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1.
Unified Local Magnitude Scale for Earthquakes of South Balkan Area   总被引：1，自引：0，他引：1  
—A homogeneous earthquake catalog spanning 1964–1995 for the southern Balkan area is presented, by expressing the size of the earthquakes in a unified local magnitude scale. The strategy followed to produce this catalog is also presented. Local magnitudes calculated by six Balkan seismological centers (ATH, THE, ISK, TIR, TTG, SKO) have been used in order to obtain relations between the estimated ML values from the Greek seismological centers (ATH and THE) and the remaining four Balkan stations. Since it was found that local magnitudes estimated by ATH and THE are almost identical, they have been used as one data set so as to correlate with the data of each one of the remaining four seismological stations. Based on the proposed relations, a unified local magnitude, MLGR, is given for each earthquake of the regional catalog. A published scaling relation between the ML values from ATH and THE networks and the corresponding seismic moment magnitude, Mw, was used in combination with the above relations, in order to enable the conversion of any ML value from any station into Mw. The catalog completeness has been checked and the b-value has been calculated for the complete data sample.  相似文献   

2.
—A new methodology for probabilistic seismic hazard analysis is described. The approach combines the best features of the "deductive" (Cornell, 1968) and "historic" (Veneziano et al., 1984) procedures. It can be called a "parametric-historic" procedure. The maximum regional magnitude mmax is of paramount importance in this approach and Part I of the authors’ work (Kijko and Graham, 1998) was dedicated to developing efficient statistical procedures that can be used for the evaluation of this parameter. In Part II the approach of a probabilistic seismic hazard assessment at a given site is described. The approach permits the utilization of incomplete earthquake catalogues. It is assumed that a typical catalogue contains two types of information historical macroseismic events that occurred over a period of a few hundred years and recent, instrumental data. The historical part of the catalogue contains only the strongest events, whereas the complete part can be divided into several subcatalogues, each assumed complete above a specified threshold of magnitude. The author’s approach also takes into account uncertainty in the determination of the earthquake magnitude. The technique has been developed specifically for the estimation of seismic hazard at individual sites, without the subjective judgment involved in the definition of seismic source zones, in which specific active faults have not been mapped and identified, and where the causes of seismicity are not well understood. As an example of the application of the new technique, the results of a typical hazard analysis for a hypothetical engineering structure located in the territory of South Africa are presented. It was assumed that the only reliable information in the assessment of the seismic hazard parameters in the vicinity of the selected site comes from a knowledge of past seismicity. The procedure was applied to seismic data that were divided into an incomplete part, containing only the largest events, and two complete parts, containing information obtained from instruments. The simulation experiments described in Part I of our study have shown that the Bayesian estimator K-S-B tends to perform very well, especially in the presence of inevitable deviations from the simple Gutenberg–Richter model. In the light of this fact value &gif1; = 6.66 - 0.44, which was obtained from the K-S-B technique, was regarded as the best choice. At an exceedance probability of 10х per annum, the median value of peak ground acceleration on rock at the site is 0.31g, and at an exceedance probability of 10ц per annum, the median peak ground acceleration at the site is 0.39g. The median value of the maximum possible acceleration at the site is 0.40g, which was calculated from attenuation formulae by assuming the occurrence of the strongest possible earthquake, e.g., with magnitude &gif1; = 6.66 at distance 10 km.  相似文献   

3.
— Adjacent stratigraphic units that have undergone an identical deformation history often show variability with regard to deformation style. We present one such example and attribute the variability in deformation to variations in host rock properties. The Aztec sandstone of southern Nevada has two distinct zones of deformation style, the Lower and Upper structural domains. The Lower domain has deformed predominantly by opening mode fractures, whereas the Upper domain has deformed predominantly by deformation band faulting. Within a Transitional domain, deformation band abundance increases toward the Upper domain. We use petrophysical data (ultrasonic velocity, elastic moduli, grain and bulk density, helium porosity, and gas permeability) to distinguish among the host rocks of the differently deformed domains. The laboratory results of 29 samples of Upper, Transitional, and Lower domain Aztec sandstone impart the following petrophysical distinctions among the structural domains: (1) the Lower and Transitional domains show similarly high VP and VS and are both well cemented; (2) the Transitional and Upper domains show similarly high porosity and permeability; and, (3) the Upper domain is poorly cemented. We demonstrate that: (1) the nature of intergranular cement controls VP and VS; and, (2) based on the petrophysical properties we have examined, deformation style is controlled by porosity.  相似文献   

