IRANIAN JOURNAL OF GEOPHYSICS
Year:2016 | Volume:10 | Issue:4 |Papers Count:13

A thorough appreciation of the dynamic effects of bottom friction on water mass flow requires an understanding of the boundary layer characteristics, known as Ekman layer. Ekman layer on a sloping bed includes upslope or downslope flows that may intensify mixing or change the thickness of the Ekman boundary layer. The bottom friction can also reduce the fluid current, thereby reducing the Coriolis force and destroying the geostrophic current, named as the process of spin down. This study is conducted to investigate the impact of bottom slope on the distribution of physical parameters of water and the resulting phenomena, in order to identify the characteristics of the Ekman layer on the bottom slopes of the western Strait of Hormuz. The research utilizes the CTD (conductivity, temperature, and depth) field data of the western Strait of Hormuz in 2005 gathered by the National Institute of Oceanography of Iran. The vertical profiles of temperature, salinity, density, tidal currents and the horizontal cross sections of density were made by using Excel and MATLAB software. The results indicate that the current on the bottom of Strait of Hormuz is of an upslope type, and the pycnoclines become almost perpendicular to the slope with the increase of slope of the bottom. The minimum Ekman flux spin down is estimated of two hr and is related to a station with the maximum slope. Greater bottom slope and stronger stratification are found to remarkably shorten the shutdown timescale of the Ekman layer. We estimated the eddy diffusivity, v k, between zero and 0/005 m2s-1. A positive correlation was also found between eddy diffusion coefficient v k and Ekman layer parameters, which is indicative of the non-stationary flow and mixing due to Ekman layer stoppage. The horizontal scale that characterizes the dimension of a boundary current is the Rossby radius of deformation. Since baroclinic flow involves a number of internal modes, there will be a Rossby radius of deformation for each mode. The average Rossby radius of deformation in the south of the island for the first baroclinic mode is estimated at about 5. 6 km, and buoyancy frequency in the Strait of Hormuz is 0. 02 ( s 1 ). Spin down time ( s t ) of the stations located in the south of Qeshm for the first baroclinic and barotropic modes were also calculated. Results show that the greater the depth of the boundary layer on the bottom, the larger the spin down time for the first baroclinic mode. When Ekman layer is arrested in the barotropic mode, it seems that the spin down time for the first baroclinic mode becomes very large; thus we can conclude that the spin down does not happen in the arrested Ekman layer. Typical thickness of the Ekman layer in the Strait of Hormuz varies between 0. 5 and 22 meters. Since the most important driving force of water up the slope is the tide, the tidal level changes about the long mean for the periods of ten years, one year, one month and one day are examined in the area. The results show that in the northern Strait of Hormuz, the maximum tidal range is about 4. 73 meters and the average neap and spring tides are 2. 15 and 3. 53 meters, respectively. The calculated tidal currents and the related profiles for all the stations indicate that the maximum tidal speed is seen at the top of the bottom mixed layer. As the bottom is approached the speed of tidal current decreases remarkably and turbulence dominates which represent the state of the surface layer.

