جستجوی مقالات مرتبط با کلیدواژه « Quality factor » در نشریات گروه « زمین شناسی »

In this study, coda wave decay parameter, coda Q, is evaluated using waveform data of three networks in the east and northeast of Iran based on the single-scattering method. The database includes 300 earthquakes with magnitudes in the range of 2.5-6.0 recorded in 2012 to 2020. The waveforms were gathered from the Iranain Seismological Center (IRSC) of the University of Tehran, National center of Broadband Seismic Network of Iran (BIN) at the International Institute of Earthquake Engineering and Seismology (IIEES) and Seismological Earthquake Research Center of the Ferdowsi University of Mashhad (FUMSN). The coda Q for the east-northeast of Iran was calculated as Qc = 125f 0.76 for a lapse time of 40s and optimal parameters of our database. We also evaluated coda Q based on parameters proposed by Havskov et al. (2016) as Qc = 90f 0.87 and as can be seen, the difference in the parameters leads to the different results. Furthermore, the effect of different parameters in estimating the quality factor of coda waves was investigated. The frequency dependence of the wave attenuation coefficient is evident in this region. Considering the 10-year interval of earthquakes used in the east and northeast of of Iran in this study, the obtained results indicate high seismicity rate and continuous seismic activity in the study area.   Recently some studies have been conducted to study the seismic wave propagation effects in this region. For example, Safari et al. (2020) investigated the attenuation of seismic waves in the Fariman region based on 122 local earthquakes recorded in a temporary dense seismic network of IIEES. Based on this research, the frequency relationship Qc = 66f 0.84 for a lapse time of 20s was obtained.The results of the quality factor studies depend on tectonic regime of the study area and various proccesing factors, such as: the lapse time, filter bandwidth, the window length, etc. Therefore, accurate comparison is only possible when similar parameters are considered in different studies.    We compared our results with several studies in different regions of  Iran and other parts of the world. The estimated values of Q0 in this area are in the same range of active regions of the world (Q0 <200) and is near to the results of studies conducted for another active regions of Iran. The results of our study are in good agreement with previous studies and indicate the strong frequency dependency in attenuation of seismic waves in northeast of Iran Plateaue. The observed small differences in coda Q estimates originate from differnet tectonic rigions and  processing parameters.

Tabriz city with 1.6 m population is located in the NW Iran. Estimation of empirical attenuation relations for the region is a key for realistic seismic hazard assessments. Recently, Motaghi et al. (2016) have employed microseismicity of the region to estimate an attenuation relationship for the NW Iran. Their regression has left significant amplitude residuals at different stations where neighboring stations have similar average residuals confirming important lateral structural variations ignored in the attenuation estimations. For instance, stations located around the NTF have systematic negative residuals consistent with location of a thick sedimentary basin in the region. Such systematic residual patterns have been reported before in Canada and North America by Atkinson (2004) or in Alborz Mountains, north Iran, by Motaghi and Ghods (2012). These observations have inspired us to conduct 2-D amplitude residual tomography similar to widely used local travel time tomography (e.g., Rawlinson and Sambridge, 2003). We assume that attenuation variations are only affected by anelastic coefficient variations generated by change of rock properties. Thus, we have discretized the study region and considered anelastic attenuation coefficient (inverse of quality factor) as an unknown parameter for the inversion procedure. 

Methodology and Approaches:

We formulized a linear relationship between the amplitude residual (as datum) and anelastic coefficient variation (as unknown parameter), and then, we analyzed seismograms recorded from 943 local earthquakes with magnitudes between 1.6 and 5.2, and azimuthal gap less than 250o . The data were gathered by 35 seismic stations located in the NW Iran to carry out the inversion. We used a weighted damped least squares approach (e.g. Menke, 1989) in which weight matrices for the data and model parameters were used. The weights for the data come from the signal to noise ratio calculated for each signal. The weights for the model parameters come from the number of rays per block. This helps to exclude model parameters associated with blocks not crossed by rays. An optimal damping value of 23 was selected from the trade-off curve between the total residuals and the weighted model variances. The inversion procedure estimated variations of intrinsic attenuation coefficient relative to the average value (0.0012). We converted the obtained values into change of quality factor, ΔQ, which was more usual in the literature.