4.
: An experimental study was carried out to investigate the resuspension of particles by a momentum jet discharging from below into a conical basin. The work was motivated by its relevance to Lake Banyoles, where sediments are suspended in different conical basins by bottom jets fed by a groundwater karstic system. Two different flow regimes were identified: a "Jet Flow" regime (JF) and a "Lutocline Flow" regime (LF). In the LF regime, the particles were resuspended, forming a well-established interface along the entire cross-sectional ara of the cone. This regime occurs when the particle Reynolds number Rep = (u-ws)dp/ n {\bf Re_p = (u-w_s)d_p/ \nu} where u is the mean velocity of the flow at the jet entry, ws is the settling velocity of particles, dp is the diameter of that particles and n {\bf \nu} is the kinematic viscosity of water) is below a critical number Rec that depends on the slope of the basin. The maximum height to which particles rise in the LF regime was also determined as a function of four non-dimensional variables: D1 = ho/(Mo1/2/ws), D2 = ho/dp, Rep and tan  b {\bf tan \, \beta} (where ho is the height of the particle bed, Mo is the kinematic momentum flux of the jet and tan  b {\bf tan \, \beta} is the slope of the basin). Application of the results to the basins of Lake Banyoles showed a good agreement provided that the aggregate properties of the suspensions are used. It is also shown that, in the LF regime, inflows to the basins can be estimated by a simple technique based on a balance between the sedimentation flux and the upward advection of sediments by the mean flow.  相似文献   

5.
v—vRegional seismic discriminants for the May 11, 1998 Indian underground nuclear test(s) and earthquakes recorded at station NIL (Nilore, Pakistan) provide new data to test strategies that can be used to monitor the Comprehensive Nuclear-Test-Ban Treaty (CTBT). Three categories of regional discriminants (ratios of P- and/or S-wave energy) were measured on short-period (0.5–6uHz) seismograms: P/S amplitude ratios (phase ratios) measured in the same frequency band, P- and S-wave spectral ratios (i.e., low frequency to high frequency for the same phase) and P/S cross-spectral ratios (i.e., low frequency S-wave to high frequency P-wave). The P/S amplitude ratios show good separation of the Indian nuclear test and regional earthquakes for Pn/Lg and Pn/Sn, however Pg/Lg does not discriminate as well. Pn/Lg and Pn/Sn discriminate well at frequencies as low as 0.5–2uHz, especially after accounting for path effects. This observation differs from previous studies that report poor separation of earthquakes and explosions at lower frequencies. The P/S amplitude ratios do not show any magnitude dependence, suggesting that forming the ratios in a fixed frequency band cancels the effects of source size-corner frequency scaling. Spatial variability of the observed discriminants arises from variations in crustal waveguide and/or attenuation structure (path propagation effects). Grouping amplitude ratios for earthquakes with paths similar to the Indian test greatly improves discrimination. Removing distance trends does not generally improve discrimination. Accounting for path effects with Bayesian kriging significantly improves discrimination. Spectral ratios (e.g., Pn [0.5–1uHz]/Pn [4–6uHz]) and cross-spectral ratios (e.g., Lg [1–2uHz]/Pn [4–6uHz]) show distance and magnitude dependence. We developed a technique for simultaneously removing the effects of distance and source size-corner frequency scaling on amplitude and spectral ratios. The technique uses a gridsearch to find several parameters that characterize the observed distance and magnitude dependence. Discrimination of the Indian test improved dramatically after the distance and magnitude trends were removed from the spectral and cross-spectral ratio data.  相似文献   

