首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 25 毫秒
1.
— A novel seismic moment tensor inversion approach is applied to microseismic events from the Kidd mine with moment magnitudes ranging between ?1.2 and 0. Data consist of 35 events recorded on 8 triaxial accelerometers installed underground. Reliable solutions are obtained for 21 events, of which 14 represent pure shear mechanisms, whereas the remaining 7 exhibit a significant positive volumetric component (72–76%), along with some pure shear failure (15–20%). Further analysis indicates that 6 of the events characterized by high volumetric components are located within a sill pillar on the 4,700 level and have subvertical P and subhorizontal T axes. This is in agreement with the presence of tensile cracks close to openings for incipient pillar bursting. The pure shear events are located outside the sill pillar, between the 4,600 and 4,800 levels, and on the 5,600 level within a highly fractured rockmass. For the latter events, the subvertical nodal planes are found to match closely the orientation of subvertical NW-SE fractures aligned parallel to the major faults in the area.  相似文献   

2.
Source parameter scaling relations are examined for microseismic events (–2.4M–0.3) occurring within higyly and moderately stressed and fractured rock masses at Strathcona mine, Sudbury, Canada. Insight into scaling is provided by waveform complexities, calculated rupture velocities, and maximum shear stresses based onin situ and numerical modelling data. The importance of normal stress on the failure process is also considered. Our results show that a strong dependence exists between stress release and seismic moment. An observed positive scaling in excess stress release (/2– a ) is consistent with the concept of overshoot. Rupture velocities ranging from 0.2 to 0.5 and waveform complexities less than 1.5 suggested that overshoot was related to healing behind a slowly advancing rupture front. Scaling in seismic efficiency paralleled that in apparent stress, implying that seismic stress release estimates are quasi-independent of the maximum shear stress. High levels of normal stress further supported the importance of high resisting stress in the observed overshoot behaviour and its role in the failure process.  相似文献   

3.
Summary Meteorological microseisms, recorded at the Prague seismic station and, similarly, at other Central Europe stations, reflect certain meteorological situations in the North Atlantic frontal zone. Meteorological elements in this zone are affected by some factors of extra-terrestrial origin, among others by the interplanetary magnetic field (IMF) sector structure. The analysis of the 68 IMF sector boundary passage effects in microseismic activity, expressed in microseismic amplitudes, showed a well-developed minimum of microseismic activity on the day of the boundary passage in winter, but no such effect in spring and autumn. In spring and autumn, however, a tendency was observed to slightly higher microseismic activity in the away (+) sector, whereas no such tendency was observed in winter. These results agree with the IMF effect found in the troposphere. Generally, the IMF sector structure is only a modulating rather than a major factor controlling the microseismic activity.  相似文献   

4.
In the summer of 1984, a three-dimensional, high-resolution microearthquake network was operated in the vicinity of two coal mines beneath Gentry Mountain in the eastern Wasatch Plateau, Utah. During a six-week period, approximately 3,000 seismic events were observed of which the majority were impulsive, higher frequency (>10 Hz), short duration (<2–3 sec) events probably associated with the caving of the roof from a longwall operation. In contrast, 234 of the largest located events appeared to occur predominantlybeneath the mines to a depth of 2 to 3 km consistent with previous studies. The magnitudes of these events ranged from less thanM c 0 to 1.6. In addition to the unusual depths of these latter events, an anomalous aspect displayed by the events was an apparent dilatational focal mechanism suggesting a non-double-couple, possibly implosional source. Implosional events have been observed in other studies of mine seismicity; however, the generally inadequate instrumental coverage of the focal sphere has cast some doubt on the validity of such mechanisms. Previously suggested source mechanisms for such implosional events have included tensional failure through strata collapse, and a shear-implosional displacement mechanism. Shear failure must be involved in the failure process of the Gentry Mountain implosional events as evidenced by well-defined shear waves in the observed seismograms. Simultaneous monitoring in the East Mountain coal mining area to the south by the University of Utah revealed typical shear failure events mixed with implosional events. The observed double-couple, reverse focal mechanisms at East Mountain were similar to mechanisms determined in previous studies and a composite focal mechanism determined in this study for a sequence outside the mining areas. This suggested that the shear events within the mining areas are being influenced by the regional tectonic stress field. Thus in addition to the seismic events associated with caving of the roof from the longwall operation, there appear to be at least two other types of mining-induced seismic events occurring in the eastern Wasatch Plateau, both submine in origin: (1) events characterized by apparent non-double-couple possibly implosional focal mechanisms and well-defined shear waves; and (2) shear events, which are indistinguishable from tectonic earthquakes and may be considered mining triggered earthquakes. The small mining-induced stress changes that occur beyond a few hundred meters from the mine workings suggest both types of seismic events are occurring on critically stressed, pre-existing zones of weakness. Topography, overburden, method of mining, and mine configuration also appear to be significant factors influencing the occurrence of the implosional submine events.  相似文献   

