Crustal thickness and Poisson’s ratio variations across the northwest Himalaya and eastern Ladakh

Crustal thickness and Poisson’s ratios are estimated across the northwest (NW) Himalaya and eastern Ladakh applying H-k stacking method on receiver functions of teleseismic earthquakes recorded at 16 broadband seismological stations. The results show significant lateral variation of crustal thickness from the Lesser and Higher Himalaya (～50 km thick) to Ladakh (～80 km thick) through the Indus Tsangpo Suture Zone (ITSZ). The Indian Moho is continuously traceable across the ITSZ which is consistent with the underthrusting of the Indian plate beyond the surface collision boundary. The estimated Poisson’s ratios in the Lesser and Higher Himalaya are low (0.249–0.253), suggesting felsic composition of the crust. The Poisson’s ratio is intermediate in the Tethyan Himalaya (0.269–0.273) and high beneath Ladakh (0.280–0.303), indicating the effect of aqueous fluid/partial melt present in the crust.     

Inversion of receiver function by wavelet transformation

Introduction Receiver function has been extensively applied in studying S wave velocity of crust and up-per mantle for about 20 years (Owens, et al, 1987; LIU, et al, 1996), which is a time series ob-tained by the deconvolution of vertical component from horizontal component for teleseismic P waveform. Receiver function represents the teleseismic P plane wave response of crust and upper mantle beneath seismic station, from which the source and propagation effects are removed. Receiver funct…     

Upper crustal Poisson’s ratio and coda-wave attenuation beneath Eastern Anatolia

The attenuation of seismic waves reflects the elastic nature of the media within which the waves propagate. In this study, we calculate the Coda-Q(Q_c), frequency dependence(η), Vp/Vs and Poisson's(υ) ratios by using 2621 vertical component seismograms generated by 987 earthquakes recorded by 13 seismic stations in Eastern Anatolia, and creat a 2-D seismic tomographic Q_c model for the region. The obtained model provides significant information for exploring the boundaries of adjacent tectonic units within the upper crust and interpreting their dynamic characteristics. The 2-D Q_c model and the other parameters are consistent with the seismotectonic features of Eastern Anatolia. Highly heterogeneous Q_c values are observed in the study area dividing it into north-south directed bands of low and high attenuation. The highest η values were obtained beneath the northwestern and eastern parts of the study region. Clear, high and low υ values are obtained in the western and eastern parts of the study area, respectively. The spatial variations in the measured parameters are consistent with many geophysical observations including low Pn velocities, efficient Sn blockage, high heat flow, and widespread volcanism. Different upper crustal thicknesses and inhomogeneous stress distribution along the East and North Anatolian Fault Zones may also contribute to the observed heterogeneities.     

The velocity structure of crust and uppermantle in the Wudalianchi volcano area in—ferred from the receiver function

The Wudalianchi volcano is a modern volcano erupted since the Holocene.Its frequent occurrence of the small earthquake is considered to be indicator of active dormancy volcano.The S wave velocity structure is inferred from the receiver function for the crust and upper mantle of the Wudalianchi volcano area.The results show that the low velocity structure of Swave is widely distributed undemeath the volcano area and part of the low-velocity-zone located at shallow depth in the Wudalianchi volcano area.The low velocity structure is related to the seismicity.The Moho interface is not clear undemeath the volcano area,which may be regard to be an nec-essary condition for the lava upwelling.Therefore,we infer that the Wudalianchi volcano has the deep structural condition for the volcano activity and may be alive again.     

The inversion of 3-D crustal structure and hypocenter location in the Beijing-Tianjin-Tangshan-Zhangjiakou area by genetic algorithm

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Correction to the crustal thickness of the northwestern China using the data of seis-mic tomography

Introduction The gravity anomaly is an indicator of the density distribution of the underground material. Therefore the gravity anomalies have been important data used for studying the deep crustal struc-ture for a long time. Many people have made detailed researches on the regional crustal structure inverted by Bouguer anomalies. In particular some empirical formulae and practical algorithms about the crustal thickness were brought forward, and a series of results were obtained (MENG, 1996)…     

Determining the thickness of the low-latitude boundary layer in the Earth’s magnetosphere

We describe a method for determining the thickness of the low-latitude boundary layer (LLBL) of the Earth’s magnetosphere at the dayside near the equatorial plane based on the data gathered by a single satellite that traverses the layer and measures the plasma velocity. The method may be applied when the position of the magnetopause and the magnetosheath parameters fluctuate. The necessity of taking the presence of outer and inner LLBL regions into account is analyzed. The developed method is tested using the analysis results of two almost simultaneous close traverses of the magnetopause completed by the THEMIS mission satellites that provided relatively precise data on the LLBL thickness. It is shown that the developed method makes it possible to determine the LLBL thickness with an accuracy of ～10%.     