6.
— The Indo-Burma (Myanmar) subduction boundary is highly oblique to the direction of relative velocity of the Indian tectonic plate with respect to the Eurasian plate. The area includes features of active subduction zones such as a Wadati-Benioff zone of earthquakes, a magmatic arc, thrust and fold belts. It also has features of oblique subduction such as: an arc-parallel strike-slip fault (Sagaing Fault) that takes up a large fraction of the northward component of motion and a buttress (the Mishmi block) that resists the motion of the fore-arc sliver. In this paper, I have examined the seismicity, slip vectors and principal axes of the focal mechanisms of the earthquakes to look for features of active subduction zones and for evidence of slip partitioning as observed in other subduction zones. The data set consists of Harvard CMT solutions of 89 earthquakes (1977–1999 with 4.8≦̸Mw≦̸7.2 and depths between 3–140 km). Most of these events are shallow and intermediate depth events occurring within the Indian plate subducting eastward beneath the Indo-Burman ranges. Some shallow events within the fore-arc region have arc-parallel Paxes, reflecting buttressing of the fore-arc sliver at its leading edge. Some of the shallowest events have nearly E-W oriented P axes which might account for recent folding and thrusting. Examination of earthquake slip vectors in this region shows that the slip vector azimuths of earthquakes in the region between 20°–26°N are rotated towards the trench normal, which is an indication of partial partitioning of the oblique convergence. It is seen that all aspects of seismicity, including the paucity of shallow underthrusting earthquakes and the orientation of P axes, are consistent with oblique convergence. The conclusions of this paper are consistent with recent geological studies and interpretations such as the coexistence of eastward subduction, volcanic activity and transcurrent movement through mid-Miocene to Quaternary period.  相似文献   

7.
Northeast India and adjoining regions (20°–32° N and 87°–100° E) are highly vulnerable to earthquake hazard in the Indian sub-continent, which fall under seismic zones V, IV and III in the seismic zoning map of India with magnitudes M exceeding 8, 7 and 6, respectively. It has experienced two devastating earthquakes, namely, the Shillong Plateau earthquake of June 12, 1897 (M w 8.1) and the Assam earthquake of August 15, 1950 (M w 8.5) that caused huge loss of lives and property in the Indian sub-continent. In the present study, the probabilities of the occurrences of earthquakes with magnitude M ≥ 7.0 during a specified interval of time has been estimated on the basis of three probabilistic models, namely, Weibull, Gamma and Lognormal, with the help of the earthquake catalogue spanning the period 1846 to 1995. The method of maximum likelihood has been used to estimate the earthquake hazard parameters. The logarithmic probability of likelihood function (ln L) is estimated and used to compare the suitability of models and it was found that the Gamma model fits best with the actual data. The sample mean interval of occurrence of such earthquakes is estimated as 7.82 years in the northeast India region and the expected mean values for Weibull, Gamma and Lognormal distributions are estimated as 7.837, 7.820 and 8.269 years, respectively. The estimated cumulative probability for an earthquake M ≥ 7.0 reaches 0.8 after about 15–16 (2010–2011) years and 0.9 after about 18–20 (2013–2015) years from the occurrence of the last earthquake (1995) in the region. The estimated conditional probability also reaches 0.8 to 0.9 after about 13–17 (2008–2012) years in the considered region for an earthquake M ≥ 7.0 when the elapsed time is zero years. However, the conditional probability reaches 0.8 to 0.9 after about 9–13 (2018–2022) years for earthquake M ≥ 7.0 when the elapsed time is 14 years (i.e. 2009).  相似文献   

8.
—A large number of geophysical applications need long-term telluric recordings. In order to realise it correctly, the use of very stable electrodes is necessary. The study of the potential variations of Pb-PbCl2 electrodes as a function of both the ionic composition and the pH of its electrolyte allows one to chose the optimal working criteria. The best stability and the minimum noise for the potential are obtained for a solution saturated in both salts PbCl2 and KCl, or PbCl2 and NaCl, in the presence of additional non-dissolved salts, with a pH from 4 to 5. The study of the salts diffusion between the inside of the electrode and the outside medium allows one to compute the time span over which the potential remains stable (tD time of dissaturation), and to know how to increase it. Two solutions are possible. The first one is to increase the electrode length, because tD is proportional to the length squared. The second one is to reduce the exchanges of salts with the external medium, by using an electrode with a narrow channel. In this case, tD is proportional to the quantity of non-dissolved salt in the electrode and to the internal electrical resistance of the electrode. The fabrication of this new electrodes design with a channel is described.  相似文献   