Air temperature is one of the principal climate variables with important direct and indirect socio-economic impacts. Among different factors affecting the climate variability of different regions, the low-frequency atmospheric phenomena have attracted the attention of climatologists because of their role in the fluctuations of the climate system in time scales on the order of years and decades. Since these phenomena affect the atmospheric conditions of regions far from their sources, they are called teleconnection patterns. Teleconnection patterns cause large-scale changes in the atmospheric circulation and affect air temperature, precipitation, storm tracks and the position and intensity of jet streams. Therefore, they are of great significance for determining regional climate anomalies. In this study, anomalies in the mean winter surface (2 m level) temperature of Iran are investigated for the period 1950 to 2010. Winter of each year is defined as January and February of that year and December of the previous year. The study is conducted in three different scales, including Iran as the whole, six different regions of Iran separated according to three latitude and two longitude bands and the grid scale, using the monthly mean NCEP/NCAR reanalysis data. First, the reanalysis data of temperature were quality controlled, and years before 1967 were excluded from the study because of the poor quality of the data. Statistical analysis showed that there are significant positive correlations between the mean winter temperatures of different regions and between the mean winter temperature of each region and that averaged over Iran. The largest positive temperature anomalies in the 44 winters occurred in 1970, 1979, 1999 and 2010 and the largest negative anomalies occurred in the winters of 1972, 1973, 1989 and 2008. Analysis of the mean winter geopotential height fields at different pressure levels in a domain covering 10– 60 ° N and 10° W – 80 ° E showed that there is a center of positive (negative) height anomaly over North-West Europe (North-East Atlantic) and a center of negative (positive) height anomaly over North-East Caspian Sea when the mean winter surface air temperature of Iran is anomalously low (high). The correlation coefficient is higher for upper atmospheric levels, and stays significant at 95% confidence level down to the 700 hPa level. In this regard, a new teleconnection index is introduced in the paper, to study the mean winter temperature anomalies in Iran, calculated as the mean winter 250hPa geopotential height anomaly over 50– 60° E, 40– 50° N minus that over 0– 10 ° E, 45– 55 ° N. This resembles a teleconnection pattern which is similar to that of the East Atlantic/West Russia (EA/WR: Barnston and Livezey, 1987; Krichak et al., 2012) and the North Sea– Caspian Pattern (NCP; Kutil and Benaroch, 2002). The new index has a significant correlation with the EA/WR, and particularly with the NCP index. We found a significant positive correlation between the average winter temperature in different regions of Iran and the introduced index which was much higher than those between the mean winter temperature of Iran and both EA/WR and NCP teleconnection indices. Comparison between the temperature and geopotential height anomalies at different levels reveals that the middle troposphere temperature is anomalously high (low) in winters that geopotential height is positively (negatively) anomalous. Also, the wind field shows a positive (negative) anomaly over the Caspian Sea and a negative (positive) anomaly over Europe when the mean winter air temperature in Iran is higher (lower) than normal.

Identification of periodic droughts can provide a scientific tool for predicting the occurrence of this environmental risk. One of the most important methods that can be used for detection of periodic droughts is the spectral analysis or frequency-domain analysis. The purpose of this study is to detect the periodic behavior of Iran’ s monthly droughts. To this aim, use has been made of the monthly rainfall statistics of 41 synoptic stations for a period of 31 years (1983– 2013) obtained from the IR of Iran Meteorological Organization (IRIMO). First, to identify considerable differences in droughts, the “ effective drought index” (EDI) was employed. Assigning a numerical value to each rainfall event on a monthly time scale is the main objective of the EDI in order to compare the areas with different climates based on it. Then the spectral analysis by Fourier transform was used to identify the dominant available periods in the stations’ drought time series. In this analysis, Radix-2 Fast Fourier Transform was used. Since Fourier transform leads to difficulties in the spectral analysis of non-stationary series, first the stationary condition of the EDI monthly time series was investigated for all of the 41 stations used in this study. In those stations with a non-stationary condition, the Box– Cox procedure was applied to make them stationary. The results were classified into five areas. The results showed that the dominant periods in the time series of Iran’ s droughts vary greatly from periods of 2 to 30 years. In addition to the dominant short-term periods, droughts in the northwest of Iran have two dominant long-term periods of 10 and 30 years. Droughts in the southern coast of the Caspian Sea have shorter periodicities. So with the exception of the two stations of Ramsar and Ardebil with the periodicities of 30 and 15 years, the other stations have periods of less than 10 years. The dominant periods in most stations in the northern districts of central Iran are short-term ones, and the longest periodicity in this area is 15 years. The southern districts of the central Iran have a more consistent spectral behavior. In this area, the dominant period with the highest power is the period of 30 years. All of the stations in this area with the exception of Mashhad station have a 30-year period in their first three components. The 10-year period, as the second dominant period, can also be seen in half of the stations in this area. The southeast area does not exhibit any specific behavior for the dominant periods. The longest periodicity in Chabahar station is a 10-year period. The two stations in Zabol and Iranshahr have shown the longest period of 30 years and the long-term period seen in Zahedan is a 15-year one. In addition to these long-term periods, a period of 2. 5 to 3 years can play the role of the dominant short-term period in this area. In general, for the long-term behavior, the two periods of 10 and 30 years can be considered as being the dominant periods across Iran. Among the long-term periods, the frequency of occurrence of these two periods is maximum in the first component and reduces in the other two components. In the third component, the minimum frequency of occurrence of these two periods is seen.

