Sediment Thicknesses and Q<Subscript>s</Subscript> vs. Q<Subscript>p</Subscript> Relations in the Kachchh Rift Basin,Gujarat, India Using Sp Converted Phases

Delineation of the top sedimentary structure and its Q_s vs. Q_p relationship using the travel-time difference of direct S and converted Sp phase is key to understanding the seismic hazard of any sedimentary basin area. We constructed filtered displacement waveforms from local ETNA Episensor acceleration recordings as well as local velocity recordings of aftershocks of the 2001 Bhuj earthquake recorded by the Kachchh seismological network of the National Geophysical Research Institute (NGRI), Hyderabad, India during 2001–2004. Stations are within 15–70km of epicenters, and the resulting displacement waveforms are generally simple, displaying prominent P, Sp, and S wave pulses. Particle motion of P and S waves suggest near-vertical raypaths consistent with preliminary depth estimates. The direct S wave on the horizontal component is characterized by lower frequency content than the converted Sp phase on the vertical component. This difference in frequency content between S and Sp phases can be explained in terms of different attenuation effects for P and S waves in the unconsolidated sediments. The Sp phase is generated by S-to-P phase conversion at the base of Mesozoic sediments of the Kachchh basin. Travel-time inversion (VELEST) of 2565 P and 2380 S arrivals from 658 well located aftershocks recorded at 8–14 three-component local seismic stations led to 1 D velocity models indicated very slow sediments in the upper 0–2 km depth range (Vp: 2.92 km/s and Vs: 0.90 km/s) and an increasing trend of velocities with depth at 2–40 km depth. The estimated sediment thicknesses beneath 12 accelerograph and 6 seismograph sites from the estimated velocity model and the travel-time difference between S and converted Sp phases reaches a maximum of (1.534 ± 0.117) km beneath Bandri (near the location of 2001 Bhuj mainshock) and attains a minimum sediment thickness of (0.858 ± 0.104) km beneath Ramvav and Burudia. The spectral ratios between Sp and S from 159 three-component accelerograms have been used to study seismic wave attenuation in the Kachchh rift basin. The estimated Q_s vs. Q_p relations for 12 accelerograph sites vary from Q_s = 0.184 Q_p (at Chobari) to Q_s = 0.505 Q_p (at Dudhai). For stations Chobari, Chopdwa, Jahawarnagar, Vondh and Tapar, the spectral ratio slopes and hence the calculated Q_s vs. Q_p relations are effectively the same, and the correlation coefficients are quite high (0.91–0.93). Stations Adhoi, Manfara, New Dudhai, Dudhai and Sikara have similar Q_s vs. Q_p relationships to each other and also have high correlation coefficients (0.78–0.87). The spectral ratios for stations Anjar and Ramvav are small and poorly constrained, resulting in less reliable Q_s vs. Q_p relations. This could be due to noisy data, fewer available waveforms, or scattering due to velocity heterogeneities and/or interface irregularities.     

Stress Rotation in the Kachchh Rift Zone, Gujarat, India

Double difference relocations of the 1402 Kachchh events (2001–2006) clearly delineate two fault zones viz. south-dipping North Wagad fault (NWF) and almost vertical Gedi fault (GF). The relocated focal depths delineate a marked variation of 4 and 7 km in the brittle-ductile transition depths beneath GF and NWF, respectively. The focal mechanism solutions of 464 aftershocks (using 8–12 first motions) show that the focal mechanisms ranged between pure reverse and pure strike-slip except for a few pure dip-slip solutions. The stress inversions performed for five rectangular zones across the Kachchh rift reveal both clockwise and anticlockwise rotation (7–32°) in the σ₁ orientation within the rupture zone, favoring a heterogeneous stress regime with an average N-S fault normal compression. This rotation may be attributed to the presence of crustal mafic intrusives (5–35 km depth) in the rupture zone of the 2001 Bhuj main shock. Results suggest a relatively homogeneous stress regime in the GF zone favoring strike-slip motion, with a fault normal N-S compression.     

