| Earthquake and its secondary disaster cause the most seriously human and economic cost of all the natural disasters. China is one of the most active regions of global inland seismic activity. From the statistic data of National Earthquake Information Center (NEIC), it is87times earthquakes occurred that magnitude exceeding8in the world since1990(as of Jan.1st2014), six of them happened in China. It is over30times earthquakes occurred at South-North Earthquake Zone that magnitude exceeding7since the16th century, and19earthquakes happened after1900. The Wenchuan and Lushan earthquake, happened in May12th2008and April20th2013respectively, give Chinese a wake-up call for the earthquake prevention and disaster reduction. And also it promotes the study of seismic origin and earthquake prediction in China.We analysis of the focus distribution of Tibetan Plateau, Longmenshan, Java island of Indonesia, the western Pacific island arc and Taiwan region, and explain the difference between the intraplate earthquake and plates boundary earthquake. We indicate that we should begin with the intraplate earthquake features for the study of continental earthquake in China.We indicate key regions of earthquake monitoring according to the position of North China asthenosphere uplift based on the telluric electromagnetic sounding and seismic tomography data. At the same time, we figured that the melt fractions is relatively concentration in the asthenosphere upwelling and formed low density (2.9～3.0g/cm3) atherosclerosis upwelling. At the depth of70-80km, the melt fractions is up to10-30%which is500-600℃hotter than that at the same depth. Based on the physical and chemical properties, we figured that the main earthquake energy came from atherosclerosis upwelling. By study of the relationship with earthquake focus among the crust structures, upper mantle and atherosclerosis upwelling in Tangshan and Fenwei rifting regions of North China area, we found that there are ductile layers beneath the epicenter in middle-lower crust, and the ductile layers is the main accumulation place of seismic energy and the energy storage bag od earthquake. The fluid and energy from the bottom of asthenosphere went up continually, so that the energy accumulating slow and long term in the storage bag and made the difference greater from the surrounding rock, and formed ’Sandwich structure’. The’Sandwich structure’changed the energy of ductile layer from thermal and chemical energy into strain energy, and it became the premise of cracking in the overlying brittle rock. The brittle faults prevalent distribute on the surface of earth is only the trigger structure and release mechanism of seismic energy. And also, it is the significant tunnel of earth surface expression of the earthquake precursory phenomena.The essential of earthquake is the geological process of the sudden releasing and long term accumulating of energy. Based on the analysis and study of background of seismic geology in North China, we made the releasing and accumulating of energy as the thread and established the Three-Level Tectonic Mode of intraplates. The basic idea is:in the deeper layer, atherosclerosis upwelling provides the main earthquake energy; in the middle layer, the ductile layer of the middle-lower crust is seismic energy accumulation structure; in the upper layer, the brittle faults is the trigger structure and release mechanism of seismic energy.Because of the increase of activities in north-south earthquake zone recently, and also to verify the Three-Level Tectonic Mode of intraplates, we apply the earthquake formation model into Longmenshan and Tianshui regions in the middle of north-south earthquake zone to study the origins of earthquake formation and the prediction of the epicenter.According to the former data of five geophysical profiles in Longmenshan region, we did research of crust-mantle structure in this area. From the results, we found that there is an atherosclerosis upwelling beneath the Longmenshan earthquake zone with about200km in width and about30km in height. The material and energy of the atherosclerosis uplift and transport that lead to the crust melting and formed10km thick ductile layer which is20-30km thick in deep. The formation of ductile layer made the energy focus on the interface or the peak of the greatest material competent difference, which means that beneath the Longmenshan fault there is a stress concentration3times as its interior stress. Under this great stress, brittle rock of upper Longmenshan fault formed the’microcracks’, and was accompanied by liquid and energy release slightly. Once it began the upper pre-existing fractures activities were increased by the releasing fluid and energy. Meanwhile, the’microcracks’developed into’crack chains’, then the strength of the energy storage bag around the brittle rock was reduced. When the accumulated stress is bigger than the strength of rocks, the earthquake happened. This process is similar with fluid inclusions burst. It could make the hard megablock rock of overlying strata fractured, oriental projected, high-speed and long-distance slide and a wide range of mountain rock comminuted fracture. Another possible way to trigger the earthquake is that the energy storage bag suddenly burst when fractured active in the brittle rock of overlying strata effect it.By analysis the crust-mantle structure features of Tianshui area on the base of high resolution seismic refraction profile, magnetotelluric sounding profile, velocity structure profile of S wave and dates of Cenozoic volcanic rocks. It is showed that there is a macromolecule mantle hot doming with the100km depth of top and300km diameters and it is the fundamental dynamic of forming the middle of north-south tectonic belt. There is a40km in depth and20km thick inter-crust ductile layer in upper mantle hot upwelling. The former research results of the geochemical and formation mechanism of kamafugite in this region provide evidence for the upper crust-mantle structure. In this region, the tectonic orientation is east-west in upper crust and that is north-south in deeper, so we call it ’overpass type’. Meanwhile, we believe that the overpass type structure of tectonic intersection in Tianshui area is the reason this area destructive earthquake-prone. The mantle upwelling material periodic uplift lead to the overlying rock periodic open and closed, it is the power source of opening-closing tectonics. When the overlying lithosphere is open, the mantle-derived