The Zagros mountain belt, situated on the northern margin of the Arabian plate, is one of the youngest continental collision belts. This belt was formed by a collision between the Arabian plate and the Central Iranian micro-continent. In this study, we used data from 38 temporary seismological stations installed on a 400 km long profile from May to November 2003. The trend of the profile is N58° E across northern Zagros and part of the Central Iran. The stations are part of Zagros03 profile (Paul et al., 2010) operated by the International Institute of Earthquake Engineering and Seismology (IIEES) of Iran in collaboration with CNRS-Université Joseph Fourier, France. We examine the structure of the lithosphere, across the profile by analysis of P-wave receiver functions and Rayleigh wave fundamental mode phase velocity dispersion curves. Joint inversion of Rayleigh wave phase velocity dispersion and receiver functions have been used to estimate the velocity structure beneath 28 seismic stations. Receiver functions are time-series computed from the three-component body-wave seismograms and are sensitive to the earth structure near the receiver station. They are composed of P-to S-wave conversions in discontinuities under the stations. These converted waves are isolated by deconvolving the vertical component of a teleseismic P-wave record from its radial component. For each event, a 120s time-window centered at the direct P arrival is selected and used for the calculation of the receiver function. The deconvolution used is the iterative deconvolution method of Ligorria and Ammon (1999). Surface waves arise from the presence (boundary conditions) of the stress-free surface of the Earth, and in the presence of layering, they are dispersed. They provide valuable information on the absolute S-wave velocity, but they are relatively insensitive to sharp velocity contrasts. On the other hand, receiver functions are sensitive to S-wave velocity contrasts, which give rise to converted phases, but allow for a substantial trade-off between the depth and velocity above an impedance change. Combining them in a joint inversion process bridges the resolution gaps associated with each data set. We jointly inverted the stacked receiver function and surface wave dispersion data. We employ the program joint96 which is available in the software package “ Computer Program in Seismology” (Herrmann and Ammon, 2003). In this study, we try to calculate the Moho depth and velocity structure in the north Zagros collision zone using the joint inversion of receiver function and surface wave dispersions. Receiver functions are calculated using teleseismic events of magnitude greater than 5. 1, located between 30◦ and 95◦ epicentral distances. The fundamental mode Rayleigh-wave group velocities are extracted from the tomographic study conducted by Rahimi et al. (2014). The 1D velocity models resolved by joint inversion are juxtaposed, and a 2D velocity model is obtained. Results obtained from the 2D model reveal that the thickness of the sediments beneath the Zagros is 12 km, the Moho depth beneath this region of Zagros is 43-57 km, which increases towards Sanandaj– Sirjan zone and Urumieh– Dokhtar magmatic arc and reaches an expanse of 62 km and then decreases in the central Iran with a depth of 42 km. The velocity model confirms the presence of a crustal root and a thick high-velocity lithosphere beneath and north of the suture. These evidences imply that the Arabian plate continues to underthrust beneath the Central Iran.

