Mengcheng National Geophysical Observatory

Mengcheng Chengguanzhen, China

Mengcheng National Geophysical Observatory

Mengcheng Chengguanzhen, China

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Ren H.,Hefei University of Technology | Ren H.,Mengcheng National Geophysical Observatory | Huang Q.,Peking University | Chen X.,Hefei University of Technology | Chen X.,Mengcheng National Geophysical Observatory
Geophysical Journal International | Year: 2016

In this work, we carry out numerical simulations of the seismo-electromagnetic fields associated with a fault in a porous medium by considering the electrokinetic effect. In addition to porous materials, the adopted layered models comprise solid materials in which the electrokinetic effect is inoperative. First, sensitivity study is performed for the evanescent and direct radiation electromagnetic (EM) waves generated by a double couple point source embedded in a porous half-space below a solid half-space. Results suggest that both the evanescent and direct radiation EM waves are sensitive to some medium properties, for example porosity, salinity, fluid viscosity, and conductivity of solid layer. Then, adopting an eight-layer halfspace model, we simulate the seismic and EM wavefields generated by the rupture process of a finite fault. It is shown that the electrokinetic effect is able to generate observable corupture EM signals, but the observability depends on some factors such as the epicentral distance, properties of the medium where the fault is located, and local activity levels of EM noise. Time synchronization coseismic EM signals are recorded when the receiver is close to the ground water level but located in a solid medium. In addition to the post-seismic electric field, our results also show the post-seismic magnetic field which has not been identified in previous simulation studies on the electrokinetic effect. The generation of the post-seismic magnetic field probably requires a sufficiently strong medium heterogeneity or fluid-pressure gradient. © The Authors 2016.


Wu Z.Q.,Hefei University of Technology | Wu Z.Q.,Mengcheng National Geophysical Observatory | Wang W.Z.,Hefei University of Technology
Science China Earth Sciences | Year: 2016

The elasticity of minerals at high temperature and pressure (PT) is critical for constraining the composition and temperature of the Earth’s interior and understand better the deep water cycle and the dynamic Earth. First-principles calculations without introducing any adjustable parameters, whose results can be comparable to experimental data, play a more and more important role in investigating the elasticity of minerals at high PT mainly because of (1) the quick increasing of computational powers and (2) advances in method. For example, the new method reduces the computation loads to one-tenth of the traditional method with the comparable precise as the traditional method. This is extraordinarily helpful because first-principles calculations of the elasticity of minerals at high PT are extremely time-consuming. So far the elasticity of most of lower mantle minerals has been investigated in detail. We have good idea on the effect of temperature, pressure, and iron concentration on elasticity of main minerals of the lower mantle and the unusual softening in bulk modulus by the spin crossover of iron in ferropericlase. With these elastic data the lower mantle has been constrained to have 10–15 wt% ferropericlase, which is sufficient to generate some visible effects of spin crossover in seismic tomography. For example, the spin crossover causes that the temperature sensitivity of P wave at the depth of ~1700 km is only a fraction of that at the depth of ~2300 km. The disruptions of global P wave structure and of P wave image below hotspots such as Hawaii and Iceland at similar depth are in consistence with the spin crossover effect of iron in ferropericlase. The spin crossover, which causes anomalous thermodynamic properties of ferropericlase, has also been found to play a control role for the two features of the large low shear velocity provinces (LLSVPs): the sharp edge and high elevation up to 1000 km above core-mantle boundary. All these results clearly suggest the spin crossover of iron in the lower mantle. The theoretical investigations for the elasticity of minerals at the upper mantle and water effect on elasticity of minerals at the mantle transition zone and subducting slab have also been conducted extensively. These researches are critical for understanding better the composition of the upper mantle and water distribution and transport in the Earth’s mantle. Most of these were static calculations, which did not include the vibrational (temperature) effect on elasticity, although temperature effect on elasticity is basic because of high temperature at the Earth’s interior and huge temperature difference between the ambient mantle and the subducting slab. Including temperature effect on elasticity of minerals should be important future work. New method developed is helpful for these directions. The elasticity of iron and iron-alloy with various light elements has also been calculated extensively. However, more work is necessary in order to meet the demand for constraining the types and amount of light elements at the Earth’s core. © 2016, Science China Press and Springer-Verlag Berlin Heidelberg.


