不同时期水库地震活动主要影响因素讨论以三峡库区微震活动为例

结合微震活动的流体作用强度检测及孔隙压扩散模拟,讨论了三峡库区不同时期微震活动的主要影响因素。以2008年9月蓄水季为界划分前、后期,前期流体渗透导致的孔隙压力增加,使裂隙或断层面强度降低,是库区微震活动的主要影响因素,这一时期微震频次及ETAS模型参数μ值有起伏地缓慢增大,与库水位加卸载过程关系不明显; 后期由于流体渗透引起的孔隙压力变化趋于零,在新的流体平衡条件下,库水位加卸载过程所导致的裂隙或断层面上的应力变化,成为库区微震活动的主要影响因素,这一时期微震频次及μ值显示出与水位变化明显的关联特征。库区小震震源深度的时间变化支持上述观点。在此基础上,进一步讨论了水库“诱发”和水库“触发”地震的力学差异,认为前者主要缘于流体渗透导致的裂隙或断层面强度的“主动”降低,后者则主要与库水加卸载所导致的裂隙或断层面上应力增强有关。进一步推论认为,流体对小地震“诱发”、“触发”皆可能发生,但中强地震缘于流体“诱发”的可能性非常小,对水库区发生的中强地震,流体仅可能对处于临界状态的断层系统起到“触发”作用。     

唐山大震前的地震空区和围空地震的机理

本文通过分析区域小震活动的变异,指出:1)唐山大震前不同震级下限的围空图象,实际上是唐山附近的地震平静和外围三支小震条带活动,二者合成的结果;2)空区边缘的应力集中和地震活动增强,主要是断裂带活动的结果.最后用强度和应力分布不均匀的断层模式讨论了围空图象形成的力学过程.      

2-D elastic FEM simulation on stress state in the deep part of a subducted slab

Introduction Geoscientists concern the explanation of deep focus earthquakes greatly. As a great progress in understanding the formation of deep focus earthquakes, Sung and Burns (1976a, b) proposed that olivine could exist in transition zone in metastable form in the core of some cold subducting slabs, which have got supports from laboratory studies (Rubie, Ross, 1994). Iidaka and Suetsugu (1992) used the seismic data recorded by the dense seismograph networks in Japan to study the travelti…     

俯冲带深部应力场的二维弹性有限元数值模拟

基于一些简化的数值模型，根据弹性本构关系，用平面应变有限元方法计算了相变引起的体积变化、板块内部温度差、密度异常及边界力产生的应力场分布情况．数值模拟结果显示，热应力能够解释俯冲带深源地震的应力场方向特征，但解释不了深源地震的深度分布特征；有亚稳态橄榄石存在时密度异常所产生的应力场特征与地震观测结果所显示的应力场特征有所偏离；虽然橄榄石——尖晶石相变体积变化所产生的应力在橄榄石——尖晶石相变过渡区附近有最大值，其数值远远超过温度差和密度异常产生的应力场的最大剪应力数值，但在相变界面附近的区域，主压应力方向垂直于相变界面的方向，与地震观测结果所显示的主压应力的方向不一致．所以，不能用弹性模拟得出的俯冲带温度变化所产生的热应力、俯冲带密度变化所产生的应力、相变体积变化所产生的应力来对深源地震进行简单化的解释．      

福建仙游震群的孔隙压扩散特征

2010年8月至2015年12月,福建仙游地区发生了一系列小震群活动。基于孔隙压力扩散机制对精定位的小震数据进行分析,结果显示仙游震群序列在空间上呈丛集分布,且具有明显的分区现象,发震时间受金钟水库水位变化的调制作用较为明显。5个分区（A—E）的流体孔隙压力扩散系数分别为0.04,0.08,0.07,0.12和0.05 m2/s,伴随着地震活动性由强至弱,孔隙压扩散系数呈先增后减的变化规律。仙游震群位于断裂构造发育区内,震中分布优势方向与石苍断裂一致,沿着断裂走向的孔隙压扩散系数最大,说明地下流体沿狭长的石苍断裂由NW向NE活动。各分区的流体孔隙压系数与扩散最大距离、最大诱发地震震级及地震释放能量均有较好的相关性。此外,当金钟水库水位下降时,各分区的扩散距离r呈现减小趋势,震群范围收缩,地震频次增加,并伴有较大震级的地震,说明水位下降时,流体孔隙压力的扩散作用在同一地点反复作用,使得触发源附近区域的应力水平更容易达到饱和或临界状态,进而触发较大震级的地震,这也解释了水位下降过程中地震更为强烈的原因。      