This study evaluates the performance of Earth system models for accurately simulating the phytoplankton productivity and bloom dynamics in the Oman Sea and the northwest of Arabian Sea. Satellite data (SeaWIFS ocean color) show two climatological blooms in this region, a wintertime bloom peaking in February and a summertime bloom peaking in September. On a regional scale, interannual variability of the wintertime bloom is dominated by cyclonic eddies which vary in location from year to year. During the wintertime, while both cooling in the winter and eddies control the blooms, variability in bloom location will arise from variability in the location of eddies, and so may not be predictable. In contrast, during the Southwest Monsoon, the dominant upwelling associated with the intense environmental forcing supersedes the effects of eddies, and the activity of the cold eddies is not pronounced. We consider numerical results from five different 3-D global Earth system models, which are denoted by CORE-TOPAZ, Coupled-TOPAZ, Coupled-BLING, Coupled-miniBLING, and the Geophysical Fluid Dynamics Laboratory (GFDL) Climate Model version 2. 6 (CM2. 6 miniBLING). Two coarse (1° grid resolution) models with a relatively complex biogeochemistry (TOPAZ: Tracers of Ocean Productivity with Allometric Zooplankton) capture the annual cycle but fail to capture both the eddies and the interannual variability. The results showed that the models differ from the observational data in terms of interannual variability. The low-resolution models (CORE-and coupled-TOPAZ) provide an almost uniform seasonal coefficient of variation, while both the data and eddy resolving CM2. 6 models show higher interannual variability and seasonal changes. The coefficients of variabilities are particularly higher during the winter and summer blooms in the observations, while the low-resolution models do not see these signals. In other words, the low-resolution models fail to attain enough variability, while the high-resolution models (i. e. CM2. 6) produce too much interannual variability. Accordingly, eddies are necessary to explain the variability in the data as opposed to the low-resolution models, but that the high-resolution model does not properly capture this variability. An eddy-resolving model (GFDL CM2. 6) with a simpler biogeochemistry (miniBLING) displays larger interannual variability, but overestimates the wintertime bloom and captures eddy-bloom coupling in the south but not in the north. The models fail to capture both the magnitude of the wintertime bloom and its modulation by the eddies in part because of their failure to capture the observed sharp thermocline/nutricline in this region. In the wintertime, this leads to the excessive convective supply of nutrients and too strong of a bloom. However, for a few cases, eddies with blooms at the center are tracked in the southern part of the domain. For the model to simulate the observed wintertime blooms within cyclones, it will be necessary to represent this relatively unusual nutrient structure as well as the cyclonic or cold eddies. Both the temperature and mixed layer biases in the northern part of the Arabian Sea may result from having too much water from the Persian Gulf in this region. This is a challenge in the northern Arabian Sea as it requires capturing the details of the outflow from the Persian Gulf, something that is poorly done in global models.

The concentration of ground-level ozone is the result of thousands of complex chemical reactions. Basically, an increase of ozone concentration occurs in the presence of NOx, VOCs, and the sun’ s radiation. This study deals with analyzing the ground-level ozone in Isfahan city from November 22 to December 21 (a full solar month) in 2009. According to the observations made by the Isfahan Meteorological Organization, photochemical smog was visible over the city during this month. The data used in this study include NO2, NO and O3 concentrations and the meteorological variables of temperature, relative humidity and wind speed which have been measured in Isfahan in 2009. The analyses were carried out for the sunshine hours in two time periods of 9 to 12 am and 6 am to 15 pm whose main characteristics are: A) During 6– 15 period: the sun rises from 6 am and by becoming closer to the dusk, i. e. about 15 pm, both the radiation intensity, and temperature decrease; B) During 9– 12 period: the higher temperature, radiation intensity, and traffic are the effective factors in the emission of pollutants when compared to the other hours of the day. The days under study are classified based on maximum, minimum and average ozone concentration. In order to analyze the tropospheric ozone and smog creation, in this study, the photochemical and semi-empirical models were used. The kinetic and mechanism of a number of photochemical reactions effective in ozone formation were taken into account in order to analyze the changes in ozone concentration. Calculations were carried out by using the Excel and Matlab software programs. Making use of the steady-state approximation method and considering oxygen atom in the steady state, the reaction rates have been computed. The differential relation obtained (d[O3]/dt = k2k1[NO2]-k3[O3][NO]) is a function of three variables of NO, NO2 and O3 concentrations. The amounts of reaction rate (d[O3]/dt) and also the rate constants k2k1 and k3 were also calculated. Analysis of the experimental relation between the activation energy and the results obtained from calculations indicate that the reactions that take place in the troposphere can be considered rank 3 reactions. In the troposphere, the quantum of energy (hν ), which is released in some reactions, is very strong. The activation energies obtained for all days of this study include negative values, and this confirms the fact that the energy of the photons of the sun is needed to change NO2 and O2 to O3 in the troposphere. Based on the negative activation energies obtained, we can consider the reaction NO2+O2+ hν → NO+O3 as the mechanism for the tropospheric ozone production in Isfahan. The creation of photochemical smog, SP(t) with t denoting the time, during the mentioned days was studied based on the Jonson’ s semi-empirical model. The relations obtained based on the changes in NO and O3 concentrations with respect to time show that smog creation follows a quadratic nonlinear relation. In general, the increase of the concentration of pollutants on the ground as a result of photolysis reactions has led to the production of ozone concentrations. The results achieved from the analysis of reaction rate, smog creation, and the resulting curves indicate that ozone concentration has not been uniformly increasing or decreasing during the studied days, but there were both the increasing and decreasing trends. In general, the photochemical reactions taken place in the atmosphere of Isfahan city have caused both production and loss of ozone. Consequently, the investigation showed that the changes of the ozone concentrations under the effects of the solar radiation followed the same pattern in autumn 2009. On the other hand, the changes in the ozone concentration on the ground level caused changes in smog creation during the studied time. With regard to the above-mentioned arguments and on the basis of the effect of nearly the same conditions, a constant process prevailed. It could be predicted that this pattern will be the same in the future years.

