Seismicity around Brazilian dam reservoirs

More than 30 cases of seismicity associated with dam reservoir sites are known throughout the world. Despite the lack of data in some areas, where seismicity occurred after reservoir impounding, there have been distinct seismic patterns observed in seismic areas after dam projects implantation. This has demonstrated that reservoir loading can trigger earthquakes. A mechanism of earthquake generation by reservoir impounding is proposed here with particular application to the Brazilian cases and to areas subject to low confining stress conditions in stable regions. Six artificial lakes are described and the associated earthquake sources are discussed in terms of natural or induced seismicity. Earthquake monitoring in Brazil up to 1967, when Brasilia's seismological station started operation, was mainly based in personal communications to the media. Therefore, there is a general lack of seismic records in relatively uninhabited areas, making it difficult to establish a seismic risk classification for the territory and to distinguish natural from induced seismicity. Despite this, cases reported here have shown an alteration of the original seismic stability in dam sites, after reservoir loading, as observed by the inhabitants or records from Brasilia's seismological station. All cases appear to be related to an increase in pore pressure in permeable rocks or fracture zones which are confined between impermeable rock slabs of more competent rock. It is apparent that some cases show some participation of high residual stress conditions in the area.     

Earthquake‐induced turbidites triggered by sea level oscillations in the Upper Cretaceous and Paleocene of Italy

Earthquakes are widely recognized as triggers for turbidites, submarine debris flows and slumps. In tectonically active areas, surprisingly small changes in stress can trigger seismic events, implying that past sea level changes may be important in controlling the timing of seismicity and the occurrence of turbidites. We apply this idea in an analysis of turbidites from the upper Cretaceous–Paleocene Scaglia Rossa Formation of the Umbria‐Marches region of Italy. These turbidites are composed of resedimented foraminiferal tests derived from fluidizing deep‐water (∼1500 m), pelagic sediments; seismic triggering is the most likely triggering mechanism given this setting and composition. The timing of these turbidites (and associated synsedimentary slumps), constrained by biostratigraphy and magnetostratigraphy, reveals an unusual, non‐random temporal pattern that appears to correlate well with proposed eustatic fluctuations. This correlation between turbidites and eustatic fluctuations leads us to suggest that stress and pore fluid pressure changes associated with changing sea level may trigger periods of increased seismicity in the geological past.     

Moment tensors and micromechanical models

A numerical modelling approach that simulates cracking and failure in rock and the associated seismicity is presented and a technique is described for quantifying the seismic source mechanisms of the modelled events. The modelling approach represents rock as an assemblage of circular particles bonded together at points of contact. The connecting bonds can break under applied stress forming cracks and fractures in the modelled rock. If numerical damping is set to reproduce realistic levels of attenuation, then energy is released when the bonds break and seismic source information can be obtained as damage occurs. A technique is described by which moment tensors and moment magnitudes can be calculated for these simulated seismic events. The technique basically involves integrating around the source and summing the components of force change at the surrounding particle contacts to obtain the elements of the moment tensor matrix. The moment magnitude is then calculated from the eigenvalues of the moment tensor. The modelling approach is tested by simulating a well-controlled experiment in which a tunnel is excavated in highly stressed granite while microseismicity is recorded. The seismicity produced by the model is compared to the actual recorded seismicity underground. The model reproduces the spatial and temporal distribution of seismicity observed around the tunnel and also the magnitudes of the events. A direct comparison between the actual and simulated moment tensors is not performed due to the two-dimensional nature of the model, however, qualitative comparisons are presented and it is shown that the model produces intuitively realistic source mechanisms. The ability to obtain seismic source information from the models provides a unique means for model validation through comparison with actual recorded seismicity. Once it is established that the model is performing in a realistic manner, it can then be used to examine the micromechanics of cracking, failure and the associated seismicity and to help resolve the non-uniqueness of the geophysical interpretation. This is demonstrated by examining in detail the mechanics of one of the modelled seismic events by observation of the time dependence of the moment tensor and by direct examination of the particle motions at the simulated source.     