Stochastic finite fault modelling of M w 4.8 earthquake in Kachchh, Gujarat, India

The modified stochastic finite fault modelling technique based on dynamic corner frequency has been used to simulate the strong ground motions of M _w 4.8 earthquake in the Kachchh region of Gujarat, India. The accelerograms have been simulated for 14 strong motion accelerographs sites (11 sites in Kachchh and three sites in Saurashtra) where the earthquake has been recorded. The region-specific source, attenuation and generic site parameters, which are derived from recordings of small to moderate earthquakes, have been used for the simulations. The main characteristics of the simulated accelerograms, comprised of peak ground acceleration (pga), duration, Fourier and response spectra, predominant period, are in general in good agreement with those of observed ones at most of the sites. The rate of decay of simulated pga values with distance is found to be similar with that of observed values. The successful modelling of the empirical accelerograms indicates that the method can be used to prepare wide range of scenarios based on simulation which provide the information useful for evaluating and mitigating the seismic hazard in the region.     

Coda Q in the Kachchh Basin, Western India Using Aftershocks of the Bhuj Earthquake of January 26, 2001

Q_C-estimates of Kachchh Basin in western India have been obtained in a high frequency range from 1.5 to 24.0 Hz using the aftershock data of Bhuj earthquake of January 26, 2001 recorded within an epicentral distance of 80 km. The decay of coda waves of 30 sec window from 186 seismograms has been analysed in four lapse time windows, adopting the single backscattering model. The study shows that Q_c is a function of frequency and increases as frequency increases. The frequency dependent Q_c relations obtained for four lapse-time windows are: Q_c=82 f^1.17 (20–50 sec), Q_c=106 f^1.11 (30–60 sec), Q_c=126f^1.03 (40–70 sec) and Q_c=122f^1.02 (50–80 sec). These empirical relations represent the average attenuation properties of a zone covering the surface area of about 11,000, 20,000, 28,000 and 38,000 square km and a depth extent of about 60, 80, 95, 110 km, respectively. With increasing window length, the degree of frequency dependence, n, decreases marginally from 1.17 to 1.02, whereas Q₀ increases significantly from 82 to 122. At lower frequencies up to 6 Hz, Q_c⁻¹ of Kachchh Basin is in agreement with other regions of the world, whereas at higher frequencies from 12 to 24 Hz it is found to be low.     

Relocation of aftershocks of the 2001 Bhuj earthquake: A new insight into seismotectonics of the Kachchh seismic zone,Gujarat, India

In view of an anomalous crust–mantle structure beneath the 2001 Bhuj earthquake region, double-difference relocations of 1402 aftershocks of the 2001 Bhuj earthquake were determined, using an improved 1D velocity model constructed from 3D velocity tomograms based on data from 10 to 58 three-component seismograph stations. This clearly delineated four major tectonic features: (i) south-dipping north Wagad fault (NWF), (ii and iii) south-dipping south Wagad faults 1 and 2 (SWF₁, SWF₂), and (iv) a northeast dipping transverse fault (ITF), which is a new find. The relocated aftershocks correlate satisfactorily with the geologically mapped and inferred faults in the epicentral region. The relocated focal depths delineate a marked variation to the tune of 12 km in the brittle–ductile transition depths beneath the central aftershock zone that could be attributed to a lateral variation in crustal composition (more or less mafic) or in the level of fracturing across the fault zone. A fault intersection between the NWF and ITF has been clearly mapped in the 10–20 km depth range beneath the central aftershock zone. It is inferred that large intraplate stresses associated with the fault intersection, deepening of the brittle–ductile transition to a depth of 34 km due to the presence of mafic/ultramafic material in the crust–mantle transition zone, and the presence of aqueous fluids (released during the metamorphic process of eclogitisation of lower crustal olivine-rich rocks) and volatile CO₂ at the hypocentral depths, might have resulted in generating the 2001 Bhuj earthquake sequence covering the entire lower crust.     

