The Study Of Deformation Characteristics Near Sorce Area Before Wenchuan Ms8. 0 Earthquake

Before the earthquake of Wenchuan, there is a variety of deformation data in the epicentral area and its nearby, such as GPS, tilt, level across the fault, gravity, etc. According to deformation observation data analysis before wenchuan earthquake study shows, compared with far source zone, deformation trend of near source area has weakened, and presents variation belt near the Longmenshan fault. Period of 3 to 8 years before wenchuan earthquake, deformation and gravity change is relatively significant, and period of 1-3 years before the earthquake change gradually flatten out. In general, it presents a feature that the more near the epicenter, the less significant of deformation, the closer before the earthquake, the gentler of deformation.According to data and theoretical analysis of the deformation field, gravity field and density field, proved that there are relatively close relationships with each other. In view that the crustal material density changes can intuitively reflect the deep motivation of Wenchuan earthquake’s "pregnant", occurrence and development process, and to reveal the seismic deformation phenomenon, we conducted the research of using gravity to inverse crustal density. Using of the mobile gravity data between 1996 and 2007 which provided by Longmenshan regional gravity network, with the help of 3 d inversion technique of gravity potential field, we get the dynamic change process of different depth of material density near the quake zone. Results show that, firstly, from the view of spatial distribution, the distribution of density change in the whole area is uneven and in some individual area is significantly, especially in the Longmenshan fault zone and its nearby where is a gradient zone in the bouguer gravity anomaly map, crustal thickness map and density variation maps, and is also a strong zone of material change and differential movement of tectonic deformation. This shows that the surface structure and deep structure are both in Longmenshan belt. Results also shows that on the time series, rates of crustal medium density change is horizontal inhomogeneity and periodic. The density change of the longmen area has the tendency of increase 10 years ago before wenchuan earthquake and appears the most dramatic change in 5-8 years before the earthquake, while the change tend to be more gentle the 1-2 years ago before earthquake. In addition, within a certain depth, the degree of density increase is more and more significant with the increase of the depth, which means deep crustal activity is much bigger than shallow activities. The last, in the short term before the earthquake, the spatial distribution of gravity and density field changes is well ordered and relatively concentrated on the Longmenshan Fault zone and its vicinity, which is accord to the phenomenon of minus entropy in deformation field before earthquake. So we conclude that the trend of density field’s dynamic change is consistent to the trends of gravity field and deformation field changes.The results of the density field’s dynamic change provide support and evidence for the selection of appropriate earthquake generation’s model and the dynamics’ evolution process. On one hand, to explain the contradictory phenomena between “small deformation” in shallow crust and significant density changes in deep, we think the Wenchuan earthquake’s "pregnant", occurrence and development are the result of the earth’s upper crust and lower crust is not in the same power source matching system which leads the upper and lower crustal movement to be not unified. On the other hand, combining the features of density changes, we select it as the suitable generation model of Wenchuan earthquake that combination- hardening model. This partly reveals the phenomenon that there was no significant deformation on short-term before the earthquake in the near-source area, and at same time, deepens the understanding of the mechanism of deformation precursory changes before the earthquake.The research ideas of this paper is that through the surface deformation and deep geophysical data to establish the evolution process of earthquake preparation and the dynamic process of earthquake near source area, and with the deep geophysical data to provide support for model of earthquake preparation, then discusses the mechanism of small deformation before the earthquake. Due to the deformation of deep underground is related to many uncertain factors, and the work of deep deformation detection is difficult, this paper’s studies failed to make perfect explanation to ground deformation mechanism, but put forward a feasible method for deep density change and crustal deformation research.