J Asadifallah; M Zare; M.R Gheytanchi
Abstract
Natural frequency of soil is an important factor in site effect studies. In order to determine this parameter, microtremor measurements were applied as a fast, simple and economic tool. For this purpose, microtremors were recorded in 74 points with time durations of 5 to 10 minutes throughout the city ...
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Natural frequency of soil is an important factor in site effect studies. In order to determine this parameter, microtremor measurements were applied as a fast, simple and economic tool. For this purpose, microtremors were recorded in 74 points with time durations of 5 to 10 minutes throughout the city of Shiraz. Natural frequency and amplification factor of soil were then estimated considering the main peak in spectral ratio between horizontal and vertical components (H/V) that is also known as Nakamura’s technique. Fourier and power spectrums were also used to determine this spectral ratio. The results demonstrate that natural frequency varies from 0.5HZ to 6HZ in different parts of the city, so that it increases in the east and north part of the city where alluvial deposits over the bedrock are thin. Also, amplification factor of the alluvial deposits obtained by this method has values between 1.3 and 5 throughout the city. In order to investigate time-dependence of the natural frequency, after 8 years of data recording new data were recorded in five stations. The results indicate acceptable correlation and consistency between old and new data results in these five stations.
M Abdetedal; Z.H Shomali; M.R Gheitanchi
Abstract
The Makran zone in southeastern Iran and southern Pakistan is the result of subduction of oceanic crust of the Arabian Plate under the Eurasian Plate. From seismic behavior point of view, there is a distinct segmentation between the western and eastern parts of the subduction zone. The western part of ...
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The Makran zone in southeastern Iran and southern Pakistan is the result of subduction of oceanic crust of the Arabian Plate under the Eurasian Plate. From seismic behavior point of view, there is a distinct segmentation between the western and eastern parts of the subduction zone. The western part of the Makran has an abnormally very low level of deep seismicity with lack of recorded great earthquakes, while the eastern part has experienced many great earthquakes. Another difference between the western and eastern parts of the Makran region is that the distance between the Quaternary volcanic arc and fore-arc setting is larger in the east than in the west. Understanding the nature of unusual behaviors of the Makran subduction zone has long been one of the biggest challenges in seismotectonic investigations of this region. The present study aims at producing high-resolution love-wave velocity structure maps of the crust and the upper mantle in the Makran subduction zone using ambient seismic noise. To achieve this purpose, a large dataset has been provided to produce tomographic maps. Empirical Green’s functions were obtained from cross-correlations of broad-band seismic noise records at different stations inside and outside the region. Love-wave velocity dispersion curves were then extracted from the ambient noise, and finally converted into a 2D group velocity image (or tomography map) for crustal and upper mantle structures of the region.
M. Nemati; D. Hatzfeld; M. R. Gheitanchi; M. Talebian; N. Mirzaei; A. Sadidkhouy
Abstract
The Shahroud fault system plays important role in seismotectonic of the eastern Alborz. In this paper we have surveyed the seismicity of the middle-eastern Alborz and its southern area. At this investigation, the data of the Geological Survey of Iran local seismological networks, the seismological networks ...
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The Shahroud fault system plays important role in seismotectonic of the eastern Alborz. In this paper we have surveyed the seismicity of the middle-eastern Alborz and its southern area. At this investigation, the data of the Geological Survey of Iran local seismological networks, the seismological networks of the Institute of Geophysics of the University of Tehran and the International Institute of Earthquake Engineering and Seismology of Iran were used for processing the focal mechanism of micro-earthquakes and the south of Damghan earthquake and its greatest aftershock. Distribution of the micro-earthquakes and the south of Damghan events epicenters indicate intense activity of the Shahroud fault system and the Toroud fault. Focal mechanisms of them shows near vertical dipping of the faults and left lateral mechanism of the western segments of the fault system and the Toroud fault. The focal mechanisms suggest the Astaneh, Chashm and Firouzkuh faults from the system fault behave in a same manner with no deference between them at depth and have seismic potential proportion to their total length. Also due to left lateral mechanism of the south of Damghan earthquakes, Toroud fault treats like of the eastern Alborz seismotectonically and this area could cover Toroud fault.