5.
Severe stress release has occurred to the surrounding rocks of the typically columnar jointed basalt after excavation at the Baihetan Hydropower Station, Jinsha River, China, where cracking, collapse, and other types of failure may take place occasionally due to relaxation fracture. In order to understand the relaxation fracture characteristics of the columnar jointed basalt in the entire excavation process at the diversion tunnel of the Baihetan Hydropower Station, real-time microseismic monitoring tests were performed. First, the applicability of a geophone and accelerometer was analyzed in the columnar jointed basalt tunnel, and the results show that the accelerometer was more applicable to the cracking monitoring of the columnar jointed basalt. Next, the waveform characteristics of the microseismic signals were analyzed, and the microseismic signals were identified as follows: rock fracture signal, drilling signal, electrical signal, heavy vehicle passing signal, and blast signal. Then, the attenuation characteristics of the microseismic signals in the columnar jointed basalt tunnel were studied, as well as the types and characteristics of the columnar jointed basalt fracture. Finally, location analysis was conducted on the strong rock fracture events, in which four or more sensors were triggered, to obtain the temporal and spatial evolution characteristics and laws of the columnar jointed basalt relaxation fracture after excavation. The test results are not only of important reference value to the excavation and support of diversion tunnel at the Baihetan Hydropower Station, but also of great referential significance and value to the conduction of similar tests.  相似文献   

6.
Some comparisons between mining-induced and laboratory earthquakes   总被引：3，自引：0，他引：3  
Although laboratory stick-slip friction experiments have long been regarded as analogs to natural crustal earthquakes, the potential use of laboratory results for understanding the earthquake source mechanism has not been fully exploited because of essential difficulties in relating seismographic data to measurements made in the controlled laboratory environment. Mining-induced earthquakes, however, provide a means of calibrating the seismic data in terms of laboratory results because, in contrast to natural earthquakes, the causative forces as well as the hypocentral conditions are known. A comparison of stick-slip friction events in a large granite sample with mining-induced earthquakes in South Africa and Canada indicates both similarities and differences between the two phenomena. The physics of unstable fault slip appears to be largely the same for both types of events. For example, both laboratory and mining-induced earthquakes have very low seismic efficiencies where a is the apparent stress and is the average stress acting on the fault plane to cause slip; nearly all of the energy released by faulting is consumed in overcoming friction. In more detail, the mining-induced earthquakes differ from the laboratory events in the behavior of as a function of seismic momentM 0. Whereas for the laboratory events 0.06 independent ofM 0, depends quite strongly onM 0 for each set of induced earthquakes, with 0.06 serving, apparently, as an upper bound. It seems most likely that this observed scaling difference is due to variations in slip distribution over the fault plane. In the laboratory, a stick-slip event entails homogeneous slip over a fault of fixed area. For each set of induced earthquakes, the fault area appears to be approximately fixed but the slip is inhomogeneous due presumably to barriers (zones of no slip) distributed over the fault plane; at constant , larger events correspond to larger a as a consequence of fewer barriers to slip. If the inequality a / 0.06 has general validity, then measurements of a =µE a /M 0, where is the modulus of rigidity andE a is the seismically-radiated energy, can be used to infer the absolute level of deviatoric stress at the hypocenter.  相似文献   

