Seismotectonic investigation of the Alborz Mountains, Northern Iran, by means of an innovative algorithm for seismic moment tensor retrieval
Final Report Abstract
For the first time it is possible to retrieve regional seismic moment tensors comprehensively in the Alborz mountains. Two main steps lead to this result. First, we set-up a 1D layered seismic velocity model appropriate for modelling the dispersion charcateristic of the surface waves and the shape of the body waves. Second, we aimed to include first-motion body-wave polarity data from shortperiod station into the inversion of broadband data. However, it turned out that a combined inversion of the entire short-period waveforms together with the broadband waveforms works surprisingly well. Thus, the short-period data can be exploited much better and the benefit for the moment tensor results is much larger. Hence, the magnitude threshold, for which data processing is possible, could be decreased from Mw 5.0 to Mw 3.0. Thus, the amount of available data could be increased from about 5 to about 65 per year on average and successful inversion is now possible where it so far failed or was not stable enough for further scientific usage. Therefore, gaps in the spatial coverage of seismic moment tensors can be closed. In addition, the inversion for moment tensors allowed to retrieve well resolved centroid depths for the analysed earthquakes. A temporal and spatial comprehensive catalogue of regional moment tensors with reliable depth information is crucial for further studies on e.g. tomography, seismotectonics, and hazard. By applying the improved velocity model and the new inversion approach a first and more comprehensive catalogue of moment tensors, comprising 40 solutions within the entire mountain range, was established. From this initial catalogue already several important information about the seismotectonics of the Alborz mountains could be retrieved. We did not expect so much understanding from these few solutions and are very satisfied with the seismotectonic results. It demonstrates the large benefit of the technical improvements developed in this project for the study of seismotectonics in this region. The now available catalogue of moment tensors can be expanded by further exploring the available data from past earthquakes and successive processing of new data from future earthquakes. It turned out that the determined velocity model is not only applicable for the Alborz mountains but also for northwestern Iran, at least within the low frequency range. The NW Iran was struck by an earthquake doublet (Mw 6.4 and 6.2) in August 2012. Because the region faced no noteworthy seismicity, neither in historical nor modern times, this doublet attracted our attention. By retrieving moment tensors for the two mainshocks and several larger aftershocks, we were able to retrieve detailed insight into the tectonics of the affected rock volume but could also derive important understandings about the tectonics of the general region. The technical developments established in this project are not only applicable within the Alborz mountains or northern Iran. Our strategy can be applied wherever only a few and insufficiently distributed broadband stations have so far prevented the successful inversion for reliable moment tensors. By reducing the magnitude threshold for data processing it also assists in completing already existing catalogues of moment tensors by adding solutions of smaller magnitude earthquakes.
Publications
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(2009), Earthquake source models for earthquakes in Northern Iran, Conference on the tectonics of Iran, 1.7.-4.7.2009 in Cambridge (Great Britain)
Donner, Stefanie, Manfred Strecker, Dirk Rößler, Abdolreza Ghods, Frank Krüger, Angela Landgraf, and Paolo Ballato
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(2011), Source mechanisms of the 2004 Baladeh (Iran) earthquake sequence from Iranian broadband and short-period data and seismotectonic implications, AG Seismologie, 27.-29. September 2011, Sankelmark
Donner, Stefanie, Dirk Rößler, Frank Krüger, Abdolreza Ghods, and Manfred Strecker
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(2013) Segmented seismicity of the Mw 6.2 Baladeh earthquake sequence (Alborz mountains, Iran) revealed from regional moment tensors. Journal of Seismology, v. 17, no. 3, pp. 925-959
Donner, S., Rößler, D., Krüger, F., Ghods, A., Strecker, M. R.
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(2013), Seismotectonic implications for the Alborz mountains, Iran, from regional moment tensors. PhD thesis, University of Potsdam
Donner, Stefanie
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(2013), The Ahar-Varzeghan (Iran) double earthquakes of August 11th, 2012, Geophysical Research Abstracts, 15(6928), EGU General Assembly, 07-12 April, 2013, Vienna
Donner, Stefanie, Abdolreza Ghods, Frank Krüger, Dirk Rößler, Angela Landgraf, Paolo Ballato, A. Zanjani, and G. Mortezanejad
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(2014), Combined inversion of broad-band and short-period waveform data for regional moment tensors: A case study in the Alborz mountains, Iran, Bulletin of the Seismological Society of America, 104(3), pp. 1358-1373
Donner, Stefanie, Frank Krüger, Dirk Rößler, and Abdolreza Ghods
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(2014), Seismotectonic implications for the Alborz mountains, N Iran, from regional seismic moment tensors, 74. Jahrestagung der Deutschen Geophysikalischen Gesellschaft (DGG), 10.-13. März 2014, Karlsruhe
Donner, Stefanie, Frank Krüger, Dirk Rößler, Abdolreza Ghods, Angela Landgraf, and Paolo Ballato
