Tectonics
Mohammad Amir Alimi
Abstract
This paper presents the results of a field study aiming to describe and to interpret origin of the vein network in the south of Birjand. Adjacent to Birjand ophiolite in eastern Iran, the Paleocene-Eocene flysch facies was deposited in the Birjand foreland area concomitant with the Alpine orogeny (Laramide). ...
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This paper presents the results of a field study aiming to describe and to interpret origin of the vein network in the south of Birjand. Adjacent to Birjand ophiolite in eastern Iran, the Paleocene-Eocene flysch facies was deposited in the Birjand foreland area concomitant with the Alpine orogeny (Laramide). The sandstone unit of this facies contain two orthogonal sets of quartzite veins. The N310-340 striking veins (set 1) are arranged parallel to the Bagheran Kuh range front and perpendicular to the vein set 2 (N215-240). The paleostress reconstruction in the Paleocene-Eocene shows that the regional compression direction N240 is perpendicular to the Bagheran Kuh range front. Structurally, to create orthogonal veins, σ1 should be perpendicular to the layering and σ2 and σ3 should be horizontal. This situation was created in the middle Eocene-Oligocene. The post-collision extensions of this period caused a decrease in regional pressure in the region. As a resault, the maximum principal stress (σ1), was changed to a vertical state and intermediate stress (σ2) was parallel to the orogenic pressure. In these conditions, orthogonal tensile openings (state I) were formed as a result of the local inversion of stress and fluid pressure in the flysch facies.
Tectonics
Fereshteh Jafari-Hajati; Maryam Agh-Atabai; Majid Nemati; Ali Biglari-Fadafan
Abstract
In this research, to understand the present-day tectonic situation of Gorgan-plain in the east of the South Caspian basin, the tectonic stress regime of this area and its adjacent areas was estimated using the inversion of the data of the earthquake focal mechanisms. For this aim, in addition to analyzing ...
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In this research, to understand the present-day tectonic situation of Gorgan-plain in the east of the South Caspian basin, the tectonic stress regime of this area and its adjacent areas was estimated using the inversion of the data of the earthquake focal mechanisms. For this aim, in addition to analyzing focal mechanisms solved by different sources, the focal mechanisms were solved for several earthquakes with appropriate and sufficient information. The results indicate the presence of various mechanisms, including thrust, normal, and strike-slip, and as a result, various orientations of kinematic P and T axes, which can indicate the complexity of the region. By analyzing the earthquakes with a minimum magnitude of 4 in the Gorgan-plain, the Kopeh-Dagh in the east of the plain, and the entire eastern region of the South Caspian, a compression regime with a NNW-SSE trend was obtained. However, by analyzing all of the earthquakes (smaller and larger earthquakes) in the Gorgan-plain, a strike-slip regime was obtained, in which the trend of the P and T axes were calculated NE-SW and NW-SE, respectively. The results indicate a local strike-slip regime in addition to the regional compressional regime in the region.
Gh. Ghorbani Rostam; M. Pakzad Pakzad; N. Mirzaei
Abstract
There are two different seismotectonic zones in around of the Zendan- Minab- Palami (ZMP) fault system and the Oman Line, in south of Iran (Makran subduction zone in the east and Zagros collision zone in the west), which led to the complexity of this region. Since studying the stress field is important ...
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There are two different seismotectonic zones in around of the Zendan- Minab- Palami (ZMP) fault system and the Oman Line, in south of Iran (Makran subduction zone in the east and Zagros collision zone in the west), which led to the complexity of this region. Since studying the stress field is important for accurate perception from elastic features of environment, surveying the exerted the tectonic stresses to the tectonics plates and their magnitude, and description the geodynamic of this region, in this study considerd to assessment of stress field and also, maximum horizontal stress (SH) in around of ZMP fault system. To receive this purpose, amount and direction of stress is calculated by iterative joint inversion of earthquake focal mechanism. From east to west of ZMP fault system, with transition from Makran subduction to Zagros collision, direction of SH is reduced from 5.09º in east to 0.9º. To surveying the strain field, we used Global Positioning System (GPS) data. Maximum variance between velocity vector and direction of SH is determined in Bandar-Abbas (BABS) station, that located in adjacent of ZMP fault system. The friction coefficients which obtained in this study show that friction in Makran zone is more than Zagros zone.