7.
We investigate the history, kinematics, principal stress orientations and geometry of deformation at the end of a bent normal fault segment of the Wasatch fault zone, Utah. Three fault types, developed in Archean crystalline rocks, reflect progressive uplift of fault-related footwall rocks. Chlorite-breccias and phyllonites reflect deep-level, reaction-assisted plastic deformation along the north-striking part of the segment. Planar, fretted faults which formed by cataclasis cut the phyllonites and breccias and are developed throughout the footwall of the segment. Youngest faults are hematitecoated, extremely narrow polished surfaces. Slip vectors and kinematic analyses of small faults developed in the footwall indicate oblique normal slip along the north-striking portion of the segment. Slip vectors and fault orientation along the northwest-striking portion of the segment reflect complexly oriented slip on faults which strike subparallel and at high angles to the main fault trace, yet slip is confined to a broad fault-parallel zone. Small faults at the southernmost tip of the segment indicate a strong influence of the north-striking Weber segment to the south. Inversion of fault data for principal stress orientations document complexly oriented principal stresses through the segment boundary zone and suggest that 3 may have reoriented approximately 60° over the life of the segment. Subsurface structure combined with small fault data indicate the segment boundary is comprised of a southwest-plunging bedrock high which is reflected by a sharp bend in the Brigham City segment. The southern end of the Brigham City segment may have started, as a straight, north-striking fault which has bent due to changes in stress orientations and/or interaction with the adjacent Weber segment.  相似文献   

8.
Summary To study the ionization equilibrium, simultancous measurements of the concentration of small ions (n), concentration of condensation nuclei (Z), and the diffusion coefficient of condensation nuclei (D) have been carried out over the sea. If we take the variation ofD into consideration, the correlation among them was found to be well expressed by the simple formula;q=n Z. The dependence of the effective attachment coefficient of small ions () upon the size of nuclei (2r) were also studied, and was found to correlate well withD orr. Slight differences of the dependence of uponD orr over the sea from that over the land seem to be well explained by the difference of charged states of nuclei over the sea and land.
Zusammenfassung Zur Untersuchung des Ionisationsgleichgewichts wurden gleichzeitige Messungen der Konzentration schneller Ionen (n), der Konzentration der Kondensationskerne (Z) und des Diffusions-Koeffizienten der Kondensationskerne (D) über dem Meere ausgefürt. Unter Berücksichtigung des Diffusions-Koeffizienten (D) konnte der Zusammenhang zwischen den beiden anderen Grössen recht gut durch eine einfache Formel ausgedrückt werden:q=n Z. Die Abhängigkeit des wirksamen Anlagerungskoeffizienten für schnelle Ionen () von der Grösse der Kondensationskerne (2r) wurde ebenfalls untersucht, und es wurde gefunden, dass recht gut mitD oderr korreliert. Kleinere Unterschiede in der Abhängigkeit von vonD oderr, welche bei einem Vergleich der Messungen über See und über Land festgestellt wurden, liessen sich gut durch den Unterschied der geladenen Zustände der Kondensationskerne über See und Land erklären.
  相似文献   