Crust and uppermost mantle structure of the Ailaoshan-Red River fault from receiver function analysis

S-wave velocity structure beneath the Ailaoshan-Red River fault was obtained from receiver functions by using teleseismic body wave records of broadband digital seismic stations. The average crustal thickness, Vp/Vs ratio and Poisson’s ratio were also estimated. The results indicate that the interface of crust and mantle beneath the Ailaoshan-Red River fault is not a sharp velocity discontinuity but a characteristic transition zone. The velocity increases relatively fast at the depth of Moho and then increases slowly in the uppermost mantle. The average crustal thickness across the fault is 36―37 km on the southwest side and 40―42 km on the northeast side, indicating that the fault cuts the crust. The relatively high Poisson’s ratio (0.26―0.28) of the crust implies a high content of mafic materials in the lower crust. Moreover, the lower crust with low velocity could be an ideal position for decoupling between the crust and upper mantle.     

S-wave crustal and upper mantle’s velocity structure in the eastern Tibetan Plateau — Deep environment of lower crustal flow

A teleseismic profile consisting of 26 stations was deployed along 30°N latitude in the eastern Tibetan Plateau. By use of the inversion of P-wave receiver function, the S-wave velocity structures at depth from surface to 80 km beneath the profile have been determined. The inversion results reveal that there is significant lateral variation of the crustal structure between the tectonic blocks on the profile. From Linzhi north of the eastern Himalayan Syntaxis, the crust is gradually thickened in NE direction; the crustal thickness reaches to the maximum value (∼72 km) at the Bangong-Nujiang suture, and then decreased to 65 km in the Qiangtang block, to 57–64 km in the Bayan Har block, and to 40–45 km in the Sichuan Basin. The eastern segment of the teleseismic profile (to the east of Batang) coincides geographically with the Zhubalong-Zizhong deep seismic sounding profile carried out in 2000, and the S-wave velocity structure determined from receiver functions is consistent with the P-wave velocity structure obtained by deep seismic sounding in respect of the depths of Moho and major crustal interfaces. In the Qiangtang and the Bayan Har blocks, the lower velocity layer is widespread in the lower crust (at depth of 30–60 km) along the profile, while there is a normal velocity distribution in lower crust in the Sichuan Basin. On an average, the crustal velocity ratio (Poisson ratio) in tectonic blocks on the profile is 1.73 (σ = 0.247) in the Lhasa block, 1.78 (σ = 0.269) in the Banggong-Nujiang suture, 1.80 (σ = 0.275) in the Qiangtang block, 1.86 (σ = 0.294) in the Bayan Har blocks, and 1.77 (σ = 0.265) in the Yangtze block, respectively. The Qiangtang and the Bayan Har blocks are characterized by lower S-wave velocity anomaly in lower crust, complicated Moho transition, and higher crustal Poisson ratio, indicating that there is a hot and weak medium in lower crust. These are considered as the deep environment of lower crustal flow in the eastern Tibetan Plateau. Flowage of the ductile material in lower crust may be attributable to the variation of the gravitational potential energy in upper crust from higher on the plateau to lower off plateau. Supported by the National Natural Science Foundation of China (Grants No. 40334041 and 40774037) and the International Cooperation Program of the Ministry of Science and Technology of China (Grant No. 2003DF000011)     

Green’s function of the linearized Saint-Venant equations in laminar and turbulent flows

In the present paper, an analytical expression of the Green’s function of linearized Saint-Venant equations (LSVEs) for shallow water waves is provided and applied to analyse the propagation of a perturbation superposed to a uniform flow. Independently of the kinematic character of the base flow, i.e., subcritical or supercritical uniform flow, the effects of a non-uniform vertical velocity profile and a non-constant resistance coefficient are accounted for. The use of the Darcy-Weisbach friction law allows a unified treatment of both laminar and turbulent conditions. The influence on the wave evolution of the wall roughness and the fluid viscosity are finally discussed, showing that in turbulent regime the assumption of constant friction coefficient may lead to an underestimation of both amplification and damping factors on the wave fronts, especially at low Reynolds numbers. This conclusion has to be accounted for, particularly in describing hyper-concentrated suspensions or other kinds of Newtonian mixtures, for which the high values of the kinematic viscosity may lead to relatively low Reynolds numbers.     

Ionization of the earth’s atmosphere by solar and galactic cosmic rays

A brief review of the research of atmospheric effects of cosmic rays is presented. Numerical models are discussed, that are capable to compute the cosmic ray induced ionization at a given location and time. Intercomparison of the models, as well as comparison with fragmentary direct measurements of the atmospheric ionization, validates their applicability for the entire atmosphere and the whole range of the solar activity level variations. The effect of sporadic solar energetic particle events is shown to be limited on the global scale, even for the most severe event, but can be very strong locally in polar regions, affecting the physical-chemical properties of the upper atmosphere, especially at high altitudes. Thus, a new methodology is presented to study cosmic ray induced ionization of the atmosphere in full detail using realistic numerical models calibrated to direct observations.     

