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.     

Seismicity induced by the filling of the Talbingo reservoir

The 162 m high Talbingo dam was the last of a number of dams to be completed as part of the extensive Snowy Mountains Hydro‐electric scheme in southeastern New South Wales. During a thirteen year monitoring period prior to the filling of the reservoir in May 1971, no seismic activity had been detected in the immediate area. Three weeks after impounding commenced, the first of a long series of events was recorded. The earthquake activity was not distinguished by one main shock, but rather by several sequences, all of which included events up to magnitude 3.5. The first activity occurred in the region of the reservoir, but over a period of three years, the events migrated to a region about 5 km downstream of the damsite. Subsequent activity, whch has been sporadic to the present day, has encompassed both regions. Fault‐plane solutions indicate normal dip‐slip faulting along the line of the reservoir, and indicate that the activity could have been triggered either by the reservoir load or by an increase in pore pressure.     

Seismicity patterns associated with the September 10th, 2008 Qeshm earthquake, South Iran

The b value of the Gutenberg-Richter relation and the standard deviate, Z, were calculated to investigate the temporal and spatial variations in seismicity patterns associated with the September 10th, 2008 (Mw?=?6.1) Qeshm earthquake. The temporal variations of b value illustrate a distinct dramatic drop preceding the Qeshm earthquake, and the spatial changes in b value highlight a zone with an abnormally low b value around the epicenter of this event. The cumulative number and Z value as a function of time show a precursory seismic quiescence preceding the 2008 Qeshm earthquake that observed for 1?year in a circle with R?=?50?km around its epicenter. The spatial distribution map of the standard deviate, Z, also exhibits an obvious precursory seismic quiescence region before the 2008 Qeshm event around the epicenter of this event. Interestingly, the precursory seismic quiescence region is approximately consistent with low b value anomaly region, and both have E–W to NE–SW trend. These two precursory anomalies took place in relatively large regions, which were possibly relevant to the preparation zone of the 2008 Qeshm event.     

Seismicity of Gujarat

Paper describes tectonics, earthquake monitoring, past and present seismicity, catalogue of earthquakes and estimated return periods of large earthquakes in Gujarat state, western India. The Gujarat region has three failed Mesozoic rifts of Kachchh, Cambay, and Narmada, with several active faults. Kachchh district of Gujarat is the only region outside Himalaya-Andaman belt that has high seismic hazard of magnitude 8 corresponding to zone V in the seismic zoning map of India. The other parts of Gujarat have seismic hazard of magnitude 6 or less. Kachchh region is considered seismically one of the most active intraplate regions of the World. It is known to have low seismicity but high hazard in view of occurrence of fewer smaller earthquakes of M????6 in a region having three devastating earthquakes that occurred during 1819 (M _w7.8), 1956 (M _w6.0) and 2001 (M _w7.7). The second in order of seismic status is Narmada rift zone that experienced a severely damaging 1970 Bharuch earthquake of M5.4 at its western end and M????6 earthquakes further east in 1927 (Son earthquake), 1938 (Satpura earthquake) and 1997 (Jabalpur earthquake). The Saurashtra Peninsula south of Kachchh has experienced seismicity of magnitude less than 6.     

Compositional changes of minerals associated with dynamic recrystallizatin

The rate of compositional and isotopic exchange between minerals may be enhanced significantly if the rock is deformed simultaneously. The enhanced exchange rate may result from a reduction in grain size (shorter distance for volume diffusion), dissolution and growth of grains by diffusion creep (pressure solution), or the movement of high-angle grain boundaries through strained grains during recrystallization in the dislocation creep regime. The migration of high-angle grain boundaries provides high diffusivity paths for the rapid exchange of components during recrystallization. The operation of the latter process has been demonstrated by deforming aggregates consisting of two plagioclases (An₁ and An₇₉) at 900°C, 1 GPa confining pressure, and a strain rate of ∼2x10^-6s^-1. The polygonal, recrystallized grains were analyzed using an analytical transmission electron microscope and have a variable but often intermediate composition. At the conditions of these experiments, the volume interdiffusion rate of NaSi/CaAl is too slow to produce any observable chemical change, and microstructural-chemical relations indicate that the contribution from diffusion creep was insignificant except for initially fine-grained (2–10 μm) aggregates. These results indicate that strain-induced recrystallization can be an effective mechanism for enhancing the kinetics of metamorphic reactions and for resetting the isotope systematics of minerals such as feldspars, pyroxenes, and amphiboles.     