Considerations in Phase Estimation and Event Location Using Small-aperture Regional Seismic Arrays

The global monitoring of earthquakes and explosions at decreasing magnitudes necessitates the fully automatic detection, location and classification of an ever increasing number of seismic events. Many seismic stations of the International Monitoring System are small-aperture arrays designed to optimize the detection and measurement of regional phases. Collaboration with operators of mines within regional distances of the ARCES array, together with waveform correlation techniques, has provided an unparalleled opportunity to assess the ability of a small-aperture array to provide robust and accurate direction and slowness estimates for phase arrivals resulting from well-constrained events at sites of repeating seismicity. A significant reason for the inaccuracy of current fully-automatic event location estimates is the use of f?k slowness estimates measured in variable frequency bands. The variability of slowness and azimuth measurements for a given phase from a given source region is reduced by the application of almost any constant frequency band. However, the frequency band resulting in the most stable estimates varies greatly from site to site. Situations are observed in which regional P- arrivals from two sites, far closer than the theoretical resolution of the array, result in highly distinct populations in slowness space. This means that the f?k estimates, even at relatively low frequencies, can be sensitive to source and path-specific characteristics of the wavefield and should be treated with caution when inferring a geographical backazimuth under the assumption of a planar wavefront arriving along the great-circle path. Moreover, different frequency bands are associated with different biases meaning that slowness and azimuth station corrections (commonly denoted SASCs) cannot be calibrated, and should not be used, without reference to the frequency band employed. We demonstrate an example where fully-automatic locations based on a source-region specific fixed-parameter template are more stable than the corresponding analyst reviewed estimates. The reason is that the analyst selects a frequency band and analysis window which appears optimal for each event. In this case, the frequency band which produces the most consistent direction estimates has neither the best SNR or the greatest beam-gain, and is therefore unlikely to be chosen by an analyst without calibration data.     

Hydrogeologic Framework of Fractured Sedimentary Rock,Newark Basin,New Jersey

The hydrogeologic framework of fractured sedimentary bedrock at the former Naval Air Warfare Center (NAWC), ¹ 1 For more information on the USGS Toxics Substances Hydrology Program at the Naval Air Warfare Center visit the NAWC website at http://nj.usgs.gov/nawc/
Trenton, New Jersey, a trichloroethylene (TCE)-contaminated site in the Newark Basin, is developed using an understanding of the geologic history of the strata, gamma-ray logs, and rock cores. NAWC is the newest field research site established as part of the U.S. Geological Survey Toxic Substances Hydrology Program, Department of Defense (DoD) Strategic Environmental Research and Development Program, and DoD Environmental Security Technology Certification Program to investigate contaminant remediation in fractured rock. Sedimentary bedrock at the NAWC research site comprises the Skunk Hollow, Byram, and Ewing Creek Members of the Lockatong Formation and Raven Rock Member of the Stockton Formation. Muds of the Lockatong Formation that were deposited in Van Houten cycles during the Triassic have lithified to form the bedrock that is typical of much of the Newark Basin. Four lithotypes formed from the sediments include black, carbon-rich laminated mudstone, dark-gray laminated mudstone, light-gray massive mudstone, and red massive mudstone. Diagenesis, tectonic compression, off-loading, and weathering have altered the rocks to give some strata greater hydraulic conductivity than other strata. Each stratum in the Lockatong Formation is 0.3 to 8 m thick, strikes N65°E, and dips 25° to 70°NW. The black, carbon-rich laminated mudstone tends to fracture easily, has a relatively high hydraulic conductivity and is associated with high natural gamma-ray count rates. The dark-gray laminated mudstone is less fractured and has a lower hydraulic conductivity than the black carbon-rich laminated mudstone. The light-gray and the red massive mudstones are highly indurated and tend to have the least fractures and a low hydraulic conductivity. The differences in gamma-ray count rates for different mudstones allow gamma-ray logs to be used to correlate and delineate the lithostratigraphy from multiple wells. Gamma-ray logs and rock cores were correlated to develop a 13-layer gamma-ray stratigraphy and 41-layer lithostratigraphy throughout the fractured sedimentary rock research site. Detailed hydrogeologic framework shows that black carbon-rich laminated mudstones are the most hydraulically conductive. Water-quality and aquifer-test data indicate that groundwater flow is greatest and TCE contamination is highest in the black, carbon- and clay-rich laminated mudstones. Large-scale groundwater flow at the NAWC research site can be modeled as highly anisotropic with the highest component of permeability occurring along bedding planes.     

