مجله ژئوفیزیک ایران
سال پانزدهم شماره 3 (پیاپی 52، پاییز 1400)

Convective systems are meso-scale atmospheric phenomena which their intensity of activity is determined by atmospheric static stability indices (convective indices). The effect of large-scale teleconnections on the increase or decrease in the potential for occurrence of convective systems in the atmosphere is of particular importance. Therefore, study of the climatological distribution of atmospheric stability indices along with teleconnections can be important in predicting convective systems. Given the lack of a comprehensive study of the relationship between the two phenomena, the purpose of this study is to investigate the climatological distribution of some atmospheric stability indices and their relationships with some teleconnections over the West Asia.In this study, first using JRA55 reanalysis data with a horizontal resolution of 1.25×1.25 degree during the period 1958 to 2018, four atmospheric stability indices including Convective Available Potential Energy (CAPE), K Index (KI), Lifted Index (LI) and Total Totals Index (TTI) were calculated. Then, in the first part of this study, the average seasonal distribution of these indices on the West Asian region in the range of 0 to 60 degrees north and 10 to 90 degrees east are presented. Next, in the second part of this study, the relationship between atmospheric stability indices and teleconnections is examined. For this purpose, by determining the critical phases of the four teleconnections, including the North Atlantic Oscillation (NAO), East Atlantic/West Russia (EA/WR), Indian Ocean Dipole (IOD) and Madden–Julian Oscillation (MJO), the difference in distribution of the average atmospheric stability indices for the positive and negative critical months of these teleconnections is investigated in spring season during the study period on the West Asia.The results of the first part of this study showed that the seasonal distribution of atmospheric stability indices on the study area is affected by the seasonal cycles of temperature and humidity associated with the ITCZ displacement. The results also showed that the maximum values of the stability indices occur in summer over lowland areas and the minimum ones in winter and highlands. The results of the second part showed that generally the area in which the atmospheric stability indices have significant values over the most parts of the study area decreases (increases) during the positive (negative) phases of the two northern indices of NAO and EA/WR, but increases (decreases) during the positive (negative) phases of the two southern indices IOD and MJO. The highest increase of CAPE index during the negative phase of NAO (EAWR) compared to its positive phase on the Oman Sea (East India) exceeds +250 (+450) J/kg and also during the positive phase of IOD (MJO) compared to its negative phase on Western India (Oman Sea) exceeds +450 (+600) J/kg.

Optically stimulated luminescence (OSL) dating directly gives the last time of the sediment exposure to light and zeroing of the luminescence signal inside the sediment. OSL has become a major tool for dating sediments over the past 40 years. This method has found a special place in Quaternary sciences in the world today and has vast applications in Iran. Its use to determine the time of occurrence of long-lasting earthquakes, fault slip rate, earthquake return period, past climatic periods including drought and wet periods, as well as determining the time of construction of ancient facilities and the growth of Iranian civilization, has expanded over time. Knowing the basics and having a strategy for site selection, choosing a location, and suitable sediment sampling method for luminescence dating have a significant effect on the age accuracy. Sampling from an inappropriate site, sampling of unsuitable materials or suitable but insufficient materials can thwart the laboratory's attempt to determine age. Every measurement begins with sampling, and sampling always contributes to the uncertainty of the measurement. Age uncertainty is the most important parameter that describes the quality of measurements. Sediment sample collection methods for luminescence dating can be divided into 3 categories: Sampling with tube, as a block or in a light bag. The most common method of sample collection for luminescence dating is to hammer a tube in the vertical surface of a sedimentary layer. If the layer is thin and the tube is too large for the layer and cannot sample the thin target; or either the grain size of the target unit is large to prevent hammering, or it is cement scale and sampling using a tube is impossible, there are two options: to manually collect sediment, or to extract a block.The higher the accuracy of dating, the more accurate the interpretations of age. Accurate sampling from the right place are the two main factors to obtain the exact age of the accident. Sampling strategies vary depending on the purpose, environment and location of the sampling. But when sampling, it is necessary to consider the relevant principles. These include the adequate knowledge of study hypotheses, source and history of sediments, presence of quartz and potassium feldspar in sediments, the role of light, the role of moisture, ensure of no biological disturbance in soil and sediment. And the presence of radioactive equilibrium around the sample. It is especially important to consider the adequacy of resetting of the luminescence signal, the ability to characterize the radioactive environment surrounding the sample (dose rate), and the lack of evidence for post-depositional mixing (bioturbation in soils and sediment).

