| 1. |
- Eken, Tuna, 1978-, et al.
(författare)
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Effects of seismic anisotropy on P-velocity tomography of the Baltic Shield
- 2012
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Ingår i: Geophysical Journal International. - 0956-540X .- 1365-246X. ; 188:2, s. 600-612
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Tidskriftsartikel (refereegranskat)abstract
- We investigate possible effects of neglecting seismic anisotropy on standard isotropic P-velocity tomographic images of the upper mantle beneath the Baltic shield. Isotropic inversions of teleseismic P- and S-wave traveltimes exhibit alternating high- and low-velocity heterogeneities down to depths of over 400 km. Differences in tomographic inversions of SV- and SH-wave traveltimes are distinct down to depths of about 200 km and are associated with anisotropy of the lithospheric mantle. Anisotropic structures of the upper mantle affect both the P and S traveltimes, shear-wave splitting as well as the P polarization directions. Joint inversion for isotropic and anisotropic velocity perturbations is not feasible due to the limited 3-D ray coverage of available data. Therefore, we correct the input traveltimes for anisotropic contributions derived from independent analyses and then perform standard isotropic inversions. These corrections are derived either directly from directional deviations of P-wave propagation or are calculated in anisotropic models retrieved by joint inversions of body-wave anisotropic parameters (P-residual spheres and shear-wave splitting). These anisotropic models are also used to fit backazimuth variations of P-wave polarization directions. General features of tomographic images calculated from the original and the anisotropy-corrected data are similar. Amplitudes of the velocity perturbations decrease below similar to 200 km depth, that is in the sub-lithospheric mantle. In general, large-scale anisotropy related to the fabrics of the continental mantle lithosphere can contaminate tomographic images in some parts of models and should not be ignored.
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| 2. |
- Eken, Tuna, et al.
(författare)
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Upper-mantle structure of the Baltic Shield below the Swedish National Seismological Network (SNSN) resolved by teleseismic tomography
- 2007
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Ingår i: Geophysical Journal International. - 0956-540X .- 1365-246X. ; 169:2, s. 617-630
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Tidskriftsartikel (refereegranskat)abstract
- Upper-mantle structure under the Baltic Shield is studied using non-linear high resolution teleseismic P-phase tomography. Observed relative arrival-time residuals from 52 teleseismic earthquakes recorded by the Swedish National Seismological Network (SNSN) are inverted to delineate the structure of the upper mantle. The network consists of 47 (currently working) three-component broad-band stations located in an area about 450 km wide and 1450 km long. In order to reduce complications due to possible significant three-dimensionality of Earth structure, events chosen for this study lay close to in-line with the long-axis of the array (±30°) . Results indicate P-wave velocity perturbations of ±3 per cent down to at least 470 km below the network. The size of the array allows inversion for structures even at greater depths, and lateral variations of velocity at depths of up to 680 km appear to be resolved. Below the central part of the array (60°–64° N), where ray coverage is best, the data reveals a large region of relatively low velocity at depths of over about 300 km. At depths less than about 250–300 km, the models include a number of features, including an apparent slab-like structure dipping gently towards the north.
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| 3. |
- Hagos, Lijam, et al.
(författare)
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Re-evaluation of focal depths and source mechanisms of selected earthquakes in the Afar depression
- 2006
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Ingår i: Geophysical Journal International. - 0956-540X .- 1365-246X. ; 167:1, s. 297-308
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Tidskriftsartikel (refereegranskat)abstract
- We present a stepwise inversion procedure to assess the focal depth and model earthquake source complexity of seven moderate-sized earthquakes (6.2 > M-w > 5.1) that occurred in the Afar depression and the surrounding region. The Afar depression is a region of highly extended and intruded lithosphere, and zones of incipient seafloor spreading. A time-domain inversion of full moment tensor was performed to model direct P and SH waves of teleseismic data. Waveform inversion of the selected events estimated focal depths in the range of 17-22 km, deeper than previously published results. This suggests that the brittle-ductile transition zone beneath parts of the Afar depression extends more than 22 km. The effect of near-source velocity structure on the moment tensor elements was also investigated and was found to respond little to the models considered. Synthetic tests indicate that the size of the estimated, non-physical, non-isotropic source component is rather sensitive to incorrect depth estimation. The dominant double couple part of the moment tensor solutions for most of the events indicates that their occurrence is mainly due to shearing. Parameters associated with source directivity (rupture velocity and azimuth) were also investigated. Re-evaluation of the analysed events shows predominantly normal faulting consistent with the relative plate motions in the region.
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| 4. |
- Heidari, Reza, et al.