库水变动下水库滑坡变形演化机理

滑坡变形演化是滑坡内动力物质系统与外动力激励系统相互作用的结果,对于库水变动影响下的水库滑坡而言,内外系统相互作用的过程则是通过地下水的力学作用效应得以体现的,为此可通过库水变动过程中滑坡地下水力学作用效应的强弱关系及其演化规律来揭示水库滑坡的变形演化机理.基于上述思路,以三峡库区巴东县李家湾滑坡为研究对象,首先利用敏...     

Chlorophyll concentration and surface temperature changes associated with earthquakes

The preparation process of an impending earthquake may leave fingerprints on the earth??s surface. Elastic strain in rocks, formation of micro-cracks, gas releases and other chemical or physical activities in the earth??s crust before and during earthquakes has been reported to cause rises in temperature, surface latent heat flux (SLHF), upwelling index and chlorophyll-a (Chl-a) concentration on the ground or sea surface. Changes in surface temperature can be monitored with thermal infrared sensors such as NOAA-AVHRR and microwave radiometers like AMSR-E/Aqua. SLHF data and upwelling indices are provided by National Centers for Environmental Prediction (NCEP) Reanalysis Project and Pacific Fisheries Environmental Laboratory, respectively. This study examines behaviors of the above four factors prior to the past three oceanic and coastal earthquakes occurred at the Pacific Ocean (Northern California of June 15, 2005, Central California of September 28, 2004, and December 22, 2003). We were successful in detecting pre-earthquake anomalies prior to all three earthquakes. Our detailed analysis revealed 1?C5?°C rises in surface temperature in epicentral areas. Considerable anomalies in Chl-a concentration, 1?C2?weeks before the day of the main earthquakes, were spotted, which are attributed to the rise in upwelling index. Time series of SLHF showed meaningful rises from 1?month to a fortnight before the earthquake events. One problem in our research was the low resolution of the data which makes the graphs that are generated from NCEP database affected by all sources of anomalies, other than seismic activities, within an about 1.8°?C2.5° (200?km) area.     

Macrostructural changes in compacted earthen construction materials under loading

There is increasing interest in the use of earthen materials for modern construction. The mechanical behaviour of these materials is strongly controlled by their internal macrostructures. Rammed earth (RE) is one example of these materials, created by in situ compaction of a wet soil mixture. Changes to the material structure occur on loading and during compaction; therefore, the nature of these changes needs to be understood if the effect on the material behaviour can be predicted. Here, the change in the macrostructure of RE on the application of compressive loading is investigated by using X-ray computed tomography and fractal analysis to monitor the changes in loaded RE specimens. The macrostructures of specimens comprising different layer thicknesses are also investigated in order to determine how layer thickness affects the compaction of the material. Results are used to recommend procedures for manufacturing specimens that are representative of the material found in full-scale RE structures.     