magmas can easily break the thin crust and form ultra-alkaline rock and carbonate. It also took out energy dominated by heat and chemical, part of them accumulated into the high-conductivity and low-velocity layer. The accumulation of thermal energy and fluid melt the crust-mantle material, changed local mechanical properties of lithosphere and formed ductile layer. It also formed ’Sandwich structure’ with the competent layers below and above it. The stress is focus on the most different physical properties direction and increasing the energy accumulation. The earthquake is not happen until the stress from the accumulated energy over the brittle layer break strength or limit of friction of upside and edge ductile layer which could make the interface breaking and decoupling.The main clue of the three-level tectonic model for intraplate earthquake is the energy supply-accumulate-release, but the fluid play an important role in the energy accumulate and release. So we discuss the source of deep fluid, its effect and monitoring approach. We indicate the deep fluid came from remain fluid of earth processes and the earth surface fluid infiltration. In deep portion, the fluid is in supercritical or subcritical state with stable properties. Due to the external disturbance, the fluid became active. Fluid upwelling mainly functions are as following: cause mass migration, cause the energy immigration, the fluid expansion can be directly induced earthquake, lubrication fault and produce the generation of seismic precursor. According to the features of fluid transport, we indicate we can notice the seismic precursor caused by fluid active from thermal infrared temperature anomaly monitoring, atmospheric composition monitoring, the earthquake cloud monitoring, geoelectric monitoring and the groundwater environmental monitoring and supplies reliable data for earthquake prediction. To test and verify these theories, we monitor and analysis the earthquake precursor information of the middle north-south earthquake zone.(1) We studied the satellite thermal infrared anomalies features of the Mianzhu Ms5.6pre-earthquake by the thermal infrared data of’Fengyun2’satellite. We found there are obviously increasing thermal anomalies along the north Xianjiang, Inner Mongolia, Hebei and Shandong areas in the satellite thermal images before the Mianzhu earthquake. The thermal anomalies grew up to about2.2million km212hours earlier before the earthquake. After that, the anomalies areas immigrated to south and the thermal anomalies front arrived at Mianzhu3hours earlier before the earthquake and stay till it happened.(2) By using the methane concentration data at top of troposphere359hPa and the ground temperature data extracted from Atmospheric Infrared Sounder (AIRS) on NASA’s Aqua satellite, we studied the synchronism between atmosphere temperature increasing and methane emission, and the relationship between the variation of methane concentration and the fault activities of the atmosphere before the Lushan earthquake (Ms7.0) in the Longmenshan area. We found methane emission is correlation with atmosphere temperature increasing to some extent before the Lushan earthquake; there are strong relevant between the methane emission before the Lushan earthquake and Longmenshan structure features and earthquake activities, and the amount of methane emission obvious grow and expound to overthrust belt while it is closed to the earthquake happened and crust activities increased. The’microcracks’is the tunnel of methane emission and the’crack chains’ promote this process. It forms the thermal anomalies by the atmosphere temperature increasing under the greenhouse effect before earthquake.(3) According to the characteristic periods of climate elements, component signals caused by climate and human factors are decomposed and recognized from the volume mixing ratio of methane in the troposphere, using wavelet analysis method. And then, extract the variation of methane concentration signal caused by geological activity (geological methane). On this basis, analyze the relationship between the variation of geological methane concentration and seismic activity, take Wenchuan and Yushu areas for examples. The result shows that there is an obviously corresponding relationship between the variation of geological methane concentration and seismic activity. In addition,13peak values of geological methane concentration occurrence in Wenchuan area in2004～2006, and14peaks values occurrence in Yushu area in2004～2005. In total,21cases (78%) of the time difference of geological methane and seismic activity peak are less than2months, and the time difference decrease firstly and then increase in February2005, in these two areas. And, there is a positive correlation between geological methane concentration and seismic activity peak values in Wenchuan area. Finally, preliminary study on the source and variation of geological methane, the results indicate that geological methane comes from upwelling of mantle and crust fluids, and its concentration correlated closely with the temperature and pressure of ductile rheological layer and "microcracks" and "Rupture chain" in mid-upper crust.(4)We studied the earthquake clouds and the earth exhaust of July8th2013Dujiangyan Ms4.1pre-earthquake. We figured that the arcuate rope clouds are formed due to the super-low vibration of the local ground surface acting on the water vapour and dust in the sky. We indicate that the Dujiangyan Ms4.1earthquake is an aftershock of the Wenchuan Ms8.0earthquake and both have the same formation mechanism. The Beichuan-Yingxiu thrusting with right-lateral slip resulted in a sudden release of the energy accumulated at the edge of the boundary between differential rheological layers, a density saltation position in the middle crust.(5)We analyzed the fingerprint characteristics and its causes of electrical signals from the SW monitor in the Minxian Ms6.6earthquake occurred in July22nd,2013. We believe that the cyclic activity of hot doming in Tianshui led to the abnormal of terrestrial electricity. This provides the basis for the success of the earthquake prediction. And the hot doming is not only the source of the material and energy during earthquake, but also brings plenty of fluids for the upper crustal structure. These fluids aggravate the physical differences of crustal rheological layers, lubricate the fault and accelerate the earthquake at the same time. |
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