关于应力触发和应力影概念在地震预报中应用的一些思考

应力触发和应力影概念及其在地震预报中的应用,是学术界关注的热点,尚有一系列需要研究的问题。例如初始应力场的确定,合理流变模型的选取和计算,孔隙压力的变化和影响,断层强度随时间的变化等。特别在估计应力场变化时,人们现今只注意到大地震对应力场造成的变化。一些新的观测事实表明,大量的小地震效应,慢地震的效应和无震滑动的效应,也不容忽视。定量考虑它们的效应,是今后应力触发和应力影研究中应该注意的课题。     

The effect of pore fluid on seismicity: a computer model

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Preliminary Research on Relation Between Field and Source in Earthquake Preparation

The "field" and "source" are two important branches in seismology and earthquake research.In this paper,we use a computer model to study the relation between the field and the source in earthquake activity.In our modeling,a network of nonlinear elements is used to simulate seismic activity of seismic zones in a seismotectonic block.Constant strain rate is enforced on the boundary of the model,cyclic seismic activity,quasi-periodic variation of stress field intensity,and strain energy are observed with high and low fluctuations.There is a main seismicity area in seismic cycles,and the main seismicity area shows the spatial migration during different seismic cycles.If the precursory area is related to high element stress,it is found that the development of precursors in our model is quite complicated.No certain relation between the precursors and earthquakes has been discovered.Anomalies show different characteristics in the seismic quiet period and active period.All of the seismic zones in the system hav     

鲜水河断裂带地震活动特征及强震发生随时间增长概率

作者等仔细分析了鲜水河断裂带从１７２５年到现在的地震资料，并利用乌莫洛夫Ｔ─Ｓ、Ｍ为参数的作图法及强震发生随时间增长概率，绘制了地震活动图件及地震发生概率曲线以及Ｍ─Ｔ图和鲜水河断裂应变释放曲线。根据这些资料可以清楚地看出鲜水河断裂带自１７２５年到现在可分为两个大的活动周期，其中６．０级以上地震有由康定依次向甘孜迁移的特点。在每一个大的地震活动周期中，地震基本上两次重复由康定向甘孜迁移的过程，而且较强地震多发生在第二次迁移过程中，１９８２年甘孜地震标志着断裂带在第二幕地震活动高潮中，中强震已经完成了最后一次由断裂带东南端向西北端迁移的过程。同时考虑到断裂带应变释放曲线的特征，估计鲜水河断裂带目前已进入新的平静阶段。前两个大的活动周期之间，平静了近一个世纪。按历史上地震定向迁移规律，估计在新的活动期地震仍将从康定方向开始，逐步向甘孜发展。     

中国大陆地震活动分期及其与构造运动的关系

利用应变积累释放曲线的方法，讨论中国各主要地震区带的平静－活跃期现象及其与构造运动之间的联系。中国西南地区的地震活动反映了由印度－欧亚板块碰撞引起的动力作用由西向东，由南向北的推过程。     

Speculation on the causes of continuing seismicity near Koyna reservoir,India

The cause for continuous induced seismicity at Koyna is not well understood. A heuristic model based on various physical parameters observed at Koyna is being proposed to explain the ongoing seismicity. This model contains two essential elements: (i) Intersecting faults near Koyna provide means of stress build-up in response to plate tectonic forces. (ii) The annual reservoir loading cycle and changes in the ground water table perturb this stress build-up by an influx of pore pressure in a fluid infiltrated medium. Hence, the spatial and temporal pattern of the pore prussure distribution and the seismicity will be governed by the location and hydromechanical properties of the faults and fractures. The predictions of the model can be tested by comparing the temporal and spatial pattern of seismicity with the changes in lake level and water table.     

Paterns and regularity of ring distribution of seismic activity before great earthquakes in China

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华北地区部分活动断裂面现今应力状态与地震活动的时域变化关系

采用我国华北地区1966～1984年原地应力测量数据49组,结合各测点附近主要活动断裂的几何要素进行了分析,发现地壳浅部断裂面上的有效正应力和剪应力的比值随时间变化与断裂附近地震活动存在某种对应关系。即使在同一断裂上同一时域内,由于不同地段地震活动性的差异,所对应的有效应力比值亦有不同的变化。分析认为,当断裂面上有效应力比值在0.5以下时,断裂附近地震活动微弱,断裂呈现稳态,大于此值时,地震活动增强,断裂失稳     

Patterns and regularity of ring distribution of seismic activity before great earthquakes in China