One of the basic steps of oil exploration is to define the hydrocarbon zone. Different methods have been used so far for defining such zones. For a specific dataset, finding the most appropriate method leads to more accurate estimates and predictions of analysis besides improving the speed of calculations. Support Vector Machine (SVM), which is one of the methods for analyzing the data, uses kernel functions. It finds a better relationship between data factors and hydrocarbon zone leading to better estimates and classifications.    In this article, hydrocarbon zone detection has been done using seismic and well data.    The purpose of facies analysis is to obtain important petrophysical parameters of the reservoir and to identify heterogeneous boundaries below the ground. The results of the interpretation of petrophysical parameters are the input of the three-dimensional reservoir modeling process and through these parameters, the reservoir parameters are distributed in three-dimensional space. This model is widely used in various sections such as exploration and drilling of new wells, overdraft from a reservoir, determination of suitable areas for overdraft, reduction of drilling risk and risk, determination of reservoir lithology and identification of key well and its extension to other wells in the region. The most important petrophysical parameters are shale volume, porosity, permeability, reservoir fluid saturation and reservoir lithology.    The study of seismic facies has been started since the 90's and due to its importance and application in reservoir description, it has always been considered by many researchers.    To perform the analysis above, first, the hydrocarbon zones were spotted across the Asmari Formation using well logs and well geology reports. Next, the SVM method was used to detect each hydrocarbon zone using well logs. There was an acceptable agreement between the results of SVM method and well geology reports. Second, hydrocarbon zones detection was done using seismic data by SVM. At this stage, seismic attributes were extracted from the seismic trace in the well location. Then, covariance matrix and cross plots of seismic attributes used to identify the most effective attributes to hydrocarbon zones detection. In order to validate the results, the seismic attributes of another trace near the well location were used for hydrocarbon zone detection. SVM results matched hydrocarbon zones with low error.

Earthquake early warning parameters play an important role in issuing of a warning and reducing casualties caused by earthquakes. Site effects are one of the factors that affecting the values of earthquake early warning parameters (, , ,  and B). In this study, in order to investigate the site effects on the values of earthquake early warning parameters, 1830 earthquake accelerograms recorded at the six KiK-net accelerometery stations in Japan between 2008 and 2020, were used. These accelerograms were recorded at the surface and borehole stations. Short time Fourier transform (STFT) as well as deconvolution tool were used to remove the site effects from the accelerograms in the time-frequency domain. After removing the site effect from the waveforms of the surface stations, the values of all the earthquake early warning parameters, especially ,  and B, were improved and became closer to the values obtained from the bedrock. The results show that the early warning parameters  and  have the minimal influence from site effects and also behave on surface and borehole stations similarly. In general, the values of the frequency-dependent early warning parameters  and  increase after modifying the site effect on the vertical component. In contrast, the parameters , and  were reduced at all stations. After modification of the site effect, the difference between the calculated values of all parameters on the surface and bedrock are decreased, which indicates the improvement of the results of earthquake early warning systems in estimating the earthquake source parameters.