Geochemistry of geopressured hydrothermal waters in the Niigata Sedimentary Basin, Japan

Abstract   Geothermal waters in the Niigata Sedimentary Basin, central Japan, are divided into four groups based on their chemical composition (i.e. Na-SO₄-type, Na-SO₄-Cl-type, Na-Cl-type and Na-Cl-HCO₃-type). The Na-SO₄-type geothermal water forms as a consequence of water–rock interaction and generally occurs in the outer part of the basin. The Na-Cl-type geothermal water is further subdivided into the original Na-Cl-type geopressured thermal water and the mixed Na-Cl-type geothermal water, in terms of its geochemical and isotopic composition. The original Na-Cl-type geopressured thermal water originates from a geopressured hydrothermal system containing the altered fossil formation waters that are sealed at depth. It moves up to the upper part of the depositional succession or the ground, and generally does not mix with groundwater that is of meteoric origin. This type of water is cooled by heat conduction. The concentration of Cl^– in this type of thermal water is very similar to that in seawater. The δD and δ¹⁸O values are approximately constant and independent of temperature. The original Na-Cl-type geopressured thermal water is distributed mainly along anticlinal axes in folded Neogene formations. The mixed Na-Cl-type geothermal water is related to the expulsion activity of the geopressured hydrothermal system and occurs mostly along active faults. It is formed by shallow groundwater of meteoric origin being mixed with geopressured hydrothermal water when the geopressured hydrothermal system was expulsed along active faults by paroxysmal tectonic events.     

Flow Estimation in a Basin by Using a Hydrological Model: Application to the Basin of Wadi Djendjen (Jijel,North-East Algeria)

The aim of this study was to transform the rain into flow in the basin of Wadi Djendjen by using a simulation model in absence of the continuous measurements of the flow. The obtained results indicated and confirmed the strong adaptability of this model with varied hydroclimatic situations. The validation process showed an interesting result, which lead to conclude that the model is well calibrated and has a good performance to be used for the basins with Mediterranean climate. The obtained results for the simulation by the GR2M model over the studied period (1973–1986) showed that there no significant difference between the obtained value for the yearly average flow (197.70 hm³) and that measured at Missa hydrometric station (200.80 hm³), which allow us to estimate quantitatively the flow in Missa hydrometric station. While, for the period of 2000–2012, the results showed that the yearly average flow value (171.90 hm³), is significantly different (reduction of 15% for 12 years), in comparison with that measured at Missa hydrometric station (200.80 hm³). This can be due to the dryness which struck the region since 2000, and the local degradation of the forest ecosystem, which has considerably affected the runoff surface.     

QLg tomography in Gujarat,Western India

We propose a novel Lg attenuation tomography model (Q_Lg tomography) for the state of Gujarat, Western India, using earthquake data recorded by the Gujarat Seismic Network, operated by the Institute of Seismological Research in Gandhinagar. The waveform dataset consist of 400 3-component recordings, produced by 60 earthquakes with magnitude (M_L) spanning from 3.6 to 5.1, recorded at 60 seismic stations having epicentral distances spanning between 200 and 500 km. Spectral amplitude decays for Lg wave displacement were obtained by generalized inversion at 17 frequencies spanning between 0.9 and 9 Hz. Lg wave propagation efficiency was measured by Lg/Pn spectral ratio categorizing as efficient ratio ≥6 for 86%, intermediate ratio of 3–6 for 10% and inefficient ratio <3 for 4% paths of total 400 ray paths. The earthquake size and quality of waveform recorded at dense network found sufficient to resolve lateral variation of Q_Lg in Gujarat.Average power-law attenuation relationship obtained for Gujarat as Q_Lg(f) = 234f^0.64, which corresponds to high attenuation in comparison to peninsular India shield region and other several regions around the world. Q_Lg tomography resolves the highly attenuating crust of extremely fractured Saurashtra region and tectonically active Kachchh region. The Gujarat average attenuation is also lying in between them. The low attenuation in Cambay and Narmada rift basins and extremely low attenuation in patch of Surendranagar area is identified. This study is the first attempt and can be utilized as pivotal criteria for scenario hazard assessment, as maximum hazard has been reported in highly attenuating tectonically active Kachchh region and in low attenuating Cambay, Narmada and Surendranagar regions. The site and source terms are also obtained along with the Q_Lg inversion. The estimated site responses are comparable with observed local geological condition and agree with the previously reported site amplifications at the same sites. The source terms are comparable with local magnitude estimated from Network. The M_w (Lg) is nearly equivalent to M_L (GSN) and the slight differences are noted for larger magnitude events.     