9.
Temporal distribution of earthquakes with M w > 6 in the Dasht-e-Bayaz region, eastern Iran has been investigated using time-dependent models. Based on these types of models, it is assumed that the times between consecutive large earthquakes follow a certain statistical distribution. For this purpose, four time-dependent inter-event distributions including the Weibull, Gamma, Lognormal, and the Brownian Passage Time (BPT) are used in this study and the associated parameters are estimated using the method of maximum likelihood estimation. The suitable distribution is selected based on logarithm likelihood function and Bayesian Information Criterion. The probability of the occurrence of the next large earthquake during a specified interval of time was calculated for each model. Then, the concept of conditional probability has been applied to forecast the next major (M w > 6) earthquake in the site of our interest. The emphasis is on statistical methods which attempt to quantify the probability of an earthquake occurring within a specified time, space, and magnitude windows. According to obtained results, the probability of occurrence of an earthquake with M w > 6 in the near future is significantly high.  相似文献   

10.
The present study aims at understanding the seismotectonic province of the Shillong Plateau (SP) by identifying the potential seismic source zones within a radius of 500 km from the centre of the SP. From existing literature and earthquake (EQ) data, the seismotectonic region is found to vary in terms of seismicity, tectonic features, geology, thickness of overburden, rupture characteristics and rate of movement. Thus, entire 500-km-radius seismotectonic region is divided into four seismic source zones: namely (1) the Shillong Plateau–Assam Valley Zone (SP-AVZ), (2) the Indo-Burma Ranges Zone (IBRZ), (3) the Bengal Basin Zone (BBZ) and (4) the Eastern Himalaya Zone (EHZ). EQ catalogues for each source zone is analysed for completeness of magnitude and time. Seismic parameter b estimated using a maximum likelihood method is found to be 0.91 ± 0.03, 0.94 ± 0.02, 0.80 ± 0.03 and 0.89 ± 0.03 for the SP-AVZ, IBRZ, BBZ and EHZ, respectively. In addition, the maximum likelihood method is used to estimate the mean annual activity rate, maximum possible magnitude (m max), return period and probability of exceedance for the four zones. The b values estimated suggest that the BBZ is seismically more active; however, the rate of occurrence of EQs is highest in the IBRZ. Findings from this study are an indication of the relative contribution from each of the four seismic source zones towards a seismic hazard of the SP.  相似文献   

11.
Since March 2014, an unusually large amount of earthquakes occur southeast of the city of Darmstadt in the northern Upper Rhine Graben. During the period, until April 2015, we have recorded 356 earthquakes with magnitudes ranging from ML?=??0.6 to 4.2. We identified two source clusters separated laterally by about 5 km. The hypocentres within these clusters are aligned vertically extending over a depth range from 1 to 8 km with a lateral extent of about 1 to 2 km. Focal mechanisms show left-lateral strike-slip movements; b values are changing with time between b?=?0.6 and b?=?0.9. This is the first time in almost 150 years that such high earthquake rates have been observed in the region. Historical accounts dating back to the nineteenth century report of over 2000 felt earthquakes over a time span from 1869 to 1871. From these, maximum intensities of VII have been estimated. Other seismic activities in the region were reported in the 1970s. The observations of the 2014–2015 earthquake series do not completely match a typical main shock–aftershock sequence or a typical earthquake swarm. Especially the activity at the beginning of the earthquake series may be considered as a mixture of a main shock–aftershock sequence and a short-lasting swarm event. Whether or not the time gap between the current seismic activity, which actually takes place at the same locations as parts of the seismic swarm in 1869–1871, and the seismic activity in the nineteenth century or the seismic activity in the 1970s can be interpreted as a seismic cycle remains unclear.  相似文献   

