Seismic wave velocity in the Tehran seismic network region
Mehdi
Rezapour
مؤسسه ژئوفیزیک دانشگاه تهران، استادیار
author
Ali
Mottaghi
مؤسسه ژئوفیزیک دانشگاه تهران، دانشجوی دکتری
author
text
article
2009
per
The velocity data of seismic waves in a region has an important role in locating occurred events. Information such as velocity ratio of VP/VS and velocity curves for Pg, Pn, Sg, and Sn phases directly affect the locating results. It is well known that the VP/VS ratio can be used to predict lithology, and that S-waves are sensitive to microcracks aligned by the present day stress field. Also, the variation of VP/VS could be used as a precursor of earthquakes.
To obtain seismic wave velocity, a travel time curve can be constructed by arranging observed records of ground motion due to the explosion or earthquake as a function of distance. On the other hand, the (local) slope of the travel time curves contain important information about the horizontal slowness, and thus about the wave speed, and also the zero offset time contains information about the layer thickness. It is clear that travel time curves cannot be expressed by a single formula, but only in so-called parametric form. In other words, we are encountering additional complications in the estimation of wave velocity, but the average estimation by using both travel times and arrival times can be useful to assess the velocity macro-variations in the study area. Thus, a significant body of research is based on the arrival times of first arriving. It should be noted that some uncertainties such as picking and location errors can cause a substantial scatter in the arrival time and travel time data, respectively. This is significantly unavoidable and unpredictable for small signal to noise ratio in the data (for instance when there is a small earthquake). Applying cross correlation techniques to phase picking and using a modern relative location method, apparently, are better ways to solve these problems.
In this research, the average VP/VS ratio of 1.727 is obtained using the arrival and travel time data of P- and S-waves. Comparison of obtained VP/VS value with other researches shows that there is not a significant difference. Also, local velocity curves for Pg, Pn, Sg, and Sn phases are obtained in the study area by using the data base 1996 through 2006. The slopes of these curves give crustal P and S velocities of 6.128±0.051 and 3.524±0.12 kms-1, and Moho P and S velocities of 8.083±0.082 and 4.752±0.08 kms-1 , respectively. This research shows that the Pn and Sn phases in the study area up to about 175 km are the first arrivals in comparison with Pg and Sg phases respectively.
Journal of the Earth and Space Physics
Institute of Geophysics, University of Tehran
2538-371X
35
v.
3
no.
2009
https://jesphys.ut.ac.ir/article_21431_0bf55f54585910008f73be3bf2071666.pdf
Processing and interpretation of airborne magnetic data for prospecting chromite deposits in the Sabzevar area
Abolghasem
Kamkar Rouhani
دانشگاه صنعتی شاهرود، استادیار
author
Majid
Beiki
دانشگاه Uppsala، دانشجوی دکتری
author
text
article
2009
per
In general, the airborne magnetic method compared to other airborne geophysical methods has more applications in prospecting and exploration for mineral deposits, and mainly leads to better results. Nowadays, geophysical prospecting for most mineral deposits, especially for metallic deposits such as base metals, iron and chromite, in large areas is often carried out using the airborne magnetic method. Three main stages in this method, like other geophysical methods, are: 1. data acquisition, 2. data processing, and 3. data interpretation. The data acquisition stage includes various methods, which can generally be classified in two categories: data acquisition using helicopters and data acquisition using aircraft. In the data processing stage, necessary corrections and also filters or transformations are applied on the data. The data interpretation stage includes different methods of interpretation, modeling and determination of earth parameters including depth of observed magnetic anomalies.
In order to prospect chromite deposits in the Sabzevar ophiolithic complex area, airborne magnetic data from the area acquired by an aircraft along a flight pattern with a flight interval of 1000 meters, have been processed and interpreted. The Sabzevar ophiolithic complex in the 1:100000 geological map or sheet of the area (provided by the Geological Survey of Iran) is seen in the north of the map as an extensive band elongated in an east-west direction with a length of approximately 300 kilometers. In this study, the geological sheet of the area with the scale of 1:100000 has been divided into 4 larger scale sheets (with the scale of 1:50000), and prospecting of chromite deposits has been made in each of these 4 sheets by tracing magnetic minerals associated with chromite, where chromite itself does not have a considerable magnetic property. Although geological and geochemical investigations of the area have already been made, there has been no attempt to carry out detailed processing and interpretation of airborne magnetic data from the area. This research work aims to prospect chromite deposits in the area by processing and interpretation of the acquired magnetic data. To fulfill this aim, in the first step, we have digitized all the magnetic data as well as the geological maps, geochemical data and other exploration information from the area.