Shi L.-W.,Hefei University of Technology | Shi L.-W.,Chinese Academy of Sciences | Shi L.-W.,Mengcheng National Geophysical Observatory | Shen C.-L.,Hefei University of Technology | And 3 more authors.
Chinese Journal of Geophysics (Acta Geophysica Sinica) | Year: 2014

Previous results show that various interplanetary structures, such as interplanetary coronal mass ejections (ICMEs) and corotating interaction regions (CIRs), may contain the southward component of the magnetic field. Thus, all these structures are thought to be the interplanetary origins of the geomagnetic storm. In this paper, all moderate and strong geomagnetic storms with Dstmin≤-50 nT between 2007 and 2012 are studied. During this period, there are total 51 geomagnetic storms, in which 9 geomagnetic storms are strong events with Dstmin≤-100 nT. The number of the geomagnetic storm during this period is much smaller than similar period of 1996-2001 in the previous solar cycle. In addition, the interplanetary origins of these geomagnetic storms are identified based on the in-situ observations from WIND and ACE. It is found that 65% geomagnetic storms with Dstmin≤-50 nT were caused by the structures related to coronal mass ejections (CMEs), and 31% were caused by corotating interaction regions (CIRs). It should be noted that in this period, the CME related structures did not produce any extreme strong geomagnetic storm with Dstmin≤-200 nT and the CIR did not cause the strong geomagnetic storm with Dstmin≤-100 nT. These results suggest that the geoeffectiveness of the CME and CIR is weak in this period. Furthermore, we found that the CIRs were the main cause of geomagnetic storms in solar minimum while CMEs-related structures were the main cause of geomagnetic storm in the rising phase and solar maximum. ©, 2014, Science Press. All right reserved.


Wu Z.,Anhui University of Science and Technology | Wu Z.,Mengcheng National Geophysical Observatory | Huang F.,Anhui University of Science and Technology | Huang S.,University of Nevada, Las Vegas
Earth and Planetary Science Letters | Year: 2015

With increasing pressure, olivine transforms to wadsleyite, ringwoodite, and finally dissociates into bridgmanite and ferropericlase, producing dramatic structure changes along these phase transformations. Here we investigated the equilibrium isotope fractionation of Mg, Si, and O among these phases using first-principles calculations. Both Mg and Si have measurable isotope fractionations among these phases, even at mantle's pressure-temperature conditions. Wadsleyite and ringwoodite are depleted in heavy Si but enriched in heavy Mg relative to olivine. Bridgmanite is depleted in heavy Si and Mg among all phases. Increasing pressure can slightly reduce the size of Si isotope fractionation but significantly depress Mg isotope fractionation among these phases. Mg and Si isotope fractionation among Mg2SiO4 polymorphs may provide a promising way to "probe" the depth and temperature of origin of mantle xenoliths. © 2014 Elsevier B.V.


Zhang Z.,Anhui University of Science and Technology | Zhang Z.,Mengcheng National Geophysical Observatory | Zhang W.,Anhui University of Science and Technology | Zhang W.,Mengcheng National Geophysical Observatory | And 2 more authors.
Geophysical Journal International | Year: 2014

In this study, we present a new method for simulating the 3-D dynamic rupture process occurring on a non-planar fault. The method is based on the curved-grid finite-difference method (CG-FDM) proposed by Zhang & Chen and Zhang et al. to simulate the propagation of seismic waves in media with arbitrary irregular surface topography. While keeping the advantages of conventional FDM, that is computational efficiency and easy implementation, the CG-FDM also is flexible in modelling the complex fault model by using general curvilinear grids, and thus is able to model the rupture dynamics of a fault with complex geometry, such as oblique dipping fault, non-planar fault, fault with step-over, fault branching, even if irregular topography exists. The accuracy and robustness of this new method have been validated by comparing with the previous results of Day et al., and benchmarks for rupture dynamics simulations. Finally, two simulations of rupture dynamics with complex fault geometry, that is a non-planar fault and a fault rupturing a free surface with topography, are presented. A very interesting phenomenon was observed that topography can weaken the tendency for supershear transition to occur when rupture breaks out at a free surface. Undoubtedly, this new method provides an effective, at least an alternative, tool to simulate the rupture dynamics of a complex non-planar fault, and can be applied to model the rupture dynamics of a real earthquake with complex geometry. © The Authors 2014.