12.
Seismicity parameters for important urban agglomerations in India   总被引：1，自引：0，他引：1  
India’s urban population has increased in the recent times. An earthquake near an urban agglomeration has the potential to cause severe damage. In this article, seismicity parameters for region surrounding important urban agglomerations in India are estimated. A comprehensive earthquake catalogue for the region (6°E–42°E latitude and 60°N–100°N longitude) including historic and pre-historic events has been compiled from various sources. To estimate the parameters, past earthquake data in a control region of radius 300 km has been assembled to quantify the seismicity around each urban agglomeration. The collected earthquake data is first evaluated for its completeness. From combined (historical and instrumental) data, the seismicity parameters b-value, seismic activity rate, λ and maximum expected magnitude (m max ) have been obtained from the methodology proposed by Kijko and Graham (1998). The obtained activity rates indicate that region surrounding Guwahati urban agglomeration is the most seismically active region followed by Srinagar, Patna, Amritsar and Chandigarh.  相似文献   

13.
The Gujarat and adjoining region falls under all four seismic zones V, IV, III and II of the seismic zoning map of India, and is one of the most seismically prone intracontinental regions of the world. It has experienced two large earthquakes of magnitude M w 7.8 and 7.7 in 1819 and 2001, respectively and several moderate earthquakes during the past two centuries. In the present study, the probability of occurrence of earthquakes of M ≥ 5.0 has been estimated during a specified time interval for different elapsed times on the basis of observed time intervals between earthquakes using three stochastic models namely, Weibull, Gamma and Lognormal. A complete earthquake catalogue has been used covering the time interval of 1819 to 2006. The whole region has been divided into three major seismic regions (Saurashtra, Mainland Gujarat and Kachchh) on the basis of seismotectonics and geomorphology of the region. The earthquake hazard parameters have been estimated using the method of maximum likelihood. The logarithmic of likelihood function (ln L) is estimated and used to test the suitability of models in three different regions. It was found that the Weibull model fits well with the actual data in Saurashtra and Kachchh regions, whereas Lognormal model fits well in Mainland Gujarat. The mean intervals of occurrence of earthquakes are estimated as 40.455, 20.249 and 13.338 years in the Saurashtra, Mainland Gujarat and Kachchh region, respectively. The estimated cumulative probability (probability that the next earthquake will occur at a time later than some specific time from the last earthquake) for the earthquakes of M ≥ 5.0 reaches 0.9 after about 64 years from the last earthquake (1993) in Saurashtra, about 49 years from the last earthquake (1969) in Mainland Gujarat and about 29 years from the last earthquake (2006) in the Kachchh region. The conditional probability (probability that the next earthquake will occur during some specific time interval after a certain elapsed time from last earthquake) is also estimated and it reaches about 0.8 to 0.9 during the time interval of about 57 to 66 years from the last earthquake (1993) in Saurashtra region, 31 to 51 years from the last earthquake (1969) in Mainland Gujarat and about 21 to 28 years from the last earthquake (2006) in Kachchh region.  相似文献   

14.
—The Himalayan region is one of the most seismic prone areas of the world. The North-East (NE) Indian peninsula and the Hindukush regions mark the zone of collision of the Indian and Eurasian plates. The probability of the occurrence of great earthquakes with magnitude greater than 7.0 during a specified interval of time has been estimated on the basis of four probabilistic models, namely, Weibull, Gamma, Lognormal and Exponential for the NE Indian peninsula and Hindukush regions. The model parameters have been estimated by the method of Maximum Likelihood Estimates (MLE) and the Method of Moments (MOM). The cumulative probability is estimated for a period of 40 years from 1964 and is ranging between 0.881 to 0.995 by the year 1995, using all four models for the NE Indian peninsula. The conditional probability is also estimated and it is concluded that the NE Indian peninsula would expect a great earthquake at any time in the remaining years of the present century. For the Hindukush region, the cumulative probability has already crossed its highest value, but no earthquake of magnitude greater than 7.0 has occurred after 1974 in this area. It may attribute to the occurrence of frequent shocks of moderate size, as seventeen earthquakes of magnitude greater than 6.0, including four greater than 6.4, have been reported until 1994 from this region.  相似文献   

