Triggered earthquakes and deep well activities

Earthquakes can be triggered by any significant perturbation of the hydrologic regime. In areas where potentially active faults are already close to failure, the increased pore pressure resulting from fluid injection, or, alternatively, the massive extraction of fluid or gas, can induce sufficient stress and/or strain changes that, with time, can lead to sudden catastrophic failure in a major earthquake. Injection-induced earthquakes typically result from the reduction in frictional strength along preexisting, nearby faults caused by the increased formation fluid pressure. Earthquakes associated with production appear to respond to more complex mechanisms of subsidence, crustal unloading, and poroelastic changes in response to applied strains induced by the massive withdrawal of subsurface material. As each of these different types of triggered events can occur up to several years after well activities have begun (or even several years after all well activities have stopped), this suggests that the actual triggering process may be a very complex combination of effects, particularly if both fluid extraction and injection have taken place locally. To date, more than thirty cases of earthquakes triggered by well activities can be documented throughout the United States and Canada. Based on these case histories, it is evident that, owing to preexisting stress conditions in the upper crust, certain areas tend to have higher probabilities of exhibiting such induced seismicity.     

Analytical and numerical analysis of pressure drawdown in a poroelastic reservoir with complete overburden effect considered

Starting from an analytical reservoir model that incorporates full interaction with an elastic overburden, a new hybrid mathematical approach is developed by combining two numerical discretization methods. A tabular reservoir (petroleum reservoir or an aquifer) in an infinite or semi-infinite domain is viewed as a macroscopic displacement discontinuity, allowing use of the efficient displacement discontinuity mathematical method to calculate stresses and displacements that arise because of pressure changes. A 3-D finite element method using a poroelastic formulation is used to discretize the reservoir itself. By coupling the displacement discontinuity and finite element methods, a 3-D large-scale poroelastic reservoir can be simulated within an infinite or semi-infinite domain. The numerical model has been verified through comparison to known solutions, and some time-dependent pressure drawdown problems are analyzed. Results indicate that including the complete overburden (reservoir surroundings) response has a significant effect on pressure drawdown in a poroelastic reservoir during pumping, and should be incorporated in appropriate applications such as well test equations and subsidence analyses.     

Stresses and excess pore pressure induced in saturated poroelastic halfspace by moving line load

This paper examines stresses and excess pore fluid pressure that are induced in a saturated poroelastic soil of halfspace extent by a concentrated line load. The line load is moving at a constant velocity along the surface of the poroelastic halfspace. The governing equations for the proposed analysis are based on the Biot's theory of dynamics in saturated poroelastic soils. The governing partial differential equations are solved using Fourier transforms. The solutions for the stresses and excess pore pressure are expressed in the forms of inverse Fourier transforms. The numerical results are obtained by performing the numerical inversion of the transform integrals. A parametric study is presented to illustrate the influences of the velocity of moving load and the poroelastic material parameters on the stresses and excess pore pressure. At a high velocity, the maximum values of the stresses in a poroelastic halfspace are smaller than those in an elastic solid, whilst at a low velocity the stresses in a poroelastic halfspace are larger than those in an elastic halfspace. The potential of diffusivity has an important influence on the stresses and excess pore pressure.     

