Tectonics
M. Kouhpeyma; M. Talebian; L. Chen; A. Ghods; M. Ghorashi
Abstract
The Kopeh Dagh and Binalud-Alla Dagh mountains are important structural elements located in the northeastern boundary of Arabia-Eurasia collision zone. Due to existence of large cities with a long history of civilization, there is a relatively rich body of data on historical seismicity in this area. ...
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The Kopeh Dagh and Binalud-Alla Dagh mountains are important structural elements located in the northeastern boundary of Arabia-Eurasia collision zone. Due to existence of large cities with a long history of civilization, there is a relatively rich body of data on historical seismicity in this area. Nonetheless, little adequate instrumental seismic data were available prior to this study. In this paper, we utilized the temporary China-Iran local seismic network data deployed in the area for 13 months. Based on this data, we determined 37 focal solutions through first motion polarity approach suggesting a combination of strike-slip, reverse and normal mechanisms. There is a significant concentration of epicenters and focal mechanisms around the eastern Alborz - Kopeh Dagh boundary along the Atrak River. The southern and western boundaries of the Binalud Mountains also show relatively high seismic activities. The Local magnitude (ML) of the events ranges from 3.5 to 4.9 with depths of up to 20 km, mostly concentrated at ~10 km. The principal stress axes and slip vectors obtained from the focal mechanisms agree well with kinematic state of main faults and tectonic regime of the area, confirming the seismogenic nature of these faults. In addition, they are consistent with right-lateral component of slip along thrust faults in eastern Kopeh Dagh and Binalud and both laft- and right-lateral motions in the central Kopeh Dagh.
Sedimentology
Najmeh Etemad-Saeed; Mahdi Najafi; Navid Zeinolabedin Qavim; Ghods Abdolreza
Abstract
The present study provides a detailed facies and depositional environment analyses of the Neogene sediments in the northern Dezful embayment, footwall of the Zagros Mountain Front fault. The Neogene sediments in this area, including the Mishan, Aghajari, and Bakhtyari formations, constitute the thickest ...
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The present study provides a detailed facies and depositional environment analyses of the Neogene sediments in the northern Dezful embayment, footwall of the Zagros Mountain Front fault. The Neogene sediments in this area, including the Mishan, Aghajari, and Bakhtyari formations, constitute the thickest Zagros foreland succession, with more than 5 km of thickness. As these sediments were deposited synchronous with the main phase of the Zagros orogeny, they have recorded the history of deformation events. During the current study, 9 lithofacies have been identified on the basis of lithology, grain size, sedimentary structures, and bed geometry, which can be classified into three facies assemblages including: gravel dominated (Gm, Gp), sand dominated (Sh, St, Sp, Sr), and mud dominated (Fm, Fl). Furthermore, two major marine ichnofacies, the skolithos and the Cruziana ichonafacies, have been identified at the base of the Aghajari Formation. As the results indicated, the vertical stacking of facies represents an overall shallowing-upward succession that was deposited in marginal marine (Mishan), siliciclastic shoreline (base of Aghajari), meandering river (top of Aghajari) and braided river (Bakhtyari) depositional environments, from base to top respectively. Considering eustatic sea-level fluctuations in the deposition period (13 to 3 Ma), these results propose that evolution of the Neogene sedimentary basin in the North Dezful likely controlled by tectonic folding and faulting.
M Nemati; A.R Ghods; M.R Ghassemi
Abstract
Due to absence of an appropriate scale for estimation of ML for the earthquakes in eastern Alborz Range, we calculated 1113 synthetic Wood-Anderson peak amplitudes from waveforms of 215 earthquakes recorded by 23 stations at local hypocentral distances. The events were recorded by two local temporary ...
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Due to absence of an appropriate scale for estimation of ML for the earthquakes in eastern Alborz Range, we calculated 1113 synthetic Wood-Anderson peak amplitudes from waveforms of 215 earthquakes recorded by 23 stations at local hypocentral distances. The events were recorded by two local temporary seismological networks installed during 2007 and 2008 by the Geological Survey of Iran (GSI) and the stations of the permanent network of the Institute of Geophysics of University of Tehran (IGUT). Both temporary networks were installed for two discontinuous periods of nine months in the eastern- middle Alborz. In order to estimate an empirical attenuation curve for ML amplitudes, A, read from the stations at very short hypocentral distances, we fit a parametric relationship to the peak amplitude readings while considering geometrical spreading, intrinsic attenuation and stations corrections. We obtained the following empirical attenuation relationship:
Log Aij=-1.986log (Rij/100)-0.00452(Rij-100)-3+Sj
Where Rij is hypocentral distance in km between the jth station pair and ith earthquake and Sj is value of station correction for the jth stationThe realtionship clearly indicates a larger attenuation for shear waves in short hypocentral distances below 20 km. Our new ML relationship implies that using ML relationship derived for hypocentral distances larger than 50 km would overestimate ML magnitude of events recorded by our local networks by about half of unit magnitude. Thus we suggest that for local networks in other regions lacking any local ML relationship, ML relationship derived in this study to be used.
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 .
