| 11. |
- 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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| 12. |
- Eken, Tuna, 1978-, et al.
(författare)
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S and P velocity heterogeneities within the upper mantle below the Baltic Shield
- 2008
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Ingår i: Tectonophysics. - : Elsevier BV. - 0040-1951 .- 1879-3266. ; 462:1-4, s. 109-124
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Tidskriftsartikel (refereegranskat)abstract
- Upper mantle structure beneath the Baltic (Fennoscandian) Shield is investigated using non-linear tomographic inversion of relative arrival-time residuals. 52 selected teleseismic earthquakes recorded by 45 broadband stations of the Swedish National Seismological Network (SNSN) provide 1532 good quality S-wave relative arrival times. SV and SH arrival-time residuals were initially analyzed independently, providing two separate models. These reveal several consistent major features, many of which are also consistent with P-wave results. Lateral velocity variations of ± 3–4% are observed to depths of at least 470 km. The correlation between the SH and SV models is investigated and shows a pattern of minor but significant differences down to around 150–200 km depth, below which the models are essentially similar. Direct cell by cell comparison of the model velocities reveals a similar pattern, with velocity differences between the models of up to 4%. Numerical tests show that differences in the two S-wave models can only be partially attributed to noise and limited resolution, and some features are attributed to the effect of large scale anisotropy. One of the significant and sharp discrepancies between the S models coincides with a presumed boundary between Archean and Proterozic domains, suggesting different anisotropic characteristics in the two regions.
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| 13. |
- Eken, Tuna, 1978-, et al.
(författare)
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Seismic anisotropy of the mantle lithosphere beneath the Swedish National Seismological Network (SNSN)
- 2010
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Ingår i: Tectonophysics. - : Elsevier. - 0040-1951 .- 1879-3266. ; 480:1-4, s. 241-258
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Tidskriftsartikel (refereegranskat)abstract
- Body-wave analysis – shear-wave splitting and P-travel time residuals - detect anisotropic structure of the upper mantle beneath the Swedish part of Fennoscandia. Geographic variations of both the splitting measurements and the P-residual spheres map regions of different fabrics of the mantle lithosphere. The fabric of individual mantle domains is internally consistent, usually with sudden changes at their boundaries. Distinct back-azimuth dependence of SKS splitting excludes single-layer anisotropy models with horizontal symmetry axes for the whole region. Based upon joint inversion of body-wave anisotropic parameters we instead propose 3D self-consistent anisotropic models of well-defined mantle lithosphere domains with differently oriented fabrics approximated by hexagonal aggregates with plunging symmetry axes. The domain-like structure of the Precambrian mantle lithosphere, most probably retaining fossil fabric since the domains’ origin, supports the idea of the existence of an early form of plate tectonics during formation of continental cratons already in the Archean. Similarly to different geochemical and geological constraints, the 3D anisotropy modelling and mapping of fabrics of the lithosphere domains contribute to tracking plate tectonics regimes back in time.
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| 14. |
- 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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| 15. |
- Enferadi, S., et al.
(författare)
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Feasibility study of earthquake early warning in Tehran, Iran
- 2021
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Ingår i: Journal of Seismology. - : Springer Nature. - 1383-4649 .- 1573-157X. ; 25:4, s. 1127-1140
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Tidskriftsartikel (refereegranskat)abstract
- In this study, we examine the scientific feasibility of an Earthquake Early Warning System in Tehran, Iran, by the integration of the Tehran Disaster Mitigation and Management Organization (TDMMO) accelerometric network and the PRobabilistic and Evolutionary early warning SysTem (PRESTo). To evaluate the performance of the TDMMO-PRESTo system in providing the reliable estimations of earthquake parameters and the available lead-times for The Metropolis of Tehran, two different approaches were analyzed in this work. The first approach was assessed by applying the PRESTo algorithms on waveforms from 11 moderate instrumental earthquakes that occurred in the vicinity of Tehran during the period 2009-2020. Moreover, we conducted a simulation analysis using synthetic waveforms of 10 large historical earthquakes that occurred in the vicinity of Tehran. We demonstrated that the six worst-case earthquake scenarios can be considered for The Metropolis of Tehran, which are mostly related to the historical and instrumental events that occurred in the southern, eastern, and western parts of Tehran. Our results indicate that the TDMMO-PRESTo system could provide reliable and sufficient lead-times of about 1 to 15s and maximum lead-times of about 20s for civil protection purposes in The Metropolis of Tehran.
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| 16. |
- Gregersen, S., et al.
(författare)
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Physical differences in the deep lithosphere of Northern and Central Europe
- 2006. - 32
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Ingår i: European Lithosphere Dynamics. - : Geological Society of London. - 9781862392120 - 1862392129 ; 32, s. 313-322
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- A number of large-scale integrated studies, including the TOR and POLONAISE’97 projects, with an emphasis on seismicmethods, have been used to elucidate the southwestern boundary (suture zone) between the East European Craton and the Phanerozoicterranes of Western Europe. Results indicate that a thick slab of mantle lithosphere below the craton thins southwestwards beneath theTrans-European Suture Zone and is not seen south of the Variscan front. The thinning is not gradual, but is interrupted by at least twoabrupt deep boundaries, the most significant of which corresponds to the surface position of the Tornquist Zone, a major fault. Thepresent geometry of the lithosphere is the result of modification of the margin of the Neoproterozoic continent Baltica by Phanerozoicprocesses, including the development of the Tornquist Zone and the stretching of the lithosphere in a broad central block SW of thiszone. Seismic results and their interpretations from the TOR tomographic project are presented and compared with results from thePOLONAISE’97 controlled source project to the SE. Both investigations have shown high-angle, non-symmetrical features extendingdeep into the mantle.