(författare)
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Magnitude-scaling relations using period parameters tau(c) and tau(max)(p), for Tehran region, Iran
- 2013
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Ingår i: Geophysical Journal International. - : Oxford University Press (OUP). - 0956-540X .- 1365-246X. ; 192:1, s. 275-284
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Tidskriftsartikel (refereegranskat)abstract
- In this study, the first step towards establishing an onsite earthquake early warning system (EWS) in the Tehran region is presented. The system uses the period parameters tau(max)(p) and tau(c) from the first 3 s of the vertical and horizontal components of a P wave, separately and combined. Various regression relations between the magnitude and period parameters were determined for different seismic networks operating in the study area. The data set used in this study contains small ground motions including 194 events with magnitudes between M-L 2.5 and 4.6 located within approximately 80 km from the epicentre in the Tehran region. The SDs of the magnitude-scaling relations for all the component categories (vertical, horizontal and total components) based on the tau(max)(p) and tau(c) approaches were estimated to be on the order of +/- 1.0 and +/- 1.1 unit of magnitude, respectively. These relations were determined from the small magnitude range of the velocity records (M-L 2.5-4.6) as input seismograms. Additional tests were conducted to verify the reliability and robustness of the determined magnitude-scaling relations using acceleration records from the 2002 June 22, M-L 6.5 Changureh-Avaj earthquake; 2004 May 28, M-L 6.1 Firoozabad-Koojour earthquake; 2009 October 17, M-L 3.9 Shahre-Rey earthquake and 2011 February 20, M-L 4.1 Sharif-Abad earthquake; the first two events (Changureh-Avaj and Firoozabad-Koojour) occurred outside the study area. Among the various regression scaling relations obtained, the estimated magnitude based on the tau(max)(p) approach using the vertical components yielded the most stable and reliable results of 6.4 (+/- 0.4), 5.9 (+/- 0.4), 3.3 (+/- 0.5) and 3.8 (+/- 0.3) for the Changureh-Avaj, Firoozabad-Koojour, Shahre-Rey and Sharif-Abad earthquakes, respectively. The magnitudes estimated using the tau(c) method exhibited more scatter with higher SDs than those using the vertical components using the tau(max)(p) approach. Our results also indicate that using the horizontal components produces larger SDs, which may be attributed to the larger site effects; however, the horizontal components can be used as auxiliary available data to provide more constrained information for a multilevels pilot alarm system and to reduce the number of missed or false alarms. The main uncertainties in the proposed magnitude-scaling relations result from the absence of any large earthquakes and poor station distributions in the study area. However, the results presented in this study can be used as a pilot onsite earthquake EWS in the Tehran region.
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| 5. |
- Riahi, Ali, et al.
(författare)
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Simultaneous retrieval of body and surface waves in the Dehdasht area, Iran, from the seismic ambient field and the observation of spurious artefacts
- 2021
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Ingår i: Geophysical Journal International. - : Oxford University Press. - 0956-540X .- 1365-246X. ; 227:2, s. 1193-1203
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Tidskriftsartikel (refereegranskat)abstract
- Recent developments in sensor technology have allowed for the low-cost deployment of dense seismic arrays which continuously record the seismic ambient noise field. Generally, the extraction of body waves from the seismic ambient noise field is more challenging than for surface waves, due to the dominant amplitude of the latter. In this study, we work with data from a dense seismic array deployment in the Dehdasht area, southwestern Iran. We show that by using the polarization properties of seismic waves, we can simultaneously retrieve both high frequency body waves and surface waves from the cross-correlated noise field. As a by-product of this study, we also observe high energy spurious artefacts, particularly those associated with the direct P-phases. Numerical simulations show that these artefacts are a consequence of art uneven distribution of noise sources, and are difficult to suppress during the simultaneous retrieval of body and surface waves.
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| 6. |
- Shad Manaman, Navid, et al.
(författare)
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New constraints on upper-mantle S-velocity structure and crustal thickness of the Iranian plateau using partitioned waveform inversion
- 2011
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Ingår i: Geophysical Journal International. - 0956-540X .- 1365-246X. ; 184:1, s. 247-267
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Tidskriftsartikel (refereegranskat)abstract
- The continental collision between the African and Eurasian plates resulted in a tectonically young and complex deformation in the Iranian plateau. The present-day Iranian plateau is characterized by diverse tectonic domains including the continental collisions (e.g. Zagros and Alborz) and the oceanic plate subduction (e.g. Makran). Partitioned waveform inversion method is used here to image the S-velocity structure of the upper-mantle and Moho-depth variations in the Iranian plateau. Of nearly 3000 waveforms originally selected for the analysis, we have fitted 974 waveforms from 101 events and 39 stations which result in 11 688 linear constraints on the upper-mantle S-velocity structure and Moho depth for the region.Our new seismic images show seismically faster upper mantle beneath the Zagros and the Arabian Plate compared to the Central Iran and Alborz. This high-velocity anomaly has descended beneath the Central Iran along the Main Zagros Thrust. Across the Zagros collision zone, the tomographic images show a slab stagnation in the transition zone (at about 410 km depth) in the form of a horizontal high-velocity zone which does not penetrate the deeper parts of the mantle. The lowest S-velocities are concentrated beneath the Central Iran and Alborz. According to our velocity models, a strong high-velocity anomaly is resolved beneath the trapped South Caspian Basin with clear indication of westward underthrusting beneath the Talesh and western Alborz. In Makran, southeast Iran, there is a clear evidence of subduction of oceanic crust of the Arabian plate beneath the Makran belt which is correlated with seismicity pattern across the Makran zone. Also, the inverted Moho map using a priori information of crustal thickness constraints shows the large crustal thickness beneath the Zagros suture zone (in some places to a maximum depth of 65 km) which indicates the influence of crustal thickening and shortening beneath Arabian-Eurasian Plate boundary. A significantly crustal thinning is observable across the South Caspian Basin compared to its margins.
