Source time function and spectra for underground explosions

Constants occurring in Seggern and Blandford's analytic expression of reduced displacement potential are computed for shot media of alluvium, salt and tuff. The amplitude spectra as well as impulse response of the source function for models of Haskell and Seggern are studied as a function of yield. It is shown that the maximum in amplitude spectrum shifts to lower frequency with increase in yield for a given model. The duration of impulse response is longer for Seggern's model as compared to Haskell's model for a given yield.     

A statistical comparison of various source formulations for explosions and earthquakes

Stochastic models are derived for two source formulations for explosions. Using this kind of model, a comparison is made between source time functions due to Blake, Haskell, Mueller and Murphy, and Von Seggern and Blandford, for explosions, and between -square (Aki, Brune)and -cube (Aki, Haskell)models for earthquakes. When seeking a stochastic model for records of Rayleigh waves from atmospheric explosions, the k-model corresponding to Haskell's time function was found to be an appropriate choice.     

Coda power and modulation characteristics of a complex P signal from underground nuclear explosions

Coda power characteristics of six underground nuclear explosions from two near by test sites of Novaya Zemlya are examined. It is shown that one of the events presenting a complex P waveform and distinctive coda power characteristics can be synthesised by superposition of two signals of simpler form from the same site. The relative P amplitudes of the two signals give a ratio of 0.7–1.0 and the relative delay of 0.8 sec. The modulation effect is clearly seen in band-pass filtered traces where a shift in the envelope maximum occurs for center frequencies between 0.6 and 0.8 Hz. The complex event is interpreted as a double explosion.     

Porewater pressure increases in soil and rock from underground chemical and nuclear explosions

A review and analysis of chemical and nuclear explosive-induced porewater pressure increases and induced rise in groundwater table elevations (groundwater mounding) is presented. Our analysis indicates that residual pore pressure increases and groundwater mounding can be induced by underground chemical and nuclear explosions to scaled distances of 879 m/(kt)^1/3. This relationship is linear over seven orders of magnitude of explosive energy ranging from a 0.01 kg chemical explosion to a 100 kt nuclear explosion and is valid for a wide variety of saturated geological profiles. Underground chemical explosions, and probably underground nuclear explosions have the potential to induce liquefaction of water-saturated soils to scaled distances of about 260 m/(kt)^1/3.     

Space and time distribution of the intermediate focal depth earthquakes in the Hellenic arc

Accurately determined focal coordinates of earthquakes which occurred in the Hellenic arc between 1964 and 1979 have been used to check the suggestion made ten years ago for a Benioff zone in the Hellenic arc. These recent data leave no doubt that the foci of the intermediate focal depth earthquakes in the Hellenic arc form a well-developed Benioff zone of amphitheatrical shape which dips from the convex (eastern Mediterranean) to the concave (Aegean) part of the arc.

Evidence, based on the time distribution of the intermediate focal depth earthquakes in the Hellenic arc during the last two centuries, indicates that this deep seismic activity follows a periodic pattern. Quiescence periods of the order of six decades are followed by very active periods of about two decades. If this periodicity, which seems to be compatible with recent ideas on the deep tectonics of the area, is true, a new period of high-rate intermediate focal depth activity is expected to break in about twenty five years.     

Quantification of SEM microtextures useful in sedimentary environmental discrimination

Scanning electron microscopic imagery is often used to identify and discriminate among environments of sedimentation with the main aim of identifying individual microfeatures, or suites of microtextures, that are considered indicative of a particular depositional environment or geologic process. Because few microtextures are considered to represent a single geologic process it is necessary to analyze a large number of quartz sands and other mineralic grains with the objective of determining the frequency of occurrence of a range of microtextures within a distinct sample suite. Using percent frequency of occurrence of different microtextures from suites of fluvial, glaciofluvial and glacial sands from sites in Estonia and Latvia, we invoked statistical comparison of different sample suites using Euclidean distances. These provide a quantitative means of measuring the differences among different sediments and processes that formed them and also a quantification tool useful in assessing microtextures as a recorder of sedimentary environments.     