Results and Conclusions

Our tomogram for the NW Iran showed a lower quality anomaly for Neogene sedimentary basin around the North Tabriz Fault in contact with high quality Cretaceous volcanic and Cambrian metamorphic basement outcrops in the north of the region. Such clear consistency between our tomogram and lithology variations at the surface disappeared in the eastern part of the tomogram where deep (depth = 20–45 km) events occurring in the oceanic-like crust of the South Caspian Basin are observed. Comparing our eastern anomalies with a teleseismic tomography across the same region, we found that low and high quality patches in eastern part of our tomogram are probably generated by thermal effects of Sabalan volcano and oceanic like crust of the South Caspian Basin, respectively. Good consistency between our results and previously reported features confirms that the amplitude residuals are proper data set to detect structural heterogeneities responsible for large amplitude variations in active seismic regions.

The shear wave quality factor is one of the key parameters for earthquake and engineering seismology studies. In the present study, this factor was investigated in the region of the 2012 Ahar-Varzaghan twin earthquakes. We used a generalized inverse method based on the shear wave windows of the aftershocks of these two earthquakes. 2860 records of broadband three-components and 540 records of short-period single-component from 1,650 aftershocks with a magnitude of 0.2 to 3.0 were used. The records of the aftershocks obtained within 29 days by a temporal seismic network by the Earthquake Research Center - Ferdowsi University of Mashhad at five broad-band and five short-period stations. Since the generalized inverse method needs a reference site to remove the source effect, the choice of the reference site was done using the results of the H/V spectral ratio method at the broad-band stations, as well as the topographic location of the stations. The frequency dependence of the was regressed in the range of 0.1 to 10 Hz and as Q_s=43 f^1.52 . This low value and high frequency-dependency can be attributed to the high heterogeneity of the crust and the active seismicity of the region.

In this study, we have used recorded local earthquakes by 17 permanet seismic stations to separate intrinsic and scattering attenuation in North-West of Iranian pleateau. Intrinsic and scattering attenuation can be applied as useful tools to study the geodynamic and tectonic characteristics of a region. They also represent thermal, compositional and deformational characteristics of the crust and upper mantle. The wave attenuation has strong correlation with seismicity and heterogeneity of medium and is regularly used in the study of tectonically active regions of the world. Single backscattering and coda normalized methods are used to estimate the coda Q (Qc) and Qs respectively, using 14,969 earthquakes which are recorded by the stations. The results show this region is very active region tectonically and seismically. Due to low values of Quality factor and thus high attenuation values of body and shear waves in North West part of Iran, amplitude of the propagated waves are decreased severely in the interested area.The intrinsic attenuation and the Coda wave attenuations curves around the North Tabriz fault are closer in comparison with entire northwestern Iran region and Tabriz city, indicating a strong attenuation of the earthquake waves around this fault system. Similarly, these curves are closer in Tabriz city than those calculated for the northwestern region of Iran which expresses the overriding intrinsic attenuation from the effect of dispersion. The attenuation effect of seismic waves reduces the damages caused by earthquakes at appropriate distances of faults at the time of earthquake occurrence.

In this study, for estimating of coda wave attenuation in the North of Sanandaj-Sirjan Zone the seismograms of earthquakes recorded at the seismic stations which located at the longitude of 48°-50° and latitude of 38°-40° have been used. The data with an epicentral distance less than 200 km and a signal to noise ratio equal to or greater than 3 were used. Due to the volumetric percolation of the coda waves from the medium, the variation in coda waves quality factor was investigated in both lateral and depth, and the results were compared with the values obtained for other regions of Iran and the world. After applying the intermediate filter on waveforms in 10 frequency bands, the quality factor was estimated, and finally, the mean values of the quality factor and frequency dependence factor in the region by using two single-back-scattering and single-isotropic-scattering obtained QC꞊149 ± 9f0.66 ± 0.03 and QC꞊152 ± 12f0.66 ± 0.03 respectively. The comparison of the results with the values have been obtained in Central Iran (Qc꞊94f0.97), Alborz (Qc꞊79f1.07) and Southeast Zagros (Qc꞊72f1.19) indicate more homogeneity in the shallow depth layers and seismicity is lower than Central Iran, Alborz and Zagros. Also, for investigating the depth variations of the quality factor in the region, 11 lapse time window lengths were used in 5 seconds increments. The window lengths are begin from 10 seconds. In the lower window lengths, the low Q0 values represent high heterogeneity in shallow depths of the earth.