Finite element models to represent seismic activity of the Indian plate

Quantification of seismic activity is one of the most challenging problems faced by earthquake engineers in probabilistic seismic hazard analysis. Currently, this problem has been attempted using empirical approaches which are based on the regional earthquake recurrence relations from the available earthquake catalogue. However, at a specified site of engineering interest, these empirical models are associated with large number of uncertainties due to lack of sufficient data. Due to these uncertainties, engineers need to develop mechanistic models to quantify seismic activity. A wide range of techniques for modeling continental plates provides useful insights on the mechanics of plates and their seismic activity. Among the different continental plates, the Indian plate experiences diffused seismicity. In India, although Himalaya is regarded as a plate boundary and active region, the seismicity database indicates that there are other regions in the Indian shield reporting sporadic seismic activity. It is expected that mechanistic models of Indian plate, based on finite element method, simulate stress fields that quantify the seismic potential of active regions in India. This article explores the development of a finite element model for Indian plate by observing the simulated stress field for various boundary conditions, geological and rheological conditions. The study observes that the magnitude and direction of stresses in the plate is sensitive to these conditions. The numerical analysis of the models shows that the simulated stress field represents the active seismic zones in India.     

大连开发区场地地震设计参数

根据工作区潜在震源区划分、潜在震源区地震活动性参数和地震动衰减关系, 进行场地地震危险性分析, 得到不同概率水平下场区相应地震烈度和基岩水平加速度峰值及其反应谱。根据场地工程地震条件划分不同地质单元及相应的场地类别, 进行不同概率水平的地震反应分析计算, 确定地震动设计参数。     

Seismicity associated with mining

The virgin state of stress in the earth's crust is disturbed by mine excavations which result in stress concentrations in the rock. Rock emits seismic pulses when subjected to stresses approaching in value the strength of the rock. Mining gives rise to seismic activity ranging from microseismic events radiating 10⁻⁵ J (M - 6) to rockbursts or tremors radiating 10⁹ J (M5).

Rock failures in mines fall into four main categories: rockfalls, in which loosened rock falls mainly under its own weight; rockbursts which are violent failures of rock and may cause damage to excavations; bumps, which are violent failures, but which do not cause damage to the excavations; and outbursts in which the rapid release of gas causes rock to be ejected into the excavation. Microseismic activity is associated with all these four categories of rock failure.

Rockbursts and bumps occur in deep metalliferous mines in which thin tabular deposits in strong, brittle siliceous rocks of igneous or metamorphic origin are worked. The seismicity associated with mining appears to have many features in common with those natural earthquakes. However, the mechanics and dynamics of mining can be analysed relatively completely and the excavations provide access to the source region of the seismic events.

Comprehensive studies of rock deformation and fractures have been made on several gold mines of the Witwatersrand System. The gold-bearing reefs are mined at depths of down to more than 3 km below surface by stoping. This creates flat voids in the quartzitic strata extending laterally for distances of up to several kilometres with an initial excavated thickness of a metre. Geodetic-type measurements in tunnels and boreholes traversing the rock mass around these excavations have shown that the rock mass responds elastically over long periods of time to the extension of the boundaries of the excavations resulting from advancing the stope faces. Failure of the rock in the regions of maximum stress concentration near the edges of these excavations is an unavoidable concomitant of mining. It gives rise to new fracture planes closely parallel to the stope faces and to continuous seismic activity. The foci of seismic events with magnitudes from less than 0 to greater than 3 are found to occur mainly within tens of metres of the advancing stope faces. Diurnal and weekly distributions of seismic activity and of deformation show that the failure of the rock is time-dependent. The seismic radiation efficiency has been found to be of the order of 0.1%.

The seismicity is related to the spatial rate of energy release associated with the extension of the excavations and the incidence of seismic events and their magnitudes can be predicted statistically from calculated values of this quantity.     