15.
: Iron-rich colloids formed at the oxic-anoxic interface of a eutrophic lake (Lake Lugano; CH) were characterized by bulk chemical methods and analytical electron microscopy. Fractionation of raw waters showed that non-dissolved iron is particulate above the oxicline and mainly colloidal in the anoxic part of the hypolimnion, while non-dissolved ortho-phosphate is mostly colloidal through the water column. Because of these differences, filtration did not prove helpful for the determination of the role of iron-rich species in the scavenging of ortho-phosphate. On the other hand, analytical electron microscopy revealed that iron-rich nano-granules (ca. 50nm) are associated to the surface of bacterially produced fibrillar polysaccharides. Iron colloids in these complex entities contain important and constant amounts of phosphorus ([PO4]part:[Fe]part = 0.48 - 0.11, n = 1096 Fe-rich entities analyzed by electron microscopy), which suggests that phosphates are stoichiometrically incorporated into the hydrous iron oxide phase, with a tentative composition Fe2[OOH]1-x[(OH)3]x[PO4], during its genesis.  相似文献   

16.
Steady convective exchange flows down slopes   总被引：2，自引：0，他引：2  
: Horizontal exchange flows driven by destabilising buoyancy fluxes through the surface waters of lakes and coastal regions of oceans are important in understanding the transport of nutrients, micro-organisms and pollutants from littoral to pelagic zones. Our interest here is in the discharge flow driven by cooling or destabilising forcing at the water surface in a water body with variable depth due to sloping bottom topography. Flow visualisation studies and measurements in a laboratory model enabled us to develop scaling arguments to predict the dependency of discharge upon surface forcing and the angle of bottom slope. The results were used to interpret both the laboratory measurements and field data from a small shallow lake with sloping sides and an essentially flat bottomed interior, as well as published results from the literature. The steady state horizontal exchange can be described by Q = 0.24 B1/3 (l tan //(1 + tan /))4/3, where Q is the discharge rate per unit length of shoreline, / is the angle of the bottom slope, B is the surface buoyancy flux and l is the horizontal length of the forcing region over the slope. The flushing timescale of the wedge shaped littoral region was given by Ff ~ l 2/3 (1 + tan /) 4/3/ (B tan / 1/3. While the buoyancy flux in the field is almost never constant in space or time and the slope from the shore is seldom uniform, we found that the exchange rate was relatively insensitive to buoyancy flux changes and only moderately sensitive to slope.  相似文献   

17.
四川盆地南部的长宁页岩气开发区附近地震频发,近年来已发生近10次ML>4.0的中型地震和万余次ML1.0~3.0的小微地震,灾害风险持续增高。由于国家地震台网的固定台站较为分散,难以捕捉到1级以下微震事件的精确信息,通过近场微震监测数据来分析页岩气开发区的地震风险演化趋势,已经成为势在必行的科学问题。本文基于专门布设的13个近场流动台站和国家地震台网固定台站2017年2月至2018年6月监测拾取到的1万余个地震事件,采用最大似然估计法计算了双差定位后地震目录的b值,详细探讨了长宁页岩气开发区的b值演化特征。基于更低的最小完整震级,估计出区域总体b值为0.98±0.02,略高于前人研究结果。拟合直线的双线性特征和错位分布指示出b值演化可能存在明显的时空差异特征。通过分析验证了这一结果,并且发现5个地震事件数量快速增加的时间段恰好伴随着b值的强振荡特征和相关空间距离SCL值的稳定低值分布,认为可能与附近的页岩气压裂开采存在关联。分析认为4个较大震级事件发生之前出现的b值下降应归因于震前的应力累积过程,类似于一般的大型天然地震事件。  相似文献   

