fault deformation mechanism, which can be understood by studying the scientific issues of earthquake preparation and occurrence. This thesis has made the following studies:1. The study based on displacement of dynamic characteristics of GPS observation data includes: Dynamic Change velocity field at different times; quantitative identification in tectonic deformation analysis for rotation parametersa. The dynamic characteristics of crustal deformation associated with the 2008 Wenchuan earthquake is analyzed based on the multi-periods of comparable GPS velocity fields of the north-south seismic zone with the unified reference datum to the Southern China block, which was established using the seven periods GPS regional data of "Crustal Movement Observation Network of China" and "Chinese continental tectonic environment monitoring network".(1) we improved primary selection criterion of the quasi-accurate detection of gross errors to exclude instability point in observation point group, which can effectively restrain the influence of the benchmark deviation on the screening of stable points.(2) The continuous distribution of GPS velocity field is fitted by using the least squares collocation based on the unified and proper covariance decay parameters in order to make sure the velocity field with the same deformation frequency and a better comparability, and solve the differences between the measured data point distribution.(3) The differences of grid velocity fields between 2007-2009 and 2004-2007 indicate that dynamic load enhanced on southern segment of the Longmenshan fault zone after the Wenchuan earthquake, which has a significant response to the SEE-orientated movement enhance, including the north Qilian and Qaida block. But response of the Ordos block closer to the fault is very small. Maybe the background of strain accumulation has already been at a high level. The effect after the Wenchuan earthquake shows a significant continual loading process on the southern segment of the Longmen Shan fault zone, while the Xianshuihe fault shows right-lateral shear, an opposite response to the strain accumulation background(4) After the Wenchuan earthquake, the SE-directed extrusion slip of Sichuan-Yunnan block did not enhanc until 2011, and then accelerated to some extent by 2013. b. Rotation parameters calculated by the strain tensor calculation reflect the limited rotational unit, but does not contain the strain information, in conjunction with the strain parameters can more objective reflect the status of surface deformation.(1) The characteristics and shear strain parameters are different, and the direction of rotation of selected parameters is independent of the coordinates. Size value indicates the rotating small unit.(2) Rotating parameter is related to the reference benchmark. Unified reference should be considered in several periods of data, and can calculate velocity field in no net rotation (NNR).(3) Rotation parameters can be represent the maximum shear strain in both directions due to the asymmetry. Rotation parameter reflects the infinitesimal deformation of the main strain axis deflection.(4) For pure strike-slip fault, the inter-sesminc displacement curve slope reflects the rotation parameters. When the fault deformation width is narrow, regional rotation parameter is nonzero narrow; when the fault deformation width is wide regional rotation parameter is nonzero wide. When shear strain in the area is large, the rotation parameter values high, indicating the degree of strain accumulation of faults may be higher. When the rotation parameter values are low, indicating the degree of strain accumulation of faults may be low, even in a creeping fault state. 2. The interseismic/coseismic surface displacement formulas of strike-slip/dip-slip faults with dip angles are deduced based on the analysis of particular fault profiles. The influence of fault dip angle on the surface displacement is analyzed from the mechanical property.(1)Generally, the largest inert-seismic deformation of a trike-slip fault is not on the surface where the fault exposes, instead lies at the upper edge of the sliding fault portion, i.e. surface projection of the dividing line between the locked section and the moving section of the fault, which depends on the fault dip angle.(2) There is a relationship of tan δ=d/doffset among the distance from the surface of the dividing line projected on the surface between the locked section and the moving section doffset, fault locking depth d and dip angle of fault δ, which reveals the relationship between the locking depth and the dip angle of the fault.(3) Due to influence of fault dip angle, the inter-seismic deformation curve centers at the on the surface projection aforementioned. When the shock occurs, the hanging wall and footwall of the fault move in opposite directions along the fault plane, leading to asymmetric coseismic displacements of two walls. The smaller fault dip angle, the greater displacement the hanging wall releases, while the less displacement on the footwall. For a blind fault, source rupture does not reach the surface, similar to the coseismic case.(4) Actual data are fitted by using the formula including fault dip on a strike-slip fault, of which the result is smaller than geological observations. The reason may be due to actual faults are curved rather than flat. On the surface, a fault is steep, but as the depth increases the fault dip can become smaller. Since a fault can slide under the locking segment, the influence of surface displacement is equivalent to the presence of a "flat fault" between the exposed surface of the ground and locking-sliding boundary. The result of tangent function fitting is just the "plane fault" inclination.3. The mechanical properties of seismogenic faults are studied using the three-dimensional numerical manifold method which has the advantage of unifying the continuum and non-continuum. The characteristics of the surface displacement field are obtained by carrying out the shear tests of loading different power forces setting the blocking of the upper fault and sliding of the lower fault. The dynamic changes of the Longmenshan fault and the east boundary of the Sichuan-Yunnan block body fault after the 2008 Wenchuan earthquake, and the locking depths of Longmenshan fault and Xiaojiang fault on the eastern boundary of Sichuan-Yunnan block body are explained using the DEFNODE method.