M Rustaei; M Rustaei; B Zamani; M Nemati
Abstract
In this study, the stress regime governing the Gorgan plain area (NE Iran) is calculated using inversion analysis based on earthquake focal mechanism solutions. In addition, a kinematic model is presented for hidden faults in this area. To gain this goal, the earthquakes occurred in this region and had ...
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In this study, the stress regime governing the Gorgan plain area (NE Iran) is calculated using inversion analysis based on earthquake focal mechanism solutions. In addition, a kinematic model is presented for hidden faults in this area. To gain this goal, the earthquakes occurred in this region and had available focal mechanism solutions were first selected for the inversion analysis. After applying the inverse analysis on the seismic data, the results indicate that there is a NW-SE (N42W)-trending tensional stress regime over the area. This is represented by the dominant local extensional structures such as shallow normal faults. Also the intermediate and minimum stress axes are close to horizontal direction, and indicate structures such as strike slip faults in the region. This is in agreement with most of the seismic dataset involved in the inversion analysis. By examining the previous studies and their results, more attempts were made to provide a kinematic model for this area. The Gharnaveh fault system comprises two sinistral faults (Marave-tappeh and Incheboroun), which are assumed to extend at eastern termination along an E-W direction under the Gorgan plain sediments up close to the Caspian Sea. Movement along these two faults under the sediments cause a clockwise rotation in the zone enclosed between them. Direction of this zone, which is affected by the faults, is consistent with the trend of regional seismicity. The proposed model can be used to account for many of the focal mechanism solutions of the earthquakes occurred, as well as the depth of the events. Most earthquakes occurred in this region are associated with the normal or left-lateral components. On the other hand, superficial structures such as mud volcanoes and fault-related folds (in the north of Aq qala) can be explained by this model, although lack of subsurface geophysical data in this area makes this model somehow speculative.
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.
M.A Shokri; M Foroutan; M Nemati; M.J Bolourchi; SH Javadipour; B Oveisi
Abstract
The Touchahi earthquake of Aug 27, 2010 (MN 5.9; IRSC- Mw 5.7; USGS) occurred at 19:23:49 UTC (23:53:49 local time on 5 Shahrivar 1389) in south of Damghan city. No foreshock were reported before this earthquake whereas 85 aftershocks (MN 1-5) were registered by IRSC until 1 month after the mainshock. ...
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The Touchahi earthquake of Aug 27, 2010 (MN 5.9; IRSC- Mw 5.7; USGS) occurred at 19:23:49 UTC (23:53:49 local time on 5 Shahrivar 1389) in south of Damghan city. No foreshock were reported before this earthquake whereas 85 aftershocks (MN 1-5) were registered by IRSC until 1 month after the mainshock. According to our field study after the event, surface rupture of causative fault was not observed but we measured some fractures related to this event with dominant strike of N120º-140º. According to our observations of 32 towns and villages that were damaged in this seismic event, maximum intensity (I0) of VIII+ in MMI scale occurred near the Touchahi village in ~85 km south of Damghan city. Unfortunately in this earthquake 4 people were killed. Focal mechanisms of the Touchahi seismic event and its greatest aftershock is solved using the first P motion method. The fault plane solution show near vertical plane for the causative fault of the earthquake and suggests a left- lateral mechnism. The mechanisms associated with the fault show mainly left-lateral strike–slip motion, on a NE –SW striking fault plane. Based on location of the earthquake epicenter, its aftershocks location, the fault plane solution (left-lateral strike-slip with N039º strike and dip direction toward NW) and field observations, the causative fault of Touchahi earthquake is one of the active fault branches that is situated in north of Darestan mountain and south of Touchahi, Koohzar and Kooshahi villages. This fault with left-lateral strike-slip mechanism by general strike of NE-SW and dip direction toward NW is indicated as Touchahi fault.