Impact of Poroelastic Response of Sandstones on Geothermal Power Production

During geothermal power production using a borehole doublet consisting of a production and injection well, the reservoir conditions such as permeability k, porosity φ and Skempton coefficient B at the geothermal research site Gross Schoenebeck/Germany will change. Besides a temperature decrease at the injection well and a change of the chemical equilibrium, also the pore pressure p _p will vary in a range of approximately 44 MPa ± 10 MPa in our reservoir at ?3850 to ?4258 m depth. This leads to a poroelastic response of the reservoir rocks depending on effective pressure p _eff (difference between mean stress and pore pressure), resulting in a change in permeability k, porosity φ and the poroelastic parameter Skempton coefficient B. Hence, we investigated the effective pressure dependency of Flechtinger sandstone, an outcropping equivalent of the reservoir rock via laboratory experiments. The permeability decreased by 21% at an effective pressure range from 3 to 30 MPa, the porosity decreased by 11% (p _eff = 6 to 65 MPa) and the Skempton coefficient decreased by 24% (p _eff = 4 to 25 MPa). We will show which mechanisms lead to the change of the mentioned hydraulic and poroelastic parameters and the influence of these changes on the productivity of the reservoir. The most significant changes occur at low effective pressures until 15 to 20 MPa. For our in situ reservoir conditions p _eff = 43 MPa a change of 10 MPa effective pressure will result in a change in matrix permeability of less than 4% and in matrix porosity of less than 2%. Besides natural fracture systems, fault zones and induced hydraulic fractures, the rock matrix its only one part of geothermal systems. All components can be influenced by pressure, temperature and chemical reactions. Therefore, the determined small poroelastic response of rock matrix does not significantly influence the sustainability of the geothermal reservoir.     

Estimation of in-situ hydraulic diffusivity of rock masses

A method of estimating in-situ hydraulic diffusivity of rock masses by means of well-injection history and frequency of induced seismicity is presented. The method is based on the diffusion of injected fluid from a spherical cavity in a poroelastic half-space and the effective stress theory, as proposed byTerzaghi (1925, 1936) andHubbert andRubey (1959). Application of the method to two different regions, one in western New York and the other in Japan, resulted in estimated diffusivities of the order 10³ and 10⁴ cm²/sec, respectively. These values lie within the range of published estimates of in-situ diffusivity by other means, a summary of which is presented in tabular form. The calculated diffusivities suggest that the characteristic time of fluid diffusion is close to 0.1, rather than unity, as is sometimes assumed in the literature.     

Poroelasticity: Finite element modelling of anomalous tilt and pore pressure caused by pumping in a sedimentary half space with fault

Extraction of groundwater or hydrocarbons causes pore pressure gradients and soil deformation due to poroelastic coupling. Recent studies show that high-resolution engineering tiltmeters installed at shallow depth between 2 and 10 m resolve this deformation. Models using poroelasticity can describe the relationship between fluid extraction, pore pressure gradients and induced tilt for homogeneous and layered sedimentary half spaces. Faults intersecting a stack of sedimentary layers, for example in the Lower-Rhine-Embayment, are of fundamental impact to the groundwater flow system of an area. However, the fault’s hydromechanical effect on pump induced tilt and the pore pressure regime is still poorly investigated. We chose a comparatively simple approach to quantify anomalous pump induced tilt and pore pressure observed near a fault and close to the surface in a sedimentary subsoil. A PC-based Finite Element software is used to model poroelastic deformation, i.e. modelling vertical tilt and excess pore pressure in response to fluid extraction through a singular well. We compare numerical solutions for models with and without faults and show that a fault can modify symmetry and amplitude of the deformation field by more than a magnitude. We conclude that tilt and pore pressure measurements also at shallow depth can thus be biased by large subsurface structures like faults. Vice versa, these measurements may provide means to quantify hydromechanical effects caused by subsurface structures. However, depending on the geological setting, i.e. if pathways are established by a fault, the anomaly caused by the fault can also be small and hard to detect. Therefore, faults and geological structures like material boundaries have to be considered in poroelastic models carefully. For tilt surveys with a limited number of instruments in geologically well constrained areas these models allow the preselection of potential positions for tiltmeters where prominent field anomalies are expected.     

Production-induced Normal Faulting in the Valhall and Ekofisk Oil Fields

—?In situ stress and pore pressure data from the Valhall and Ekofisk oil reservoirs indicate that at the onset of production in both fields an incipient state of normal faulting existed in the crest of the anticlinal structures. In contrast, on the flanks of the structures the initial least principal stress values indicate an almost isotropic state of stress. Oil production from both fields caused marked pore pressure reductions as well as poroelastic reductions of the least principal stress in both the crest and flanks of the two structures. We demonstrate that as a result of production-induced pore pressure and stress changes, normal faulting appears to have spread out from the crests of the structures on to the flanks. Further evidence of a normal faulting stress state at Valhall has been found using data from a passive seismic monitoring experiment. Numerous microearthquakes were recorded during a six week monitoring period that are located at the very top of the reservoir or in the shale caprock immediately above it. An inverse/composite focal plane mechanism of these microearthquakes is consistent with a normal faulting stress regime.     