In the primary processing stage, necessary corrections, such as parallax, leveling, microleveling and diurnal corrections have been made on the airborne magnetic data acquired from the Sabzevar area. Then, the total magnetic map of the area from the corrected magnetic data has been presented in which the Sabzevar ophiolithic complex has generally higher magnetic intensity in comparison with other parts of the area that are mainly covered by sediments and sedimentary rocks. Furthermore, various filters or transformations such as reduction to the pole, vertical first and second derivatives, analytical signal, upward continuation and downward continuation have been applied to the magnetic data, and as a result, magnetic anomalies have been detected. Applying downward and upward continuation filters on the magnetic data, we increase the detection of shallow and deep magnetic anomalies, respectively, and hence, these anomalies will be discriminated. The filter of reduction to the pole causes magnetic induction to be made vertically, and thus, the magnetic data are changed so that they have been acquired in the pole (i.e. in the magnetic latitude of 90 degrees). The detection and resolution of shallow magnetic anomalies are increased by applying vertical derivatives on the magnetic data. Finally, applying the analytical signal filter on the magnetic data, we determine the location and shape or geometry of the detected magnetic anomalies. To remove or considerably reduce regional effects from, and thus increase residual anomalies, on the magnetic data, we can use vertical derivative or downward continuation maps in which this condition has been provided.
Qualitative interpretations of the total magnetic and filtered magnetic maps indicate the existence and location of sedimentary basins in the area as they can be recognized by low magnetic relief zones. These sedimentary basins have been surrounded by high magnetic relief zones. Also, magnetic lineaments are determined as the boundaries between the zones having different magnetic relief. Geological faults are an example of such magnetic lineaments. Note that the existence of a magnetic lineament in a location does not necessarily indicate the existence of a geological fault in the location.
By accurately marking the locations of active chromite mines of the area on the total magnetic map (or other magnetic maps) of the area, we can easily observe that these active chromite mines have been located on the places where magnetic intensity is relatively moderate (or even relatively low). This indicates that these active chromite mines are not mainly situated on the places having high magnetite contents. Moreover, as a result of accurately studying the locations of the geochemical samples of chromite taken from the area, and investigating the results of chemical analysis of the chromite samples, and also considering the locations of the active chromite mines on the interpreted magnetic maps of the area, we have been able to obtain a suitable magnetic pattern or model from the area. The magnetic pattern, and integration of this magnetic pattern with geological and geochemical information, has led to the determining of 20 prospective chromite areas, which will be considered for the next exploration stages.
Journal of the Earth and Space Physics
Institute of Geophysics, University of Tehran
2538-371X
35
v.
3
no.
2009
https://jesphys.ut.ac.ir/article_21432_524bac37542a6bb70055a14fe7d613de.pdf
The effect of pore geometry on seismic wave velocities in carbonate rocks from hydrocarbon reservoirs
Jafar
Vali
پژوهشگاه صنعت نفت، مربی
author
Ezatollah
Kazemzadeh
پژوهشگاه صنعت نفت، استادیار
author
hesam
Aloki Bakhtiari
پژوهشگاه صنعت نفت، مربی
author
Mohammad Reza
Esfahani
پژوهشگاه صنعت نفت، مربی
author
text
article
2009
per
The goal of this laboratory study is to investigate the effect of pore shapes on seismic wave velocities in carbonate rocks under reservoir conditions. In this research, 41 core plugs of carbonate rocks from oil fields of the southwest of Iran were prepared. The compressional and shear wave velocities were measured in both dry and brine saturated samples under several pressures especially reservoir pressure. The results from XRD and thin section studies were used to determine minerals, features and pore types of the samples. The cross plots of velocity versus porosity, density, pressure, mineralogy, and especially pore types have been investigated. These cross plots showed that the pore shapes are the main reason for variation in velocities and dispersion of the data points, so that for a constant porosity the variance of elastic wave velocity is about 1500 ms-1 and also the variance of porosity is about 20 percentage for a constant velocity. The velocity is higher in samples with vuggy porosity and lower in samples with small size vuggy porosity than the velocity determined from the time average equation.