"空间技术揭示的古吉拉特地震的地球系统过程"国际研讨会

“空间技术揭示的古吉拉特地震的地球系统过程”国际研讨会于2003年1月27－29日在印度坎普尔举行。来自印度、美国、中国、德国和俄罗斯约80位专家参加了该研讨会。会计收到约70篇研究报告。文章介绍了会议概况、学术报告和专题辩论的情况。     

Deep Injection of Waste Water in the Western Canada Sedimentary Basin

Injection of wastes into the deep subsurface has become a contentious issue, particularly in emerging regions of oil and gas production. Experience in other regions suggests that injection is an effective waste management practice and that widespread environmental damage is unlikely. Over the past several decades, 23 km³ of water has been injected into the Western Canada Sedimentary Basin (WCSB). The oil and gas industry has injected most of this water but large amounts of injection are associated with mining activities. The amount of water injected into this basin during the past century is 2 to 3 orders magnitude greater than natural recharge to deep formations in the WCSB. Despite this large‐scale disturbance to the hydrogeological system, there have been few documented cases of environmental problems related to injection wells. Deep injection of waste appears to be a low risk activity based on this experience but monitoring efforts are insufficient to make definitive statements. Serious uncharacterized legacy issues could be present. Initiating more comprehensive monitoring and research programs on the effects of injection in the WCSB could provide insight into the risks associated with injection in less developed sedimentary basins.     

Over-exploitation of an alluvial aquifer in Gujarat,India

Abstract

Over-exploitation of an alluvial aquifer in India has occurred since tubewells were drilled into deeper aquifers. Field information concerning-the fall in both the water table and the piezometric heads in the deeper aquifers has been examined and a numerical model has been developed to represent a heavily exploited area. Predictions using the model suggest that the annual declines in the water table and the piezometric heads are accelerating as the water table moves into a zone having a lower specific yield. Careful monitoring of the movement of the water table is proposed.     

Sedimentary and geochemical evidence of Eocene climate change in the Xining Basin,northeastern Tibetan Plateau

The northeastern Tibetan Plateau began to grow during the Eocene and it is important to understand the climatic history of Asia during this period of so-called ‘doubthouse' conditions. However, despite major advances in the last few decades,the evolutionary history and possible mechanisms of Eocene climate change in the northeastern Tibetan Plateau remain unclear.The Xining Basin in the northeastern Tibetan Plateau contains a continuous sequence of Early to Late Eocene non-marine sediments which provides the opportunity to resolve long-term climate changes during this period. In this study, we report the results of analyses of lithofacies, sediment color and geochemistry of bulk samples collected from the Xijigou section of the Xining Basin. An abrupt lithofacies change between the Early(~52–40 Ma) and Late Eocene(~40–34 Ma) indicates a change in the depositional environment from a shallow lake to a playa lake in response to a significant climatic shift. During ~52–40 Ma,higher values of sediment redness(a*), redness/lightness(a*/L*) and higher modified Chemical Index of Weathering(CIW′)indicate a relatively warm and humid climate, while from ~40–34 Ma the lower values of a*, a*/L*and lower CIW′ imply subhumid to semi-arid climatic conditions. The paleoclimatic records indicate a long-term(~52–34 Ma) trend of decreasing chemical weathering, consistent with global climate change. An abrupt sharp excursion of the proxy records during ~42–40 Ma suggests a relatively brief warm interval, corresponding to the Middle Eocene Climatic Optimum(MECO). We suggest that global cooling substantially reduced humidity in inner Asia, resulting in sub-humid to semi-arid climatic conditions after 40 Ma in the Xining Basin, which may have been responsible for the long-term trend of decreasing chemical weathering during the Eocene.     