Faults are among the most important geological events for hydrocarbon and mineral exploration and geological studies. They are considered as the major hydrocarbon traps whose detection in seismic reflection data have important roles in hydrocarbon reservoir characterization and geomechanical studies of reservoirs. According to previous studies, approximately 75% of the oil fields are associated with faults. Faults can influence the efficiency of hydrocarbon reservoirs by improving the permeability of a porous medium. Faults are defined as a discontinuity along a geological layer or geological event which is the result of the failure of that layer or event against the tension exerted on it. Reflection seismology is one of the best geophysical methods for hydrocarbon exploration. Various methods have been introduced to identify faults in seismic reflection data. In seismic reflection data, faults can be seen as discontinuities along the reflectors. One of the conventional methods of interpretation of the faults is a manual interpretation of seismic horizons which is a highly time-consuming process. Due to the nature of the seismic data, identification of faults plane in this data is a difficult process. Seismic attributes are considered as useful tools that will reveal hidden information in seismic data which can help the interpreters to detect faults. In fact, a seismic attribute can be used as a filter that makes the structural and stratigraphic information more apparent from the seismic data. There are several seismic attributes for faults detection in reflection seismic data such as coherency, curvature, chaos, and variance. The seismic coherence attribute is one of the most common attributes for fault detection. This attribute is calculated by different criteria such as cross-correlation, semblance, eigenstructure, gradient structure tensor. In this paper, we used a new form of a seismic attribute for edge detection. It is the Sobel filter which is a widely used tool in image processing, computer vision, and edge detection problems. It is the first derivative of the image and is sensitive to amplitude changes. It is a discrete differentiation operator, computing an approximation of the gradient of the image intensity function by convolving the isotropic 3 3 operators. These operators are extensible to higher sizes and dimensions. Therefore, the Sobel attributes can be used in two-and threedimensional seismic data. It can be used in seismic data for identification of the geological events such as faults, salt dome, and buried channels. The efficiency of dip guided Sobel attribute is evaluated by applying it to both synthetic and real seismic data. We compared the obtained results with the results of conventional seismic coherency attributes. In this paper, we used the eigenstructure-based coherency attribute. Comparison of the results of synthetic models in both noisefree and noisy cases in two and three dimensions show that the dip guided Sobel filter can be a good alternative for coherence attributes. In comparison with the coherence attributes, dip guided Sobel attribute is a short run time process and has large stability against noise. In real seismic tests in two and three dimensions, the obtained results from two attributes show that the dip guided Sobel attribute performs better than the eigenstructure-based coherency attribute.

