IRANIAN JOURNAL OF GEOPHYSICS
Year:2017 | Volume:11 | Issue:|Papers Count:11

The proper parameterizations of exchange processes between air and sea are critically important in better predictions of the atmosphere and ocean characteristics using numerical simulations. These exchanges mainly include sensible, latent and momentum fluxes between the two media. Using numerical weather prediction models is a common way to prepare input data for the numerical ocean models. The meteorological data obtained by this method are often used as forcing for the ocean models. For semi-enclosed seas like the Persian Gulf, using meso-scale numerical weather prediction models are preferred. The Weather and Research Forecasting (WRF) model is one of the most popular scientific and operational numerical weather prediction models that has been widely used in many studies and projects. In the present work, sensitivity to the choice of different physical parameterizations in WRF model simulations have been studied over the Persian Gulf and Oman Sea for the period of 2011 summer monsoon. Monsoon is the most important phenomena that affects the meteorology of the Oman Sea and the Persian Gulf. The main domain of the model is selected from Arvandrood in the northern Persian Gulf, to the northern part of the Indian Ocean. To provide the initial and boundary data for the WRF model simulations, the FNL data from NCEP are used. The Simulations are carried out for a period starting from the beginning to the end of summer monsoon of the Indian Ocean in 2011. To run the model for a given period of time, namely 7 days, the time period is divided into daily periods. Then, the model is run for every 1. 25 days (30 hours) with 6 hours of spin up. When the daily (1. 25 days) simulations are done, the first 6 hours of individual simulations are discarded and the resulting daily simulations are concatenated to form a pseudo-continuous dataset. Different choices of physical parameterizations are used to create nine WRF model configurations. The parameters simulated include temperature, humidity and wind velocity for nine different configurations of parameterization schemes. Then the results are compared with the meteorological observations of the coastal and island synoptic stations of I. R. of Iran Meteorological Organization (including Abumoosa, Bushehr, Jask, Qeshm, Khark, and Chabahar), Chabahar buoy and WINDSAT satellite data. To compare results of the model with the satellite data, six points (three in Persian Gulf and three in Oman Sea) are selected. The time interval between two successive observations is three hours for the synoptic stations, one hour for the Chabahar Buoy, and the satellite data are gathered two times per day. The amount of absolute, relative and “ root mean square errors” (RMSE) of wind speed at 10 m height, the dry bulb and dew point temperatures at 2 m height are calculated. The results show that configuration No. 3 including Lin microphysics, MRF planetary boundary layer, Kain– Fritsch cumulus convection, RRTM longwave radiation, Goddard shortwave radiation, Revised-MM5 surface layer and NOAH land surface produce less error for temperature and humidity parameters, and configuration No. 2 including Lin microphysics, ACM2 planetary boundary layer, Kain– Fritsch cumulus convection, RRTM longwave radiation, Goddard shortwave radiation, Pleim– Xiu surface layer and land surface had the least error for simulation of surface wind speeds.

Tehran, the capital of Iran, is located in the foothills of Alborz Mountains in a very seismically active region and is surrounded by many active quaternary faults with the potential for devastating earthquakes. This city has a daytime population of 12 million people and is the political and economical capital of Iran as well. These facts together with the existence of neighborhoods with old buildings that are poorly constructed increases the importance of different studies to better characterize the nature of ground shaking from future probable earthquakes in the city. With the increasing computational power in recent years, the seismic waveform simulation has become one of the preferred methods for studying the seismic hazard in regions like Tehran. The topography effect, in this regard, is one of the components of site effects that need to be included in hazard assessment studies. It is a known fact that the surface topography has significant effects on the earthquake ground motion, especially in mountainous areas with ridges and valleys. As has been observed in the annals of past earthquakes and numerical simulations, topography, In general, increases the ground shaking amplitude on mountain tops and reduces the ground motion amplitude in valleys. The Spectral Element Method, combining the power of Pseudo Spectral methods with the geometrical flexibility of Finite Element Method, is one of the best methods for modeling the seismic wave propagation in regions such as Tehran, with notable surface topography. In the present research, the Spectral Element Method was employed in order to simulate three point sources and three extended source earthquake scenarios both within and around Tehran city and to investigate the role of surface topography on the ground shaking inside the city. The topography effect was investigated by comparing the results of simulations with and without incorporating the surface topography in meshes; the resulting amplification was presented as color maps in the region. Simulations were performed via SPECFEM3D software package which implements the Spectral Element Method to simulate the seismic wave propagation in the region. The first step in using the Spectral element Method was to create quality meshes in the study area; in this regard, CUBIT program is employed so as to create the hexahedral meshes in the model area. The extent of the model area was 100 x 60 kilometers horizontally and, vertically, from the ground surface to the depth of 60 kilometers with limits of latitude and longitude of 35. 5 to 36. 5 degrees and 51 to 52 degrees, respectively. The peak ground acceleration amplification maps were presented for the topography effect in the frequency range of 0. 01 to 1 Hz. The findings indicated that the topography effect inside Tehran city is dependent upon the earthquake scenario and the resulting amplification from topography effect inside the city is generally low and negligible. On average, the amplification resulting from the topography inside the city was between-10% to +10% which could reach as high as ± 30% in certain earthquake scenarios at certain locations within the city. In mountainous areas near the city, we observed amplification on the peaks and de-amplification in the valleys; the amplification fell between-50% to +50 %.