Introduction: The parameters which have effect on velocity are divided into two classes. The first class are the parameters that are related to the natural character of the rock, lithology and rock physics such as porosity, pore type, grain size and a combination of them. The second class is the parameters which are affined to depositional environment and they are not physically related to rock structures. These parameters are such as depth of burial, confining stress and age of deposition.
The effect of rock properties, such as porosity, type of porosity, minerals and pressure on P and S wave velocities are investigated by laboratory measurements of compressional and shear wave velocities for both dry and brine saturated rock under different confining pressure.
The effect of pore shapes has been either overleaped or not suitably used in theoretical equations, therefore seismic inversion analysis, AVO and pore volume calculations, which are based on these equations are highly inconclusive.
Pore types were classified into five groups by Anselmity and Eberli (1993), which are inter crystalline and interparticle porosity, micro porosity, moldic porosity, inter grain porosity and low porosity with high cementation. They also studied the effect of pore types on velocity measurements.
Burial depth, compressibility, saturation, wettability, hysteresis of saturation and frequency of wave velocity are other factors which have an effect on velocity.
In this paper, first the factors affecting seismic wave velocity in carbonate rocks were reviewed and then flow work of this study consisting of preparation of samples, determination of pethrophysical properties (porosity, permeability) and compressional and shear wave velocities measurements were performed. Seismic wave velocity performed on 41 dry and brine saturated core plug samples under reservoir temperature and pressure. The diameters of the plugs were 3.7 centimeter for 34 samples and 5 centimeter for 7 samples.
Effective factors on seismic wave velocities in carbonate rocks Porosity: Porosity is one of the important parameters that has an effect on velocity, so that normally with an increase of porosity the velocity is decreased. Prediction of porosity just from seismic velocity is difficult, because in carbonate rocks seismic wave velocity is dependent on too many other parameters.
Minerals: Though the velocity differs in different minerals, the type of mineral is not the main factor that controls velocity in carbonate rocks.
Density: Generally a direct relation between density and velocity is found but there isn't any experimental equation for the relation between density and velocity. Therefore to increase certainty coefficient, laboratory measurements and determination of the relation between density and velocity are necessary.
Pressure: Pressure is one of the important factors that controls velocity in fractured rocks, which are brine saturated. Porosity reduction and better contact of grains in rock is the reason for the increase in velocity by the increase in pressure.
Pore geometry: The results of microscopic studies by Anselmiti and Eberli (1993) showed five different pore geometries in carbonate reservoir rock. They analyzed different types of pore geometries and compared data points in velocity-porosity cross plot with time average equation.
Flow work: Flow work consisted of plugging, cleaning, petrophysical properties and acoustic velocity measurements.
Conventional core properties such as porosity, permeability and grain density provide the fundamental data set for well bore and reservoir characterization.
The core plugs were cleaned to remove residual hydrocarbons, formation brine, salts and other contaminants by using toluene and then they were dried in a conventional oven.
Porosity and grain density of samples were measured under ambient conditions using helium expansion and the application of Boyles’s law to quantify grain volume by Ultraporosimeter 200A.
Air permeability was measured in ambient conditions by Ultrapermeameter, which uses the Darcy equation to calculate air permeability.
Acoustic velocity was measured in dry and brine saturated samples in reservoir temperature and pressure from 4400 psi to 800 psi by non equal steps.
Laboratory study of factors that control seismic wave velocity:The data points in the porosity- velocity cross plot, which resulted from laboratory measurements, were too scattered. Pore shape and cementation of pores are the reason for this scattering. The density-velocity cross plot shows that seismic wave velocity isn't controlled by the type of minerals. In all states mentioned above, the variability of velocity vs pressure is followed by V=a*Pb.
Results: * In carbonate rocks acoustic wave velocity is dependant on some parameters such as diageneous, mineralogy, pore structure, type of fluid, pressure, temperature and also in no dense carbonate the wave velocity is related to grain to matrix ratio, shape, size and sorting of grains.
* Cross plot of velocity versus porosity showed that for a constant porosity the variance of velocity is about 1500 ms-1 and also for a constant velocity the variance of porosity is about 20 percentage.
* The velocity is higher in samples with vuggy porosity and lower in samples with small size vug porosity than the velocity determined from the time average equation.
