Analysis of the polar vortex oscillations in a shallow water model of the stratosphere using quasi−Lagrangian diagnostics
Seyed Majid
MirRokni
مؤسسه ژئوفیزیک دانشگاه تهران
author
Alireza
Mohebalhojeh
مؤسسه ژئوفیزیک دانشگاه تهران، ایران
author
text
article
1389
per
Polar vortex oscillations are investigated using the long−term run of the potential-vorticity-based shallow water (SW) models for the stratosphere. In the SW models examined here, the combined effects of mechanical forcing, thermal forcing, and dissipation are the factors that can cause polar vortex oscillation. The mechanical forcing is provided by a time-independent topography, mimicking tropospheric excitation of the stratosphere. The thermal forcing is provided by a linear relaxation of the mass field to a time-independent equilibrium state mimicking the radiative relaxation taking place in the stratosphere. The SW equations in the potential vorticity (PV), velocity divergence and acceleration divergence representation are solved for a range of resolutions using the "diabatic contour-advective semi-Lagrangian" (DCASL) algorithm and a standard pure semi-Lagrangian (SL) algorithm. Using vastly different numerical algorithms makes it possible to uncover the degree of numerical sensitivity and the properties of the zonal vacillations with much greater accuracy than previous studies based on the SW model of the stratosphere. The equations for velocity and acceleration divergence are solved using spectral transform in longitude and compact fourth-order finite differencing in latitude. The spatial resolution is indicated by M Ï N, M and N being the number of grid points in the longitudinal and latitudinal directions, respectively. The results for the pure SL algorithm with spatial resolutions of 256 Ï 256, 512 Ï 512, and 1024 Ï 1024 are presented and compared with the corresponding results for the DCASL algorithm with a spatial resolution of 256 Ï 256. The results for the quasi-Lagrangian diagnostics indicate the superiority of the DCASL algorithm, since it can give results in 256 Ï 256 resolution comparable with those of the SL in the vastly more expensive 1024 Ï 1024 resolution. This accuracy occurs despite a clear difference in the topology of the quasi-equilibrium state to which the long-term solutions tend to approach, a strong vortex for DCASL as opposed to a diffuse vortex for the SL, indicating the possibility of multiple equilibrium solutions depending on the degree of diffusion. The present research focuses on a Lagrangian viewpoint to the evolution of the polar vortex by looking at the behavior of the quasi-Lagrangian diagnostics of the equivalent latitude, the mass enclosed by PV contours and the terms involved in its time evolution. The PV field forms the basis for calculating the quasi-Lagrangian diagnostics. The time evolution of the mass enclosed by PV contour is associated with nonconservative processes such as diabatic heating, friction, and irreversible small-scale mixing. Generally, the mass of the polar vortex increases (decreases) due to the action of diabatic (dissipative) mass flux. The results of this study are in contrast with the results reported at T42 resolution by Rong and Waugh in 2004, where the spectral transform algorithm was used to solve the SW equations in vorticity, divergence, and mass representation, wherein dissipation was produced by explicitly damping vorticity using hyperdiffusion. Except for the first large-amplitude oscillation, there is no sign of a clear, systematic phase shift between the dissipative and diabatic mass fluxes across the edge of the polar vortex, though such a shift is proposed by Rong and Waugh as the main mechanism responsible for the vacillations. Concomitant with the absence of the phase shift, the oscillations tend to be decaying and occur rather intermittently. Rather than the phase shift, the inherent fluctuations in both the diabatic and dissipative mass fluxes across the edge of the polar vortex seem to play the dominant role in generating the vacillations. Further diagnostics and numerical experiments are needed to assess the latter mechanism.
Iranian Journal of Geophysics
انجمن ژئوفیزیک ایران
2008-336
4
v.
2
no.