NW Iran is part of the complex tectonic system caused by the interaction between Arabian plate, Anatolia and Eurasia. The North Tabriz Fault (NTF) is one of the main structural features of the region and is considered to be the eastern termination of the Gailatu-Siah-Chesmeh-Khoy fault (Karakhanian et al., 2004), which merges with the Maku and the Nakhichevan dextral strike-slip faults and continues to move farther east. Part of the northward motion of Arabia is transferred to Anatolia by this complex system of faults and, the oblique orientation of the motion relative to the Zagros mountain range, results in the partitioning of the motion between shortening and thickening in the Caucasus and right-lateral strike-slip motion along the NTF. In this research, we investigated the laterally two-dimensional velocity structure in the upper crust of NW Iran (mainly around the NTF) using local earthquake P-waves tomography. Several data sets were utilized, including Pg phase pickings of the Tabriz Network permanent stations governed by Institute of Geophysics, University of Tehran (1996 to 2013), temporary seismic stations installed around the North Tabriz Fault by International Institute of Earthquake Engineering and Seismology (IIEES) (April to July 2004) and temporary seismic stations installed by Institute for Advanced Studies in Basic Sciences (IASBS) (2009 to 2011) that merged with our data set so as to improve ray coverage in the eastern parts of the study area. The merged data set, recorded by 72 stations, consisted of more than 20, 000 local earthquakes out of which, only 940 earthquakes were good enough to be selected for the local earthquake tomography. The velocity structures were resolved via a simultaneous solution of the coupled hypocenter and velocity model programmed in SIMULPS14. The tomographic images obtained from the linearized inversion are dependent on the initial velocity models and hypocenter locations. We primarily calculated the initial velocity model through the use of the 940 earthquake selected datasets. The time difference between the observed phase arrival time and predicted arrival time was then calculated and called travel time residuals. The residuals were further used as inputs for SIMULPS14 simulator to be converted into velocity model, which would in turn be used to adjust earthquake location parameters. Following four iterations for our inversion process, we obtained a 2D velocity tomogram that clearly showed different velocity structures on the two sides of the NTF. The velocity contrast across the NTF might have been caused by existence of different kinds of rocks on the two side of the fault trace. The North Tabriz Fault is an active and steep strike-slip fault generating strong structural differences around its surficial trace. It is a WNW– ESE trending fault in which the motion is concentrated on the fault at a rate of 7 mm/year. Such a strong rate of sliding explains the clear structure difference on the two sides of the fault. An anomalous low velocity feature can be seen in the central part of NTF. Comparing the velocity tomogram with the geological map of the region, one can observe that there exists a thick sediment basin in the same area. The low velocity anomaly is probably related to the thick, low velocity sediments deposited in that area.