Wu B.,Anhui University of Science and Technology | Wu B.,Mengcheng National Geophysical Observatory | Chen X.,Anhui University of Science and Technology | Chen X.,Mengcheng National Geophysical Observatory
Geophysical Journal International | Year: 2016

We propose an adaptive root-determining strategy that is very usefulwhen dealingwith trapped modes or Stoneley modes whose energies become very insignificant on the free surface in the presence of low-velocity layers or fluid layers in the model. Loss of modes in these cases or inaccuracy in the calculation of these modes may then be easily avoided. Built upon the generalized reflection/transmission coefficients, the concept of 'family of secular functions' that we herein call 'adaptive mode observers' is thus naturally introduced to implement this strategy, the underlying idea of which has been distinctly noted for the first time and may be generalized to other applications such as free oscillations or applied to other methods in use when these cases are encountered. Additionally, we have made further improvements upon the generalized reflection/transmission coefficient method; mode observers associated with only the free surface and low-velocity layers (and the fluid/solid interface if the model contains fluid layers) are adequate to guarantee no loss and high precision at the same time of any physically existent modes without excessive calculations. Finally, the conventional definition of the fundamental mode is reconsidered, which is entailed in the cases under study. Some computational aspects are remarked on.With the additional help afforded by our superior rootsearching scheme and the possibility of speeding calculation using a less number of layers aided by the concept of 'turning point', our algorithm is remarkably efficient as well as stable and accurate and can be used as a powerful tool for widely related applications. © The Authors 2016.


Huang F.,Anhui University of Science and Technology | Wu Z.,Anhui University of Science and Technology | Wu Z.,Mengcheng National Geophysical Observatory | Huang S.,University of Nevada, Las Vegas | Wu F.,Anhui University of Science and Technology
Geochimica et Cosmochimica Acta | Year: 2014

Silicon isotope fractionation factors for mantle silicate minerals, including olivine, wadsleyite, ringwoodite, pyroxenes, garnet (pyrope), majorite, and Mg-perovskite, are calculated using density functional theory. Our results show that equilibrium fractionations of Si isotopes are negligible among pyroxenes, olivine, and pyrope, but are significant between olivine and its polymorphs (wadsleyite and ringwoodite). There is also significant Si isotope fractionations between mantle minerals with different Si coordination numbers (CN), such as Mg-perovskite (CN=6) and olivine polymorphs (CN=4). When in equilibrium with each other, 30Si/28Si decreases in the order of olivine>pyroxenes>wadsleyite>majorite>ringwoodite>Mg-perovskite.Our calculation predicts significant Si isotope fractionation between mantle minerals, e.g., perovskite vs. ringwoodite, majorite vs. pyroxene, and olivine vs. its polymorphs even at high pressure and temperature conditions of deep mantle. The Si CN in silicate melt increases with increasing pressure, implying that Si isotope fractionation between silicate and metal could be a function of pressure. Our results suggest that Si isotopic fractionation factor between silicate and metal may decrease with increasing pressure; consequently, Si isotopic fractionation factor obtained from low pressure experiments may not be applicable to Si isotope fraction during core formation which occurred at high pressure. Finally, Si isotopes could also be fractionated between perovskite-rich mantle and residual melt during magma-ocean cooling in the lower mantle because of their different Si CNs. If such primordial signature is not destroyed and partially preserved through the Earth's history, significant Si isotope heterogeneity could still exist between the upper and lower mantle. © 2014 Elsevier Ltd.