The attenuation quality parameter (Q) is a phenomenological quantity depending on the observations and on the underlying theoretical models. Attenuation of seismic waves is expressed with inverse quality factor (Q-1) and helps understand the physical laws governing the propagation of seismic waves in the lithosphere. Attenuation is often found to be anisotropic (directionally dependent) due to a variety of factors such as the intrinsic anisotropy of the material, the presence of aligned fluid-fractures (Batzle et al., 2005), or interbedding of thin layers with different properties (Zhu et al., 2007). The magnitude of attenuation anisotropy can be much higher than that of velocity anisotropy, and the symmetry of the attenuation coefficient can be different than that of the velocity function (Liu et al., 2007). The observed seismic-wave amplitudes usually decay exponentially with increasing travel distance after the correction for geometrical spreading, and decay rates are proportional to Q−1 that characterizes the spatial attenuation for SH-wave. Qeshm Island, the largest island of the Persian Gulf, is important because of various aspects such as population, economics and some oil and gas reservoirs.
Since the most destructive part of the elastic waves, is the horizontal component of the shear waves, estimation of attenuation of the horizontal component will provide us with very useful information. Horizontal components of shear waves are also affected by the structure of the earth.
In this study, 661 well-located aftershocks are selected and 18342 seismograms are used to the calculation. by rotation of the components, the horizontal part of the shear waves are separated and horizontal shear wave quality factor was determined by using the Coda normalization method in five frequency bands 1-2, 2-4, 4-8, 8-16, 16-32 (Hz) with a central frequency of 1.5, 3, 6, 12, 24 Hz, in the lapse time of 30 seconds. Based on the calculations, the frequency dependence relation for shear waves: QSH = (11 ± 1.2)f(1.2±0.105).
The relationship between the frequency dependence for horizontal shear waves shows the attenuation in the Qeshm Island is very high and consequently the region is seismically active. Besides, a small amount of the quality factor for horizontal shear waves associated with the low velocity of shear wave propagation in the crust that may relate to the presence of gas and oil fluids and some salt dome. In the azimuthal study, the attenuation of horizontal shear waves are calculated in two directions: northeast-southwest and northwest-southeast. For low frequencies, the attenuation in the northeast-southwest direction is close to northwest-southeast direction, which seems the horizontal component of the shear waves are not affected by tectonic structures, so it seems mostly to be dependent on the material of the earth, whereas for high frequencies greater than 6 Hz, there are significant differences between two azimuthal attenuation, that can be due to some small-scale heterogeneity of the region.

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 (

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.

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 short-period 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 was 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 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. .09 for Central Alborz: Motaghi and Ghods, 2012; .2 for North Iran: Motazedian, 2006; .2 for SE Canada and the NE United States: Atkinson, 2004; .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 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, , obtained equal to for frequencies greater than 1.5 Hz. In fact, the values show a U-shaped behavior in all of the frequency ranges and the function that describes it is defined as .
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 short-period 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 was 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 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. .09 for Central Alborz: Motaghi and Ghods, 2012; .2 for North Iran: Motazedian, 2006; .2 for SE Canada and the NE United States: Atkinson, 2004; .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 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, , obtained equal to for frequencies greater than 1.5 Hz. In fact, the values show a U-shaped behavior in all of the frequency ranges and the function that describes it is defined as .

The energy of a seismic wave decays while passing througha “real” medium such as the earth which is not completely elastic. Scattering and attenuation of high-frequency seismic waves are substantial parameters to quantify and to physically characterize the earth medium and which useful information on medium properties can be inferred. The coda waves in seismograms are one of the most prominent observations supporting the existence random heterogeneities in the earth. Determination of source parameters must take into account the proper attenuation characteristic of the wave path. Moreover, it is essential for seismic risk studies and seismic hazard assessment, and consequently for seismic risk mitigation and engineering seismology. Many researchers used coda waves small earthquakes to determine local attenuation properties of the crust. The S-coda has a common amplitude decay curve for lapse time greater than the twice the S-wave travel time.The shape of this decay curve is quantified by using a parameter knows coda attenuation Qc-1. The time domain coda decay method of a single back scattering model is employed to estimate frequency dependence of the quality factor of coda waves modeled using, is the coda quality factor at frequency of 1 Hz and is the frequency parameter. The purpose of this study is to determine the coda quality factors recorded events at 17 stations in the NW of Iranian plateau, using the single backscattering method (Aki and Chouet 1975). Scattering models have been developed in order to infer physical properties of the lithosphere observations of seismic codas. In this study, the coda quality factors of seismic waves have been estimated by using local earthquakes with recorded in NW of Iranian plateau. This region includes major faults such North Tabriz Fault and two volcanoes (Sahand and Sabalan) and many thermal units. The data used in this study consists more than 13000 earthquakes and 26724 high-quality waveform recorded between 2006-2013 by Iranian National Seismic Network (INSN) and Iranian Seismological Center (IRSC) stations to estimate lateral variations of coda wave quality factor. By using these data set, Qc and its frequency dependency were estimated, in NW of Iranian plateau. We also investigated lateral and depth variation of Qc in this region. The average frequency relations for NW of Iranian plateau and around North Tabriz Fault (NTF) are , and, respectively. These values show this region is very active region tectonically and seismically. To investigate the attenuation variation with depth, Qc value was calculated for 18 lapse-times (5, 10, 15, 90s) for two data sets comprising epicentral distance range R