Investigation of Crustal Thickness in Eastern Anatolia Using Gravity,Magnetic and Topographic Data

The tectonic regime of Eastern Anatolia is determined by the Arabia-Eurasia continent-continent collision. Several dynamic models have been proposed to characterize the collision zone and its geodynamic structure. In this study, change in crustal thickness has been investigated using gravity, magnetic and topographic data of the region. In the first stage, two-dimensional low-pass filter and upward analytical continuation techniques were applied to the Bouguer gravity data of the region to investigate the behavior of the regional gravity anomalies. Next the moving window power spectrum method was used, and changes in the probable structural depths from 38 to 52 km were determined. The changes in crustal thickness where free air gravity and magnetic data have inversely correlated and the type of the anomaly resources were investigated applying the Euler deconvolution method to Bouguer gravity data. The obtained depth values are consistent with the results obtained using the power spectrum method. It was determined that the types of anomaly resources are different in the west and east of the 40° E longitude. Finally, using the obtained findings from this study and seismic velocity models proposed for this region by previous studies, a probable two-dimensional crust model was constituted.     

浙江永康盆地朝川组泥灰岩段沉积特征及其古气候意义

位于浙江省东部的永康盆地是典型的白垩纪陆相红盆.本文作者发现在永康盆地北东部的朝川组红层沉积中发育较深水暗色泥灰岩沉积,与较浅水红色细碎屑沉积呈互层或夹层产出,具渐变式湖相碳酸盐岩与陆源碎屑岩混合沉积的特征.控制该混合沉积形成的主要因素是湖盆浪基面之下的静水环境及干旱与潮湿气候的交替变化.通过对朝川组泥灰岩段沉积环境及成因进行分析,结合该泥灰岩段中保存的孢粉和植物化石组合,笔者认为该套湖相碳酸盐岩与陆源碎屑岩的混合沉积反映了浙东地区早白垩世晚期总体可能处于干热气候环境,但存在短时间内的干湿气候交替.     

Sedimentological and petrochemical studies of Jurassic clastic rocks,Habo Dome Basin,Kachchh Mainland,Northwest India: Implications for depositional environment,provenance, and tectonic setting

Mesozoic rocks are extensively and excellently preserved in the western Indian shield in several basins. The Kachchh Mainland Basin (KMB), comprising six small sub‐basins, is the main repository of these sediments. Habo Dome Basin, situated in the easternmost part of KMB and largest among the six basins, hosts clastics of the Chari Formation of Jurassic age. The fluctuating transgressive–regressive facies cycle, developed during the Callovian and Late Early Oxfordian in the Habo Dome Basin, was mainly controlled by local tectonics and not by global eustatic fluctuations. Near magmatic relationships are displayed by various elements of the clastic rocks of Habo Dome Basin. Two litho‐chemical groups have been identified in Habo Dome Basin, which are cyclically repeated over entire lithostratigraphic sequence, indicating alternate pulses of sediment inputs from two different sources under palpitating tectonic conditions. Provenance indicator elements and their ratios coupled with source modeling indicate predominantly felsic source with basic and alkalic components. Integrated analysis of petrograhic and geochemical characteristics suggests two source terranes for these rocks: a granitoid source with significant basic volcanics (Banded Gneissic Complex) and a granite–gneissic source with minor alkaline volcanics (Nagarparkar Massif) lying to northeast and NNW respectively. The petrochemistry of Habo Dome clastics suggests their deposition in a fault controlled sink which was influenced by sea level changes. Drifting of the Indian plate resulted in the opening of series of rifted basins in the Kachchh Mainland during Late Triassic/Early Jurassic, which were closed later during collision of Indian plate with Eurasia at early Eocene. The Habo Dome Basin which opened up as a half graben in response to the initial stress regime, remained tectonically unstable until the cessation of pre and post collisional stress regimes.     

带保温层管道腐蚀状态的X射线检测方法研究

随着无损检测技术的飞速发展,X射线实时成像技术获得了广泛的应用。本文论述了带保温层管道的X射线实时成像检测方法,重在对管道腐蚀缺陷深度如何定量的实验方法研究,和管道缺陷处厚度与图像灰度的数学模型建立,从而对管道腐蚀厚度进行定量,管道缺陷的定量研究有助于管道安全运行的评价和隐患问题的及时发现。