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| 17. |
- Gregersen, S., et al.
(författare)
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Uniqueness of modeling results from teleseismic P-Wave tomography in Project Tor
- 2010
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Ingår i: Tectonophysics. - 0040-1951 .- 1879-3266. ; 481:1-4, s. 99-107
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Tidskriftsartikel (refereegranskat)abstract
- Within Project Tor. which is about Teleseismic Tomography across the Tomquist Zone in Germany-Denmark-Sweden, we have confirmed very significant deep lithosphere differences And modeling is substantiated via completely independent methods. In 1996-1997 our 130 seismographs constituted the largest seismic antenna ever in Europe. The Tor area was chosen along a well studied crustal profile of an earlier project, and the modeling efforts were concentrated on the deep lithosphere and asthenosphere differences to depths around 300 km The Tor data have been subjected to P-wave travel time tomography. surface wave and receiver function analysis as well as anisotropy and scattering measurements An important goal of the project was to make several independent inversions of the tomography data. and compare the results in an attempt to evaluate uniqueness, resolution and accuracy of these inversions. The comparisons of this paper involve more diversity in methods than any previous comparison. The geological outcome is a substantiation of earlier statements that, "The transition is interpreted to be sharp and steep in two places It goes all through the lithosphere at the northern rim of the Tornquist Zone near the border between Sweden and Denmark, and here the lithosphere difference is large to depths more than 200 km. The other lithosphere difference. of smaller scale, is found near the southern edge of the Ringkobing-Fyn High near the border between Denmark and Germany Also this transition is sharp and steep. and goes all through the lithosphere to depths around 120 km. These two sharp transitions divide the Tor region into 3 different lithosphere structures distinguishable in P-wave travel time tomography. surface wave dispersion. P- and S-wave anisotropy and partly in P-wave scattering" The mentioned broad-scale features are judged to be unambiguously determined, with well-described resolution and accuracy Unfortunately a detail like the slope of the subcrustal lithosphere transition right under the Tronquist Zone cannot be constrained even if this is where the resolution is best. and the curiosity largest.
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| 18. |
- Hagos, Lijam, et al.
(författare)
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An application of relative moment tensor inversion to aftershocks of the June 1998 Hengill earthquake in southwest Iceland
- 2008
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Ingår i: Bulletin of The Seismological Society of America (BSSA). - : Seismological Society of America (SSA). - 0037-1106 .- 1943-3573. ; 98:2, s. 636-650
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Tidskriftsartikel (refereegranskat)abstract
- Using spectral amplitudes from the South Iceland Lowland (SIL) seismic network, we conduct a relative moment tensor inversion (RMTI) on aftershocks of the June 1998 M-w 5: 4 event that occurred at the Hengill triple junction, southwest Iceland. Three distinct groups of spatially clustered events are observed in the region for 25 selected events that occurred during the period from 4-5 June 1998. These clusters have previously been relocated with very high accuracy using cross-correlation techniques. We use the RMTI method to determine the focal mechanisms of these events and compare our results with the SIL network mechanisms obtained using spectral amplitudes. Most focal mechanisms obtained in this study show a predominantly right-lateral strike-slip motion, similar to those obtained by the SIL network, but more consistently in agreement with the orientations of the surface faults in the area. The spectral amplitude grouping method was used to investigate discrepancies between some of the focal mechanisms obtained using RMTI and the method used in the SIL network. This resolved apparent differences in the focal mechanism solutions for two of the studied events. Cluster alignment across the presumed fault and the individual event mechanisms agree well, suggesting the occurrence of the events along a fault plane dipping steeply towards the east. Consistency in the pressure and tension axes of the focal mechanisms suggests that the region was under northeast-southwest-oriented compression during the activity. Decomposition of the moment tensors into double-couple and isotropic components and the resulting insignificant isotropic component also suggests that the styles of failure for the analyzed events was mainly due to shearing.
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| 19. |
- 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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| 20. |
- Hagos, Lijam Zemichael, 1968-
(författare)
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Earthquake Sources, the Stress Field and Seismic Hazard : A Study in Eritrea and its Surrounding
- 2006
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Presented in this thesis are some basic concepts and applications of seismic hazard analysis and the elements that influence the amplitude and geometric attenuation of earthquake ground motion. This thesis centers on the identification of the styles of failure, focal mechanisms, and the state of regional stress in the study area. Seismic hazard is a complex problem often involving considerable uncertainties. Therefore it is reasonable to consider different seismic hazard analysis approaches in order to as robustly as possible define zones of different levels of hazard. With the aim of characterizing and quantifying hazard in the east African region of Eritrea and its surroundings, a study is included in the thesis presenting hazard maps constructed using two non-parametric probabilistic seismic hazard analysis (PSHA) approaches. Peak ground acceleration (PGA) values for 10% probability of exceedence in 50 years are computed at given grid points for the whole selected area and results from both methods are compared. Other aspects addressed in the thesis include the determination of source parameters of selected earthquakes that occur in the Afar region. The styles of faulting, the mechanisms involved during the rupture process and the states of stress along the major tectonic features are also highlighted. Source parameters for selected events in the region were re-evaluated and improved solutions obtained. An aftershock sequence in the Hengill volcanic area in SW Iceland, following the major event that occurred on June 4, 1998, was used to investigate improved methodologies for moment tensor using a relative approach. The sensitive and spatially dense seismic network in this area reveals large sets of clustered events allowing the power of the new methodology to be demonstrated and providing greater insight into the tectonic implications of the activity in the area.
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