The Madden-Julian Oscillation (MJO) is the dominant mode of tropical intraseasonal variability, characterized by its planetary spatial scale, 30– 60 day period, and eastward propagation. The extra-tropical links with the MJO are well established (Barlow et al., 2005). Such oscillation has an evident seasonality, with larger amplitude during boreal winter and spring than summer (Zhou et al., 2012). MJO can significantly modulate variations in weather and climate in the far-reaching subtropics and midlatitudes (Handerson et al., 2016). In this study, we examined the infl uence of the MJO on precipitation and large-scale circulation anomalies based on a 10-years daily data from 2000– 2010. We further analyzed the atmospheric circulations, Iran precipitation data and their anomalies with respect to the real-time multivariate (RMM) Index phases of the MJO whose employed index was that developed by Wheeler and Hendon (2004), downloaded from the website of the Australia Meteorological Bureau. The MJO index data used in this research, considering that at least five consecutive days in a phase remained constant and its value was equal or more than 1, was extended from December 2000 to February 2010. Daily precipitation data were obtained from 47 meteorological synoptic stations in Iran from December 2000 to February 2010. Furthermore, the grid point data were extracted from NCEP-NCAR reanalysis dataset. Daily anomalies of precipitation and other variables were calculated by subtracting their 11-year means from the original data. Composites of mean daily anomalies were computed for each of the eight phases of the MJO on the basis of the RMM index with the same MJO phases in boreal winter during 2000-2010. Composites of the regional fl ow associated with the MJO phases during winter seasons were calculated by averaging fields of data over lists of dates obtained from the analysis of the MJO indices. Figures 2 to 6 display the composites of large-scale atmospheric circulations and precipitation anomalies for the eight MJO phases. It is very clear that winter large-scale atmospheric circulations and precipitation anomalies in Iran show meaningful and significant variations when the MJO propagates from the western Indian Ocean (phase 1) into the central Pacific Ocean (phase 8). As is observed in the figures, winter precipitation in certain parts of Iran is higher than the 1981-2010 climate normals when the MJO is in phase 2, which is close to the mean; positive anomalies weaken over the country as we get closer to phase 3. When the MJO is in phases 4 and 5, precipitation anomalies are less than normal, with the maximum negative anomalies reaching around 100% relative to the mean. During such phases (4 and 5), associated with positive geopotential height anomalies over the eastern Mediterranean Sea, precipitation negative anomalies occur in the country. Winter precipitation in certain regions is much higher than the climate normal when the MJO is in phases 6 and 8. Further observed in these phases were the precipitation positive anomalies over the vast parts of western and southern slopes of the Alborz (from 30 to 150%). So, in these situations which associated with zonal dipole of geopotential heights anomalies over Eurasia where strong negative anomalies of geopotential heights were located over the eastern Mediterranean Sea and its neighborhoods areas, the precipitations over the most part of Iran are more than the climate normals. Extrapolated from the foregoing data is the fact that the MJO infl uence on Iran precipitation is signifi cant during northern winter season. When the MJO is in phases 6 and 8 (the convection increases over central and eastern Pacific Ocean), more precipitation is observed in the western regions of Iran. On the other hand, as enhanced tropical convection shifts over the Maritime continent, less precipitation is seen over the country. Therefore, owing to the broad tropical and extratropical impacts of the MJO on interaseasonal timescales, a better understanding of the MJO is potentially conducive to the amelioration of the extended range forecast of week-two and beyond, practically when there is an on-going MJO event. Numerical and empirical model experiments have shown the potential predictability of MJO up to 4 weeks. In this study, the objective was to develop composites in order to provide a compendious, large-scale overview of MJO impact on the winter season circulations and precipitation in Iran.

