Geophysics
Mojtaba Naghavi; Habib Rahimi; Ali Moradi
Abstract
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. ...
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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.
Geophysics
Jafar Jaghouri; Hossein sadeghi; Sayyed Keivan Hosseini
Abstract
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 ...
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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.
Reza Emami; Reza Rezaei
Abstract
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 ...
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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.
M Naghavi; H Rahimi; A Moradi
Abstract
The energy of a seismic wave decays while passing through a “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 from which useful ...
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The energy of a seismic wave decays while passing through a “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 from 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 from 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, where 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 from 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 from 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 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 < 100 km (data set 1) and 100 < R < 200 km (data set 2). As the quality factor is related to the heat flow, as the mechanisms show, it decrease with increasing temperature, because active region greater absorption than stable region. We should note that in this study the results, are taken as mean values of each propagation-path. It is observed that generally with increasing coda wave lapse-time, Q0 (quality factor at 1 Hz) and n (frequency dependence factor) values show increasing and decreasing trend, respectively.
Determinations of and n in the attenuation relationships for different tectonic regions, have been the focus of many studies. Both these parameters appear to represent the level of tectonic activity of a seismic region. According to the results, we observed well correlation between reported lithosphere thickness and trends of and n in longer lapse-times (larger depths). The lateral variation of correlates well with the large scale tectonic units of the studied area. According to the results obtained in this study NTF, Sahand volcano and its surrounding regions are characterized by relatively low and a high gradient of can be observed in the region. Furthermore NW Iran is a region of significant geothermal activity and anomalously high crustal temperatures. These geothermal activities result in smaller values for quality factor and higher attenuation of seismic waves. Such of the and n variations can be attributed to variability in the depth and severity of the crustal velocity gradient.
Kh. Motaghi; A. R. Ghods; H. R. Siahkoohi
Abstract
Study of ground motion attenuation in Tehran region is a very important aspect of determining a more precise hazard map of the city. For the last 10 years, three short period seismic networks have been operating in the study region by the Institute of Geophysics, University of Tehran (IGUT). We ...
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Study of ground motion attenuation in Tehran region is a very important aspect of determining a more precise hazard map of the city. For the last 10 years, three short period seismic networks have been operating in the study region by the Institute of Geophysics, University of Tehran (IGUT). We have selected 47 events recorded by IGUT stations during 1996-2004 to estimate attenuation parameters for the study area. The selected events have provided 480 records with good spatial resolution. Only records with signal-to-noise ratio of greater than 4 have been selected. To find the distances at which the nature of geometrical spreading attenuation (R-b) changes significantly, we use a local regression smoothing method called Robust Lowess. It is found that a tri-linear function having hinges at distances about 106±10 and 191±10 km describes the geometric spreading attenuation with distance. Using a tri-linear regression analysis, we found that b1=1.1±0.1, b2=-0.4±0.1, b3=0.5 minimize the average absolute value of the residuals at a frequency of 4 Hz. The remaining attenuation is assumed to be caused by anelasticity. Using anelastic attenuation at different frequencies, the quality factor in Tehran region is obtained as .
H. Ghasemi; E. Zabihi; A. Roshandel Kahoo; M. Zare
Abstract
Strong motion records from the 2002 Bam Earthquake are used to investigate the shape of attenuation function in several frequencies ranging between 0.8 to 18 Hz. Amplitude spectra of the records are inverted to find source scaling factor and attenuation parameters using Singular Value Decomposition ...
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Strong motion records from the 2002 Bam Earthquake are used to investigate the shape of attenuation function in several frequencies ranging between 0.8 to 18 Hz. Amplitude spectra of the records are inverted to find source scaling factor and attenuation parameters using Singular Value Decomposition method (SVD). After correcting the obtained attenuation functions for geometrical spreading effect, the values of follow the frequency dependent relation, , which is in good agreement with similar results in other tectonically active regions in the world and could be served as a basic input parameter for future seismic hazard studies in the region of bam.