Long-term prediction of intermediate depth earthquakes in the southern Aegean region based on a time-predictable model

Repeat times of strong intermediate depth (60 km h 180 km) earthquakes have been determined by the use of instrumental and historical data for six seismogenic sources in the Benioff zone of the southern Aegean area. For four of these sources, at least two interevent times (three mainshocks) are available for each source. By using the repeat times for these four sources, the following relation has been determined: logT _t = 0.20M _min + 0.19M _p +a, whereT _t is the repeat time (in years),M _min the surface wave magnitude of the smallest earthquake considered,M _p the magnitude of the preceding mainshock and a parameter which varies from source to source. A multilinear correlation coefficient equal to 0.91 was determined for this relation, which indicates that the time predictable model holds to a satisfactory degree for the strong mainshocks of intermediate focal depth in the southern Aegean.By assuming that the ratioT/T _t, whereT is the observed andT _t the calculated repeat time, follows a lognormal distribution, the conditional probabilities for the occurrence of strong (M _s 6.5) and very strong (M _s 7.5) earthquakes during the period 1991–2001 in these four seismogenic sources have been calculated. These probabilities are very high (P > 0.9) for the strong and high (P > 0.5) for the very strong intermediate depth earthquakes which occur in the three sources of the shallower (h < 100 km) part of the Benioff zone where coupling occurs between the front parts of the Mediterranean lithosphere (downgoing) and the Aegean lithosphere.     

活动断裂带附近地下水中的氢同位素变化与地震关系研究

本文主要研究活动断裂带附近地下水中的氢同位素变化与地震的相关性。2009年1月初,我们在汶川地震的发震断裂带——龙门山断裂带南缘的雅安进行考察,并开始采集地热深井的地下水样品,到5月初结束。在采样期间,龙门山断裂带发生4~5级余震多次。对所采地下水样品氢同位素δD值的分析结果表明,发震断裂带附近地下水中的氢同位素异常与该断裂带上4.0级以上地震有一定相关性。从氢同位素δD值随时间变化曲线可看出,(1)震前氢同位素δD值由背景值逐渐降低,震后升高,然后再逐渐回归背景值;(2)地震的级别越大,氢同位素δD值变化越显著;(3)从氢同位素δD值开始系统地降低到地震发生,这个过程大约可持续一周左右时间,属于临震预报的时间范畴。从本研究结果可得出地下水中氢同位素δD值异常能够反映断裂带的地震活动性,可作为5.0级以上地震临震预测的一种方法。     

Prediction of hollows in abandoned underground workings at shallow depth

Composite statistical analysis of the lithological composition of the rock mass above underground workings in coal seams and experimental work (laboratory mechanical tests and boring for the determination of hollows) in Donetsk city (Ukraine) have been used to develop a method for predicting the presence of hollows. The empirical criteria K₁–K₂>0 and K₃2.5 which relate to the physical characteristics of the overlying strata were found to predict the existence of such hollows in abandoned underground workings at shallow depth. The values of K₁–K₂ and K₃ depend on the thicknesses of the different rock layers and the uniaxial compressive strength of the immediate roof over underground openings. The different layers e.g. sandstones, argillites, aleurolites and alluvium in the rock mass are shown to influence the existence of hollows in abandoned workings. Large thicknesses of sandstones in the rock mass or relatively high uniaxial compressive strength rock in the immediate roof contribute to the existence of hollows in abandoned workings. On the other hand, sandstones of small thickness, weak alluvium, argillites and aleurolites only give additional weight on immediate roof of the opening and allow collapse of the rock mass. The method of prediction for hollows was developed from the back-analysis of data from 41 boreholes which were drilled from the ground surface above underground openings.     