Mechanics of seismic instabilities induced by the recovery of hydrocarbons

We review earthquake distributions associated with hydrocarbon fields in the context of pore pressure diffusion models, poroelastic stress transfer and isostasy theory. These three mechanisms trigger or induce seismic instabilities at both local scale (D5 km) and at regional scale (D20 km). The modeled changes in stress are small (1 MPa), whatever the tectonic setting. Each mechanism corresponds to different production processes. (1) Local hydraulic fracturing due to fluid injection induces seismic-slip on cracks (M _L3) within the injected reservoir through decreasing the effective stress. (2) Pure fluid withdrawal causes pore pressure to decrease within the reservoir. It triggers adjustments of the geological structure to perturbations related to the reservoir response to depletion. Poroelastic mechanisms transfer this stress change from the reservoir to the surrounding levels whereM _L5 seismic instabilities occur either above or below the reservoir. (3) Massive hydrocarbon recovery induces crustal readjustments due to the removal of load from the upper crust. It can induce larger earthquakes (M _L6) at greater distance from the hydrocarbon fields than the two other mechanisms.Due to the mechanical properties of the shallow rock matrices involved, seismic slip triggered either by mechanism (1) or (2), is a second-order process of the main elastoplastic deformation. for a minimum of 80% of commercially productive basins, most of the local deformation is reported as aseismic, i.e., there is no evidence forM _L3 earthquakes. Nevertheless, the induced stresses vary as a function of time in a manner that depends on the hydraulic diffusivity (i.e., permeability) of the reservoir and surrounding rocks. Because small earthquakes (M _L3) indicate changes in stress and pore pressure, monitoring of seismicity is a means of assessingin situ reservoir behavior.The less constrained seismic response to hydrocarbon recovery is the possible connection between local fluid manipulations, triggered earthquakes and major regional earthquakes. Positive feedback mechanisms suggest that the region of seismic hazard changes is much larger than the area where hydrocarbons are extracted. These observations and models testify that fluid movement and pore pressure changes (increase or decrease) play important roles in the mechanics of earthquakes and in the triggering of natural earthquakes.     

Poroelastic Loading of an Aquifer Due to Upstream Dam Releases

Short-term changes in the hydraulic head of surface water bodies are known to influence the shallow response of hydraulically connected groundwaters. Associated with these fluctuations is the physical increase in stream water creating a mechanical load on the ground surface. This load is supported by the geologic materials (sediment or rock) and the pore fluid contained within the pores. Changes in this surface load have a direct effect on the total stress of the aquifer causing either a change in effective stress or fluid pressure. This response, predicted by the framework of linear poroelasticity, is a well-understood phenomenon in geologic materials. Currently, field measurements of the hydraulic response (i.e., fluid pressure) of aquifer materials are undergoing poroelastic loading due to dam releases in the Deerfield River Watershed in Massachusetts. An increase in stream stage from upstream dam releases causes an instantaneous pore fluid pressure increase at multiple depths and locations in the aquifer. This increase lasts anywhere from 15 to 40 minutes depending on the magnitude of the rise in the stream stage. Pore-pressure changes are well correlated to stream stage fluctuations for all of the recorded events. Poroelastic models created using basin stratigraphy and hydraulic properties of the aquifer response match the field observations well. Model results suggest that the overall stratigraphy is important in controlling the magnitude and duration of the poroelastic response. An improved understanding of responses such as these can be used to constrain uncertainties in model calibration and simulations of the contaminant migration in low permeability fine-grained (compressive) materials.     