* The test results for non visible vuggy samples show negative deviation from the time average equation for calcite and dolomite, and they have lower velocity than vuggy and small size vuggy porosity samples.
* The density - velocity cross plot showed that the effect of the type of minerals to control elastic properties is negligible.
* Pore shape is the main factor which causes scattering of data points in velocity-porosity cross plot for carbonate rocks.
Journal of the Earth and Space Physics
Institute of Geophysics, University of Tehran
2538-371X
35
v.
3
no.
2009
https://jesphys.ut.ac.ir/article_21433_d900edb26fe5dc531ff3e2b831c083dd.pdf
Using relation figures of horizontal and vertical gradient in quantitative interpretation of gravity data
Mohammad Ali
Ahmadi
مؤسسه ژئوفیزیک دانشگاه تهران، دانشجوی کارشناسی ارشد
author
Vahid
Ebrahimzade Ardestani
مؤسسه ژئوفیزیک دانشگاه تهران، دانشیار
author
text
article
2009
per
In this paper, it has been shown that there is a special relation between horizontal and vertical gradients of gravity data and the vertical and horizontal components of magnetic data for some of 2-D sources. It has been shown that the Hilbert transform is useful in calculating the vertical gradient of gravity and magnetic anomalies from the horizontal gradient for transformation of gravity and magnetic anomalies and to estimate the parameters of sources.
The plot of horizontal component versus vertical component in Cartesian coordinates is named the relation figure which is introduced by Werner (1953) as the polar plot of the vertical and the horizontal component of the field. The relation figure is used for qualitative interpretation of source parameters.
We have shown the relation figures by plotting the horizontal gradient of gravity versus its Hilbert transform. We have presented here the properties of relation figures of some two- dimensional models of simple geometry such as; thick dike, dip step, vertical step, horizontal step. The relation figures for these models are found to be ellipse or circle with different properties. In the first step, these properties may be used to distinguish whether the source is a dike or other model and then depth, width (for dike), dip, and radius (for horizontal cylinder) of the models. Finally, synthetic and real data is examined. Real data has been measured institute geophysics of Tehran University. There are some differences in the results of the synthetic and real data which are interesting and noticeable. In conclusion these differences and their reasons have been explained.
Journal of the Earth and Space Physics
Institute of Geophysics, University of Tehran
2538-371X
35
v.
3
no.
2009
https://jesphys.ut.ac.ir/article_21434_0d592cdbb7defc2dd3f8e6a241480922.pdf
Estimating depth and model type due finite step, thin and thick dikes using the continuous wavelet transform
Arezo
Gholghasi
، دانشگاه آزاد اسلامی ، واحد علوم و تحقیقات، دانشجوی دکتری
author
Hossein
Zomorodian
دانشگاه آزاد اسلامی ، واحد علوم و تحقیقات، استاد
author
mohsen
Ovaisi
دانشگاه رازی کرمانشاه، استادیار
author
Mohammad
Saeidi
دانشگاه آزاد اسلامی ، واحد علوم و تحقیقات، دانشجوی دکتری
author
text
article
2009
per
The continuous wavelet transform has been proposed recently for the interpretation of potential field anomalies. Using Poisson wavelets, which are equivalent to an upward continuation of the analytic signal, this technique allows us to estimate the depth of the buried homogeneous field source and to determine the nature of the source in the form of a structural index.
The interpretation of potential field data is not a straightforward process because of the many models capable of explaining the observed field. For this reason, the interpretation method selected must relate to the geology. Furthermore, if we wish to design an automatic interpretation technique that is to be applied over a large area with many anomalies, we prefer the model to be as simple as possible.
We propose an alternative technique that estimates the depth and the structural index from the ratio of wavelets of successive orders. We show how to interpret these results over multiple (non homogeneous) sources, such as a finite step, and thin and thick dikes. This method is an extension of the technique of Hsu et al. (1998) and Sailhac et al. (2000), who both consider cases of extended sources with one finite dimension such as thin dikes of finite depth extent and thick dike models.
Summary of the implementation: We propose an implementation of this algorithm that includes the following steps:
1) Start with a magnetic field profile.
2) Compute the horizontal and vertical derivatives.
3) Compute the analytic signal of orders 0 and 1 and normalize by the dilations to obtain the wavelet transforms.