(1) The three-dimensional regional block with the interior fault setting is established using the three-dimensional numerical manifold method. The regional blocks pushed by others and dragged by the soft flow substances at the bottom of brittle layer are simulated by loading "push" and "drag" forces. The results of the surface displacement field computed show more accurate arctangent function, which is in good agreement with the results obtained with the dislocations theory. The shear characteristics of the strike slip faults is first simulated exactly using the numerical simulation method, which shows the accuracy and superiority of the three-dimensional manifold method in computing continuous and non-continuous problems simultaneously.(2) The surface displacement field is calculated for the accurate arctangent function curve by using two kinds of force input with the same fault. However, the displacement curve obtained by loading the different natures of the "push force" shows the different characteristics:the arctangent curve is relatively flat in the distal fault for the mechanical properties of transmitting along the horizontal direction to the "push force". The distal curve basically reflects the heaped capacity of "push force", the broadband smaller distortion curve, the locking depth fitted using the arctangent function is smaller than the pre-set value. The arctangent curve is up warping in the distal fault for the mechanical properties of transmitting along the vertical direction to the "drag force". The distal curve cannot reflect the heaped capacity of "drag force" due to the decay when the force is transferred in the vertical direction. The locking depth fitted using the arctangent function is greater than the pre-set value.(3) The actual block should be not only pushed by the other blocks in the shallow subsurface, but also dragged by the soft flow material in the deeper subsurface. Two kinds of forces are loaded at the same time according to the different ratios. The results show that the surface displacement exhibits the characteristics of "push force" or "drag force" if mainly loaded "push force" or "drag force". When the "push force" or "drag force" loaded increases, surface displacement resulted show the characteristics close to the "push force" or "drag force". The movement characters of the upper and lower crust after the Wenchuan earthquake and the results calculated by DEFNODE negative dislocation theory can be explained based on above knowledge. After the Wenchuan earthquake, the Bayan Har block moved southeastward increasingly in the southern part of Longmenshan fault due to the coseismic release of the brittle layer in the hanging wall and footwall of the Longmenshan fault, which can be understood that the locking depth of the southern Longmenshan fault calculated became shallower caused by the "push" action of blocks. The abrupt acceleration of the upper brittle layer provided the loading power for the lower soft flow material. The locking depths calculated of the Xiaojiang fault increased resulted from the "drag force" on the brittle layer at the upper portion of the Sichuan-Yunnan block provided by the soft floe material entered from the lower part of the Bayan Har block obtained loading power.(4) The displacement results of faults at different depths calculated with loading "push" and "drag" forces show that the displacement below the locking segment of the fault gradually increases with depth, while not the abrupt boundary between the locking and sliding portions in the dislocation theory, which is consistent with the previous work.(5) The lower soft flow substance should gradually vary. If the "drag" force is improved so that it can gradually be loaded, the nearer to the fault, the smaller the load is. The surface displacement results showed that the deformation width of the surface displacement curve loading gradually "dragging force" is wider than loading not-gradually one. That means the surface deformation may become wider due to the gradual drag of soft fluid material.(6) The middle north-south seismic belt is modeled using the three-dimensional numerical manifold method. The results show that the direction of the principal stress increment and principal strain increment are consistent under the condition of elastic constitutive model and horizontal displacement of mathematical nodes. The Longmenshan fault is in the state of stress accumulation before the Wenchuan earthquake. The epicenter this earthquake is located on the edge of the high value area of compressive stress rates, and such rates in the southern Longmenshan fault (south of the Wenchuan earthquake epicenter) is greater than that in the northeastern part. The middle and northern Longmenshan fault was in the adjustment process and did not reflect the real stress accumulation, while its southern portion was in the state of stress accumulation during 2009 to 2013 after the 2008 Wenchuan earthquake. Tensional stress accumulated in the middle-northern section of the Xianshuihe fault, while tensile shear stress accumulated in its middle-southern section. The epicenter of 2013 Lushan earthquake is located in the weakening zone with maximum shear stress and rapid accumulation of strain in middle-northern section of the Longmenshan fault and the southern section of Xianshuihe fault. While the 2014 Kangding earthquake occurred in high value area of maximum shear stress accumulation in the southern section of the Xianshuihe fault. The maximum shear stress accumulation rates of the Anninghe fault was less than that of the Xianshuihe fault and Zemuhe fault.In a nutshell, the mechanical properties of seismogenic faults and surface dynamic changes are analyzed. Firstly, the dynamic changes of the surface velocity field are described quantitatively. Secondly, the improved fitting formulas are presented and analyzed from mechanical properties according to the surface displacement affected by the geometry of faults (mainly the dip angle). Finally, the surface displacement affected by the mechanical properties of the upper and lower fault is studied using the three-dimensional numerical manifold method. With the knowledge above, the relationship between the dynamic changes of surface displacement and the strain accumulation state of seismogenic fault is understood better. Further research is needed to carry out to address how to recognize the strain accumulation state of faults from the surface dynamic changes and assess the seismic risk using the three-dimensional numerical manifold method.