The probabilistic fault displacement hazard analysis method is one of the new methods in estimating the amount of possible displacement in the area at risk of causal fault rupture. In this study, using the probabilistic approach and earthquake method, the surface displacement of the North Tabriz fault has been investigated, and the probable displacement in different return periods has been estimated as contour maps. Assuming a strike-slip mechanism of the North Tabriz fault and earthquake method, to estimate the probability of displacement due to surface rupture, according to the surface ruptures caused by earthquakes of 1721 and 1780 North Tabriz fault, which were associated with 50 and 60 km of surface rupture respectively, a 50-60 km long section of North Tabriz fault was selected as the source of possible surface rupture. Due to a lack of data on large-scale earthquakes in northwestern Iran, the trace of North Tabriz fault is assumed to be a simple trace. This leads to a great epistemic uncertainty in the obtained possible displacement values    Owing to the passage of the North Tabriz fault through the residential area of Tabriz and destructive historical earthquakes, it is essential to estimate the possible future displacements of this fault. According to paleoseismic studies, probabilistic displacements were considered between zero to 4.5 and zero to 7.1 m, respectively. Using the paleoseismic studies and the catalog of historical earthquakes, the return period and the probable magnitude of the North Tabriz fault are 645 years and Mw~7.7. In the case of exceedance rate of 5% in 475 and 2475 years, the maximum displacement is estimated up to a distance of 70 and 100 meters from the site. The attenuation relationships used in this study were derived from the fitting of seismic data occurred in different parts of the world. To reduce the uncertainty in this hazard analysis and the values of possible displacement, the data of surface rupture of strike-slip earthquakes in Iran can be used to fit and obtain local attenuation relationships. In this research, considering the attenuation relationship of Petersen, the estimated maximum probability displacement of the North Tabriz fault at an exceedance rate of 5% in 50 years, for 4.5 and 7.1m displacements, is 186 cm. Moreover, the estimated maximum probability displacements in 475 and 2475 years are 469 cm and 655 cm, respectively.

Geophysical techniques have been successfully used by law enforcement agencies to locate graves and forensic evidences. Nonetheless, more controlled research is needed to better understand the applicability of this technology. Ground-Penetrating Radar (GPR) is a non-invasive geophysical method that uses radar pulses to image the subsurface. This method can be used in a variety of media, including rock, soil, ice, fresh water, pavements and structures. Different applications of GPR as a convenient geophysical tool have been studied in near-surface assessments for diverse media and targets. Snow is one of the low-loss medium and relatively suitable for GPR studies. Considering geophysical approaches in such environments, we will deliberate how body of a mountain climber was detected in snow due to avalanche occurrence. This kind of study is part of forensic geophysics. First step in such works is to estimate the position of the buried body by inspecting the place cautiously, which is different than routine works. Important points that assist in such cases are avalanche path, and rocks/hills and obstacles in the way after avalanche incident to mountaineer which react as a trap. Against the mountaineer body’s coverings which are formed by non-conductive materials, the body involves highly conductive textures. This phenomenon along with turbulences in snow layers occurred by the avalanche, increase complexity of these kind of studies. However, for investigation in places without avalanche incident and with more homogenous target, the procedure is simpler and straight forward. Looking at the data of mentioned in the case study, initially processed radargrams have no obvious sign of the buried body. Building a synthetic model based on environment and target properties can provide better vision for processing procedure. Therefore, besides forward modelling, some advanced methods were used. The applied advanced process created remarkable changes in radargrams especially when continuous wavelet transform (CWT) is used. It seems that application of some processing parameters leads to higher amplitudes in radargrams. Eventually, more apparent hyperbola related to the target, were appeared that helped to separate snowy layer from beneath rock. In this direction, excavation a trench and laying a survey inside of it which was a convenient place to conduct data acquisition, was helpful to find probable indications to mountaineer’s body location. At the end, excavations revealed the body of climber at the depth around 1m where substantiated the results achieved from advanced processes and interpreted radargrams. Overall, advanced processing approach along with commonly used processes can reap suitable results for data interpretation.