A systematic study on “ring phenomena” frequently occurring before great earthquakes has made in this paper, which has analyzed the features of ring distributions before 16 great earthquakes and part of large earthquakes in China and its boundary areas, and discussed their features of generality, regularity and predictive meaning. The results have showed that moderate earthquakes or larger earthquakes distribute around the epicenter like a ring from decades to hundred years before the great earthquakes of magnitude more than 7, which is a general phenomenon of great earthquakes without an exception. The active ring generally occurs in the areas from hundreds to thousands of kilometers from the epicenter (according to the magnitude). The seismicity in the ring has three basic stages with different features. in the first stage, seismicity remains at low level and the earthquakes distribute scatteredly, while the source area of the future great earthquake remains quiet; in the second stage, the seismicity strengthens, whose frequency, intensity, concentrated degree, released rate of strain and ratio of distributed area etc. increase, while the quiet area decreases or disappears; in the third stage, the seismicity is weaker than in the former stage, and the quiet area appears again. The source area surrounded by the active ring might have three periods of activity (called as early-term, medium-term and late-term foreshocks activity). The length of the quiet area undergoes the process from large to small, then to large. Therefore, we can estimate the occurring place, magnitude and seismogenic stage of great earthquake according to the area, length and the seismicity in the active ring, which is valuable to make a long-term prediction of great earthquakes. At last, we had a preliminary discussion on the mechanism of active ring formation.     

INFLUENCE OF PRESSURE HEAD CHANGE AND ITS CHANGE RATE ON RESERVOIR TRIGGERED SEISMICITY -A CASE STUDY OF ZIPINGPU RESERVOIR

For analyzing the role that reservoir impounding plays in triggering earthquake, the process of diffusion of pore pressure and its mechanism of action should be understood firstly. The temporal distribution of seismicity, which occurred before the M_S8.0 Wenchuan earthquake, following the impoundment of Zipingpu reservoir is studied in this paper. Then the mechanisms of the occurrence and development of reservoir triggered seismicity are discussed. A comparative analysis of the temporal distribution of seismicity and the submerged area by reservoir impounding is carried out firstly. Then the influence of various factors on modeling is analyzed in detail. After calculating, the pore pressure change by the Zipingpu reservoir impoundment is obtained. The following observations are made:(1)Conspicuous swarms of earthquakes, of which the sources are located on the same fault of the M_S8.0 Wenchuan earthquake, occurred orderly with the impoundment of Zipingpu reservoir.(2)Because of the influences of the terrain and the medium, the range of effect of pore pressure change by the impoundmemt is limited and anisotropic. Hydraulic diffusivities(D)of 0.7 and 0.35m²/s along the fault strike and the fault dip are reached respectively by a semi-quantitative assessment. Of course, the qualitative pressure constraints on the surface are also applied for the modeling.(3)The calculation results show that the temporal distribution of seismicity near the Zipingpu reservoir is related with the pore pressure change. After the pore pressure reached the threshold of triggering earthquake, whether the pressure head change is high or not, the change rate of pressure head change plays a key role in the decrease or increase of seismicity. It means that the triggered seismicity by pore pressure is a dynamic triggering process.     

Pore fluid pressure effects in anisotropic rocks: Mechanisms of induced seismicity and weak faults

Many observations and studies indicate that pore fluid pressure in the crustal rocks plays an important role in deformation, faulting, and earthquake processes. Conventional models of pore pressure effects often assume isotropic porous rocks and yield the nondeviatoric pressure effects which seem insufficient to explain diverse phenomena related to pore pressure variation, such as fluid-extraction induced seismicity and crustal weak faults. We derive the anisotropic effective stress law especially for transversely-isotropic and orthotropic rocks, and propose that the deviatoric effects of pore fluid pressure in anisotropic rocks not only affect rock effective strength but also cause variation of shear stresses. Such shear stress variations induced by either pore pressure buildup or pore pressure decline may lead to faulting instability and trigger earthquakes, and provide mechanisms for the failure of crustal weak faults with low level of shear stresses. We believe that the deviatoric effects of pore fluid pressure in anisotropic rocks are of wide application in studies of earthquake precursors and aftershocks, oil and gas reservoir characterization, enhanced oil recovery, and hydraulic fracturing.     