18.
This paper investigates the suitability of a three-parameter (scale, shape, and location) Weibull distribution in probabilistic assessment of earthquake hazards. The performance is also compared with two other popular models from same Weibull family, namely the two-parameter Weibull model and the inverse Weibull model. A complete and homogeneous earthquake catalog (Yadav et al. in Pure Appl Geophys 167:1331–1342, 2010) of 20 events (M ≥ 7.0), spanning the period 1846 to 1995 from north–east India and its surrounding region (20°–32°N and 87°–100°E), is used to perform this study. The model parameters are initially estimated from graphical plots and later confirmed from statistical estimations such as maximum likelihood estimation (MLE) and method of moments (MoM). The asymptotic variance–covariance matrix for the MLE estimated parameters is further calculated on the basis of the Fisher information matrix (FIM). The model suitability is appraised using different statistical goodness-of-fit tests. For the study area, the estimated conditional probability for an earthquake within a decade comes out to be very high (≥0.90) for an elapsed time of 18 years (i.e., 2013). The study also reveals that the use of location parameter provides more flexibility to the three-parameter Weibull model in comparison to the two-parameter Weibull model. Therefore, it is suggested that three-parameter Weibull model has high importance in empirical modeling of earthquake recurrence and seismic hazard assessment.  相似文献   

19.
The Reviewed Event Bulletin (REB) of the International Data Center (IDC) has been used in order to investigate the seismicity of the Northwest Himalaya and its neighboring region for the time period June 1999 to March 2015 within the geographical coordinates 25–40° N latitude and 65–85° E longitude. We have used a very precisely located earthquake dataset recorded by the International Monitoring System (IMS) Network containing 7,583 events with body wave magnitudes from 2.5 to 6.3. The study area has been subdivided into six regions based on the Flinn-Engdahl (F-E) seismic and geographical regionalization scheme, which was used as the region classifications of the International Data Center catalog. The examined region includes NW India, Pakistan, Nepal, Xizang, Kashmir, and Hindukush. For each region, Magnitudes of completeness (Mc) and Gutenberg-Richter (GR) recurrence parameters (a and b values) have been estimated. The Gutenberg-Richter analysis is preceded by an overview of the seismotectonics of the study area. The obtained Mc values vary from 3.5 to 3.9. The lower value of Mc was found mainly in Xizang region whereas the higher Mc threshold is evident in Pakistan region. However, the b values vary from 1.19 to 1.48. The lowest b value is recorded in Xizang region, which is mostly related to the Main Karakoram Thrust (MKT) fault, whereas the highest b values are recorded in NW India and Kashmir regions, which are mostly related to the Main Frontal Thrust (MFT) fault. The REB for the selected period has been compared to the most renowned bulletin of global seismicity, namely that issued by the National Earthquake Information Center (NEIC) of the United States Geological Survey (USGS). A study of 4,821 events recorded by USGS in the study region indicates that about 36 % of seismic events were missed and the catalog is considered as complete for events with magnitudes ≥4.0. However, both a and b values are obviously higher than those of IMS catalog. The a and b parameters in the Gutenberg-Richter magnitude–frequency relationship have been utilized to forecast the probability of future earthquakes of different magnitudes and returned periods (recurrence intervals).  相似文献   

20.
v—vAt the German NDC initial work on seismic event identification has focused on the application of spectral amplitude ratios for Lg in order to discriminate naturally occurring seismic events from other events associated with mining and quarry activities. Only about 10% of all seismic events occurring in Germany and adjacent areas are due to natural seismicity and are mostly constrained to the Alpine regions and areas along the Rhinegraben, Rhenish massif, Swabian Jura, and the Bohemian massif (Vogtland region). Using data from the broadband GRSN network, spectral amplitude ratios are calculated from maximum trace amplitudes in the 1–2uHz and 6–8uHz frequency bands, which are within the passbands of the deployed STS-2 instruments and the recorded 20uHz data streams. These amplitude ratios then must be corrected with an appropriate attenuation model in order to remove propagation paths effects. For event identification, a scoring scheme is applied across the GRSN network, based on station-dependent scoring thresholds. In a case study aimed at testing the identification scheme, events are investigated from a quarry in southern Germany that provided ground-truth information for six events in 1997 to demonstrate the suitability of this identification approach. Except for one event with a rather strong earthquake signature, i.e., a low spectral Lg ratio, these events could be screened out from the earthquake population by their large Lg ratios. In a second step, aimed at applying the identification scheme, all events in Germany and neighboring areas that occurred in 1995 were processed, with approximately 800 out of more than 1200 events showing explosion-type Lg ratios, while only 10% remain in the earthquake population. However, specific mining areas appear to consistently produce earthquake-type spectral ratios indicative of particular blasting practices.  相似文献   

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