Seismicity of the Cyprus region

We have used a combination of macroseismic and instrumental techniques to re-evaluate the seismicity of Cyprus and the surrounding region for the period 1890–1900. We identified 56 events which we believe to have a surface-wave magnitude (Ms) of 4.5 or greater. Early events are located by felt effects alone, and macroseismic information is used to improve the location of many later events. We find the broad pattern of seismicity follows that given by standard listings: an area of weak seismicity to the north-west of Cyprus in the Gulf of Antalia includes intermediate-depth events as are found in the Hellenic Arc to the west; a gap of reduced activity separates this activity from that of the main region of Cyprus, where we could find no evidence for sub-crustal foci; there appears to be no present seismological connection along the presumed plate boundary to the north-east to the junction of the East Anatolian fault and the extension of the Dead Sea rift system. We find a much greater level of activity in the first part of this century than is given in the standard listings, such as that of the ISC. Up to 1963 we identified 47 events of Ms > 4.5, compared with only 10 given in the ISC files. Recent improved station coverage locates more events in the region, but these have not been of large magnitude, and the moment release in recent decades has been appreciably less than earlier in the century. For the events in our list the total moment release is 2.7x 1019 Nm with 75% of this originating in the main Cyprus region. We reaffirm the danger of uncritical use of standard earthquake listings for tectonic interpretation and in particular for hazard evaluation in areas such as this, where neither the historical nor the instrumental seismicity record is complete.     

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.     

Physico—Mechanical Property Changes Associated with mineral Phase Transition in Granite

Variations in elastic wave velocity, low-frequency internal friction and acous-tic emission in granite were experimentally studied as a function of temperature.It is found that the wave velocity and Young‘s modulus tend to decrease with temperature.In association with the α-β transition of quartz a sharp internal friction peak can be recognized accompanied by a drastic drop in wave velocity and modulus and by a second peak of acoustic emission rate.     

Seismicity of Victoria to 1980

Since 1976 a number of analogue and digital seismographs, which record high frequency seismic waves in the range 4 to 20 Hz, have been installed in Victoria. These enable the detection and location of microearthquakes with Richter magnitude Ml down to less than zero. Because there are many more smaller than larger earthquakes, about 250 events are now being located in Victoria each year, compared with about 25 per year between 1960 and 1975, and an average of two per year between 1840 and 1959. Despite an uneven statewide seismograph coverage, the new instruments and new lithospheric models have led to substantial improvements in earthquake location accuracy. All Victorian earthquakes with reliable depth determinations have been found to lie within the crust, most at a depth of less than 18 km. Some very small but shallow microearthquakes have been felt or heard. The isoseismal map for such an event (of magnitude Ml 1.3) which occurred at Preston during July 1976, is given.

The greater range of magnitudes now recorded permits more reliable estimates of seismicity. On average, the recurrence of earthquakes in Victoria is represented by: log₁₀ (P) = 0.92M_L —2.40, where P is the return period of an earthquake with magnitude greater than M_l in an area of 100 km square. It is suggested that attenuation of seismic wave amplitudes with distance is quite high in Victoria.     

Seismicity of the Tanzania region

The seismicity of the East African Rift system within the region bounded by latitudes 2°N and 12°S and longitudes 28°E and 40°E has been studied as far as all available instrumentally based material permits. An earthquake catalogue is presented and the data contained therein are used for tectonophysical investigations, including frequency—magnitude relations and time and space distribution of the seismicity within the region. In addition, earthquake engineering aspects are discussed.     

Carbon dioxide injection and associated hydraulic fracturing of reservoir formations

The storage potential of subsurface geological systems makes them viable candidates for long-term disposal of significant quantities of CO₂. The geo-mechanical responses of these systems as a result of injection processes as well as the protracted storage of CO₂ are aspects that require sufficient understanding. A hypothetical model has been developed that conceptualises a typical well-reservoir system comprising an injection well where the fluid (CO₂) is introduced and a production/abandoned well sited at a distant location. This was accomplished by adopting a numerical methodology (discrete element method), specifically designed to investigate the geo-mechanical phenomena whereby the various processes are monitored at the inter-particle scale. Fracturing events were simulated. In addition, the influence of certain operating variables such as injection flow rate and fluid pressure was studied with particular interest in the nature of occurring fractures and trend of propagation, the pattern and magnitude of pressure build-up at the well vicinity, pressure distribution between well regions and pore velocity distribution between well regions. Modelling results generally show an initiation of fracturing caused by tensile failure of the rock material at the region of fluid injection; however, fracturing caused by shear failure becomes more dominant at the later stage of injection. Furthermore, isolated fracturing events were observed to occur at the production/abandoned wells that were not propagated from the injection point. This highlights the potential of CO₂ introduced through an injection well, which could be used to enhance oil/gas recovery at a distant production well. The rate and magnitude of fracture development are directly influenced by the fluid injection rate. Likewise, the magnitude of pressure build-up is greatly affected by the fluid injection rate and the distance from the point of injection. The DEM modelling technique illustrated provides an effective procedure that allows for more specific investigations of geo-mechanical mechanisms occurring at subsurface systems. The application of this methodology to the injection and storage of CO₂ facilitates the understanding of the fracturing phenomenon as well as the various factors governing the process.     