紫坪铺水库造成孔隙弹性耦合变化及其对2008年汶川地震触发作用

位于川西地区龙门山断裂带附近的紫坪铺水库于2005年9月开始蓄水.约2.7年后,2008年5月12日,M_w7.9级汶川地震在龙门山断裂带上发生,两事件在时间和空间上的接近,揭示其可能相互关联,但前人的诸多研究给出了不同甚至是相反的结果.本研究基于完全耦合孔隙弹性理论,利用二维有限元模型(FEM),模拟水库蓄水造成的区域孔隙压力场和应力场的演化过程,基于库仑应力演化探讨其对龙门山断裂带活动的影响.模拟结果显示紫坪铺水库蓄水打破了原来的区域孔隙压力平衡,形成孔隙压力梯度源,向周围地壳传播;进而造成龙门山断裂带上库仑应力正值范围不断扩大,由浅入深影响到整条断层,尤其对浅层范围的加载作用明显,达上百千帕,为整个断层面的失稳提供了基础.震源区域库仑应力呈持续增长趋势,汶川地震发震前,增长了约数千帕~数十千帕,即使初期库仑应力为负,在未来某时刻库仑应力仍可能由负转正,并不断增大.通过计算汶川地震震源及其附近区域内相对于多种可能断层倾角的库仑应力,发现库仑应力随断层倾角增大而增加.因此整体来看,紫坪铺水库蓄水对龙门山断裂带起加载作用,有可能触发地震.对紫坪铺库区周围的小震分析也显示,蓄水以来小震明显增多、且不同时段小震发生密集区与水库距离逐渐增大,与孔隙压力扩散趋势一致.以上结果表明紫坪铺水库的蓄水增加了汶川地震的危险性.     

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.     

Time harmonic point load and dynamic contact problem of contacting fluid and poroelastic half-spaces

The dynamic response of contacting fluid and fluid-saturated poroelastic half- spaces to a time-harmonic vertical point force or a point pore pressure is investigated. The solutions are formulated using the boundary conditions at the fluid-porous medium interface. The point load solutions are then used to solve the dynamic problem of the vertical vibration of a rigid disc (both permeable and impermeable discs are included) on the surface of the poroelastic half-space. The contact problems are solved by integrating the point force and point pore pressure solutions over the contact area with unknown discontinuous force and pore pressure distributions, which are determined from the boundary conditions. The solutions are expressed in terms of dual integral equations, which are converted to Fredholm integral equations of the second kind and solved numerically. Selected numerical results for the vertical dynamic compliance coefficient for the cases with or without fluid overlying the poroelastic half-space are presented to show the effects of the fluid. The influence of the permeability condition of the disc on the compliance of the poroelastic half-space is investigated. The displacement, vertical stress, pore pressure in the poroelastic half-space and water pressure in the fluid half-space are also examined for different poroelastic materials and frequencies of excitation. The present results are helpful in the study of the dynamic response of foundations on the seabed under seawater.     

不同注水方式下断层动力学响应数值模拟研究

工业开采注水能导致现存断层活化,从而诱发大量的破坏型地震。因此,研究注水作用下断层的动力学响应对探索诱发地震的力学机理具有重要的意义。本文基于孔弹性弹簧-滑块模型,采用多孔介质弹性耦合数值模拟,计算分析了三类典型注水方式(上升型、迅速上升/下降型和间歇型)对断层稳定性的影响。研究结果表明:随着流体的不断注入,断层内部流体压力会经过缓慢上升、迅速上升和稳定上升三个阶段。针对于不同的注水方式,这三个阶段并不完全相同,体现形式存在差异;在注水方式相同的条件下,储层的渗透率越小,井口附近流体压力越大,断层处流体压力越小,两者间的流体压力差值越大;注水过程中断层临界刚度的变化与是否发生滑移并引发地震密切相关,数值越大越易诱发地震,其数值与注入储层流体的流体压力呈负相关,与流体压力变化率呈正相关;临界刚度由于流体压力变化率的增加在前期呈现快速增长趋势,后期则是由于流体压力的影响开始减小。迅速上升/下降型注水方式极大增加了注水前期诱发地震的可能性,间歇性注水方式在注水后期引起的临界刚度变化值较大,增大了诱发地震的可能性。该研究可以为注水诱发地震的危险性评价提供定量的科学依据。     

