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Microseismicity and seismotectonics of the South Caspian Lowlands, NE Iran

Nemati, Majid and Hollingsworth, James and Zhan, Zhongwen and Bolourchi, Mohammad Javad and Talebian, Morteza (2013) Microseismicity and seismotectonics of the South Caspian Lowlands, NE Iran. Geophysical Journal International, 193 (3). pp. 1053-1070. ISSN 0956-540X. doi:10.1093/gji/ggs114.

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This paper is concerned with the microseismicity and seismotectonics of the eastern South Caspian Sea region, where the East Alborz mountains descend to meet the South Caspian Lowlands of NE Iran. To better understand the present-day tectonics and seismicity of this region, which includes the cities of Gorgan and Gonbad-e-Qabus (combined population 500 000), we installed a temporary local seismic network across the area for 6 months between 2009 and 2010. We analysed the seismicity and focal mechanisms together with data from the permanent networks of the Institute of Geophysics, University of Tehran (IGUT) and the International Institute of Earthquake Engineering and Seismology (IIEES), based in Tehran. Microseismicity is focused primarily on the Shahrud fault system, which bounds the east Alborz range to the south. Relatively few earthquakes are associated with the Khazar thrust fault, which bounds the north side of the range. A cluster of shallow microseismicity (<15 km depth) occurs 40 km north of the Khazar fault (within the South Caspian Lowlands; SCL), an area typically thought to be non-deforming. This area coincides with the location of three relatively deep thrust earthquakes (M_w 5.3–5.5) which occurred in 1999, 2004 and 2005. Inversion of teleseismic body waveforms allows us to constrain the depth of these earthquakes at 26–29 km. Although significant sedimentation throughout the SCL obscures any expression of recent fault activity at the surface, focal mechanisms of well-located events from the shallow cluster of micro-seismicity show a significant component of left-lateral strike-slip motion (assuming slip occurs on NE–SW fault planes, typical of active faults in the region), as well as a small normal component. Inversion of traveltimes for well-located events in our network yields a velocity structure for the region, and a Moho depth of 41 km. The pattern of deep thrust and shallow normal seismicity could be explained by bending of the rigid South Caspian crust as it underthrusts the East Alborz mountains and Central Iran. Late Quaternary reorganization of drainage systems in the SCL may be the result of shallow normal fault activity within the SCL.

Item Type:Article
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URLURL TypeDescription DOIArticle
Hollingsworth, James0000-0003-0122-296X
Zhan, Zhongwen0000-0002-5586-2607
Additional Information:© 2013 The Authors. Published by Oxford University Press on behalf of The Royal Astronomical Society. Accepted 2012 December 18. Received 2012 December 17; in original form 2012 January 24. First published online: March 26, 2013. We thank the Geological Survey of Iran for organizing all aspects of the fieldwork, and providing seismological instruments. We also thank the IGUT and the IIEES in Iran, who kindly provided the records from their seismological networks. We are particularly grateful to the people and governor of Golestan province, who supported our study and wished to learn more about the seismic hazard posed by active faults in this region. We are grateful to James Jackson and an anonymous reviewer for their careful reviews. This study benefited from useful discussions on the geology and seismology of the region with Hamid Nazari, Mohammad Tatar and Shahryar Soleimani Azad. Ali Moradi provided important technical guidance. JH would like to thank the Gordon and Betty Moore Foundation and the Tectonic Observatory at Caltech for financial assistance. Various images in this paper were created using the public domain Generic Mapping Tools (GMT) software (Wessel & Smith 1998). Quickbird satellite images were obtained from GoogleEarth, Landsat7 images from the Global Land Cover Facility at the University of Maryland, and SRTM digital topography from the CGIAR Consortium for Spatial Information. This is Tectonic Observatory contribution number 224.
Group:Caltech Tectonics Observatory, Seismological Laboratory
Funding AgencyGrant Number
Gordon and Betty Moore FoundationUNSPECIFIED
Caltech Tectonics ObservatoryUNSPECIFIED
Subject Keywords:Earthquake source observations; Seismicity and tectonics; Continental neotectonics; Tectonics and landscape evolution; Crustal structure; Asia
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Other Numbering System NameOther Numbering System ID
Caltech Tectonics Observatory224
Issue or Number:3
Record Number:CaltechAUTHORS:20130715-105740216
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:39360
Deposited By: Tony Diaz
Deposited On:12 Aug 2013 21:28
Last Modified:09 Nov 2021 23:44

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