Zhang Z.,Anhui University of Science and Technology | Zhang Z.,Mengcheng National Geophysical Observatory | Zhang W.,Anhui University of Science and Technology | Zhang W.,Mengcheng National Geophysical Observatory | And 2 more authors.
Geophysical Journal International | Year: 2014

Perfectly matched layer (PML) is an efficient absorbing technique for numerical wave simulations. Since it appeared, various improvements have been made. The complex frequency-shifted PML (CFS-PML) improves the absorbing performance for near-grazing incident waves and evanescent waves. The auxiliary differential equation (ADE) formulation of the PML provides a convenient unsplit-field PML implementation that can be directly used with high order time marching schemes. The multi-axial PML (MPML) stabilizes the PML on anisotropic media. However, these improvements were generally developed for Cartesian grids. In this paper, we extend the ADE CFS-PML to general curvilinear (non-orthogonal) grids for elastic wave modelling. Unlike the common implementations to absorb the waves in the computational space, we apply the damping along the perpendicular direction of the PML layer in the local Cartesian coordinates. Further, we relate the perpendicular and parallel components of the gradient operator in the local Cartesian coordinates to the derivatives in the curvilinear coordinates, to avoid mapping the wavefield to the local Cartesian coordinates. It is thus easy to be incorporated with numerical schemes on curvilinear grids. We derive the PML equations for the interior region and for the free surface separately because the free surface boundary condition modifies the elastic wave equations. We show that the elastic wave modelling on curvilinear grids exhibits anisotropic effects in the computational space, which may lead to unstable simulations. To stabilize the simulation, we adapt the MPML strategy to also absorb the wavefield along the two parallel directions of the PML. We illustrate the stability of this ADE CFS-MPML for finite-difference elastic wave simulations on curvilinear grids by two numerical experiments. © The Authors 2014. Published by Oxford University Press on behalf of The Royal Astronomical Society.


Zhang Z.-G.,Hefei University of Technology | Zhang Z.-G.,Mengcheng National Geophysical Observatory | Sun Y.-C.,Hefei University of Technology | Sun Y.-C.,Mengcheng National Geophysical Observatory | And 6 more authors.
Chinese Journal of Geophysics (Acta Geophysica Sinica) | Year: 2014

An Ms6.5 earthquake occurred at Lundian county, Yunnan province on August 3, 2014. With the available three-dimensional velocity model and topography data, as well as the preliminarily inverted kinematic source model (Zhang et al. 2014), we simulated the seismic wave propagation of Ludian earthquake with 3D curved grids finite-difference method, and calculated the distribution of seismic intensity for the modeling region. The simulated results reveal that the highest seismic intensity caused by this earthquake is about degree 7, mainly in Lundian County and the boundaries between Ludian County and Qiaojia and Huize County. The simulation result also implies that ground motion has large amplitudes at the mountain peaks and ridges. The numerical modeling indicates that the amplification effect of low velocity basin at the northeastern of the earthquake fault may aggravate the seismic hazard.


Zhang X.,Anhui University of Science and Technology | Zha X.,Anhui University of Science and Technology | Zha X.,Mengcheng National Geophysical Observatory | Dai Z.,Anhui University of Science and Technology | Dai Z.,Mengcheng National Geophysical Observatory
Journal of Asian Earth Sciences | Year: 2015

We invert the background stress fields of the Longmenshan and its adjacent regions before and after the 2008 great Wenchuan earthquake from focal mechanism data. Our results show that the stress orientations after the 2008 great Wenchuan earthquake are more coherent than those before this event at the epicentral zone. This result includes twofold implications: one is that the Wenchuan event altered the local stress field, the other is that the strength of the Wenchuan fault zone is very high and the characteristic of the seismogenic fault is consistent with the asperity model. We also estimate the stress changes caused by the Wenchuan event using an accurate source model and exhaustive receiver faults. The stress changes of >10. kPa mainly occur in the nearby areas of the Longriba, east Kunlun, Qingchuan, Lushan faults and the southeastern section of the Xianshuihe fault, indicating that there is high earthquake hazard in these areas. The seismicity before and after the great Wenchuan earthquake in the Longmenshan and its adjacent regions shows that some subsequent earthquakes in the stress shadow area may be delayed and the 2013 Lushan earthquake may be advanced. © 2014 Elsevier Ltd.

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