Analytical investigation of hydraulic fracture-induced seismicity and fault activation

More recent public discourse has taken place regarding the potential correlation between seismic activity and hydraulic fracturing in shale gas reservoirs. Public fears about the risk of seismicity stem mainly from past earthquakes induced by conventional deep injections because the two types of projects share similar mechanisms of rock failure and fault activation. Although previous earthquake risks associated with fluid injection were not serious, the situation would be far more problematic if hydraulic fracturing in a shale gas reservoir triggered a similar-sized earthquake due to potential environmental issues. In fact, almost all documented injection-induced earthquakes have been associated with long-duration and high-volume injection rather than short-term (hours) pressurization (e.g., hydraulic fracturing). In general, hydraulic fracturing operations mostly induce microseismic events through rock failure and activation of small fractures. Although shale reservoirs in tectonically active zones pose a high risk of inducing large-magnitude seismic activities, the internal geological conditions and external stimulation conditions are impossible to be satisfied simultaneously to trigger activation of an entire fault and to result in a destructive earthquake during hydraulic fracturing operations.     

河南下汤水库诱发地震预测

文章简介了下汤地区区域地质及地震地质条件,分析了下汤地区岩体渗透性、现代地应力场特征及车村—鲁山断裂现代活动性。从地质条件分析、工程地质地震地质类比、危险度估算及统计检验等诸方面,论证了下汤水库诱发地震的可能性;结合岩石断裂力学研究成果预测了下汤水库诱发地震的可能震级和震中位置。     

Hydraulic Fracturing-induced Seismicity at the Hot Dry Rock Site of the Gonghe Basin in China

Hot dry rock is becoming an important clean energy source. Enhanced geothermal systems (EGS) hold great promise for the potential to make a contribution to the energy inventory. However, one controversial issue associated with EGS is the impact of induced seismicity. In August 2019, a hydraulic stimulation experiment took place at the hot dry rock site of the Gonghe Basin in Qinghai, China. Earthquakes of different magnitudes of 2 or less occurred during the hydraulic stimulation. Correlations between hydraulic stimulation and seismic risk are still under discussion. Here, we analyze the hydraulic stimulation test and microseismic activity. We quantify the evolution of several parameters to explore the correlations between hydraulic stimulation and induced seismicity, including hydraulic parameters, microseismic events, b-value and statistical forecasting of event magnitudes. The results show that large-magnitude microseismic events have an upward trend with an increase of the total fluid volume. The variation of the b-value with time indicates that the stimulation experiment induces small amounts of seismicity. Forecasted magnitudes of events can guide operational decisions with respect to induced seismicity during hydraulic fracturing operations, thus providing the basis for risk assessment of hot dry rock exploitation.     

Seismic behavior in central Taiwan: Response to stress evolution following the 1999 Mw 7.6 Chi-Chi earthquake

Following the 1999 M_w 7.6 Chi-Chi earthquake, a large amount of seismicity occurred in the Nantou region of central Taiwan. Among the seismic activities, eight M_w ⩾ 5.8 earthquakes took place following the Chi-Chi earthquake, whereas only four earthquakes with comparable magnitudes took place from 1900 to 1998. Since the seismicity rate during the Chi-Chi postseismic period has never returned to the background level, such seismicity activation cannot simply be attributed to modified Omori’s Law decay. In this work, we attempted to associate seismic activities with stress evolution. Based on our work, it appears that the spatial distribution of the consequent seismicity can be associated with increasing coseismic stress. On the contrary, the stress changes imparted by the afterslip; lower crust–upper mantle viscoelastic relaxation; and sequent events resulted in a stress drop in most of the study region. Understanding seismogenic mechanisms in terms of stress evolution would be beneficial to seismic hazard mitigation.     

华北与东南沿海地区岩石介质差异性研究

通过地震活动性特征研究了华北与东南沿海地区岩石介质差异性，以唐山、海域及福建地区为例，计算了上述地区的视应力降和环境应力背景值，并用分形理论中的标度揭示了这种差异性的定量特征。结果表明，在华北地区（唐山与海城），环境应力值有强烈的涨落，而东南沿海地区（福建）其涨落并不明显，从而揭示了华北与东南沿海地区地壳岩石介质的差异性。这意味着上述地区的动力学特征在板块构造力作用下存在明显的差异。最后，应用动力学观点讨论了上述变化的复杂性和可能存在的问题。     

Is Ziarat a Potential Site for Conventional or Unconventional Energy Resources?