The purpose of this study is to estimate compressional and shear wave quality factors of seismic waves by using local earthquakes occurred in the NW of Iranian Plateau. In seismological engineering studies, quality factor estimation of body and shear waves plays an important role in seismic risk assessment of different areas, determining the exact magnitude of the earthquake, strong ground motion simulation and study of destructive energy of earthquake from near to intermediate region. In this study, earthquakes recorded in the Iranian Seismological Center (IRSC) and Iranian National Broadband Seismic Network (INSN) for the longitudinal band from 43° E to 53° E and the latitude band from 36° N to 40° N were used. Among the 17 stations, 14 stations belong to the IRSC and the rest belong to the INSN. Due to the presence of some big cities in the northwestern part of Iranian Plateau, quality factors of body and shear waves were estimated by using the data of 17 seismological stations including 13000 recorded earthquakes of the IRSC and INSN. This region of intense deformation is situated between two thrust belts of the Caucasus to the north and the Zagros Mountains to the south. The NW of Iranian Plateau is a part of Turkish– Iranian plateau and includes historical and destructive earthquakes and two volcanoes with lots of thermal springs around. The North Tabriz Fault (NTF) is one of the active faults in NW Iran that has a clear surface expression. Seismic quiescence and large historical earthquakes in the region in more than the two last centuries have increased the seismic risk of this region. To estimate seismic hazard in an area, a two-step process is needed. First, we must understand the nature of the earthquake sources that generate potentially hazardous ground motion. This includes knowledge of the distribution of seismic source zones, predominant fault mechanisms and return times of large events. Second, we must understand the effects of the transmitting medium (the Earth) on the seismic waves. A synthesis of the source and path effects will allow us to calculate the ground motion at a given site. Seismic attenuation is also caused by intrinsic mechanisms that convert the wave energy to heat through friction, viscosity, and thermal relaxation processes. Scattering redistributes wave energy within the medium but does not remove energy from the overall wavefield. In contrast, intrinsic attenuation mechanisms convert the wave energy to heat through friction, viscosity, and thermal relaxation processes. Energy loss caused by inelastic behavior is called inherent or internal attenuation and is determined by the inverse of the Q parameter. Large values of quality factor mean that attenuation is low and when Q is equal to zero, attenuation is very high. Aki (1980) used the normalized Coda for the first time in order to estimate absorption amplitude of the S waves. Since then, this method has frequently been used in seismological studies for estimation of the absorption parameters of seismic waves (see, for example, Yoshimoto, 1993; Hatzidimitriou, 1995). For three categories of data with epicentral distances less than 100 km, from 100 to 200 km and 0 to 200 km, attenuation variation investigation of body waves was carried out in 9 frequency bands with central frequencies of 3, 5, 7, 10, 14, 20, 28, 38 and 47 Hz and the quality factor was estimated in different frequencies for each station, separately. For the northwesern part of Iran, the frequency dependence of the body and shear wave quality factors in all stations were estimated so that their average values are quantified as Qp=55f 0. 84 and Qs=38f 0. 93, respectively. Due to the low values of the Q parameter and thus high attenuation values of body and shear waves in North West of Iranian Plateau, the amplitude of the propagated waves are decreased severely in the interested area when these waves pass through it. The attenuation effect of seismic waves would reduce the damages caused by the earthquakes at appropriate distances from the faults at the time of probable earthquake occurence.

The southeast region of Iran includes the western part of Makran as an active subduction zone on the North side of the Oman Sea. The velocity structure of this region is not well understood because of its low-level seismicity and a small number of permanent seismic stations in this region. The main purpose of this study is the estimation of shear wave velocity structure and Moho depth variations in the southeast of Iran. To this end, we apply the joint inversion process of Rayleigh and Love waves dispersion curves and the “ P-wave receiver function” (PRF) around four permanent broadband seismic stations of the region named ZHSF, KRBR, CHBR, and BNDS. To find the group velocities of surface waves in the southeast of Iran, the “ frequency– time analysis” (FTAN) is applied to the waveforms of 40 local earthquakes, which occurred in Rigan region and recorded by the four permanent broadband seismic stations. These local earthquakes include two main shocks of 20 December 2010 (ML=6. 4) and 27 January 2011 (ML=6. 2) accompanied by their foreshocks and aftershocks in Rigan region. Therefore, the group velocities of fundamental modes of Rayleigh and Love waves are calculated in the period range from 5 to 60 sec in the paths among the four seismic stations and the sources of Rigan earthquakes. Also, to calculate P-wave receiver functions around the four permanent broadband seismic stations, 485 teleseismic earthquakes with suitable signal-to-noise ratios and epicentral distances between 30° and 95° related to the region, are selected. The radial PRFs are computed by deconvolving the vertical component from the radial component based on the iterative deconvolution method (Ligorria and Ammon, 1999). After preparing these two groups of data, we can determine the shear wave velocity structure and Moho depth vicinity of each seismic station by applying the joint inversion process to the dispersion data and the PRF data related to each seismic station (using the joint96 program; Herrmann and Ammon, 2007). Based on the results obtained in this study, the average group velocity of surface waves was estimated at less than 3. 5 kms-1 in the period range from 5 to 60 sec. The lowest average group velocity of surface waves was obtained in the paths between the CHBR station and the sources of the Rigan earthquakes. Also, the Moho depths beneath the ZHSF, KRBR, CHBR, and BNDS stations were estimated 38± 4, 46± 6, 26± 2 and 56± 5 km, respectively. The minimum thickness of crust beneath CHBR station as well as the higher velocity of shear wave estimated beneath this station, are consistent with the shallow subduction of a high-velocity oceanic crust of Arabian plate beneath the south side of Makran. Furthermore, the thicker crust beneath the KRBR station and the lower velocity of shear wave estimated in this area, when compared with the area encompassing the CHBR station, is due to the existence of magmatic assemblage in the vicinity of the KRBR station. These results are consistent with the crust thickening from the south to the north of Makran. The maximum thickness beneath the BNDS is due to the location of this station being in the southeast of Zagros mountain belt, where the thick continental Arabian plate collides with Zagros. This collision leads to thickening of crust in Zagros.