1389
1
12
http://www.ijgeophysics.ir/article_40844_cf3fb5f3dc6507406fa4d222a056a580.pdf
Non-linear inversion of magnetic data using Gradient subspace method
Ali
Nejati Kalateh
دانشگاه صنعتی شاهرود، ایران
author
Hamid Reza
Siahkoohi
مؤسسه ژئوفیزیک دانشگاه تهران، ایران
author
Mahmood
Mirzaei
دانشگاه اراک، ایران
author
Nasser
Hosseinzadeh Guya
مؤسسه ژئوفیزیک دانشگاه تهران، ایران
author
text
article
1389
per
Inverse theory was developed by scientists and has been used in different scientific applications, such as geophysical tomography, image enhancement, curve fitting and determination of earth structure from geophysical data. Inverse theory provides mathematical techniques to obtain useful information about measurements (data). The information resulting from inversion usually reveals some specific properties of the geological structures, called model parameters. Inverse theory, in contrast to forward theory, which predicts results of measurements on the basis of a suggested model relevant to the problem, uses models that are adjusted and estimates the model parameters by using the data and some general principles. It should be noted that inverse theory provides information about unknown model parameters directly using measured data. In contrast to forward theory, inverse theory doesn't provide a basis for the model itself. Recently, considerable effort has been devoted to the explanation of gravity and magnetic anomalies by employing data inversion in the spatial domain. Three major types of gravity and magnetic data inversion are discussed in geophysical literature. The first is "Inverting data for solving both physical and shape parameters". In this case, the inverse problem is completely non-unique. The non-uniqueness of the problem is visible in the two-dimensional section as a large number of well-defined local minima, some of which are distinguished as unfeasible. In this class, unacceptable solutions can be confined by specifying some of the model parameters. The second type of gravity and magnetic data inversion is "Inverting data for solving physical parameters". In this approach, the earth is divided into a limited number of cells of fixed size with unknown physical parameters, such as density and magnetization. The non-uniqueness of the solution is evident and algorithms have been developed to produce a single model by minimizing an objective function. The third type of gravity and magnetic data inversion is "Inverting data for solving shape parameters". In this class, physical parameters are assumed to be known and nonlinear operators must be design to determine geometry of the geophysical sources. However, geophysical inversion methods are most effective when a linear operator is applied; thus, the problem is usually linearized about some initial model and the inverse problem is solved iteratively. This paper presents a robust, flexible and efficient algorithm to solve large scale non-linear inverse problems in geomagnetic surveys (the third type of gravity and magnetic data inversion). Considering the sensitivity of inverting magnetic data and the high level of noise in data acquisition, the inversion of magnetic data should be performed using advanced methods. These methods have high performance to handle noise data. The method is iterative, and at each iteration a perturbation of model parameters in a P-dimensional subspace of an M-dimensional model space are sought (the primary model is updated using perturbation values of the model parameters at each iteration). This style of iterative subspace procedure is well adapted to non-linear inverse problems with many parameters and can be successfully applied to a variety of geopotential problems. The gradient subspace algorithm utilizes a model of parameterization in which the depth of each block is described as an unknown parameter. Model parameters are allocated to separate subspaces on the basis of different physical dimensionality (in this case, model parameters have the same physical dimension). Basis vectors of P-dimensional subspace are extracted by the SVD of a Hessian matrix (the second derivation of model parameters). M-dimensional model space is projected onto P-dimensional subspace using basis vectors. If effective basis vectors are chosen for inversion procedures, the projected matrix is accurate with respect to original one. In new and small dimensions, inversion can be performed with great speed and is stable against noise. This procedure is very effective in accelerating convergence and obtaining a more accurate solution. Also, the inversion is robust with respect to data errors and poor initial estimations. The efficiency of the method is compared with one of the conventional methods of inversion of non-linear problems (Marquardt-Levenberg); the results show that the gradient subspace has fast and stable convergence in comparison to its performance in the conventional method. The practical effectiveness of this method is demonstrated by inversion of synthetic and real examples. The real magnetic data is acquired over the MOGHAN area, in the northwest of Iran. The results compared with those of seismic interpretation at the study area.
Iranian Journal of Geophysics
انجمن ژئوفیزیک ایران
2008-336
4
v.
2
no.