M Abdetedal; Z.H Shomali; M.R Gheitanchi
Abstract
The Makran zone in southeastern Iran and southern Pakistan is the result of subduction of oceanic crust of the Arabian Plate under the Eurasian Plate. From seismic behavior point of view, there is a distinct segmentation between the western and eastern parts of the subduction zone. The western part of ...
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The Makran zone in southeastern Iran and southern Pakistan is the result of subduction of oceanic crust of the Arabian Plate under the Eurasian Plate. From seismic behavior point of view, there is a distinct segmentation between the western and eastern parts of the subduction zone. The western part of the Makran has an abnormally very low level of deep seismicity with lack of recorded great earthquakes, while the eastern part has experienced many great earthquakes. Another difference between the western and eastern parts of the Makran region is that the distance between the Quaternary volcanic arc and fore-arc setting is larger in the east than in the west. Understanding the nature of unusual behaviors of the Makran subduction zone has long been one of the biggest challenges in seismotectonic investigations of this region. The present study aims at producing high-resolution love-wave velocity structure maps of the crust and the upper mantle in the Makran subduction zone using ambient seismic noise. To achieve this purpose, a large dataset has been provided to produce tomographic maps. Empirical Green’s functions were obtained from cross-correlations of broad-band seismic noise records at different stations inside and outside the region. Love-wave velocity dispersion curves were then extracted from the ambient noise, and finally converted into a 2D group velocity image (or tomography map) for crustal and upper mantle structures of the region.
M Mirzaei; L Soheili; V Ebrahimzadeh Ardestani; A Teymorian Motlagh
Abstract
The main objective of interpretation of acquired gravity data on the Earth's surface is to determine the contrasts in density or shape/dimension of mass anomalies. Interpretation of gravity data can be done through an inversion process. In this research, a block model has been considered for the subsurface ...
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The main objective of interpretation of acquired gravity data on the Earth's surface is to determine the contrasts in density or shape/dimension of mass anomalies. Interpretation of gravity data can be done through an inversion process. In this research, a block model has been considered for the subsurface anomalous mass. By considering a constant initial density (about 2.6 gr/cm3) for all blocks and by using inversion method, distribution of density of the anomalous mass was estimated and interpreted. In this research, Occam method is used to invert 246 gravity data collected in 2007. Results of the gravity data inversion show sufficient fit between observed and calculated gravity data. Using this inversion method, distribution of density in the subsurface layers related to sediments and basement are estimated in this area. Since there is a density contrast between sedimentary layers and basement, the estimated density distribution can help to explore the lithology of formations as well as the discontinuities in them. Densities less than 2 gr/cm3 in horizontal and vertical sections obtained from the inversion are attributed to the alluviums. The depth of these sediments, which include sand, silt and clay of different percentages, is estimated to be less than about 200 m. Unequal density distribution along the layers is taken to indicate fractures. In fact, these fractures are associated with part of the Tabarteh fault in this area, which caused numerous earthquakes (but less than 5 Richters in magnitude) around the Arak and Dawood Abad cities in past years.
S Ansari; F Yaminifard; M Tatar
Abstract
In this study, 23 focal mechanisms were calculated by moment tensor inversion of small regionalـlocal earthquakes in the Western Alborz and surrounding areas using wave-form inversion method. Calculated reverse-fault focal mechanisms around the Khazar and Alborz Faults in the Central Alborz, considering ...