A structural interpretation of the Ziarat block in the Balochistan region (a part of the Suleiman Fold and Thrust Belt) has been carried out using seismic and seismological data. Seismic data consists of nine 2.5D pre‐stack migrated seismic lines, whereas the seismological data covers the Fault Plane Solution and source parameters. Structural interpretation describes two broad fault sets of fore and back thrusts in the study area that have resulted in the development of pop‐up structures, accountable for the structural traps and seismicity pattern in terms of seismic hazard. Seismic interpretation includes time and depth contour maps of the Dungan Formation and Ranikot group, while seismological interpretation includes Fault Plane Solution, that is correlated with a geological and structural map of the area for the interpretation of the nature of the subsurface faults. Principal stresses are also estimated for the Ranikot group and Dungan Formation. In order to calculate anisotropic elastic properties, the parameters of the rock strength of the formations are first determined from seismic data, along with the dominant stresses (vertical, minimum horizontal, and maximum horizontal). The differential ratio of the maximum and minimum horizontal stresses is obtained to indicate optimal zones for hydraulic fracturing, and to assess the potential for geothermal energy reservoir prospect generation. The stress maps indicate high values towards the deeper part of the horizon, and low towards the shallower part, attributed to the lithological and structural variation in the area. Outcomes of structural interpretation indicate a good correlation of structure and tectonics from both seismological and seismic methods.     

A Pragmatic Approach to Seismic Parameters in a region with Low Seismicity: The Case of Eastern Iberia

In intra-plate regions with low-seismic activity, seismic cycles last between 10³ and 10⁵ years and, consequently, quiescent faults may be capable of producing catastrophic earthquakes. Paleoseismic studies, which are necessary to define capable faults in these regions and to establish their seismic parameters, are scarce and not always possible. In order to overcome the scarcity of paleoseismic data, this paper proposes an approach to evaluate the capability and seismic parameters of faults. This approach takes into account instrumental, historical, paleoseismological and geological data. Assuming that in a given structural region with definite climatic and geological characteristics similar geomorphic features reflect a similar structural-seismic evolution, the capability of faults is established by comparing the fault related geomorphic features of the few proven seismogenic faults with those of the rest of the faults. The seismic parameters were estimated using some mathematical relationships from geological and geomorphological data, where the slip rate of the faults was determined from geological and geomorphological criteria. The case of eastern Iberia is presented to illustrate this approach. This area is characterized by low seismicity, few historical destructive earthquakes, and only one fault with good paleoseismological data. Of the 249 potential faults (longer than 10 km and adequately oriented with respect to the current stress field), 23 were regarded as capable faults. Some of these were located in seismic quiescent areas, and their main seismic parameters were estimated.     

Mechanism of Underground Fluid Injection Induced Seismicity and Its Implications for CCS Projects

CO₂ capture and storage projects must consider the potential possibility ofinjection induced seismicity. Moderate earthquakes and strong earthquakes may endanger human life and property, and even felt earthquakes and microquakes also pose a threat to seal integrity of CO₂ reservoir and increase the risk of leakage. Underground fluid injection induced seismicity usually happens in some geoengineering projects such as waste fluid disposal, EOR and EGS, and it occurs when fluid is injected along the fault. Therefore, it can be studied through stress analysis. The density of supercritical CO₂ is smaller than water, which may develop density flow in the deep strata or water-rock interactions in pre-existing structures, and cause the variation in permeability and pressure to induce a seismic activity. In this paper, we reviewed the mechanism of underground fluid injection induced seismicity with the focus of CCS, combined with fluid injection projects and seismic monitoring analysis in both commercial scale and experimental scale, to investigate its impact on the integrity of the cap rock of the reservoir. Finally, we summarized the appropriate site selection, injection methods and monitoring programs to prevent the occurrence of induced seismicity.     

