A Study On Deformation Localization Of Strike-slip Fault Zones And The Process Of Earthquake Nucleation

Modern plate boundaries are the zones with the most intense tectonic activities, as well as the largest earthquake intensity and the highest earthquake frequency. Interaction between plates, combined with Eura-Asia plate internal deep dynamic action result in different types of active tectonics and control large earthquake distribution in space in mainland China. The most obvious characteristic is large faults in late Quaternary which cut mainland into fault-blocks in different levels.Active fault-block boundary tectonic zones produce large intraplate earthquakes, bcause boundaries of active fault-blocks act as discontinuous zones with the most intense differential movements during the processes of active tectonic deformation, stress releases, strain localizations, and displacement occurrence. Boundary tectonic zones of regional fault-block zones and fault-blocks can generate large earthquakes. Strike-slip fault activity and earthquakes of strike-slip type are obvious in these tectonic zones.One of the important issues in geology is how deformation concentrates on a fault or some part of the segment (deformation localization). Because earthquake nucleation is valuable in earthquake forecast, it is one of the focused issues in earthquake research as well. In fact, both of them reflect the process of seismic source forming, developing and rupturing in some place. This process is complicated and has not realized yet. This work studies the pivotal movement pattern as one part of the process.This thesis is on the basis of the pivotal movement model and characteristics of tectonic geomorphology, deformation, seismogenic process and mechanical analysis. The research objects are the Zemuhe fault Daqing segment and Fuyun fault, whose pivotal movements are typical. Combined with tectonic geomorphology measurements, trench analysis and numerical simulation, pivotal movement is analyzed quantitatively. Then this work further analysed and described the characteristics of deformation and seismogenics of strike-slip faults, to enrich the research content and try to understand the process of seismogenics and occurrence of earthquakes, which support the further understanding generation and occurrence process of earthquakes.According to summarized examples of pivotal movement, theory models, field observations and numerical simulation, the results on kinematic characteristics and process, stress, strain evolution and deformation localization of strike-slip faults are as follows:1.There are two different stages in strike-slip process:pivotal movement stage and big stick-slip stage. In pivotal movement stage, stress accumulates, strain localizes and centralizes to generate earthquakes, and vertical displacement is notable. The stage will be a very long time. In big stick-slip stage, stress is released, earthquakes occurred, and horizontal displacement is dominant. This stage is very short.2.Pivotal movement is an important form of structure and geomorphology deformation of strike-slip faults. There are two types ofthis motion:(1)transverse type, there is transverse structure intersect to fault plane;(2)slip type, there is no transverse structure. There are two types of slip as well, one is simple titling, just like the Fuyun fault, for example. The other is more complex, not only titling but also rotating. The Zemuhe fault Daqing segment is this type.Tectonics and geomorphology are distributed as four quadrants resulted from local stress field caused by pivotal movement, which is not regional deformation. Pivotal axis is the point of support of two walls’s movement,which is the aera where earthquakes occur, and strike-slip is blocked.In the whole stage of pivotal movement, the axis is the place where compression is the most intense,stress is localized and released.3.There are three factors which can influence the pivotal movement:(1) Dip angle of fault plane.When the angle increases, vertical displacement increases as well and titling is going on;(2) The area of theblock region. When the area increases, vertical displacement decreases and titling is blocked.(3) Block type. Fuyun and Daqing have two block types, movement block (friction block) and structure block(inclination of fault plane changes). Fuyun is controlled by the movement block, and Daqing is controlled by both. Movement block and structure block make vertical displacement decrease. Movement block is the major block factor, and structure block can not generate pivotal movement alone.4.In pivotal movement stage, stress on fault plane centralizes from all directions to the block region. There will be one stress centralization area to keep accumulating stress, and stress centralizes from outside to the core, like a concentric circle,growing large all the time. The largest stress in the whole stage always occurs in the core, and the core is much smaller than the block region. This process exists in a long period, in the whole stage of pivotal movement, different from earthquake nucleation in short-term in geophysics.5.In big stick-slip stage, stress values exceed the limit of the core, and the stress will release in a very short time. On the one hand shear stress increases obviously, large horizontal displacement appears and replaces vertical displacement. On the other hand, the stress in initial rupture releases slower than the hypocenter, and the largest horizontal displacement occurs.