Joint hypocentre determination of intermediate depth earthquakes in Fiordland, New Zealand

Relocation of well observed, intermediate depth earthquakes in the Fiordland region by the method of joint hypocentre determination has revealed some fine structure in the Benioff zone. The earthquakes occur in three groups. The central group is the largest and occupies a planar volume less than 15 km thick striking N40°E and dipping at 80°. The deepest events in the region, at depths of 150 km, occur at the northeast end of this group. The two smaller groups lie to the northeast and to the south of the main group. The focal mechanism of the majority of the main group is that of thrust faulting. We suggest that the main group lies within a section of Indian plate lithosphere which has been broken off and rotated into its observed position and that the northern edge of the unbroken subducted Indian plate is indicated by the southern group. We suggest that the small northeastern group has quite a different tectonic origin and is similar to a group of earthquakes further north which are at a similar distance from, and presumably related to, the Alpine Fault.Use has also been made of the travel-time information which is a by-product of the joint hypocentre method to construct upper mantle velocity models for P and S waves in the South Island. The features of this model are a high-velocity region in the vicinity of the Benioff zone, and a subcrustal zone of high seismic velocities running east-west across the center of the South Island in an otherwise normal mantle.     

Estimating the depth of underground coal fires using data integration techniques

Underground coal fires in China cause serious environmental problems, in addition to the loss of valuable coal resources. The present study aims at developing a quick and practical method to estimate the depth of coal fires using data integration techniques.
  In coal fields which have underground coal fires, the subsurface fires are associated with surface thermal anomalies. Airborne thermal infrared scanner data and colour infrared photographs were used in this study to depict the coal fire front and the outcrop of coal seams, respectively. A Digital Elevation Model (DEM) of the study area was produced from topographic maps at 1:25 000 scale. Finally, taking into account the spreading direction of the coal fires and the relationships between thermal anomalies, relief factors and the occurrence of coal seams, the depths of the coal fires were obtained automatically by means of integration of remote sensing data and GIS techniques. This helped to target the fire fighting operations and made them more cost effective.     

Source parameters of selected earthquakes on the central and western margin of Afar

Spectral analysis of 196 short-period and long-period vertical- and horizontal-component seismograms from ten earthquakes on the central and western margin of Afar is performed to determine source parameters and discuss their tectonic implications. For the earthquakes in the regions under study, the stress drop varies from 2 to 31 bar while the seismic moment varies from 2 × 10²⁴ to 154 × 10²⁴ dyn cm. In general, low stress-drop values are obtained indicating the presence of softer material (especially for central Afar) at a shallower depth. It is observed that there is an increase in stress drop with the increase in moment-magnitude which in turn is obtained from the calculated average seismic moment. Energy estimates show that the mode of energy release is different in the two regions indicating that different tectonic processes are involved in the two regions. The slip rate obtained for the Serdo area is of the order of 1.6 cm/yr and is in close agreement with the spreading rate already obtained for central Afar. Spreading rates obtained earlier and that of the present study show a low spreading rate for Afar and neighbouring regions as compared to those of the other regions of the world.     

Source parameters of some recent earthquakes in the Gulf of Aqaba, Egypt

Gulf of Aqaba is recognized as an active seismic zone where many destructive earthquakes have occurred. The estimation of source parameters and coda Q attenuation are the main target of this work. Fifty digital seismic events in eight short-period seismic stations with magnitude 2.5–5.2 are used. Most of these events occurred at hypocentral depths in the range of 7–20 km, indicating that the activity was restricted in the upper crust. Seismic moment, M _o, source radius, r, and stress drop, Δσ, are estimated from P- and S-wave spectra using the Brune’s seismic source model. The average seismic moment generated by the whole sequence of events was estimated to be 4.6E?+?22 dyne/cm. The earthquakes with higher stress drop occur at 10-km depth. The scaling relation between the seismic moment and the stress drop indicates a tendency of increasing seismic moment with stress drop. The seismic moment increases with increasing the source radius. Coda waves are sensitive to changes in the subsurface due to the wide scattering effects generating these waves. Single scattering model of local earthquakes is used to the coda Q calculation. The coda with lapse times 10, 20, and 30 s at six central frequencies 1.5, 3, 6, 12, 18, 24 Hz are calculated. The Q _c values are frequency dependent in the range 1–25 Hz, and are approximated by a least squares fit to the power law [ $ {Q_c}(f) = {Q_o}{(f/{f_o})^\eta } $ ]. The average of Q _c values increases from 53?±?10 at 1.5 Hz to 700?±?120 at 24 Hz. The average of Q _o values ranges from 13?±?1 at 1.5 Hz to 39?±?4 at 24 Hz. The frequency exponent parameter η ranges between 1.3?±?0.008 and 0.9?±?0.001.     