The Iran plateau is located in the seismic belt of the world. Sustainable development in seismic regions of the world depends on having a comprehensive bank of all destructive seismic historical events, seismic events of recent centuries and their careful analysis for the reliable design of important buildings and structures. Owing to the occurrence of historical and instrumental earthquakes in the Central Alborz region, this region is known as a high seismic active region.    In this study, the waveforms of all earthquakes recorded at existing seismic stations in Central Alborz were used in order to determine the location of earthquakes and their causative faults, accurately. The studies conducted in this area show that activity of faults in Tehran results in the occurrence of significant earthquakes. Furthermore, the calculated focal mechanisms are consistent with the geometry of the faults. The density of morphological evidence, features and landforms observed in the northern and northeastern regions were higher than the southern and northwestern regions. Moreover, accumulation of the epicenters in the northern and northeastern regions is more than the other areas. This indicates that in these areas, there are high tectonic activities which affects on the morphology of the areas strongly. The folds created between the faults indicate the compressive zone of the reverse faults and the remaining deposits indicate the existence of tension between the normal faults. Focal mechanism of high quality and reliable seismic events of the area show different compressive, strike-slip and normal mechanisms. Compressive and strike-slip mechanisms have the dominant trends in the region. Strike-slip components have a more pronounced characteristic than the other components or have always been seen alongside the other components. Small components of traction are also observed among the obtained mechanisms. This shows the existence of traction mechanisms in this area. Therefore, the effect of tensile stresses should be considered in future studies about the tectonic seismic regime of this area. Considering the seismic complexity of fundamental trends in Alborz region and a lack of common opinion among researchers about the focal mechanism of faults in the region, calculating the focal mechanisms of more seismic events can complement and confirm the results of this study. It provides more accurate interpretation of seismic characteristics and geomorphology of the region. The results of this study confirm the importance of proper station coverage in the region and the need to monitor seismic events. Installation of dense seismic networks, even temporarily and with longer deployment times around faults in the area, is necessary to collect more seismic data and record micro-earthquakes.

We performed a 3D teleseismic tomography to image the lithosphere and upper mantle structures in the northwest of the Zagros collision zone and the Iranian plateau. The Iranian Plateau is a high relief region that has formed as a result of continental collision between the Arabian plate and Eurasia in the latter part of the Cenozoic. The Zagros and Alborz active tectonic belts are situated on the southwestern and northern margins of the Plateau, respectively. Our aim was to investigate the lithospheric structure and the geometry of the subducted oceanic slab in a region encompassing the Zagros and Alborz mountain ranges in NW Iran. For this purpose teleseismic data recorded at two temporary networks in NW Iran and several stations of the Iranian permanent networks were used in the ACH tomography scheme developed by Aki et al. (1977). A total number of 8164 seismic rays where used in the tomography. Checkerboard synthetic tests were performed to insure that the tomography had adequate resolution power in order to have reliable interpretation of the results. Our seismic tomograms distinguish three major anomalous regions in terms of velocity variation with adequate resolution in the study area. They are: 1) a high velocity anomaly corresponding to the Zagros lithosphere, 2) two low velocity anomalies underneath the Sahand and Sabalan volcanos in NW Iran, 3) and a deep high velocity perturbation delineating position and geometry of the subducted oceanic slab in the upper mantle. Our results show the lithosphere beneath the Zagros Mountains has a thickness almost twice as that in central Iran and the Alborz Mountains. NW Iran shows no high velocity character at shallow depths, indicating a thin and possibly warm lithosphere. In NW Iran the lithosphere reaches its minimum thickness anywhere throughout the Iranian Plateau. Two low velocities in our model indicate anomalously warm crust beneath the Sahand and Sabalan volcanos. These crustal anomalies link with deeper low velocity regions in the lithosphere and shallow upper mantle. The presence of low velocities at this depth range in NW Iran can either be related to partial melting associated with the mantle wedge region above the subducted slab, or to the possibility of lithospheric delamination. We have traced the Tethyan slab down to 650 km depth in the central part of the model. A discontinuity in the structure of the subducted slab has been mapped in the depth range of 250 km at the base of the Zagros lithosphere, which can be the location of a slab detachment in the central part of the model. The location and depth of the velocity discontinuity point to a post-collisional and relatively young slab breakoff of 10-5 Ma age in NW Zagros

