S Ansari; F Yaminifard; M Tatar
Abstract
In this study, 23 focal mechanisms were calculated by moment tensor inversion of small regionalـlocal earthquakes in the Western Alborz and surrounding areas using wave-form inversion method. Calculated reverse-fault focal mechanisms around the Khazar and Alborz Faults in the Central Alborz, considering ...
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In this study, 23 focal mechanisms were calculated by moment tensor inversion of small regionalـlocal earthquakes in the Western Alborz and surrounding areas using wave-form inversion method. Calculated reverse-fault focal mechanisms around the Khazar and Alborz Faults in the Central Alborz, considering southward-dipping nodal plane as the fault plane, are consistent with relatively low-angle thrusts. It also implies dominant compressionsl regime in the north of the Central Alborz. Focal mechanisms in 1990 Rudbar-Tarom Earthquake region show a combination of strike-slip mechanisms and a complex fault system in the middle of the Western Alborz. A major region of dominant strike-slip mechanisms is observed in the Talesh area, located in the west of the SouthCaspianBasin, and around the Masuleh, Sangavar and Bozqush faults. The only calculated focal mechanism close to the southern margin of the Western Alborz, considering the western part of the North Tehran fault as the fault nodal plane, implies leftـlateral motion in this area. In the south of the western AlborzMountains approaching the Central Iran, two calculated mechanisms indicate dominant reverse movement, similar to the 2002 Changoreh-Avaj Earthquake. Five focal solutions close to the Kushk-e Nosrat and Soltanieh Faults, considering these faults as the fault nodal planes, are consistent with right-lateral motion along them. Depth of the earthquakes in the studied region is in the range of 2 and 20 km, indicating the brittle upper crust in the region.
M Tatar; S.M Momeni; F Yaminifard
Abstract
The V shape kink of the AlborzMountains at its southern end reaches to the Garmsar city located 100 km southeast of Tehran metropolis. We investigated seismicity and seismotectonic features of the Garmsar area by precisely locating of microearthquakes recorded by our local dense seismological network ...
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The V shape kink of the AlborzMountains at its southern end reaches to the Garmsar city located 100 km southeast of Tehran metropolis. We investigated seismicity and seismotectonic features of the Garmsar area by precisely locating of microearthquakes recorded by our local dense seismological network and by the Iranian Seismological Center (IRSC). Our results indicate high seismic activity at the central and western parts of the Garmsar fault. Three computed focal mechanisms revealed compressional movements of the central part of this fault. Very little seismic activity is observed on the Eyvanekey and the Pishva faults. The only computed focal mechanism for the northern hills of the Garmsar fault shows tensional movements in this area, which refers to strain release among the Garmsar and Sorkheh reverse faults. Most of the calculated focal mechanisms in the Garmsar area indicate compressional and strike slip motions with overall P axis direction of 10° to 35°. The calculated P axis with NW-SE trend, close to the Sorkheh fault, is different from the other calculated P axes that show NNE-SSW direction. This is probably due to rotating of structures in this area, as revealed by recent GPS measurements in this region.
Mohammad Tatar; M. R. Ebrahimi; F. Yamini Fard
Abstract
Masjed Soleyman reservoir is located in Zagros Mountain of western Iran, which is one of the most seismically active zones of the Alpe-Himalaya belt. So, it seems to be necessary to carry out widespread studies, especially on the impact of this reservoir with 177 m height and 261 million m3 capacity ...
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Masjed Soleyman reservoir is located in Zagros Mountain of western Iran, which is one of the most seismically active zones of the Alpe-Himalaya belt. So, it seems to be necessary to carry out widespread studies, especially on the impact of this reservoir with 177 m height and 261 million m3 capacity on occurrence of induced seismicity in the surrounding region. During impounding of the reservoir regards to national and international catalogues such as International Institute of Earthquake Engineering and Seismology (IIEES) and EHB catalog, an incensement in seismic rate is observed in the Masjed Soleyman region, and 90 days after completing the impounding, an earthquake with magnitude Mw=5.6 is occurred in the close vicinity of the Masjed Soleyman reservoir. In order to better understanding of the impact of this reservoir on seismic activity, a local seismic network of 5 seismological stations was installed in the area on June 2006. Seismic events recorded during a period of 15 month were used in this study. The largest recorded earthquakes during the monitoring of Masjed Soleyman reservoir have magnitudes of ML=3.9 and ML=3.6. Statistical methods such as correlation of the water level changes with variation of the regional seismicity, the foreshocks and aftershocks pattern and decay rate of aftershocks reveal the existing of induced seismicity in the Masjed Soleyman region. In absence of a local network before 2002 September, 25 earthquake (Mw=5.6), our observations indicate this earthquake is truly the first and largest induced earthquake that have been occurred and recognized in Iran.