N. Afsari; F. Sodoudi; M. R. Gheitanchi; A. Kaviani
Abstract
Characterization of the detailed structure of the crust and upper mantel is an important continuing goal of geophysical studies. Teleseismic body waveforms have been used to infer crust and upper mantel structure. In this study we use teleseismic receiver function method to determine the crustal thickness ...
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Characterization of the detailed structure of the crust and upper mantel is an important continuing goal of geophysical studies. Teleseismic body waveforms have been used to infer crust and upper mantel structure. In this study we use teleseismic receiver function method to determine the crustal thickness and VP/VS ratio under Kermanshah network in north-west of Zagros using teleseismic data (30°<D<95°, mb≥5.5) which have been recorded at five short-period three component stations (2003-2007). The differential travel time between the incident P wave and P to S converted wave (delay time) is used for computation of crustal thickness. Moho depth is not sensitive to crustal P velocity but there is a trade off between Moho depth and Vp/Vs ratio. The ambiguity can be reduced by incorporating the later multiple converted phases, namely, PpPs and PpSs+ PsPs. We use the method of Zhu and Kanamori (2000). This method performs a grid search through the H and Vp/Vs space and searches for the largest summed amplitudes at the predicted times of direct conversions and multiples. By stacking receiver functions from different distances and directions, effects of lateral structural variation are suppressed and an average crustal thickness is obtained. Applying this method to five short period stations in Kermanshah reveals that the Moho depth is 42 km and varies between 36 and 51.5 km. The thinnest crust was found beneath DHR, whereas the deepest crust was observed beneath VIS. We observed that Moho depth increase from west to east (from DHR to VIS) then decrease to KOM. The average VP/VS ratio as estimated is about 1.76.
F. Taghizadeh-Farahmand; F. Sodoudi; M. R. Gheitanchi; A. Kaviani
Abstract
We compute p receiver functions (RF) to investigate the crustal thickness and Vp/Vs ratio beneath the northwest of Iran and map out the lateral variation of Moho depth under this region. We selected data from teleseismic events (Mb≥5.5, 30˚>r > 95˚) recorded since 2000 to present at ...
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We compute p receiver functions (RF) to investigate the crustal thickness and Vp/Vs ratio beneath the northwest of Iran and map out the lateral variation of Moho depth under this region. We selected data from teleseismic events (Mb≥5.5, 30˚>r > 95˚) recorded since 2000 to present at 8 three component short period stations from Tabriz teleseismic network. RF method is now a well-known tool for studying crustal thickness when such a complete data set is available. As the p-to-s conversion points at the Moho is laterally close to the stations, the Moho depth estimation is less affected by lateral velocity variations and thus provides a good point measurement. First of all, we calculated RFs for each station and then the Moho depth will be estimated only from the delay time of the Moho p-to-s conversion phases. Then we used an H-Vp/Vs stacking algorithm (Zhu & Kanamori ,2000 ) to estimate crustal thickness and Vp/Vs ratio under each station from P-to-S converted waves in receiver functions. The best value for H and Vp/Vs ratio is found when the three phases (Ps and crustal multiples) are stacked coherently. The average Moho depth is approximately 48 km and varies from 38.51 to 53.51 km. Deeper and shallower Moho is found under the western and eastern stations beneath SHB and SRB stations respectively. The northwest Iran's crust has an average Vp/Vs ratio of 1.76, with higher ratio of 1.820.03 in Tabriz station and lower ratio of 1.73 0.03 in Azarshahr station.
M. Eskandari; M. R. Gheitanchi
Abstract
In this article, we studied the dynamic fracture process of Bam earthquake. In two presented models stress heterogeneity on the fault plain was modeled as barrier or asperity and friction included as slip-weakening relationship. Results of models were constrained by near field ground motion recorded ...
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In this article, we studied the dynamic fracture process of Bam earthquake. In two presented models stress heterogeneity on the fault plain was modeled as barrier or asperity and friction included as slip-weakening relationship. Results of models were constrained by near field ground motion recorded in Bam station. In the first model, fracture starts form a weak asperity which its waves surround the neighbor barrier and break it down. In the second model, another asperity is included in southern part of the fault. Breaking barrier releases two fracture fronts traveling in two different regimes. One of them travels faster than shear waves and goes to the intersonic velocity. The other front travels with 0.74 shear wave velocity and makes the largest pulse of the record. Both models predict the slip rate successfully, but the second model is more consistent with the real data.