Seismicity induced by gas production: II. Lithology correlated events,induced stresses and deformation

A 3D relocation technique permits precise locations of induced earthquakes. Geostatistical processing using the data of 87 boreholes provides the basis of a precise 3D structure, with a dome geometry. Conventional laboratory mechanical tests performed on deep rock samples (1000 m to 5000 m) define the rock properties at depths similar to those of the seismic events (1<M _L<4.2) that range from 1 to 7 km.In the studied period, most (85%), of the events were located above the gas reservoir, with very few located in the reservoir itself. Because the production parameters (50 MPa depletion of the gas pressure reservoir) are homogeneous throughout the gas field, the lateral inhomogeneity of the seismic rupture locations are a consequence of variations in the rheological response of the dome to the deformation induced by gas production.Here a ratio of two is found between the elastic modulus of the seismic rock matrix and the elastic modulus of the aseismic rock matrix. The contrast in strength is at least as great, if not greater. Repeated measured surface deformations involve the whole structure. Spatial and temporal deformations indicate that aseismic deformation is quantitatively the main process of this structural deformation. The heterogeneous stress pattern inferred fromP-axes of induced earthquakes disagrees with the tectonic regional stress field. The radial distribution ofP-axes towards the gas reservoir probably reflects the production induced deformation. The inferred deformation of the dome occurs in response to weak induced stresses.     

Steady-state harmonic response of a rigid plate bearing on a liquid-saturated poroelastic halfspace

Soil–structure interaction problems are typically modelled by assuming subgrade behaviour to be either elastic or viscoelastic. Herein, compliance functions that may be used to solve soil–structure interaction problems are evaluated by treating the subgrade as a liquid-saturated poroelastic material whose behaviour is governed by Biot's theory. The compliances are evaluated for the harmonic rocking and vertical motions of rigid permeable and impermeable plates bearing on a poroelastic halfspace. Comparisons are made with elastic solutions which assume the subgrade to be either completely drained or undrained. Also, solid and fluid contact stresses are reported for the poroelastic case and compared to the solid contact stresses for the elastic cases.     

Fluid Pressure Variation in a Sedimentary Geothermal Reservoir in the North German Basin: Case Study Groß Schönebeck

The Rotliegend of the North German basin is the target reservoir of an interdisciplinary investigation program to develop a technology for the generation of geothermal electricity from low-enthalpy reservoirs. An in situ downhole laboratory was established in the 4.3 km deep well Groβ Schönebeck with the purpose of developing appropriate stimulation methods to increase permeability of deep aquifers by enhancing or creating secondary porosity and flow paths. The goal is to learn how to enhance the inflow performance of a well from a variety of rock types in low permeable geothermal reservoirs. A change in effective stress due to fluid pressure was observed to be one of the key parameters influencing flow properties both downhole and in laboratory experiments on reservoir rocks. Fluid pressure variation was induced using proppant-gel-frac techniques as well as waterfrac techniques in several different new experiments in the borehole. A pressure step test indicates generation and extension of multiple fractures with closure pressures between 6 and 8.4 MPa above formation pressure. In a 24-hour production test 859 m³ water was produced from depth indicating an increase of productivity in comparison with former tests. Different depth sections and transmissibility values were observed in the borehole depending on fluid pressure. In addition, laboratory experiments were performed on core samples from the sandstone reservoir under uniaxial strain conditions, i.e., no lateral strain, constant axial load. The experiments on the borehole and the laboratory scale were realized on the same rock types under comparable stress conditions with similar pore pressure variations. Nevertheless, stress dependences of permeability are not easy to compare from scale to scale. Laboratory investigations reflect permeability variations due to microstructural heterogeneities and the behavior in the borehole is dominated by the generation of connections to large-scale structural patterns.     