Study of crustal seismic velocity in the Weihe fault depression basin by seismic tomographic imaging

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Study on the pattern and mode of vertical crustal deformation during the seismogenic process of intraplate strong earthquakes

Ｓｔｕｄｙｏｎｔｈｅｐａｔｔｅｒｎａｎｄｍｏｄｅｏｆｖｅｒｔｉｃａｌｃｒｕｓｔａｌｄｅｆｏｒｍａｔｉｏｎｄｕｒｉｎｇｔｈｅｓｅｉｓｍｏｇｅｎｉｃｐｒｏｃｅｓｓｏｆｉｎｔｒａｐｌａｔｅｓｔｒｏｎｇｅａｒｔｈｑｕａｋｅｓ杨国华，桂昆长，巩曰沐，杨春花，韩...     

Simulation of ground motion in the Moscow region using the empirical Green’s function

Historically, the Moscow region regularly experienced rather weak but quite perceptible seismic vibrations produced by intermediate-depth earthquakes of the Vrancea zone (Romania), located at a distance of 1400 km from Moscow. The coincidence of a number of unique factors such as a slowly varying focal depth, predominant source mechanisms, weak attenuation of seismic radiation in the north-northeast direction provide favorable conditions for application of the empirical Green’s function method. Using the digital seismogram of the Vrancea Mw-5.8 earthquake recorded at the Moscow seismic station, we simulated synthetic seismograms of a scenario (expected maximum) earthquake with Mw = 8.0, by application of the empirical Green’s function method adjusted for the given conditions. The calculation procedure was verified using analog records of strong earthquakes available at the Moscow seismic station. Digital records of the Obninsk seismic station included in the Incorporated Research Institutions for Seismology (IRIS) system were used for additional control. Here, the scenario earthquake was modeled using the data on a much stronger earthquake of 1990 (MW = 6.9). It is shown that, despite a certain scatter (quite adequately assessed in the scope of the method), the ultimate estimates of expected seismic impacts are quite reliable and can be recommended for practical use.     

Use of Green’s function for determining the disturbing potential of an ellipsoidal Earth

A spherical approximation makes the basis for a majority of formulas in physical geodesy. However, the present-day accuracy in determining the disturbing potential requires an ellipsoidal approximation. The paper deals with constructing Green’s function for an ellipsoidal Earth by an ellipsoidal harmonic expansion and using it for determining the disturbing potential. From the result obtained the part that corresponds to the spherical approximation has been extracted. Green’s function is known to depend just on the geometry of the surface where boundary values are given. Thus, it can be calculated irrespective of the gravity data completeness. No changes of gravity data have an effect on Green’s function and they can be easily taken into account if the function has already been constructed. Such a method, therefore, can be useful in determining the disturbing potential of an ellipsoidal Earth.     

Deep tectonic setting of the 2008 Wenchuan M_s8.0 earthquake in southwestern China ―Joint analysis of teleseismic P-wave receiver functions and Bouguer gravity anomalies

Teleseismic P-wave receiver functions at 20 broadband seismic stations in the Longmenshan fault zone (LMFZ) and its vicinity were extracted, and the crustal thickness and the P- and S-wave velocity ratio were calculated by use of the H-k stacking algorithm. With the results as constraints, the S-wave ve-locity structures beneath each station were determined by the inversion of receiver functions. The crustal structure of the Rear-range zone is similar to that of the Songpan-Garze Block, whereas the velocity...     

Effect of the magnetic field on measurements of the electron density and temperature by cylindrical probes in the Earth’s ionosphere

In the 1960s and 1970s, quite simply produced cylindrical Langmuir probes were used in the USSR both on satellites (Kosmos-378, Intercosmos-2, -4, -8, -10, -19) and to measure the electron density and temperature on vertical launched rockets (Vertical’-4, -6, -10) within the Intercosmos program. These measurements were first made at middle latitudes. With increasing inclination of the orbits of launched satellites (satellites had no stabilization), falling sections were sometimes observed on probe characteristics in the electron saturation region. The Intercosmos-Bulgaria-1300 satellite, which was stabilized along three axes and was equipped with a cylindrical probe whose longitudinal axis was always directed downward to the Earth, was launched in 1981. This satellite allowed definite conclusions on the effect of the geomagnetic field on the form of the probe characteristic and, hence, on the determination of the electron density and temperature. Probe characteristics with falling sections are presented. These measurements are compared with those performed in a laboratory plasma. The appearance of negative sections on the probe characteristics is shown to be due to the effect of the geomagnetic field. The degree of this effect depends both on the electron density and temperature and on the probe voltage.