Summary: Southeastern Iran، in Makran region، due to movement of the Arabian plate toward Eurasia، the oceanic crust is subducted beneath the continental crust forming a subductionzone. This movement and subduction has given a special situation to seismicity and geology of this zone. Covering the lake of seismological studies and the effect of geological structure on seismic wave propagation in this region، we have studied effects of the medium on propagated seismic waves from the source to the receiver using recorded data on stations of International Institute of Earthquake Engineering and Seismology from 2005 to 2011. The coda quality factor، Qc، in addition to engineering applications، could be used and provide remarkable information for seismicity studies and analyzing how the subduction of oceanic crust is treating. Regarding the extent of the studying area، it has been divided into three subareas of Southeastern of Central Iran،Southeastern of Zagros and Makran، then for each، the variation of quality factor with depth is evaluated. The “Single Back-Scattering Method” is used for estimate. The frequency dependence of coda wave quality factor for events up to 100 km epicentral distance at each three subareas is determined. Also، the lateral and depth variation of Qc are computed and discussed. In this study، the Qc values are calculated for 12 lapse times (5 to 60s with a step of 5s) for three regions. The frequency dependent relationships of Qc، for SE Zagros varies from Q  (12 1. 1) f (1. 260. 02) c at 5s to Q  (1251. 1) f (1. 070. 014) c at 60s lapse time windows. Similarly، for SE Central Iran، the relationship varies from Q (17 1. 3) f (1. 130. 051) c at 5s to Q  (147 1. 3) f (0. 890. 038) c at 60s lapse time windows; and for Makran region varies from Q  (12 1. 1) f (1. 30. 015) c at 5s to Q  (137 1. 1) f (1. 030. 018) c at 60s lapse time windows. In all regions، the value of Q is less than 200، which implies، beside a highly tectonically and seismically behavior، a highly heterogeneous medium. The results show an increase in Qc value with increasing lapse time window. Among three studied regions، SE Zagros has the minimum Qc and so is more tectonically active compared with the two others. The North Makran and SE Central Iran are places in thenext ranks، respectively. Estimated Qc at bigger lapse time window indicates less attenuation at bigger depth. In Makran region at a depth of ~97 km the variation rate of Q suddenly increases. Based on available geological cross sections، for the eastern and western Makran، depth of about 100km of oceanic crust is in the northern region of the western Makran، which shows good agreement with our observation that oceanic crust has higher velocity، so attenuation is less than continental crust.