Estimation of seismic wave attenuation due to anelasticity and geometrical spreading has attracted major interests among earthquake engineering community in recent decades. The choice of ground-motion model has a significant impact on hazard estimates in an active seismic zone such as the NW Iran. Estimation of ground motion for a typical frequency range of 0. 5– 10 Hz is required for the proper design of earthquake resistant structures and facilities and is considered as input for engineering stochastic ground motion relationships. For seismological purposes, appropriate attenuation models make it possible to calculate more accurately the source parameters such as magnitude and seismic moment. The NW Iran has experienced very few large events during the operation of the accelerometer network of the Building and Housing Research Center (BHRC). The BHRC network has been operating since 1973 but has recorded ground acceleration for few events in the study area, because of the low seismicity rate. The availability of the abundant weak-motion waveform data from the shortperiod local seismograph network of the Institute of Geophysics of the University of Tehran (IGUT) provides an opportunity to derive a new and more reliable ground-motion relationship for small events to complement those of strong-motion results. In this study, we analysed 3514 records of 943 small and moderate events that were recorded by 8 permanent stations of Tabriz network (belonging to the IGUT) and 16 temporary stations of the Institute for Advanced Studies in Basic Sciences (IASBS) to prepare a dataset including week ground-motion spectral amplitudes for different magnitudes and hypocentral distances. We graphically found the distance at which the nature of geometrical spreading attenuation changes significantly using a locally weighted scatter-plot smoothing called robust LOWESS. A bilinear function with a hinge at distance of about 70 km describes the geometric spreading attenuation with distance. Geometrical spreading and intrinsic attenuation coefficients were calculated using nonlinear regression in different frequencies and an average value of b1   1. 1 0. 28was found as geometrical spreading coefficient for distance range of 10– 70 km. This value is consistent with geometrical spreading in a layered Earth. The average geometrical spreading coefficient of 2 b   0. 44  0. 27 was found for the frequency range 0. 79– 5 Hz and the distance range of 70– 200 km. This value is smaller than the values reported for other regions in the world (e. g. +0. 09 for Central Alborz: Motaghi and Ghods, 2012; +0. 2 for North Iran: Motazedian, 2006; +0. 2 for SE Canada and the NE United States: Atkinson, 2004; +0. 1 for SE Australia: Allen et al., 2007) and indicates that the velocity contrast in the Moho discontinuity is smaller than that in the other regions. The low-velocity uppermost mantle in NW Iran was manifested by different types of tomographic results obtained for the region. The geometrical spreading coefficient 2 b does not change before and after 70 km distance for frequencies ≥ 5 Hz. Thus, the attenuation relationship in this frequency range changed from bilinear to linear function. Using anelastic attenuation coefficients calculated at different frequencies, the shear-wave quality factor, Q, obtained equal to Q 96 f 0. 84 for frequencies greater than 1. 5 Hz. In fact, the Q values show a U-shaped behavior in all of the frequency ranges and the function that describes it is defined as   2   logQ  1. 39 log f  0. 63 log f  2. 26.