新疆库尔勒活动断裂和断层破裂段探讨

新疆南疆重镇库尔勒市及附近地区，至少分布着９条颇具规模的活动断裂。尤其斜切库尔勒市区北部的库尔勒断裂，新活动十分强烈。库尔勒断裂东段与兴地断裂西段很可能是连通的一条地震破裂段，两者均存在着丰富的多期次古地震事件的遗迹，为较稳定的断层“破裂单元”。该破裂段上现代地震活动异常平静，但发现了明显的蠕滑错开形迹。这种地震的发生与断层滑动不协调现象，说明断层运动受到了抑制，能量在某处得以积累，有酿成大震的条件和危险。     

Analysis of induced seismicity for stress field determination and pore pressure mapping

The focal mechanisms of some one hundred microseismic events induced by various water injections have been determined. Within the same depth interval, numerous stress measurements have been conducted with the HTPF method. When inverted simultaneously, the HTPF data and the focal plane solutions help determine the complete stress field in a fairly large volume of rock (about 15×10⁶ m³). These results demonstrate that hydraulically conductive fault zones are associated with local stress heterogeneities. Some of these stress heterogeneities correspond to local stress concentrations with principal stress magnitudes much larger than those of the regional stress field. They preclude the determination of the regional stress field from the sole inversion of focal mechanisms. In addition to determining the regional stress field, the integrated inversion of focal mechanisms and HTPF data help identify the fault plane for each for each of the focal mechanisms. These slip motions have been demonstrated to be consistent with Terzaghi's effective stress principle and a Coulomb friction law with a friction coefficient ranging from 0.65 to 0.9. This has been used for mapping the pore pressure in the rock mass. This mapping shows that induced seismicity does not outline zones of high flow rate but only zones of high pore pressure. For one fault zone where no significant flow has been observed, the local pore pressure has been found to be larger than the regional minimum principal stress but no hydraulic fracturing has been detected there.     

Numerical simulation of the influence of lower-crustal flow on the deformation of the Sichuan-Yunnan Region

On the basis of distribution of active fault and regional rheological structure, a three-dimensional finite element model of Sichuan-Yunnan region, China, is constructed to simulate contemporary crustal motion and stress distri- bution and discuss the dynamic mechanism of crustal motion and deformation in the Sichuan-Yunnan region. Lin- ear Maxwell visco-elastic model is applied, which includes the active fault zones, the elastic upper crust and vis- cous lower crust and upper mantle. Four different models with different boundary conditions and deep structure are calculated. Some conclusions are drawn through comparison. Firstly, the crustal rotation about the eastern syntaxis of the Himalaya in the Sicuan-Yunnan region may be controlled by the special dynamic boundary condition. The drag force of the lower-crust on the upper crust is not negligible. At the same time, the main active fault zones play an important role in the contemporary crustal motion and deformation in Sichuan-Yunnan region.     

An earthquake instability model based on faults containing high fluid-pressure compartments

It has been proposed that large strike-slip faults such as the San Andreas contain water in seal-bounded compartments. Arguments based on heat flow and stress orientation suggest that in most of the compartments, the water pressure is so high that the average shear strength of the fault is less than 20 MPa. We propose a variation of this basic model in which most of the shear stress on the fault is supported by a small number of compartments where the pore pressure is relatively low. As a result, the fault gouge in these compartments is compacted and lithified and has a high undisturbed strength. When one of these locked regions fails, the system made up of the neighboring high and low pressure compartments can become unstable. Material in the high fluid pressure compartments is initially underconsolidated since the low effective confining pressure has retarded compaction. As these compartments are deformed, fluid pressure remains nearly unchanged so that they offer little resistance to shear. The low pore pressure compartments, however, are overconsolidated and dilate as they are sheared. Decompression of the pore fluid in these compartments lowers fluid pressure, increasing effective normal stress and shear strength. While this effect tends to stabilize the fault, it can be shown that this dilatancy hardening can be more than offset by displacement weakening of the fault (i.e., the drop from peak to residual strength). If the surrounding rock mass is sufficiently compliant to produce an instability, slip will propagate along the fault until the shear fracture runs into a low-stress region. Frictional heating and the accompanying increase in fluid pressure that are suggested to occur during shearing of the fault zone will act as additional destabilizers. However, significant heating occurs only after a finite amount of slip and therefore is more likely to contribute to the energetics of rupture propagation than to the initiation of the instability.We present results of a one-dimensional dynamic Burridge-Knopoff-type model to demonstrate various aspects of the fluid-assisted fault instability described above. In the numerical model, the fault is represented by a series of blocks and springs, with fault rheology expressed by static and dynamic friction. In addition, the fault surface of each block has associated with it pore pressure, porosity and permeability. All of these variables are allowed to evolve with time, resulting in a wide range of phenomena related to fluid diffusion, dilatancy, compaction and heating. These phenomena include creep events, diffusion-controlled precursors, triggered earthquakes, foreshocks, aftershocks, and multiple earthquakes. While the simulations have limitations inherent to 1-D fault models, they demonstrate that the fluid compartment model can, in principle, provide the rich assortment of phenomena that have been associated with earthquakes.