9.
Scattering of seismic waves can be shown to have a frequency dependenceQ –1 3–v if scattering is produced by arrays of inhomogeneities with a 3D power spectrumW 3D(k) k –v. In the earth's crust and upper mantle the total attenuation is often dominated by scattering rather than intrinsic absorption, and is found to be frequency dependent according toQ –1 , where –1<–0.5. IfD 1 is the fractal dimension of the surface of the 3D inhomogeneities measured on a 2D section, then this corresponds respectively to 1.5<D 11.75, since it can be shown that =2(D 1–2). Laboratory results show that such a distribution of inhomogeneities, if due to microcracking, can be produced only at low stress intensities and slow crack velocities controlled by stress corrosion reactions. Thus it is likely that the earth's brittle crust is pervaded by tensile microcracks, at least partially filled by a chemically active fluid, and preferentially aligned parallel to the maximum principal compressive stress. The possibility of stress corrosion implies that microcracks may grow under conditions which are very sensitive to pre-existing heterogeneities in material constants, and hence it may be difficult in practice to separate the relative contribution of crack-induced heterogeneity from more permanent geological heterogeneities.By constrast, shear faults formed by dynamic rupture at critical stress intensities produceD 1=1, consistent with a dynamic rupture criterion for a power law distribution of fault lengths with negative exponentD. The results presented here suggest empirically thatD 1-1/2(D+1), thereby providing the basis for a possible framework to unify the interpretation of temporal variations in seismicb-value (b-D/2) and the frequency dependence of scattering attenuation ().This is PRIS contribution 046.  相似文献   

10.
Data from four A3 radio wave circuits in central Europe are used to analyse the representativeness of the ionization and gravity wave activity pattern in the lower ionosphere (85 – 100 km) in the CRISTA experiment interval (3 – 12 November 1994) for the given conditions. It has been found that the CRISTA experiment interval was run under conditions, which are highly representative both of October – November 1994 and autumn, low-moderate solar activity, the descending phase of solar cycle conditions, i.e. CRISTA measurements may be considered to provide values identical with, or close to climatological values (at least for central Europe, h = 85 – 100 km).  相似文献   

11.
A ray-tracing computer program is described for a two-dimensional velocity distribution defined by the local wave velocitiesv i, j in points at arbitrary depthv i, j below the surface points with the horizontal coordinatesx i . The velocity variation is assumed to be linear in the triangles formed by three neighbouring points. Travel times and rays are then calculated after the exact analytic formulae for any position of the source within the model. No assumptions other than of a piecewise linear velocity structure are made. A first-order discontinuity can be approximated by a thin layer with a strong velocity gradient and refracted waves or wide-angle reflections obtained in this way. As an example,P-wave rays were computed for section No. 05 of the Alpine Longitudinal Profile. The model includes a low-velocity channel which is cut off on the eastern side, first-order discontinuities and a sediment basin.Paper presented at the ESC-Workshop Meeting Seismic Waves in Laterally Inhomogeneous Media, Liblice, 1978.  相似文献   

12.
A sequence of moderate shallow earthquakes (3.5M L5.3) was located within the Vercors massif (France) in the period 1961–1984. This subalpine massif has been a low seismic area for at least 5 centuries. During the period 1962–1963, 12 shallow earthquakes occurred in the neighborhood (10 km) of the Monteynard reservoir, 30 km south of the city of Grenoble. The latest fourM L4.0 earthquakes occurred in 1979–1984 either at larger distance (35 km) or greater depth (10 km) from the reservoir. Two triggering mechanisms are suggested for this sequence: (i) the direct effect of elastic loading through either increased shear stress or strength reducing by increased pore pressure at depth; (ii) the pore pressure diffusion induced by poroelastic stress change due to the reservoir filling.The weekly water levels, local balanced geological cross sections, and focal mechanisms argue for two types of mechanical connection between the earthquake sequence and the filling cycles of the Monteynard reservoir. The seismic sequence started with the 1962–1963 shallow earthquakes that occurred during the first filling of the reservoir and are typical of the direct effect of elastic loading. The 1979 deeper earthquake is located at a 10 km depth below the reservoir. This event occurred 16 years after the initial reservoir impoundment, but one month after the previous 1963 maximum water level was exceeded. Moreover the yearly reservoir level increased gradually in the period 1962–1979 and has decreased since 1980. Accordingly we suggest that the gradual diffusion of water from reservoir to hypocentral depths decreases the strength of the rock matrices through increased pore pressure. The transition between the two types of seismic response is supported by the analysis ofM L3.5 earthquakes which all occurred in the period 1964–1971, ranging between 10 and 30 km distance from the reservoir. The three other delayed earthquakes of the 1961–1984 seismic sequence (M L4 during the 1979–1984 period) are all located 35 km away from the reservoir. Based on the seismic activity, the estimates for the hydraulic diffusivities range between 0.2–10 m2/s, except for the first event that occurred 30 km north of the reservoir, the filling just started. The lack ofin situ measurements of crustal hydrological properties in the area, shared by most of the Reservoir-Induced-Seismicity cases, prevents us from obtaining absolute evidence for the triggering processes. These observations and conceptual models attest that previous recurrence times for moderate natural shocks (4.5M L5.5) estimated within this area using historical data, could be modified by 0.1–1 MPa stress changes. These small changes in deviatoric stress suggest that the upper crust is in this area nearly everywhere at a state of stress near failure. Although the paucity of both number and size of earthquakes in the French subalpine massif shows that aseismic displacements prevail, our study demonstrates that triggered earthquakes are important tools for assessing local seismic risk through mapping fault zones and identifying their possible seismic behavior.  相似文献   