1389
13
31
http://www.ijgeophysics.ir/article_40845_cd7bfc49394d61b87f46a3067a5fb8df.pdf
A comparison among different methods in the evaluation of torsional ground motion
Gholam Reza
Nouri
دانشگاه محقق اردبیلی، اردبیل، ایران
author
Mohammad Reza
Ghayamghamian
پژوهشگاه بینالمللی زلزلهشناسی و مهندسی زلزله، تهران، ایران
author
Majid
Hashemifard
دانشگاه محقق اردبیلی، اردبیل، ایران
author
text
article
1389
per
Rotational motions (torsional and rocking) induced by seismic waves have been essentially ignored for a long time, first because rotational effects were thought to be small for man-made structures, and second because sensitive measuring devices were not available until quite recently. The benefits of the determination of rotational motion in seismology and engineering are still under investigation. In seismology, rotational motions can provide accurate data for arrival times of SH waves and, in the near-source distance range, rotational motions might provide more detailed information on the rupture processes of earthquakes. Rotational motions could also be used to better estimate the static displacement from seismic recordings, identifying translational signals caused by rotation. In engineering, the dynamic response estimation of structures subjected to earthquake-induced base excitations is often simplified by ignoring the rotational components. This has been a widely accepted practice in the engineering community, mainly caused by the lack of recorded strong motion accelerograms for these motions. Many structural failures and the damage caused by earthquakes can be linked to differential and rotational ground motions. The torsional responses of tall buildings in Los Angeles during the San Fernando earthquake in 1971 could be ascribed to torsional excitation, while rotational and longitudinal differential motions may have caused the collapse of bridges during the San Fernando (1971), Miyagi-ken-Oki (1978) and Northridge (1994) earthquakes. Several studies have shown the importance of torsional components in seismic analysis and design of structures. The seismic design codes also prescribe ‘Accidental Eccentricity’ in design force calculations to account for unknown torsional inputs and unpredictable eccentricities. Since then, many researchers have studied the dynamic and accidental eccentricities of structures. For the first time, Newmark (1969) established a simple relationship between the translational and torsional components of the ground motion. He presented a deterministic procedure for estimating the increase in the displacement of symmetric-plan buildings caused by rotational ground motions at the base due to the horizontal propagation of plane waves with a constant velocity and further explored in the other studies. Three main approaches have been developed to incorporate rotational motions in engineering applications: one is a numerical simulation of the field of radiation from the source mechanism. It requires an appropriate model of the faulting mechanism, without considering the effects of path and local site conditions. The second approach is based on a theoretical formulation of the spatial distribution of ground motion. In this method, some information on the source, path and local layering are required. All of these studies are based on assumed models for ground motion and none of them has the benefit of being tested against field measurements. The third approach is the application of recorded strong motion data from seismic arrays. In this paper, data were collected from the Chiba dense array, which consists of 44 accelerometers with inter-station spacing in the range of 5 to 300 meters, located 30 Km east of Tokyo, that are employed to estimate the torsional ground motion. This provides a unique opportunity to examine accuracy in the estimation of torsional motion. To this end, three methods, namely, time derivation, finite difference and geodetic methods were employed. The geodetic method could be used as the criterion for accuracy of torsional motion, since it has second order accuracy for an array with regular accelerogram patterns. The results showed that the peak torsional ground motion as computed by the time derivation method is larger than those computed by the geodetic method. Peak torsional ground motion values estimated by the finite difference method show smaller values than those computed by time derivation for long separation distances (>20). However, they showed close values for short separation distances. Finally, the effects of peak ground acceleration and magnitude of earthquake on the torsional motions have been investigated. The results reveal that there is a linear relationship between peak horizontal ground acceleration and peak torsional acceleration.
Iranian Journal of Geophysics
انجمن ژئوفیزیک ایران
2008-336
4
v.
2
no.
1389
32
48
http://www.ijgeophysics.ir/article_40846_c36d1c02fa2a817de908af9fa958baa4.pdf
Comparison of the efficiency of the sixth-order super compact and combined compact methods for spatial discretization of two-layer shallow water model: presentation of linear inertia-gravity and Rossby waves
Sarmad
Ghader
مؤسسه ژئوفیزیک دانشگاه تهران، ایران
author
Farhang
Ahmadi-Givi
مؤسسه ژئوفیزیک دانشگاه تهران، ایران
author
Hakim
Golshahy
دانشگاه آزاد اسلامی واحد علوم و تحقیقات، تهران، ایران
author
text
article
1389
per
Many types of atmospheric and oceanic motions possess an oscillatory structure in both space and time, such as inertia-gravity waves and Rossby waves. This paper concentrates on these waves. Two-dimensional shallow-water models are usually used to describe these kinds of waves. The advantages of the shallow-water equations are their computational and mathematical simplicity relative to more complicated three-dimensional models. The single-layer shallow water models are extensively used in the numerical study of large scale atmospheric and oceanic motions. These simple models, however, provide no information regarding vertical motions in the atmosphere or oceans. The simple multilayer shallow-water models are usually employed to resolve this issue. In some regions of the oceans and seas, two-layer shallow-water equations are sufficient to account for the dynamics of fluids (e.g., the Strait of Gibraltar in the Atlantic Ocean, the Mediterranean Sea and the Strait of Hormoz in the Persian Gulf and the Oman Sea). Two-layer models are not only simple models of rotating-stratified fluid dynamics, but they are also proper models for the simulation of many phenomena in the ocean and atmosphere. The compact