The Iranian Plateau is one of the most seismically active regions in the world, where the recurrence time of large-magnitude events is often more than a thousand years. The Mw 7.3, 1990 Rudbar earthquake, which caused 40,000 deaths and 500,000 homeless, and the Mw 6.5, 2003 Bam earthquake, which caused around 26,000 losses and 30,000 wounded, are two of the largest and the most destructive earthquakes in the region. Seismic hazard assessment is useful in the classification of areas that are more prone to earthquake losses. The earthquake occurrence rate is an important factor in seismic hazard analysis, which is commonly based on the earthquake catalogs. Hence, providing complete and reliable catalogs is necessary to achieve more accurate estimates. Unfortunately, factors such as the incompleteness of catalogs, the long-term recurrence time of large earthquakes, and the inadequate short-term instrumental record of about 100 years have resulted in unreliable earthquake occurrence rates estimates. To reduce uncertainties, some models have been developed for some places in the world such as California, Canada, Japan, New Zealand, and Italy based on the combination of various data as inputs, such as seismicity information, geological data such as fault slip rates, and geodetic information such as GPS data. The utilization of these models can increase the knowledge about the spatio-temporal distribution of earthquakes and reduce the uncertainty of results. The purpose of this study is to convert the strain rate into the earthquakes occurrence rate for some zones in Iran. Strain rates are derived from the available comprehensive deformation model of the Iranian Plateau, in which the long-term crustal flow of the Iranian Plateau is computed by using various data sets, including the latest fault traces, geologic fault offset rates, GPS velocities, principal stress directions, and velocity boundary conditions. In the comprehensive deformation model of the Iranian Plateau, based on the existing information on relative displacement of geologic features, the long-term geological offset rates for 33 of 171 fault traces were collected as input. Moreover, geodetic velocities of 239 GPS benchmarks were considered. Comparison with the results of the existing catalogs shows that for the whole Iranian Plateau, the occurrence rate based on strain is higher than the occurrence rate based on the catalog. We expect that utilizing the occurrence rate based on strain in the hazard model in further studies can have a significant effect on the ground motion parameters for Iran in comparison with previous catalog-based seismic hazard assessments.

Seas are one of the most important sources of protein in the world. For a long time, it was thought that the oceans are so vast that the amount of their pollutants could be ignored. Nowadays, seawater pollution is a very important issue due to its direct and indirect effects on human life.    Assuming the relatively similar behavior of important pollutants and their different levels of importance, nitrate pollution is considered as a pollutant indicator in the Caspian Sea and is modeled as a conservative parameter in this study. Although these initial assumptions may cause some errors in results, owing to the main purpose of the study which is to determine the amounts of the contribution of main sources of pollution (and not their absolute amounts), the assumptions are largely acceptable.By modeling different sources of pollutant, the contribution of these sources on spatial distribution of the pollutant in the Caspian Sea has been estimated. The main source of pollution in the Caspian Sea is the Volga River with the Kura River in the second place.    In order to estimate the contribution of different pollutants in the concentration of the pollution in the Caspian Sea, the main sources of pollution are separated and introduced to the model as individual contaminants. However, in conventional methods for simulating a type of contaminant from different sources that enters an aquatic environment, all sources of pollutants are treated in the same way. The modeling of different sources of pollution as different contaminants leads to the fact that superimposing the effects of contaminants emitted from different sources, estimates total concentration of the pollutant indicator in each region, and also, the contribution of each source in different regions can be easily identified. For this purpose, 10 points of pollution sources were considered. Each source has a contaminant with a different name and amount, but with the same characteristics. The result obtained from the advection and dispersion of these sources of pollution is analyzed later to estimate the total concentration of nitrate in the Caspian Sea, and also, to calculate the contribution of each source.Comparing the concentration of total nitrate of the observed data with the results of numerical model, it is shown that the model has a good accuracy in the Southern Caspian Sea. It seems that it is the first research that considers the contribution of each polluting source in the pollution of different regions of the Caspian Sea. So, the results can be used in other studies about pollution in the Caspian Sea.