Most physical systems in nature have nonlinear dynamics and the system linearization of these physical systems is only a simplified assumption. The atmospheric motions, with the same philosophy, have a turbulent structure and ommiting the turbulent motions in the atmosphere is only a problem simplification assumption. When a buoyant jet of a chimney enters the atmosphere, it behaves like a turbulent flow, in which the atmospheric turbulence and self-generated turbulence of the plume play major parts. The present survey aimed at demonstrating the effects of turbulence on plume dynamics through computer simulation. The well-known turbulent flow simulation methods commonly used to simulate plume dynamics and atmospheric processes are: Direct numerical simulation (DNS), Reynolds averaged Navier-Stokes (RANS) and large eddy simulation (LES), which most distinguishing feature is their way of parameterizing the turbulence. As far as computational requirements, accuracy and turbulence simulation, the former two models are the two extremes while LES occupies an intermediate position between them, directly simulating the large-scale eddies and parameterizing the less important sub-grid scale (SGS) dissipative processes using sub-grid models (SGM). Most often, LES can predict the unsteadiness and intermittency of the turbulence structure, which is the most important feature of a buoyancy-driven jet. It should be noted that it is not efficient to employ full LES method when tackling an issue with certain unimportant zones. Furthermore, in the case of strong turbulent motions, the scale of flow structures near the rigid bodies are small and LES method method requires very fine grids that can increase its computational cost as large as DNS. To surmount this drawback, a hybrid RANS-LES method with a new mixed scale sub-grid parameterization model was applied to simulate the turbulent plume dynamics in ANSYS Fluent 14. 5 software. The effectiveness and the accuracy of the mentioned turbulence simulation method was demonstrated through simulation study and experimental data in the neutral atmospheric conditions. Comparing the simulation results of the RANS method, the default hybrid RANS-LES method with static sub-grid scale parameterization and the new RANS-LES method with dynamic mixed scale parameterization indicated that the mean temperature profile at stack downstream was more accurately predicted by the new hybrid method. The root mean square error of plume rise estimation of the RANS method, the default RANS-LES method and the new hybrid RANS-LES method were 0. 0437, 0. 054 and 0. 0323, respectively. It was further demonstrated that the Briggs integral plume rise model could not properly predict the plume rise in the presence of turbulence, because it does not consider the updraft and downdraft turbulence-induced motions in the atmosphere. Ultimately, we checked the capability of the new hybrid method to resolve the substantial parts of the turbulent motions. The turbulent energy transfer from the energy containing scales to inertial sub-range followed the well-known law. The capability of the new hybrid method in predicting the mean profile and the turbulent structure can be employed in the study of the effects of turbulent parameters on plume rise in different atmospheric stability classes.

The impacts of potential climate change on the surface climate variables can be appraised through the projections of the global circulation onto the target variables, considering that the observed climate shifts are commonly associated with the changing patterns of the general circulations (Fraedrich et al, 2001). Since a combination of climate variables are utilized to develop climate classification schemes, they are useful for validating the outputs of the general circulation models (GCMs). The Kö ppen climate classification (Kö ppen, 1936), as the widely used climate classification method, is well suited for simultaneously validating the temperature and precipitation model outputs, considering that it takes into account both precipitation and the near-surface air temperature as the major input variables, as well as their annual cycles and linkage with the natural vegetation patterns (Kalvova et al, 2003). Therefore, such a climate classification allows for an outlook on the possible future shifts in the climate zones under a changing climate. Many researchers applied the Kö ppen climate classification to the general circulation model outputs in order to assess the shifts in the climate zones caused by the foreseen climate changes, represented by the GCMs (Fraedrich et al, 2001; Diaz and Eischeid, 2007; De Castro et al, 2007; Ruble and Kottek, 2010; Chen and Chen, 2013; Chan et al, 2016; Engelbrecht and Engelbrecht, 2016). The present work aimed at investigating the shifts in the Iranian climate zones induced by the possible climate changes in the 21st century. Monthly total precipitation of the Global Precipitation Climatology Centre (GPCC) and the mean monthly temperature of the Climatic Research Unit (CRU) of the University of East Anglia, both having 0. 5 degree spatial resolution, were used for computing present time (1951-2000 time period) Kö ppen-Geiger climate classification for Iran. Following Rubel and Kottek (2010), the global temperature and precipitation projections corresponding to the period 2001 to 2100 were also taken from the Tyndall Centre for Climate Change Research dataset, TYNSC2. 03 (Mitchell et al., 2004), to compute Kö ppen-Geiger climate classification for the 21st century. The TYNSC2. 