4) Search for the maxima along the wavelet coefficient profiles and find the associated maxima for successive heights.
5) Apply equations to estimate the depth and structural index from different pairs of dilations.
6) Select the best estimates for the variation of the estimates with dilations.
In this research, first, we use technique over multipole sources, such as a finite step thin and thick dikes. Then we test the proposed technique on a profile data in area located in Khoramabad in Iran. The anomaly on this profile has a structural index 0.1 which indicates a step in 1120 m depth. At the end this method was compared with the Euler Deconvolution method.
Both Euler and wavelet techniques give satisfactory results, but the wavelet approach is preferable for three reasons : (1) it allows the estimation of both depths zo and structural index N in a simple manner without the need primary information; (2) it includes an upward continuation procedure which provides useful symmetries between the wavelet transform domain and buried sources , and also reduces high-frequency noise present in the data which may produce strong artifacts in Euler Deconvolution; and (3) the wavelet approach simplifies the characterization of sources of finite extent.
Journal of the Earth and Space Physics
Institute of Geophysics, University of Tehran
2538-371X
35
v.
3
no.
2009
https://jesphys.ut.ac.ir/article_21435_5d013e3462a7ab453c66e7e70e65c70a.pdf
The seismotectonic model of main recent fault between 33 and 35°N
Reza
Heidari
دانشگاه آزاد اسلامی- واحد علوم و تحقیقات، دانشجوی دکتری
author
Norbakhsh
Mirzaei
مؤسسه ژئوفیزیک دانشگاه تهران، دانشیار
author
text
article
2009
per
The Zagros fold- thrust belt as a part of Alpine- Himalayan orogenic belt, is one of the most active continental collision zones on the earth, which extends from the Tarus mountains in south eastern Turkey to the Minab fault in the east of the Strait of Hormoz in southern Iran. Structurally, its formation is related to the continuing convergent movement between the Arabian plate to the southwest and the Central Iranian Microcontinent to the northeast, resulting from the north- northeastward drift of Afro- Arabia against Eurasia.
The northeastern boundary of the Zagros coincides with the Main Zagros Reverse Fault and the Main Recent Fault. The Main Zagros Reverse Fault has a NW- SE strike from western Iran to the area north of Bandar Abbas. To the northwest, the boundary feature consists of a series of right-lateral strike-slip faults called the Main Recent Fault. The Main Recent Fault is a major structure broadly parallel but quite distinct from and younger than the Main Zagros Reverse Fault which transects it in several places. Earthquakes of larger magnitudes mostly nucleate along different segments of the Main Recent Fault with a prominent northwest trending right-lateral strike-slip mechanism along the northeast margin of the Zagros ( For example, the Silakhor earthquake of 23 January 1909, Ms=7.4, on Dorud Fault segment, is the largest event recorded in the Zagros).
The most recent tectonic deformation, and in particular the seismicity along the MRF between latitudes 33 and 35ْ N, is summarized in the context of the seismotectonic history of the region. The relation between the seismicity and the individual fault segments forming the MRF is studied and interpreted in terms of a continuing right-lateral strike- slip deformation. The Main Recent Fault is not a single structure but a narrow zone formed by a succession of individual fault segments, often arranged in a right-lateral en-echelon pattern. Thus in the southeast, the subsidence of the Silakhor Valley, probably a consequence of Quaternary right-lateral movements on the Dorud fault and on small subparallel faults, was renewed during the 1909 and 2006 earthquakes. Near the center of the region, the Nahavand plain is similarly limited by two strike-slip faults, the Nahavand and Garun Faults, both of which were reactivated, at least along their northern sections, during the 1958/08/16 (Ms=6.6) and 1963/03/24 (Ms=5.8) earthquakes. In the northwest, the Sahneh Fault with a long history of seismic activity (the Dinavar earthquakes of 1008, Ms=7.0; 1107, Ms=6.5; and 2002/04/24 and 2002/12/24, by Mw=5.4 and Mw=5.2 respectively is characterized by its exceptional direction, which is at about 20ْ to the other faults studied here, but contained in a region limited by the extensions of the Nahavand and Morvarid Faults. Practically all these segments fall into the three categories of Riedel shears, P shears and tension structures. The Riedels which were formed during the first stages of the deformation are represented by the Dorud, Nahavand and the Morvarid Faults. The P shears, which were formed at a later stage in the structural evolution, are represented mainly by the three sections of the Sahneh Fault and Ghilabad Fault. The Sahneh Fault is a good example of Positive Flower Structure (palm-tree structure) on the MRF. The tension structures, of which the Qaleh Hatam Fault, and the subsidence of the Silakhor Valley and Nahavand plain are the best examples, are much shorter and less numerous, and seem to be located near the intersections of the Riedels and P shears. The partitioning of oblique regional convergence into effectively pure thrusting and pure strike-slip is the most likely explanation for the different mechanisms of the 2002 mainshocks on the Sahneh fault.