Ring seismicity in different depth ranges before large and great earthquakes in subduction zones

Doklady Earth Sciences -     

Application of a BP neural network in predicting destroyed floor depth caused by underground pressure

Predicting the destroyed floor depth caused by the mining of coal seams is of great importance in judging whether the mining of a deep coal seam can be safely performed above a confined aquifer and to prevent the inrush of water from the floor. Thirty sets of coal mining data on destroyed floor depth were selected for study. A comprehensive analysis of the factors that influence the depth of destruction of coal seam floor strata was performed and combined with the ability of a BP neural network to address dynamic nonlinear information. Then, a set of test samples was assembled and used to construct a predictive model using a BP neural network. The model was then used to predict the destroyed floor depth of the 7105 working face of the Baizhuang Coal Mine in the Feicheng coal field. To verify the effectiveness of the model, the depth of the destroyed strata comprising the coal seam floor was measured using equipment called the “Double Sided Sealed Borehole Water Injection Device.” By comparing the predictions made by the BP neural network with actual measurements, the conclusion was reached that a BP neural network model can effectively be used to predict the destroyed floor depth caused by the mining of a coal seam.     

Characteristics of ring seismicity in different depth ranges before large and great earthquakes in the Sumatra region

Characteristics of the seismicity in depth ranges 0–33 and 34–70 km before ten large and great (M _w = 7.0−9.0) earthquakes of 2000–2008 in the Sumatra region are studied, as are those in the seismic gap zones where no large earthquakes have occurred since at least 1935. Ring seismicity structures are revealed in both depth ranges. It is shown that the epicenters of the main seismic events lie, as a rule, close to regions of overlap or in close proximity to “shallow” and “deep” rings. Correlation dependences of ring sizes and threshold earthquakes magnitudes on energy of the main seismic event in the ring seismicity regions are obtained. Identification of ring structures in the seismic gap zones (in the regions of Central and South Sumatra) suggests active processes of large earthquake preparation proceed in the region. The probable magnitudes of imminent seismic events are estimated from the data on the seismicity ring sizes.     

确定地下管线深度的方法原理研究

从理论上阐述了利用倾斜磁场最大值点、利用垂直磁场分量峰值点和利用水平磁场分量半极值点确定地下管线深度的原理方法,指出了利用倾斜磁场最大值点确定管线深度时在原理上存在的问题和不实用性,同时从原理上和实用中筛选出了最佳的定深方法-水平磁场分量三角法.     

Thermal infrared anomalies as a precursor of strong earthquakes in the distant future

Satellite thermal infrared images contain valuable earthquake precursor information. Past studies concluded that such information appeared only a few days or dozens of days before an earthquake would occur. In our study, though, we observed that the time intervals between the thermal infrared precursor and an earthquake??s occurrence can be up to 10?years. An infrared image can also synchronously indicate the locations of additional future earthquakes with different epicenters within a region. The shape, area, intensity, and movement of thermal infrared anomaly areas are a combination of all the future strong earthquakes within a region. These distant future earthquakes are generally located near the edges, endpoints, or corners of the main structure, fine structures or periphery structures of a thermal infrared anomaly area and play a role in confining the anomaly area. There have not been any exceptions among the strong earthquakes we analyzed, which have included the 2011 Japan M _w 9 event, the 2010 Yushu M _S 7.1 event, the 2008 Wenchuan M _S 8 event, and many other strong events following the 2004 Sumatra M _S 9 event. Surprisingly, some of the earthquakes can outline an area of elevated temperature observed many months ago. If we can roughly locate these potential epicenters through the analysis of thermal infrared images and combining the analysis with other information, and then dynamically monitor them, it may be easier to observe the precursor of an earthquake and predict its occurrence.