03 consists of a total of 20 GCM runs, combining 4 possible future worlds of emission scenarios (A2, B1, B2, A1F1) described by SRES (ARNELL et al., 2004) with 5 state-of-the-art climate models, namely the Hadley Centre Coupled Model Version 3 (HadCM3), the National Center for Atmospheric Research-Parallel Climate Model (NCARPCM), the Second Generation Coupled Global Climate Model (CGCM2), the Industrial Research Organization-Climate Model Version 2 (CSIRO2) and the European Centre Model Hamburg Version 4 (ECHam4). The Kö ppen-Geiger climate types of the present time were computed on the basis of the GPCC precipitation and CRU temperature datasets for 1951-2000 and 1976-2000 time periods. The 21st century Kö ppen-Geiger climate types were further computed for 1901-1925, 1926-1950, 1951-975 and 1976-2100 time sections of the TYNSC2. 03 datasets. The comparison of the climate classifications of 1951-2000 and 1976-2000 time periods highlighted certain signals of change in the Iranian climate zones in the latter half of the 20th century. The most obvious changes were the tendency of northwestern Iran to a drier climate and the extensive retreatment of BWk climate type in the central and eastern Iran in favor of BWh climate type in the last quartile of the 20th century. Shifts and changes in the climate zones of Iran were more profoundly observed in the climate classification maps of the 21st century, particularly in the final quarter of the century. Except for A2 and A1F1 pessimistic scenarios which showed the maximum climate shifts in Iran, all the scenarios considered in this study more or less agree in displaying moderate changes in the climate zones of Iran. In general, based on the pessimistic scenarios, northwestern Iran is extremely susceptible to an extensive climate shift in the future. Nevertheless, all the scenarios indicate that northwestern Iran tends to have a much drier and warmer climate in the second half of the 21st century. Moreover, the Csb climate type, currently the main climate type of most parts of the Zagros mountainous areas of western Iran, will be replaced by Bsk and Bsh climate types at the end of the 21st century. Obviously, the BWk climate type will disappear from the country at the end of the century due to the widespread invasion of the BWh climate type in the central and eastern Iran, indicating an anticipated widespread desertification in almost all parts of the country, particularly in the northwest of Iran, under a changing climate.

Ever increasing attention is paid to numerical weather prediction (NWP) models with the purpose of providing high-resolution precipitation forecasts. In such applications, which are based on both the theoretical analysis and numerical experiments, the prediction accuracy is closely related to the errors in the initial conditions and in the physical parametrization schemes. In the present research, the potential of data assimilation in improving precipitation forecasts was investigated in a case study on an active weather system in the western regions of Iran. Various data assimilation experiments were designed by running the weather research and forecasting (WRF) model and its data assimilation package (WRF-DA). In each data assimilation experiment, we applied the three-dimensional variational data assimilation (3DVAR) method. A heavy rainfall event caused by a strong synoptic system in western Iran was selected in order to study the influence of data assimilation on precipitation forecast. So as to carry out this study, the initial atmospheric and lateral boundary conditions were taken from three data categories: NCEP global forecast system (GFS), real-time forecasts at 3-h intervals, which are gridded to horizontal resolutions of 1̊ ×1̊ and 0. 5̊ ×0. 5̊ , NCEP FNL (Final) Operational Global Analysis data on 1̊ ×1̊ grids prepared operationally every six hours and ERA-Interim reanalysis dataset of ECMWF, gridded to horizontal resolution of approximately 80 km at 6-h intervals. Simulations were divided into control runs and data assimilation runs, with the former runs being based on three sets of data as initial conditions. The data assimilation runs were conducted utilizing GFS data as the background and two sets of obeservations, namely the surface observations of Iran Meteorological Organization (IRIMO) and the NCEP observations. The observation data showed a significant impact on the initial conditions of 2m temperature and 10m zonal and meridional wind components, such that in certain parts of the simulation domain, the background temperature was estimated to be up to +3C° relative to the analysis and the wind field was revised by up to ± 3 meters per second in some areas. The comparison between the scatter plots of the background and observations relative to the analysis corroborates the fact that the scatter and errors were decreased after using 3DVAR. The findings indicated that the accuracy of forecasts depends directly on the type of data employed as initial conditions for WRF model and the physical parametrization schemes, hence the fact that the simulations demonstrate significant differences. The bias analysis of precipitation for stations with precipitation records in the west illustrated that the assimilation of IRIMO surface data in one of the physical configurations decreased the forecast bias to a minimum of 73% of the cumulative 24-hour precipitation forecast. The impact of data assimilation, on the other hand, decreased in the cumulative 48-hour precipitation forecasts. Correlation analysis of the forecasted precipitation patterns and the observed values demonstrated that data assimilation generates a higher correlation coeffcient, implying that it had a discernible, though limited, positive impact on the case examined. In addition, the maximum impact of data assimilation on the correlation between data assimilation runs and control runs for precipitation was approximately 8%. Specifying a precipitation threshold for quantitative precipitation forecasts (QPF), the binary analysis was done, while the proportion correct score (PC) of each threshold was employed in order to investigate the forecasts quality. In conclusion, using the skill score of binary analysis is not a proper method to compare forecasts quality in different experimental runs when the number of forecasts and observational stations are limited.