Journal of the Earth and Space Physics
Institute of Geophysics, University of Tehran
2538-371X
35
v.
3
no.
2009
https://jesphys.ut.ac.ir/article_21436_b0ba65914eb896017235bcfef3ffd14a.pdf
Numerical study of convective cloud development using Explicit Time-dependent Tilting cloud Model (ETTM)
Maryam
gharaylo
مؤسسه ژئوفیزیک دانشگاه تهران، دانشجوی دکتری
author
Abbas Ali
Ali Akbari Bidokhti
مؤسسه ژئوفیزیک دانشگاه تهران، استاد
author
Majid
Mazrae Farahani
مؤسسه ژئوفیزیک دانشگاه تهران، استادیار
author
text
article
2009
per
Cumulus parameterization in numerical weather prediction models can significantly affect severe weather forecasts, such as hurricanes, flash floods, and winter storms. The role of convection is essential in stabilizing an unstable atmosphere through vertically transferring moisture, energy, chemical species and momentum. Cumulus parameterization schemes use simple one-dimensional convective cloud models to represent convection in the vertical direction. The cloud model is a fundamental determinant of vertical mass flux, heating and drying profiles, and precipitation rate.
The research presented in this paper is based on the cloud model developed by Chen and Sun (2004). This Explicit Time-dependent Tilting cloud Model (ETTM) features detailed processes for an updraft and a downdraft, both governed by the same dynamic and thermodynamic equations. The updraft is initiated with a thermal bubble, while the downdraft is maintained by evaporative cooling and the drag force of precipitation. Both up- and down-downdrafts employ non-hydrostatic pressure, entrainment, cloud microphysics, and lateral and vertical eddy mixing. A tilting angle for the cloud is specified to separate a portion of the downdraft from the updraft cell to account for vertical wind shear.
Since the ETTM described by Chen and Sun (2004) is not available as a community code, a slightly different algorithm was developed independently. The ETTM coordinate system is an axis-symmetric cylindrical with a constant radius mapped on where r is radius, ? is tangential angle, and Z is height in tilting coordinates. Prognosed variables are; vertical velocity, ice equivalent potential temperature, mixing ratios of water classes including cloud water, water vapor, ice water, rain water, snow and graupel.
The main purpose of this research is to examine the simulation of the development of a cumulus cloud by ETTM. An idealized sounding is used for environmental temperature, relative humidity and pressure. This sounding was measured on 20 May 1997 over Del City, Oklahoma during a storm. Time-stepping is determined according to Courant-Fredrich-Lewy (CFL) criteria, here we use 1s time-stepping. Vertical resolution is set to 500 m for each of the 34 vertical levels, placing the top of the domain at 17 km. The model is integrated for 70 minutes. ETTM also requires input for the radius and tilting angle of the up- and down-draft cells, these are based on the 3-D simulations with a mesoscale model simulation. The radius of the updraft and of the downdraft are set to 4000 and 1600 m respectively (radius of the downdraft being 40% of the updraft as described above). The tilting angle is set to 11.2?. In the model the effect of vertical diffusion and also non hydrostatic pressure gradient force are included. The governing equations of our model are exactly the same as those in Chen and Sun (2004). Results show that the ETTM is able to simulate a complete lifecycle for a cloud cell, featuring comparable zones of maximum vertical velocity, and overshooting layers on the cloud top and that this model can confidently be used in cumulus parameterization.
Journal of the Earth and Space Physics
Institute of Geophysics, University of Tehran
2538-371X
35
v.
3
no.