13.
Deep seismic sounding in the region of the Mirnyi kimberlite field indicates that the boundary velocity of the uppermost mantle is elevated (v b=8.6–8.8 km/sec) and extremely variable near the Mir kimberlite pipe. These velocity heterogeneities are probably associated with the kimberlite magmatism and may be useful in the identification of other kimberlite fields.  相似文献   

14.
Five samples from historical lava flows on Mt. Etna, which had previously been used in a palaeointensity study, were examined using a combination of rock magnetic and microscopic techniques to elucidate the causes of failure of palaeointensity determination. The samples were characterised using a combination of low-temperature susceptibility (LT-), Bitter pattern imaging and scanning electron microscope (SEM). High-temperature susceptibility curves and hysteresis loops had been previously measured by Calvo et al. (2001). Of the five samples only one gave an accurate palaeointensity. This sample was deuterically oxidised and consisted of large exsolved ore grains. It was only possible to distinguish this sample either optically or magnetically from a similar sample by the LT- warming curves; the unsuccessful sample displayed alteration in the LT- warming curves, which was not readily observed in the high temperature susceptibility curves. It is proposed that the measurement of LT- curves before and after heating could be a more sensitive method of determining suitability for palaeointensity determination than previous rock magnetic pre-selection techniques.  相似文献   

15.
The largest earthquake (M0=4.9·1027 dyn·cm) of the 20th century in the territory of Greece occurred south of Amorgos Island, causing extensive destruction in the southern Aegean area. It occurred on an ENE–trending normal fault that is seated parallel to the Islands southern coastline. Changes in the rates of moderate–size earthquakes (M 5.0) that occurred before and after the Amorgos earthquake, within circular regions centered on its epicenter with radii of 100, 150 and 200 km, are investigated. The rate for moderate–size events just before the main shock appears to be considerably increased when compared to those of either preceding or subsequent periods. Further inspection reveals that more evident seismicity fluctuations are attributed to distances exceeding 100 km. These changes may be indicative of a broad region that is approaching a high stress state prior to an eventual large earthquake. Close to the main event, that is, within the 100–km radius, a remarkable quiescence period lasting about two decades before its occurrence was observed. Changes in seismicity are discussed in combination with static stress changes calculated by the application of the stress evolutionary model that takes into account the coseismic slip associated with the larger events (M 6.5) since the beginning of the 20th century and the tectonic loading on the major faults in the study area. These larger events, as with the intermediate magnitude seismicity taking place at distances exceeding 100 km and which encircled the quiescent area observed during the last 22 years before the Amorgos earthquake, are well correlated with stress-enhanced areas in each stage of the evolutionary model.  相似文献   