Neotectonic and Seismotectonic activities have caused major seismicity in Iran. Dargahan city, located in the southeast of Iran, runs a high risk of earthquakes, hence its selection for the research. A large earthquake (Mw = 6. 5 28Jun2006) was chosen for the study due to its provision of a suitable geological and velocity structure in this region. To calculate the amplification factor of the soil, it is indispensable to know its elastic properties, especially above the bedrock. One of the best ways to achieve this goal is to implement ambient noise. The spectral ratio between the horizontal and vertical components (H/V ratio) of microtremors measured at the ground surface was employed to estimate the resonance frequency of the site. The purpose of this paper was to retrieve the Rayleigh wave so as to estimate resonance frequency with higher accuracy and offer velocity structure for a single station record. The main problem with the ellipticity curve of Rayleigh wave retrieved from H/V ratio is the existence of the body and love waves in the waveform of the microtremor. The P-SV part of a waveform contribution is only in the vertical component while the SH part is in the horizontal component of the motion. If the SH-part is removed, the H/V ratios better indicate the ellipticity of the fundamental mode of Rayleigh wave, because the interest frequency band of the P-SV is dominated by the fandamental mode. In this paper, the SH removal is done by time-frequency analysis of all three components of the ambient noise. Continuous wavelet transform (CWT) was employed for this task by using the modified Morlet wavelet. The higher the value of the Morlet wavelet parameter (m) is, the narrower the wavelet in the spectral domain, and the better the frequency resolution will be, which leads to more accuracy of the H/V technique. In this article, first-time location of the most energetic time-frequency coefficient of the vertical component was identified and the corresponding local frequency spectrum was obtained. Then, a local horizontal spectrum at the same time location was calculated from an average time-frequency representation of the two horizontal components. Finally, the H/V ratio is calculated by dividing these two local spectra. We selected 12 stations in Dargahan which data (ambient noise) were recorded by CM6TD seismometers over a period of 30 minutes. The frequency-time analysis was done for all three components in the 12 stations. Primarily, Morlet parameter (m) was changed in the range (0. 5-16), and the (H/V) curve was subsequently obtained according to this parameter, where the best value for the Morlet parameter was found to be 8. The results obtained for resonance frequency were compared with those of classic methods, showing a shift toward lower frequency in the proposed method. It was also shown that there existed two peaks in the reliable resonance frequency of the M03 station. So as to address the issue, the theoretical ellipticity curve of the Rayleigh wave was obtained from borehole data by the spectral ratio between horizontal and vertical components of its fundamental mode. The theoretical ellipticity curve of the Rayleigh wave and the log (H/V) curve were compared, upon which comparison we found out that the second peak was related to the resonance frequency in site M03. The values of the elliptic curve for near borehole stations which is in good agreement with the H/V curve, are suggested as the shear wave velocity structure for each single stations. By this method, one can estimate the shear wave structure for 8 stations close to the borehole.