finite-difference schemes are simple and powerful ways to reach the objectives of high accuracy and low computational cost. Compared with the traditional explicit finite-difference schemes of the same-order, compact schemes have proved to be significantly more accurate along with the benefit of using smaller stencil sizes, which can be essential in treating non-periodic boundary conditions. Applications of some families of the compact schemes, in particular the super compact finite difference method and the combined compact finite difference method, to spatial differencing in some idealized models of the atmosphere and oceans show that compact finite difference schemes can be considered as promising methods for the numerical simulation of atmosphere–ocean dynamics. Most of these studies apply compact finite difference methods to single-layer models of the atmosphere and oceans, but application to more complicated multi-layer model is lacking. The linearized single-layer shallow-water equations have been used in many research studies as a tool for numerical accuracy assessment of different numerical schemes in a linear extent. In the present work, the extension of this idea to two-layer shallow-water equations is used. To this end, two general discrete dispersion relations, those of inertial-gravity and Rossby waves, for the linearized two-layer shallow-water equations on different numerical grids are derived. These general discrete dispersion relations can be used for the evaluation of the performance of any numerical scheme. This paper is also focused on accuracy assessment of the sixth-order super compact (SCFDM) and sixth-order combined compact (CCFDM) finite difference schemes for spatial differencing of the linearized two-layer shallow-water equations on different numerical grids (i.e., Arakawa's A-E and Randall's Z grids). General discrete dispersion relations derived for the inertial-gravity waves and Rossby waves on different numerical grids are used to evaluate the accuracy of the sixth-order SCFDM and sixth-order CCFDM schemes for spatial differencing of the linearized two-layer shallow-water equations. In general, for both inertia-gravity and Rossby waves, the minimum error occurs on the Z grid using either the sixth-order SCFDM or the sixth-order CCFDM method. For Randall's Z grid, it is observed that the sixth-order CCFDM method exhibits a substantial improvement in measuring the frequency of linear inertia-gravity waves of the two-layer shallow-water model on the sixth-order SCFDM method. This property is not observed for other numerical grids. For Rossby waves, the sixth-order CCFDM shows improvement on the sixth-order SCFDM method on Arakawa’s C grid. In addition, for Arakawa’s C grid it can be observed that the baroclinc and barotropic modes of the inertia gravity waves in the under-resolved case show dissimilar behavior.
Iranian Journal of Geophysics
انجمن ژئوفیزیک ایران
2008-336
4
v.
2
no.
1389
49
69
http://www.ijgeophysics.ir/article_40847_223240a34f70852a8fc703a9a566613a.pdf
Estimation of depth, structural index and location of the magnetic sources by using combined method of AN-EUL
Jamaledin
Baniamerian
مؤسسه ژئوفیزیک دانشگاه تهران، ایران
author
Behrooz
Oskooi
مؤسسه ژئوفیزیک دانشگاه تهران، ایران
author
Mehrdad
Bastani
دانشکده علوم زمین، دانشگاه اپسالا، سوئد
author
text
article
1389
per
AN-EUL is a new automatic method for simultaneous estimation of depth, location and geometry of magnetic and gravity sources. The principle of this method is a combination of both the analytic signal and Euler Deconvolution methods. The derivation of the main equations of this method is based on the substitution of the appropriate derivatives of the Euler homogeneous equation into the expression of the analytic signal of the potential field. Location of source (Epi-centre) can be approximately estimated based on the position of the maximum value of the amplitude of the analytic signal, and the formulas of depth and structural index (SI) estimation is calculated at this point. This new method is applicable on data along profiles and/or girds. It is one of the basic characteristics of the analytic signal applied on the responses of the two dimensional magnetic sources, such as dike and infinitely long horizontal cylinders, that the shape of the signal amplitude and its location are independent of the magnetization direction. For these types of sources, the shape of the amplitude of the analytic signal is symmetrical, whereas for 3-dimensional sources, like spherical sources, the maximum value of the amplitude of the analytic signal is not always located directly over the body, and, for these sources, the shape of the amplitude of the analytic signal depends on the direction of magnetization and is asymmetric. Therefore, there will be some errors in determining the location of the magnetic source based upon the location of the maximum value of the amplitude of the analytic signal for these types of sources. An important advantage of the AN-EUL method is that it is not restricted only to idealized sources (i.e. having integer structural index). This wider applicability means that SI can be a fractional number that describes sources with arbitrary shapes. Because of the existence of high order derivatives in the AN-EUL method formula, this method is very sensitivity relative to noises and shallow sources; thus, the effects of noises and shallow sources can be reduced by applying an upward continuation filter. To study the resolution of the AN-EUL, the method has been applied on synthetic data generated from various magnetic models, including a thin dike, a magnetic sphere and a drum shape source. In the next stage, the simulation of real cases, the data were contaminated by random noise. For all of these models, with regard to the models parameters, the results have good accuracy. Finally, the method was applied to an aeromagnetic data set acquired over an area in Sweden to estimate the depth, location, and shape (structural index) of some of the anomalies. According to the geological studies in this region, there exists a granite intrusive body with certain fractures in which Diabase veins have penetrated. Results of this study show the nature of anomalies very well and give good estimations of the depth and shape of the magnetic sources causing these anomalies. The results agree well with the geological information found by other methods (e.g. MT, Gravity, field observations). All of the processing steps in this paper were performed by using codes wrote in Matlab.