大断面综放沿空巷道煤柱合理宽度与围岩控制

确定合理的区段煤柱宽度及巷道支护型式和参数,对于提高资源回采率和巷道安全性及实现综放开采高产高效意义重大。以王家岭煤矿20103区段运输平巷为工程背景,采用FLAC3D数值分析了不同煤柱宽度下围岩主应力差、变形及破坏演化规律,认为合理煤柱宽度为6～10 m,并结合实际地质和生产条件确定试验巷道煤柱宽度为8 m。采用理论分析和现场钻孔窥视方法综合确定基本顶断裂线位于距采空区约7 m处,认为由于综放沿空巷道围岩性质结构和应力分布沿巷道中心线呈明显非对称性,将引发煤柱侧顶板严重下沉和肩角部位煤岩体错位、嵌入、台阶下沉等非对称破坏特征,靠煤柱侧顶板及肩角部位是巷道变形破坏的关键部位。在此研究基础上,针对性地提出了以高强锚梁网、不对称锚梁、锚索桁架为主体的综合控制技术,详细阐明了具体支护措施的控制机制,并进行现场应用。工程实践表明,8 m煤柱宽度合理,该支护技术能够保证窄煤柱沿空巷道围岩稳定,并已在王家岭煤矿大面积推广应用,对类似工程条件的支护技术具有一定的理论意义和实用价值。     

Aeromagnetic data over Harrat Lunayyir and surrounding areas,western Saudi Arabia

Harrat Lunayyir is one of the smaller lava fields in western Saudi Arabia that is of current interest due to a dike intrusion episode in 2009, an ongoing swarm of earthquakes and the possible hazard that pose. In addition to seismology, other geophysical data have been used to study the structure of the area, and the available aeromagnetic information is shown and discussed here. The reduced to the pole (RTP) magnetic grid and its enhancements have been used to define some of the main anomalies, and these have been correlated with the known geology, including the Red Sea coastal dike system. There are numerous linear features that are possibly related to the dikes or faulting, but within the area of the harrat, details due to the underlying structure are largely obscured by the magnetic surface lavas. Northeasterly trends in the magnetic data may indicate old zones of weakness that intersect one of the main coastal dikes at the location of the recent seismic activity and surface fissure, suggesting that this is a point of weakness resulting in the volcanism and seismicity that also appears to be largely limited to two of the NE trends. The association of the recent seismicity with a known geological and aeromagnetic feature is important in determining the seismic hazard for the region, especially if the location of future activity can be used to reduce overall uncertainty in the analysis by identifying potential fault sources. Here, the seismicity appears to lie on one of the NNW-trending coastal dikes that have been reactivated recently along a section between two NE-trending older faults.     

煤柱下底板偏应力区域特征及案例

选取某煤矿近距离煤层为工程背景,采用FLAC3D模拟了8#煤层残余煤柱底板偏应力场分布特征。结果表明：（1）底板的偏应力呈扩散状向底板传递,距离煤柱越远扩散范围越广,煤柱边缘偏应力呈45°向底板传播;（2）煤柱较窄时,中线和边缘处偏应力影响深度浅,随煤柱宽度增加,底板偏应力变化和影响深度较大,当煤柱宽度足够大时,影响深度又变浅,中部趋于原岩应力;（3）同一水平面上,偏应力呈马鞍状分布,随煤柱宽度增加,煤柱中线处和边缘处偏应力经历了先增大后减小的过程,煤柱边缘处偏应力峰值位置变化不大;（4）同一煤柱宽度,煤柱边缘偏应力峰值向深部递减且趋势减慢,同时,峰值远离煤柱且趋势加快。在自由边界受均布载荷、底板垂直应力、水平应力、切应力解析解的基础上,推导了底板偏应力解析公式,解析与模拟结果基本吻合。具体到该工程的地质条件,9205轨道巷距离煤柱边缘20 m、9205回风巷在煤柱边缘、9205运输巷在煤柱中线处,9205轨道巷维护效果最好,证明了内错式巷道且距离煤柱足够远时,偏应力较小,宏观应力环境更适合巷道围岩自稳。     