2009
https://jesphys.ut.ac.ir/article_21437_d9b8e3eb4694c9b84e36a8388714fb30.pdf
Study of variations of critical frequency of ionospheric F2-region as a temporal precursor for the Qom earthquake
Sarmad
Ghader
مؤسسه ژئوفیزیک دانشگاه تهران، استادیار
author
Nazli
Saba
مؤسسه ژئوفیزیک دانشگاه تهران، کارشناس
author
text
article
2009
per
Many recent researches have been devoted to investigating the connection between seismic activity and ionospheric variations. It has been shown that the behavior of characteristic parameters of the ionosphere can be used as temporal and spatial precursors of strong earthquakes ( ).
In the present work variations of critical frequency of ionospheric F2 layer as a temporal precursor for the Qom earthquake are studied. Ionospheric data measured by vertical sounding technique at the Institute of Geophysics, University of Tehran (in Iran) are used to perform the analysis. The deviation of critical frequency of the F2 layer is the key parameter which is used to do the analysis.
To discriminate ionospheric precursors due to seismic activity from other causes of ionospheric variations, we also calculated the deviation of critical frequency of the F2 layer for some magnetic storm days and quiet days. It can be seen that during the quiet days the negative phase (negative value of the deviation of critical frequency of the F2 layer) is dominant and during the magnetic storm days the positive phase (positive value of the deviation of critical frequency of the F2 layer) is dominant. But two days before the earthquake the pattern of anomalies for the deviation of critical frequency of the ionospheric F2 layer is different. In this case the negative and positive phases are both present. This finding is in agreement with existing researches and shows the possibility of using ionospheric variations (measured by vertical sounding) as a temporal precursor for strong earthquakes.
Journal of the Earth and Space Physics
Institute of Geophysics, University of Tehran
2538-371X
35
v.
3
no.
2009
https://jesphys.ut.ac.ir/article_21438_8a3c0f3c9355f9c5a9f70b03c818f2db.pdf
Focal mechanisms of Mw 6.5, March 31. 2006 Iran- Silakhor earthquake
using data from the Iranian Seismic Network
Mohammad Reza
Hatami
مؤسسه ژئوفیزیک دانشگاه تهران، استادیار
author
Zaher Hossein
Shomali
مؤسسه ژئوفیزیک دانشگاه تهران، استادیار
author
Gholam
Javan Doloei
پژوهشگاه بینالمللی زلزلهشناسی و مهندسی زلزله، استادیار
author
text
article
2009
per
The source parameters and focal depth of the destructive Mw 6.5, 31 March. 2006 Iran- Silakhor earthquake, the two largest foreshocks and a major aftershock are determined using broadband data from a seismic network in Iran based on a time-domain linear moment tensor inversion. The earthquakes occurred in the Zagros Suture zone in western Iran. Waveform inversion of the selected events estimated focal depths in the range of 15–20km which limits the brittle–ductile transition zone beneath parts of the studied area. The dominant double-couple part of the moment tensor solutions for most of the events indicates that the analysis is working reasonably well and that the amplitudes at the individual stations are not too disturbed by local structure or noise. Re-evaluation of the analyzed events shows predominantly right-lateral strike-slip faulting consistent with the relative motions of the significant faulting in the region.
Journal of the Earth and Space Physics
Institute of Geophysics, University of Tehran
2538-371X
35
v.
3
no.
2009
https://jesphys.ut.ac.ir/article_21439_d36d797b101b5b35c4b9317dd85785be.pdf
Modeling of bedrock topography in an urban area through micro-gravity data
Vahid
Ebrahimzade Ardestani
مؤسسه ژئوفیزیک دانشگاه تهران، دانشیار
author
text
article
2009
per
Micro gravity data was collected in a crowded street in Tehran where a subway terrain tunnel is to be excavated. The data is corrected for the gravity effects of the surrounding densely built up area with diverse buildings and the Bouguer gravity anomalies are computed. The residual gravity anomalies are prepared by removing a trend surface. Upward continuation is used to remove the gravity effects of shallow synthetic anomalies such as subsurface galleries and canals. Then the up-ward residual gravity anomalies are used to determine the depth of the bedrock (Hezar Darreh formation). A 2D inversion algorithm is applied on the data and the model is constrained based on prior geology information provided from the bore holes.
Journal of the Earth and Space Physics
Institute of Geophysics, University of Tehran
2538-371X
35
v.
3
no.
2009
https://jesphys.ut.ac.ir/article_21440_a0677e97e2436e39f90ebd12057c1344.pdf