Iranian Journal of Geophysics
انجمن ژئوفیزیک ایران
2008-336
4
v.
2
no.
1389
70
88
http://www.ijgeophysics.ir/article_40848_6fe618cc631b7a4a3da5a36ae728d896.pdf
Interseismic deformation study on the collision zone of Arabian and Eurasian tectonic plates in the Middle-East region using an analytical model
Asghar
Rastbood
دانشکده مهندسی نقشهبرداری، دانشگاه صنعتی خواجه نصیرالدین طوسی، تهران، ایران
author
Behzad
Voosoghi
دانشکده مهندسی نقشهبرداری، دانشگاه صنعتی خواجه نصیرالدین طوسی، تهران، ایران
author
text
article
1389
per
Modern crustal deformation measurements of fault systems have had a significant impact on present day tectonic studies. Unfortunately, while these measurements reveal a wealth of information about howthe Earth is presently deforming, they neglect to provide the answers regarding why. In addition, current measurements alone cannot determine future tectonic behavior of the Earth any more than they can account for deformation of the past. For these reasons, many disciplines of Earth science rely on the use of mathematical, physics-based modeling. Applied to crustal deformation studies, fault models constrained by geologic, geodetic, and seismic data can provide valuable insights into the characteristics of faults and their behaviors over time. Based on observations of the past, models can also provide estimates of future deformation and seismic hazards, a vital resource for communities living near active fault zones. In this research, fracture mechanic concepts and dislocation modeling are used to examine the type of motion of Arabian plate relative to Eurasia based on a Eurasia-fixed velocity field from GPS observables processing. Among different inter-seismic faulting models, the Okada analytical model is used for modeling. In conventional inter-seismic models, a vertical fault, extending to infinite depth, is embedded in an elastic half-space (e.g., Savage and Burford, 1973). Below the locking depth, the fault undergoes steady-state creep with a slip rate equal to the secular plate velocity. Along a semi-infinite vertical strike-slip fault, steady-state, sub-seismogenic fault creep during the inter-seismic period results in fault-parallel displacements at the surface of the Earth that are continuous across the fault. For a single vertical dislocation, as fault length and vertical width approach infinity, the fault-parallel motion approaches the solution of Savage and Burford (1973). The elements used in this modeling include rectangular elements that extend from 15 km average locking depth to 100,000 km (effective infinite depth) in an elastic half-space with a Poisson ratio of 0.25 and Young modulus of 800,000 bars. These elements show vertical structures in lower crust and upper mantle, up to the asthenosphere (Savage and Burford, 1973). Deformations are calculated using rectangular dislocations (Okada, 1985). First, the velocity vectors are produced by introducing structures that approximate the large-scale tectonics of the region. It is very important to note that primary results show the anti-clockwise rotation of Arabia, Iran and Anatolia relative to Eurasia and even show good conformity with GPS results. Hence, there is no special need to use Reilinger (2006) and Flerit (2004) dislocation elements data in modeling, and DeMets (1994) data are sufficient to prove the anticlockwise rotation of Arabia, Iran and Anatolia relative to Eurasia. To obtain more precise results, observed features of the smaller scale fault systems are progressively included with slip amplitudes and locking depths adjusted to fit the GPS data. After verification of GPS processing results with model results, geodynamic quantities, such as displacement and strain, are computed to investigate the type of motion of Arabian plate relative to Eurasian plate. Results show an opening in the Rea-sea and oblique collision in Iran, and an extrusion of the Anatolia, and a general anti-clockwise rotation of Arabia, Iran and Anatolia with a dilatation rate in the range of per year relative to Eurasia. Regarding the complexity and uncertainty of geological data, using numerical methods, especially boundary elements, is proposed for modeling. Using a boundary element method called displacement discontinuity, it is possible to introduce the slip rate of faults as an unknown and by applying boundary conditions such as stress, strain, displacement or any combination of these, kinematical modeling is extended to mechanical modeling.