In this study, a complete waveform database of Alborz and its surroundings was processed so as to ameliorate the locations of the earthquakes and obtain an enhanced picture of the past decade’ s seismicity distribution. In the first step of this study, P-and S-wave arrival times were manually re-picked at 41 stations extending from 33° N to 37° N and from 48° E to 54° E. Our initial locations, including 4152 events, were implemented using Jackknife resampling method, normally employed for statistical inference. This up-to-date technique reliably estimates hypocentral errors by deleting one observation at a time. In order to ensure that the relocation would provide valid results, only events that met certain criteria were selected. The selection criteria were (1) largest primary azimuthal gap between stations less than 210° , (2) arrival time residuals less than 1 s, (3) number of recording stations no less than 6, and (4) initial event uncertainty in epicenter and depth of less than 10 km. The second step of this study focused on improving the arrival time pickings of event pairs utilizing P-wave cross-correlation-based time delays. Correlated events are those occurring within a few kilometers of one another to generate similar waveforms. All event pairs with separation distances less than 10 kilometers were processed. The differential times of event pairs with corresponding travel time residuals for all observations were combined into a system of linear equations and weighed based on the quality of arrival time picks. We computed a total of more than 280000 P-wave differential times and selected waveform pairs with coefficients of 0. 7 or larger. In the third step, to minimize the effect of inaccurate velocity structure, we applied the double-difference location approach. The algorithm, hypoDD, determines relative locations within clusters of closely spaced events using double-difference method developed by Waldhauser and Ellsworth (2000). By relocating merely closely spaced events, this algorithm ameliorates relative location accuracy along with reducing the effects of unmodeled velocity structure. The nearest neighbor approach was applied so as to link events using a maximum search radius of 10 km and a minimum number of 8 links. Event linkage strongly controls how the dataset is broken into clusters for relative relocation in hypoDD. For example, a single link between two closely spaced events, but perhaps occurring along different faults, causes all linked events to collapse into a single cluster rather than forming two clusters. Because of the relatively small number of stations recording each event and due to the closely spaced known faults in Alborz region, we, instead, visually prescribed cluster identification. In this way, we used such essential documentary sources as seismotectonic maps, the hypocenter locations of seismic events in the initial locating procedure, and the expansion of the major faults. The distribution of 2409 relocated events delineated more coherent features, and in general, the relative relocations increased the agreement with major active faults. The absolute and relative relocations discussed in the present research are an improvement because of either the carefully re-picked P-and S-wave arrival times or the applied appropriate waveform phase-picking algorithm.

As seismic energy propagates through the earth medium, its energy (amplitude) decays due to geometrical spreading, intrinsic attenuation and scattering. Owing to anelastic absorption, intrinsic attenuation converts the seismic energy to heat while scattering redistributes the energy at random heterogeneities. Knowledge of the relative contributions of scattering and intrinsic attenuation is important for appropriate subsurface material identification, tectonic interpretations and quantification of the ground motion. Besides, investigating seismic wave attenuation inside lithosphere allows for a more thorough knowledge as to Earth’ s deep structures. The attenuation of short-period S waves, expressed as the inverse of the quality factor (Q− 1), helps fathom the physical laws related to the propagation of the elastic energy of an earthquake through the lithosphere. Coda wave attenuation is considered as the combination of scattering and anelastic attenuation. In this study, the quality factor of coda wave was estimated in NW Iran making use of single back scattering method of Aki and Chouet (1975). For this purpose, we analyzed 3720 waveforms recorded by 8 short-period stations of Tabriz network from 1996 to 2013. So as to calculate the frequency relationships for Qc, nine frequency bands with central frequencies of 1. 5, 2, 3, 4, 6, 8, 12, 16 and 20 Hz were considered and the lateral and depth variations of Q0 (Qc in 1 Hz) were investigated in the research area. In order to study the lateral variations, we chose coda waves recorded in epicentral distances less than 80 km, in a lapse time window of 30 s. The reason for the selection of such short distance (< 80 km) and narrow lapse time (30 s) was to avoid coda waves reflected from deep scatterers, which ultimately helps compare and contrast the attenuation of shallower structures in the study area. Investigation of lateral attenuation variation demonstrated that in the northwest of Sahand volcano (in station AZR), in the northwest of Sabalan volcano (in station SRB) and around Marand (station MRD), the attenuation underwent a faint increase relative to other areas. Because of the shortage of significant lateral variations in the study area, we presented an average frequency relationship for coda quality factor in a lapse time window of 30 s as Qc = 68± 1 f 0. 84± 0. 01. The low amount of the quality factor (= 68) in the mentioned lapse time window reveal the thermal effects of the study area on the estimation of the quality factor. In order to investigate the depth variation of Qc, seventeen lapse time windows from 30 s to 100 s (time interval of 10 s) were extracted for two different datasets, one including an epicentral distance <= 80 km, the other comprised of a distance range of 80-150 km. The Qc factor was calculated for each lapse time in both datasets. The obtained quality factor indicated that Q0increased with the augment in the lapse time due to the effects of wave propagation inside the deeper parts. Frequency relationship parameter presented unexpected variations; it increased with the increase in the lapse time which is the opposite of typically-observed trends. Anomalous variations in frequency relation parameter versus the lapse time show heterogeneous uppermost mantel beneath the study area. The average frequency parameter obtained in this research was ~ 1. 0, a value indicating that the frequency dependency of lithospheric attenuation is negligible in NW Iran.