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In this study, 23 focal mechanisms were calculated by moment tensor inversion of small regionalـlocal earthquakes in the Western Alborz and surrounding areas using wave-form inversion method. Calculated reverse-fault focal mechanisms around the Khazar and Alborz Faults in the Central Alborz, considering southward-dipping nodal plane as the fault plane, are consistent with relatively low-angle thrusts. It also implies dominant compressionsl regime in the north of the Central Alborz. Focal mechanisms in 1990 Rudbar-Tarom Earthquake region show a combination of strike-slip mechanisms and a complex fault system in the middle of the Western Alborz. A major region of dominant strike-slip mechanisms is observed in the Talesh area, located in the west of the SouthCaspianBasin, and around the Masuleh, Sangavar and Bozqush faults. The only calculated focal mechanism close to the southern margin of the Western Alborz, considering the western part of the North Tehran fault as the fault nodal plane, implies leftـlateral motion in this area. In the south of the western AlborzMountains approaching the Central Iran, two calculated mechanisms indicate dominant reverse movement, similar to the 2002 Changoreh-Avaj Earthquake. Five focal solutions close to the Kushk-e Nosrat and Soltanieh Faults, considering these faults as the fault nodal planes, are consistent with right-lateral motion along them. Depth of the earthquakes in the studied region is in the range of 2 and 20 km, indicating the brittle upper crust in the region.
H Mardomi; B Oskooi
Abstract
During this project we tried to show high capability of the SIS scheme for 1D magnetotelluric (MT) sounding data inversion. The scheme transforms the nonlinear problem of estimating layer resistivities and thicknesses into a linear problem of estimating the coefficients of power series of the new response ...
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During this project we tried to show high capability of the SIS scheme for 1D magnetotelluric (MT) sounding data inversion. The scheme transforms the nonlinear problem of estimating layer resistivities and thicknesses into a linear problem of estimating the coefficients of power series of the new response function. We have used this scheme for prospecting conductive structures in southeastern shore of the Caspian Sea. This area is located in the GolestanProvince in northeastern part of Iran at the border with Turkmenistan. Detecting conductive sediments in this area, which are supposed to bear iodine is economically of interest. We provided data from three MT sites and inverted the data by the SIS scheme. According to our results there is a conductive layer in the depths deeper than 600 m at all three sites. We have compared our results with the previous results, also compared the results with the geological witnesses, which accordingly existence of a conductive layer has been proved at about 700 m depth. This conductive layer is proposed to bear iodine in economic meanings.
A. Nejati Kalate; V. Ebrahimzadeh Ardestani; E. Shahin; S. H. Motavalli Anbaran; Sh. Ghomi; E. Javan
Abstract
Determination of the geometry of bedrock, by nonlinear inverse modeling of gravity data, is the aim of this paper. In this method, reliable geological structures can be obtained by minimum geology priori information. The usual practice of inverting gravity anomalies of two-dimensional bodies replaced ...
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Determination of the geometry of bedrock, by nonlinear inverse modeling of gravity data, is the aim of this paper. In this method, reliable geological structures can be obtained by minimum geology priori information. The usual practice of inverting gravity anomalies of two-dimensional bodies replaced by n-sides polygon for determining location of the vertical that best explain the observed anomalies. In this method, the geometry of the bedrock is replaced by a series of juxtaposing prisms. Finally the length of each prism is the depth of the bedrock at that point.
The algorithm uses a nonlinear iterative procedure for simulation of bedrock geometry. At the first step, the nonlinear problem changes to a linear problem by a proper approximation and standard method. The second step is the parameterization of the model. Finally, an initial model is suggested on the basis of geological and geophysical assumption and using the numerical analysis, Jacobean matrix is calculated. In each iteration the inversion will improve the initial model, considering the differences between observed and calculated gravity anomalies, based on Levenberg-Marquardt's method.
The practical effectiveness of this method is demonstrated by inversion of synthetic (free noise and noise contaminated data) and real examples. The real data is acquired over the Moghan area and the results compared with the geological information.
A. Arzani; S.A. Alavi; M. Pooladzadeh; A.R. Golalzadeh; Z. Mehdipoor
Abstract
The area that will be discussed in this study is located between Zagros and Arabian structural zones in Iran in which full fold 3D seismic data has been acquired and interpreted using sophisticated techniques. This paper covers the role of structural sensitive seismic attributes including first and Second ...