Iranian Journal of Geophysics
انجمن ژئوفیزیک ایران
2008-336
4
v.
2
no.
1389
89
102
http://www.ijgeophysics.ir/article_40849_6678bad83c15953743315564088b45be.pdf
P-wave azimuthal seismic anisotropy across the Zagros
Forough
Keshvari
مؤسسه ژئوفیزیک دانشگاه تهران، ایران
author
Zaher Hossein
Shomali
مؤسسه ژئوفیزیک دانشگاه تهران، ایران
author
text
article
1389
per
The Zagros Fold and Thrust Belt (ZFTB), a part of the Alpine-Himalayan mountain chain, is an orogenic response to the ongoing northward convergence of the Arabian plate towards the Iranian micro continent. This young and active deforming belt located in western and southwestern Iran is a remarkable place to study the processes occurring in convergence zones during early stages of continent collisions. From northeast to southwest, the tectonic units of the Zagros collision zone consist of 1) the Uromieh-Dokhtar Magmatic Arc (UDMA), 2) the Sanandaj-Sirjan Zone (SSZ), and 3) the ZFTB. Main Zagros Thrust (MZT), a suture between the Iranian and Arabian plates separates the ZFTB and SSZ units. Physical properties in an anisotropic media, in contrast to those in an isotropic media, depend on direction; that is, they vary as a function of orientation. Seismic anisotropy occurs when seismic waves propagate faster in one direction than another. The presence of seismic anisotropy in the upper mantle normally depends on the lattice-preferred orientation (LPO) of mineral crystals. Asthenospheric convection flow beneath continents and olivine mineral LPO are the main reasons for anisotropy in this part of mantle. Olivine crystals, as a dominant mineral in the upper mantle, tend to align with the mantle convection. Models obtained for Earth anisotropy show that anisotropy has an axis of cylindrical symmetry. Anisotropy with a horizontal axis of symmetry is called Horizontal Transverse Isotropy (HTI). In such a medium, there is no anisotropy in directions perpendicular to the symmetry axis. In this paper, azimuthal anisotropy was studied in the upper mantle beneath a profile across the Zagros (Zagros profile) to a depth of 460 km using teleseismic P-wave relative residuals. Fifty-six teleseismic earthquakes were selected with epicentral distances between 30 and 90 and with magnitudes greater than 5.5. The data were corrected for the effect of crustal structure before inversion. Using P-residuals (residual spheres), attempts were first made to group 66 seismic stations along the Zagros profile, based on the directional dependence of the data. The stations were divided into seven groups, and rose diagrams were constructed for these data confirmed the result of residual spheres. It is necessary to note that when two rays propagate in opposite directions along the same ray path, it is expected that they have similar relative travel times. Thus, subtracting 180 from back-azimuths between 180 and 360, they are mapped in back-azimuths between 0 and 180 and conduced to data augmentation. The relative residuals obtained were plotted related to back-azimuths beneath each station and then a 4-degree polynomial curve was fit to the data from tt = P0 + P1θ + P2 θ2 + P3 θ3 + P4 θ4, where is the arrival-time relative residuals (s), is the back-azimuth (degree), and P0, P1, P2, P3 and P4 are the curve coefficients for the 4-degree polynomial curve. The fast velocity direction is analogous to the minimum of relative residuals in the curve and vice versa; the maximum of the relative residuals is correlated with the slow axis of anisotropy. The results indicate that the orogen-parallel fast velocity direction (NW-SE) in the upper mantle beneath Central Iran and the UDMA change to orogen-normal (NE-SW) beneath ZFTB and SSZ.
Iranian Journal of Geophysics
انجمن ژئوفیزیک ایران
2008-336
4
v.
2
no.
1389
103
112
http://www.ijgeophysics.ir/article_40850_00ecd7a1e87289494cda4e2126bfe677.pdf