Compressional- and shear-wave velocities as a function of confining stress in dry sandstones

A laboratory study was carried out to investigate the influence of confining stress on compressional- and shear-wave velocities for a set of rock samples from gas-producing sandstone reservoirs in the Cooper Basin, South Australia. The suite of samples consists of 22 consolidated sublitharenites with helium porosity ranging from 2.6% to 16.6%. We used a pulse-echo technique to measure compressional- and shear-wave velocities on dry samples (cylindrical 4.6 × 2 cm) at room temperature and at elevated confining stress (≤ 60 MPa). Compressional- and shear-wave velocities in samples increase non-linearly with confining stress. A regression equation of the form V = A ? Be^?DP gives a good fit to the measured velocities with improved prediction of velocities at high confining stresses compared with equations suggested by other studies. The predicted microcrack-closure stresses of the samples show values ranging from 70 MPa to 95 MPa and insignificant correlation with porosity, permeability or clay content. There is a positive correlation between change in velocity with core porosity and permeability, but this association is weak and diminishes with increasing confining stress. Experimental results show that pore geometry, grain-contact type, and distribution and location of clay particles may be more significant than total porosity and clay content in describing the stress sensitivity of sandstones at in situ reservoir effective stress. The stress dependence of Cooper Basin sandstones is very large compared with data from other studies. The implication of our study for hydrocarbon exploration is that where the in situ reservoir effective stress is much less than the microcrack-closure stress of the reservoir rocks, the variation of reservoir effective stress could cause significant changes in velocity of the reservoir rocks. The velocity changes induced by effective stress in highly stress-sensitive rocks can be detected at sonic-log and probably surface-seismic frequencies.     

Seismic attenuation and velocity dispersion in fractured rocks: The role played by fracture contact areas

The presence of fractures in fluid‐saturated porous rocks is usually associated with strong seismic P‐wave attenuation and velocity dispersion. This energy dissipation can be caused by oscillatory wave‐induced fluid pressure diffusion between the fractures and the host rock, an intrinsic attenuation mechanism generally referred to as wave‐induced fluid flow. Geological observations suggest that fracture surfaces are highly irregular at the millimetre and sub‐millimetre scale, which finds its expression in geometrical and mechanical complexities of the contact area between the fracture faces. It is well known that contact areas strongly affect the overall mechanical fracture properties. However, existing models for seismic attenuation and velocity dispersion in fractured rocks neglect this complexity. In this work, we explore the effects of fracture contact areas on seismic P‐wave attenuation and velocity dispersion using oscillatory relaxation simulations based on quasi‐static poroelastic equations. We verify that the geometrical and mechanical details of fracture contact areas have a strong impact on seismic signatures. In addition, our numerical approach allows us to quantify the vertical solid displacement jump across fractures, the key quantity in the linear slip theory. We find that the displacement jump is strongly affected by the geometrical details of the fracture contact area and, due to the oscillatory fluid pressure diffusion process, is complex‐valued and frequency‐dependent. By using laboratory measurements of stress‐induced changes in the fracture contact area, we relate seismic attenuation and dispersion to the effective stress. The corresponding results do indeed indicate that seismic attenuation and phase velocity may constitute useful attributes to constrain the effective stress. Alternatively, knowledge of the effective stress may help to identify the regions in which wave induced fluid flow is expected to be the dominant attenuation mechanism.     

Plate boundary deformation and man-made subsidence around geothermal fields on the Reykjanes Peninsula,Iceland

We present Interferometric Synthetic Aperture Radar (InSAR) data from 1992–1999 and 2003–2008 as well as GPS data from 2000–2009 for the active plate boundary on the Reykjanes Peninsula, southwest Iceland. The geodetic data reveal deformation mainly due to plate spreading, anthropogenic subsidence caused by geothermal fluid extraction and, possibly, increasing pressure in a geothermal system. Subsidence of around 10 cm is observed during the first 2 years of production at the Reykjanes geothermal power plant, which started operating in May 2006. We model the surface subsidence around the new power plant using point and ellipsoidal pressure sources in an elastic halfspace. Short-lived swarms of micro-earthquakes as well as aseismic fault movement are observed near the geothermal field following the start of production, possibly triggered by the stresses induced by geothermal fluid extraction.     

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。