亚洲大陆逃逸构造与现今中国地震活动

2008年5月12日汶川地震让中国地学界强烈感受到深入研究地震地质与构造变形的重要性和肩负防震减灾巨大的社会责任。本文作者从构造地质学家的角度对中国大陆地震分布、成因规律以及发展趋势做了一些讨论。按地震分布,中国大陆可以粗分为两个区域,其交界是一条过渡带。该过渡带的东界是郯庐断裂及其和海南岛的连线,西界是齐齐哈尔—北京—邯郸—郑州—宜昌—贵阳—(越南)河内连成的线,后者其实就是松辽盆地的西界(大兴安岭的东界、太行山的东界、大娄山的东界）。我们不妨将上述两线所夹过渡带称之为“地震区分界线”。分界线以西的广大地区,活动断裂、活动褶皱、活动盆地都与印度板块楔入欧亚大陆造成的青藏高原隆升、快速侧向扩展、亚洲大陆逃逸构造活动有关。流变性较好的造山带(如青藏高原和天山)和流变性较差的古老地块(如塔里木、准噶尔、阿拉善、鄂尔多斯、四川盆地等)在其边界强烈对抗,形成强震。地震区分界线以东的中国沿海地区受太平洋和菲律宾海板块运动的影响也会发生地震,但其强度和频度与该线以西的青藏高原周边、天山、鄂尔多斯地块周缘以及张家口渤海断裂带上地震低得多。由太平洋板块在日本海沟向西深俯冲形成的地震在中国仅分布在吉林省珲春—汪清一带,这些深源地震对地面工程建筑破坏性不大。处于欧亚、菲律宾海和南海3个板块的交汇部位的我国台湾地震不断。受我国台湾地震的影响,闽粤沿海NW和NE向断裂往往被激活,形成地震。总之,虽然中国大陆的现代地震受太平洋、欧亚、印度和菲律宾海四大板块联合作用控制,但最主要、最直接、影响最大的还是印度板块楔入欧亚大陆造成的青藏高原隆升、快速侧向扩展和大陆逃逸。因此,对中国的地震研究不能仅局限于某区域或某条断裂,而应把整个亚洲大陆逃逸构造作为整体的、统一的“一盘棋”看待。     

Seismicity features of the caucasus in comparison with its tectonic structures

Various parameters of seismic field are considered for the Caucasian region. The most informative appeared to be the “sigma-Σ” parameter. A sketch map of the geotectonic zoning of the Caucasus is shown in which the σ parameter is represented by isolines for all periods under consideration. A comparative analysis of the values of the sigma-Σ parameter was conducted in each elemental area for three successive decades. Zones which are characterized by an unstable behaviour of the parameter can be distinguished.     

Seismicity and seismoactive faults of Cuba

The first catalogue of active regional faults of Cuba is presented. The seismotectonic map of Cuba is a base for studying the seismicity in this region. Of the 30 faults studied, only twelve are active. The main seismotectonic structure is the Bartlett-Cayman fault system, which borders the eastern and southeastern seismotectonic units in this region. Approximately 70% of Cuban seismicity is concentrated here. The Cauto-Nipe, Cochinos and Nortecubana faults border other seismotectonic units. The Nortecubana fault is the only one associated with a tsunami. All the faults are segmented. The faults described are related to the current tectonic stress regime of the Northern Caribbean. All the available information (maps, sections and profiles, photos, geological and neotectonic data on seismicity and focal mechanisms) is supported by a GIS.