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| 7. |
- Shirzad, Taghi, et al.
(författare)
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Shallow crustal structures of the Tehran basin in Iran resolved by ambient noise tomography
- 2014
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Ingår i: Geophysical Journal International. - : Oxford University Press (OUP). - 0956-540X .- 1365-246X. ; 196:2, s. 1162-1176
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Tidskriftsartikel (refereegranskat)abstract
- In this study, we present an application of the ambient noise tomography (ANT) to study the near-surface geological structures of the metropolitan Tehran/Iran region. Short-period fundamental mode Rayleigh wave Green's functions were estimated using cross-correlations of the vertical component of the ambient noise from 2009 October to 2011 May using a variety of seismic sensors, for example, accelerometers and seismometers, deployed in the Tehran area. Standard common low frequency processing procedures were applied to the cross-correlations, and shorter time-windows comprising 10-min segments were used in the processing step to enhance the time resolution of the signal in the frequency range of interest (1-10 s). Stacking was also conducted using the rms of the estimated empirical Green's functions. Our results demonstrate that ambient seismic noise tomography is a viable technique at periods of 1-10 s in length, even when different sensor types are present. Analysis of the empirical Green's functions indicates that the dominant sources of ambient seismic noise originated from the same origin, and no significant seasonal or spatial variations in the ambient noise sources were observed. Multiple-filter analysis was used to extract the group velocities from the estimated empirical Green's functions, which were then inverted to image the spatially varying dispersion at periods of lengths between 1 and 7 s using tomographic inversion of the traveltimes estimated for each frequency. The resulting group velocity maps show high correlations with known geological and tectonic features of the study region. In general, most of the Tehran basin, with certain exceptions, could be clearly resolved with low group velocities, whereas the mountain ranges were found to be correlated with high group velocities. In the Tehran basin, for 2 and 3 s periods, the low-velocity zone deepens towards the south-southwest, which reflects thicker sediments in the southern part of the basin than in the north. This feature has also been observed in other geological studies. The Vs models also show that bedrock depth varies between 400 and 1400 m from north to south within the Tehran basin. At longer periods main faults are associated with abrupt transitions between regions of high- to low-velocity anomalies. In general, our results indicate that ANT can be a flexible and effective approach for studying near-surface heterogeneity using short-period surface wave data.
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| 8. |
- Shomali, Zaher Hossein, et al.
(författare)
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Lithospheric structure beneath the Zagros collision zone resolved by non-linear teleseismic tomography
- 2011
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Ingår i: Geophysical Journal International. - 0956-540X .- 1365-246X. ; 187:1, s. 394-406
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Tidskriftsartikel (refereegranskat)abstract
- The upper-mantle structure across the Zagros collision zone, in southwest Iran, is investigated using a non-linear weighted damped least-squares teleseismic tomography approach. The resolution of the structures/transitions in the upper mantle is enhanced significantly by correcting the teleseismic relative arrival time residuals for an a priori crustal velocity model and then performing the inversion with fixed crustal blocks. To investigate whether or not the lithospheric blocks and major transitions in the resulting model are required by the data or are artefacts of the inversion, the data were inverted using two different inverse methods (singular value decomposition and a quadratic programming method). New high-quality seismic velocity models show apparent correlation between surface geological features and seismic velocity structures at lithospheric depth across the Zagros collision zone. The image shows a sharp lithospheric boundary at the Main Zagros Thrust between 100 km and 250 km depth with P-wave velocity about 3 per cent faster within the Arabian Shield to the south. A step-like increase in lithospheric thickness across the Zagros collision zone is assumed to separate two different mantle structures namely the Arabian (to the south) and the Eurasian (to the north) domains. The most striking feature resolved is a north-dipping slab-like positive velocity anomaly.
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