| Non-elastic tectonic deformation of heterogeneous crustal media with complicated structure is a research subject attracting much attention in recent years. In addition to activities in the shallow crust, earthquakes and volcanic eruptions are associated with stress relaxation of visco-elastic deformation at depth with time and its transfer to the elastic upper crust. On this issue, heterogeneities of the lithosphere in both vertical and horizontal directions should be considered.A volcanic system, including magma chambers, country rocks, magma channels, fissures, and other media, represents a complicated earth material. Magma chambers are active low-viscosity bodies in the earth, and their surroundings are visco-elastic transitions which are important in the stress transfer. Generation of magma channels and fissures as well as earthquakes in the volcanic area are resulted from stress transfer and release. The Tianchi volcanic area of the Changbaishan Mountains has an activity period from 2002 to 2004, and many earthquake swarms occurred in the area. The GPS result also showed the volcano inflates. Volcano activity not only is related with deep activity in earth, but also comes under the influence of the shallow structure deformation. Therefore, besides understanding magmatic activity, we needs to understand the characteristic of upper crust activity in the volcanic area under the joint action of the dominate stress field and the secondary stress fields.This thesis chooses the Tianchi volcanic area in the Changbaishan Mountains as the target of the case study. The primary purpose is to analyze features of the stress and deformation fields in a medium which is inhomogeneous in both horizontal and vertical directions. Based on a conceptual model, upper crust of the volcanic area is modelled by an inhomogeneous combination of viscous and elastic materials. This work analyzes the stress distribution, transfer and its physical mechanism in the shallow crust containing a magma chamber. Meanwhile, stress adjustment in the condition of an existing fault is simulated. Using a 3D visco-elastic finite element model, and combined conditions of the regional tectonic stress and local magma force, the stress field of the study area is simulated. The result indicates that the viscidity difference can produce local anomalies of stress, which are related with the earthquakes in the volcanic area. It means that the viscidity of magma plays an important role in the process of the regional stress accommodation.1,This thesis analyzes the influence of inhomogeneous combination of viscous and elastic materials on the stess field in horizontal direction in the situation without magmatic activity. With the condition of constant velocity, the analysis of a model containing a low-viscosity body shows that the stress transfer is non-uniform in both vertical and horizontal directions. It is generated by the strain difference between the low-viscosity body and its surrounding elastic medium. Such a heterogeneity of stress will exhibit in the elastic layers near the surface. Because of visco-elastic coupling, stress can concentrate rapidly in the elastic layers, while existence of the low- viscosity body has effect on the distribution of the stress in the elastic layer. The modelling result indicates that the low-viscosity body close to the surface can produce notable change of the stress field, while the low-viscosity body at depth far from the surface has little impact on the stress field. Besides, the effect of depth on the stress is larger than that of the shape of the low-viscosity body. Many times of model tests reveal that a smaller depth of the low-viscosity body can make the relaxation time of the crust longer, hence the thickness above the low-viscosity body determines the relaxation time of the crust.2,The relationship between the low-viscosity body and stress field is influenced by faults. With models of faults and low-viscosity, which are combined in different manners, i.e. the fault lies above, on one side, and both sides of the low-viscosity body, respectively, this work performs numerical modelling of stress distribution. The result shows that the fault can weaken the stress field. When the faults lie on the both sides of the low-viscosity body, outside range of a distance from the low-viscosity body, the weakening effect is very large; and with increasing distance between the faults and low-viscosity body, it becomes small. In addition, more faults will produce stronger weakening on the stress field.3,A frequency spectrum analysis is made to the Ms>l earthquake data recorded by permanent and temporary stations in the Changbaishan Mountains area since 2002. The result shows that the principal frequencies of earthquakes are 2Hz and 5Hz, with b values 0.84 in the summer of 2002, and 0.93 in the summer of 2003. Usually earthquakes in volcanic areas are of a dilatation type, while the observed events of the study area is of shear mechanism, implying that these shocks are tectonic events rather than volcanic earthquakes. The relationship between the magnitude and time shows that there are four types of seismic swarms in the Changbaishan Mountains area, and the seismic swarms occurred about every half year. But these earthquakes did not continue very long. Since 2005, the seismicity of this area tended to be quite. It is speculated that the earthquakes in the period 2002-2004 is likely a short process which was triggered by the M7.2 deep-focus earthquake at Wangqing on 29 June 2002. Such triggering is associated with the viscid stress relaxation in the crust and upper mantle. Because stress relaxation can produce redistribution of stress through coupling deformation, in conjunction with differential stress around the magma chamber, triggering fault slips and earthquakes.4,This work designs two models, one with regional stress field, and another without it. The modelling result shows that the maximum shear stress of the first model is larger than that of the second model, meaning the joint action of the regional stress and local secondary stress can produce larger shear stress, which is a partial reason for the earthquakes of the volcanic area.5,Viscidity of magma can influence volcanic processes, such as its eruption forms and uplift of the surface. The larger the viscidity of magma, the more obvious the surface uplift by magma upwelling, and more possible of large-scale eruption when a critical condition is reached. This thesis uses a 3D visco-elastic model to simulate the stress field under the condition of dilatation of a magma chamber, and makes comparison with Mogi's model. The resulting maximum shear stress on the surface appears above the flank of the magma chamber.6,This model can also simulate the sudden change of pressure in the magma chamber which is related with deformation of the upper crust. It can be divided into three stages: (1) Magma rises and accumulates in the magma chamber, pressure increasing but unable to deform the upper crust. (2) The surface begins to uplift when the pressure exceeds the strength of crust. and (3) Uplift stops as soon as the pressure within the chamber is no larger than the outside pressure. These three stages consist a cycle. The effect of depth of the magma chamber on vertical displacement of the surface is larger than that of the radius of the chamber.The innovative points of this thesis are summed up below:It studies the stress distribution in inhomogeneous crust under the pressure of magma by using 3D finite element model, and explains the temporal-spatial distribution and mechanism of earthquakes in the Tianchi volcanic area of the Changbaishan Mountains. It discusses the role of magma viscidity, magma chamber and magma activity, in the tectonics of the volcanic area. With a visco-elastic model containing low-viscosity bodies, faults and other structural elements, numerical simulation is made to demonstrate that the non-uniform distribution of stress and abrupt change of differential stress can trigger intraplate earthquakes.
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