Study of Influence of Regional Geodynamic Background on Strata Behaviors in China's Tongxin Mine

Strata behaviors are mainly affected by regional geodynamic background. The influence of rock mass stress and energy distribution on strata behaviors in the Tongxin mine is studied in terms of regional tectonic movement, seismic activity and tectonic stress field. The results show that the extrusion lifting movement of Kouquan fault adjacent to the Tongxin mine results in the stress concentration in the rock of the Carboniferous coal bed and accumulation of a large amount of elastic energy and forms structural background of Tongxin mine. Due to various seismic activities in the mine area, the strain energy is known to reach much higher levels, up to 0.5×10⁸ J^1/2. Since the stratigraphic structure is sensitive to the mining operation, the strain energy could cause strong strata behaviors. A special geological structure model of the Tongxin mine is established based on the geodynamic division method. The distribution of regional structure stress field is determined by the rock mass stress analysis system. Based on this model, Tongxin mine is divided into five areas with high stress, eight areas with low stress and eight areas with gradient stress. The strong strata behaviors mostly occur in high stress areas. These results could provide guidance to predict the strength of regional or mine pressure and control strata behavior in different areas.     

Upwarped high velocity mafic crust, subsurface tectonics and causes of intraplate Latur-Killari (M 6.2) and Koyna (M 6.3) earthquakes, India – A comparative study

A unique attempt is made to understand the genesis of intraplate seismicity in the Latur-Killari and Koyna seismogenic regions of India, through derived crustal structure by synthesizing active and passive seismic, magnetotelluric, gravity and heat flow data. It has indicated presence of relatively high velocity/density intermediate granulite (and amphibolite) facies rocks underneath the Deccan volcanic cover caused mainly due to a continuous geodynamic process of uplift and erosion since Precambrian times. These findings have been independently confirmed by detailed borehole geological, geochemical and mineralogical investigations. The crystalline basement rock is found to contain 2 wt% of carbon-di-oxide fluid components. The presence of geodynamic process, associated with thermal anomalies at subcrustal depths, is supported by a high mantle heat flow (29–36 mW/m²) beneath both regions, although some structural and compositional variations may exist as evidenced by P- and S-wave seismic velocities. We suggest that the stress, caused by ongoing uplift and a high mantle heat flow is continuously accumulating in this denser and rheologically stronger mafic crust within which earthquakes tend to nucleate. These stresses appear to dominate over and above those generated by the India–Eurasia collision. The role of fluids in stress generation, as advocated through earlier studies, appears limited.     

Controls on intra-plate seismicity in southwestern Australia

Although the Southwest Seismic Zone (SWSZ), located about 150 km to the east of Perth in southwestern Australia, is one of the most seismically active areas in Australia, there is little understanding as to why the earthquakes are occurring.An analysis of geophysical, geological and geodetic data from the area suggests that the SWSZ coincides with a Precambrian terrane boundary. Seismic data show that the terrane boundary zone dips at a shallow angle in a northeasterly direction. Reactivation of this ‘zone of weakness’ in the contemporary stress field (east–west maximum horizontal stress) is interpreted to be the first-order control on seismicity in the region.Gravity data show that the terrane boundary is offset by near-orthogonal structures, which are interpreted as faults. At least one of these trends corresponds with a linear zone of epicentres. The temporal and spatial distributions of epicentres associated with the 1968 Meckering earthquake (ML 6.9) suggest that the second-order distribution of seismicity in the SWSZ can be explained by the ‘intersection model’, whereby stresses are amplified by space problems associated with displacements on crosscutting faults.It is speculated that a zone of high density and high seismic velocity in the lower crust may also be a second-order control on the local seismicity. However, confirmation awaits better delineation of the extent of this zone.