16.
We describe slip-rate dependent friction laws based on the Coulomb failure criteria. Frictional rate dependence is attributed to a rate dependence of cohesionc and friction angle . We show that differences in the stress states developed during sliding result in different Coulomb friction laws for distributed shear within a thick gouge layer versus localized shear within a narrow shear band or between bare rock surfaces. For shear within gouge, shear strength is given by =c cos + n sin, whereas for shear between bare rock surfaces the shear strength is =c cos + n tan, where and n are shear and normal stress, respectively. In the context of rate-dependent Coulomb friction laws, these differences mean that for a given material and rate dependence of the Coulomb parameters, pervasive shear may exhibit velocity strengthening frictional behavior while localized shear exhibits velocity weakening behavior. We derive from experimental data the slip-rate dependence and evolution ofc and for distributed and localized shear. The data show a positive rate dependence for distributed shear and a negative rate dependence for localized shear, indicating that the rate dependence ofc and are not the same for distributed and localized shear, even after accounting for differences in stress state. Our analysis is consistent with the well-known association of instability with shear localization in simulated fault gouge and the observation that bare rock surfaces exhibit predominantly velocity weakening frictional behavior whereas simulated fault gouge exhibits velocity strengthening followed by a transition to velocity weakening with increasing displacement. Natural faults also exhibit displacement dependent frictional behavior and thus the results may prove useful in understanding the seismic evolution of faulting.  相似文献   

17.
为将小波去噪方法应用于大尺度岩体结构微震监测信号的去噪研究,首先在MATLAB环境下进行仿真,验证了使用Symlet6小波进行小波去噪的可行性;利用4种自适应阈值规则对含噪信号进行去噪对比,结果表明4种阈值去噪后的信号在均方差较小的情况下都极大地提高了信号的信噪比,有效地去除了噪声,对不同的含噪信号,无偏似然原则阈值去...  相似文献   

18.
Summary Approximate expressions for the Hilbert transform of the functionf(t)=exp(- 0 2 t 2/2) cos( 0 t+v) are determined. This function, given a suitable choice of the three parameters 0, and v, approximates a wide class of seismic signals very well. The approximate expressions for the Hilbert transform enable very simple formulae to be given for the elementary seismograms of the individual seismic body waves (in the zero approximation of the ray theory). This accelerates the computation of ray theoretical seismograms considerably.  相似文献   

19.
We present results of processed microseismic events induced by hydraulic fracturing and detected using dual downhole monitoring arrays. The results provide valuable insight into hydraulic fracturing. For our study, we detected and located microseismic events and determined their magnitudes, source mechanisms and inverted stress field orientation. Event locations formed a distinct linear trend above the stimulated intervals. Source mechanisms were only computed for high‐quality events detected on a sufficient number of receivers. All the detected source mechanisms were dip‐slip mechanisms with steep and nearly horizontal nodal planes. The source mechanisms represented shear events and the non‐double‐couple components were very small. Such small, non‐double‐couple components are consistent with a noise level in the data and velocity model uncertainties. Strikes of inverted mechanisms corresponding to the nearly vertical fault plane are (within the error of measurements) identical with the strike of the location trend. Ambient principal stress directions were inverted from the source mechanisms. The least principal stress, σ3, was determined perpendicular to the strike of the trend of the locations, indicating that the hydraulic fracture propagated in the direction of maximum horizontal stress. Our analysis indicated that the source mechanisms observed using downhole instruments are consistent with the source mechanisms observed in microseismic monitoring arrays in other locations. Furthermore, the orientation of the inverted principal components of the ambient stress field is in agreement with the orientation of the known regional stress, implying that microseismic events induced by hydraulic fracturing are controlled by the regional stress field.  相似文献   

20.
The feasibility of gaining valuable geomechanical information derived from seismological data and its specialist interpretation for utilisation in the area of assessing hazards due to mining tremors and rockbursts has become a development of signal importance in mining seismology. Undoubtedly of particular interest is a certain knowledge of the directions of the principal stresses 1, 2, 3 of the regional stress tensor. For their determination, use is made of a set of parameters from the mining tremors' regional focal mechanism solutions (angular parameters of nodal planes and axes of principal stresses in the tremor focus —P andT). Results of research conducted at the Szombierki and Wujek mines and analysis of calculated results for parameters of regional stress tensor show that there exist appreciable differences between values of these parameters and also a clear correlation with local extraction conditions that is of significance from the point of view of seismic hazard.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号