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The area that will be discussed in this study is located between Zagros and Arabian structural zones in Iran in which full fold 3D seismic data has been acquired and interpreted using sophisticated techniques. This paper covers the role of structural sensitive seismic attributes including first and Second Derivative attributes, Azimuth attribute, Variance Cube attribute and Seismic Inversion attributes as an advanced stage of the interpretation program in delineating and resolving structural ambiguities in the mentioned field. The result of this study using integrating these attributes shows the interference structural features such as superimposed folding and structural basin in NW nose of the field of study. Hence it seems that an interference folding event to be most probably occurred between Zagros and Arabian structures and made a very gentle refolding belt. Complementary geophysical studies also show a reservoir of good potential in some parts of this belt that located in this field of study.
Y. Sharghi; F. Alinia; H. R. Siahkoohi
Abstract
Seismic travel time tomography uses seismic waves to image velocity distribution inside the ground. This method has been widely applied to practical applications in many fields including earth sciences. In this method, first-arrival times of seismic waves due to artificial sources, observed at receivers ...
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Seismic travel time tomography uses seismic waves to image velocity distribution inside the ground. This method has been widely applied to practical applications in many fields including earth sciences. In this method, first-arrival times of seismic waves due to artificial sources, observed at receivers in the vicinity of the exploration target are analyzed to obtain the velocity distribution within the propagation area. This paper describes the seismic tomography investigation performed at the abutments of Bakhtyari Dam site. The study aimed to find the low-velocity zones that may correspond to jointed media where seepage most likely happens. We performed 3-D tomographic inversion on Bakhtiari dam seismic data. To perform ray tracing and travel-time computation at each iteration, we used Um and Thurber (1987) method and for inversion step we used SIRT method. In most of the tomograms, average velocity was 3km/s. According to the Barton relation (2006) the seepage potential in dam sit abutments, is low. A low velocity zone has been detected above the 690 m level in right abutment. It shows that the weak zones extended from surface down to this level. Despite the presence of some weak zones, the rock mass quality is good, and the design of the grout curtain could help to bypass the reservoir water away from these seepage paths
M. R. Ghayamghamian; A. Fereydouni
Abstract
The shear wave velocity structure in Bam city is estimated in this paper. A double circle array observation of microtremors was conducted in south-east of Bam. Spatial Auto-correlation Coefficient (SPAC) method was utilized to determine phase velocity dispersion curve of Rayleigh wave in frequency range ...
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The shear wave velocity structure in Bam city is estimated in this paper. A double circle array observation of microtremors was conducted in south-east of Bam. Spatial Auto-correlation Coefficient (SPAC) method was utilized to determine phase velocity dispersion curve of Rayleigh wave in frequency range from 1.6 to 2.3 Hz. An inversion scheme based on Genetic Algorithm was employed to estimate the VS profile up to 400 m at the site under study. The inverted shear wave velocities are compared with the available down-hole data, and are found to be in good agreement. It verified the reliability of the estimated shear wave velocity profile and confirmed the use of SPAC method as a good alternative to the conventional approaches due to relatively low costs, ease of operation and analysis, accuracy, and ability to explore soil profile up to large depth.
A. Gholami; M. Javaherian
Abstract
In experimental sciences we often need to solve inverse problems. That is, we want to obtain information about the internal structure of a physical system from indirect noisy observations. Information about the errors in the observations is essential to solve ...
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In experimental sciences we often need to solve inverse problems. That is, we want to obtain information about the internal structure of a physical system from indirect noisy observations. Information about the errors in the observations is essential to solve any inverse problem; otherwise it is impossible to say when a feature ‘fits the data’. In practice, however, one seldom has a direct estimate of the data errors. Here, we exploit the trade-off between data prediction and model or data structure to determine model based estimates of the noise characteristics from a single realization of the data. Noise estimates are then used to characterize the set of reasonable models that fit the data. By intersecting set of prior model parameter constraints with the set of data fitting models, we obtain a set of models that fit the data and are in agreement with prior constraints. This prior information can also be used to set bounds on the bias. We illustrate our methods with a synthetic example of vertical seismic profiling (VSP).