The attenuation of seismic waves is one of the basic physical parameters used in seismological studies and earthquake engineering, and is closely related to the seismicity and regional tectonic activity of a particular area. Seismic wave attenuation is caused by two major factors: scattering at heterogeneities in the earth and intrinsic absorption by anelasticity of the earth. The inverse of quality factor represents the attenuation. There are different methods for estimating the quality factor of Shear and Coda waves. In this study, the quality factors of Shear waves (QS) and Coda waves (QC) have been estimated in the Hormuzgan regionin the south of Iran. This region is located in the southeastern Zagros seismotectonic region. Several faults exist in this region, including the Main Zagros Reverse Fault (MZRF), High Zagros Fault (HZF), Zagros Foredeep Fault (ZFF), Mountain Front Fault (MFF) and Minab Fault. Recordings from the Bandar-Abbas (BNDS) station (located in the Hormuzgan region, 27.40º N_56.17º E) of the Iranian National Seismic Network (INSN) of local earthquakes with signal-to-noise ratios greater than 3were used for this study. These events were recorded from June 2004 through August 2009 and registered magnitudes of between 2.5 and 5.1 (ML), epicentral distances of less than 100 km and average focal depths of about 15 km. In this study, the Coda Normalization Method (Aki, 1980) and the Single Back-Scattering Method (Aki & Chouet, 1975) are used for the estimation of QS and QC at the seven central frequencies of 1.5, 3.0, 4.5, 6.0, 9.0, 12.0 and 18.0 Hz. The Shear waves on 183 North-South (N-S) components and 142 East-West (E-W) components, and the Coda waves on 200 Vertical (U-D) components, have been analyzed to estimate QS and QC, respectively. Time windows of the Shear waves were determined by the Kinoshita algorithm and the velocity of the Shear waves was estimated at 3.5 km/s. The estimated frequency-dependent relationships of QS on N-S and E-W components are and, respectively. The QC values were computed at five lapse time windows (20, 30, 40, 50 and 60 sec), starting at double the time of the primary Shear wave from the time of origin. The frequency-dependent relationships of QC vary from at 20 sec to at 60 sec lapse time window. The results show an increase in QC value with increasing lapse time windows. The estimated QC at a greater lapse time window indicates attenuation at a greater depth. In the Hormuzgan region, the values of Q at 1.0 Hz are less than 200 for the frequency-dependent relationships of QS and QC. Therefore, the Hormuzgan region is a highly tectonically and seismically active region; also, the medium is highly heterogeneous. The results reflect sedimentary deposits and salt domes in the Hormuzgan region. The QS frequency-dependent relationship in the Hormuzgan region is similar to that of the Ardabil and Avaj regions in northwestern Iran, and of the Strait of Messina in the south of Italy. Moreover, the QC frequency-dependent relationship in the Hormuzgan region is similar to that of the Alborz and Zagros regions in Iran and in the northwest of the Himalayan region. These regions are all tectonically and seismically active.

Seismic waves lose energy by traveling through the earth. Attenuation refers to the loss of energy which is caused by parameters other than geometrical spreading, and depends on the characteristics of the transmitting medium. Generally, attenuation is determined by quality factor (Q) which is a dimensionless parameter and has a reverse relation with attenuation coefficient. Experiments show that seismic wave quality factor depends on the fluid content of formation and its elastic properties. Hence, quality factor (Q) (or attenuation) is one of the most important attributes in seismic exploration used as a direct hydrocarbon indicator. Attenuation coefficient is usually calculated in frequency domain based on power spectrum and statistical methods. Because of the Fourier transform's limitations in analyzing non-stationary signals, this paper proposes an approach using three different harmonic analysis methods and two different attributes, namely the centroid of scale and the centroid of frequency, to determine the seismic quality factor (Q). The three methods consist of the continuous wavelet transform (Mallat, 1999) with modified Morlet wavelet, the smoothed pseudo Wigner-Ville distribution, and the reassigned form of the smoothed pseudo Wigner-Ville distribution (Auger and Flandrin, 1995). Because the Wigner-Ville distributions are energy distribution type, they were used in this study to determine the seismic quality factor. Since both the traditional Wigner-Ville distribution and pseudo Wigner-Ville distribution suffer from cross terms, the smoothed and reassigned forms of them were implemented to overcome the cross terms. Smoothing extends the auto terms in Wigner-Ville distribution which is not desirable for the purposes of this study. Therefore, by using the reassigned method we solved the problem of auto terms extension in the smoothed pseudo Wigner-Ville distribution. According to the results of this study, the reassigned smoothed pseudo Wigner-Ville distribution indicates fewer cross terms than traditional Wigner-Ville and provides better resolution than smoothed pseudo Wigner-Ville distributions. We can verify that for a spike, as an ideal seismic source wavelet, the centroid of scale is inversely proportional to the quality factor (Equation (1)), while the centroid of frequency has a direct relation with the quality factor: (1) where is the centroid of scale, is time, is the modulation frequency, and is the quality factor. For a band-limited seismic wavelet, the estimated quality factor using Equation (1) differs from the true value. In such cases, there will be a relative reverse relation between the centroid of scale and quality factor as well as relative direct relation between the centroid of frequency and quality factor. Therefore, it is preferred to use the centroid of scale or centroid of frequency as a qualitative attribute for Q-factor determination. The efficiency of the introduced methods was investigated on both synthetic and real seismic data. The results showed that the quality factor estimated by the reassigned smoothed pseudo Wigner-Ville distribution method has better resolution than that of the other two transforms. Furthermore, the frequency-based results for the estimated quality factor show the low frequency shadow properties beneath the true position of the anomaly while the same results based on amplitude show the anomaly at its true position. Moreover, the results indicate that the existence of noise in the data do not affect the efficiency of the methods.