| 1. |
- Amini, Samar, et al.
(författare)
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Tomographic upper-mantle velocity structure beneath the Iranian Plateau
- 2012
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Ingår i: Tectonophysics. - : Elsevier BV. - 0040-1951 .- 1879-3266. ; 554-557, s. 42-49
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Tidskriftsartikel (refereegranskat)abstract
- The Iranian plateau is one of the most structurally complex and tectonically inhomogeneous regions in the world. In this study, we analyze Pn arrival-times from regional seismicity in order to resolve lateral velocity variations within the uppermost-mantle under the Iranian Plateau. More than 48,000 Pn first arrival times selected from the EHB catalog were used with epicentral distances of 200 to 1600 km. We used regularized isotropic and anisotropic damped least-squares inversion to image lateral velocity variations in the upper mantle. Our velocity model, with high lateral resolution, shows positive anomalies in the Zagros mountain belt with a distinct transition approximately along the Main Zagros Thrust to the lower mantle velocity zone of Central Iran. Anomalously low velocities are observed predominantly beneath NW Iran and eastern Turkey, suggesting a zone of relatively weak mantle. Low velocity region under the Damavand volcano reveals the hot upper mantle beneath the central Alborz mountains.
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| 2. |
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| 3. |
- 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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| 4. |
- 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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| 5. |
- 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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| 6. |
- 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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| 7. |
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| 8. |
- Keshvari, Forough, et al.
(författare)
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Upper-mantle S-velocity structure across the Zagros collision zone resolved by nonlinear teleseismic tomography
- 2011
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Ingår i: Journal of Seismology. - : Springer Science and Business Media LLC. - 1383-4649 .- 1573-157X. ; 15:2, s. 329-339
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Tidskriftsartikel (refereegranskat)abstract
- Non-linear teleseismic S-phase tomography across the Zagros collision zone in southwestern Iran is used to determine a high-resolution image of the upper-mantle structure. The inversion was done using 41 high-quality earthquakes recorded by 19 broad-band and medium-band stations along a 620 km long profile across the collision zone. Smearing from strong crustal velocity anomalies into the upper-mantle is suppressed by travel-time corrections calculated based on a 3D crustal model for the study area. Our results show that the relatively old and cold Arabian shield has a higher velocity (up to 6% faster, at depths between 70 and 300 km) than the younger lithosphere farther north in Central Iran. These two upper-mantle domains are separated by a sharp near-vertical transition whose surface expression coinciding with the Main Zagros Thrust.
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| 9. |
- Kind, R., et al.
(författare)
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Scandinavia : A former Tibet?
- 2013
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Ingår i: Geochemistry Geophysics Geosystems. - : American Geophysical Union (AGU). - 1525-2027. ; 14:10, s. 4479-4487
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Tidskriftsartikel (refereegranskat)abstract
- The Himalaya and the Tibetan Plateau are uplifted by the ongoing northward underthrusting of the Indian continental lithosphere below Tibet resulting in lithospheric stacking. The layered structure of the Tibetan upper mantle is imaged by seismic methods, most detailed with the receiver function method. Tibet is considered as a place where the development of a future craton is currently under way. Here we study the upper mantle from Germany to northern Sweden with seismic S receiver functions and compare the structure below Scandinavia with that below Tibet. Below Proterozoic Scandinavia, we found two low-velocity zones on top of each other, separated by a high-velocity zone. The top of the upper low-velocity zone at about 100 km depth extends from Germany to Archaean northern Sweden. It agrees with the lithosphere-asthenosphere boundary (LAB) below Germany and Denmark. Below Sweden it is known as the 8 degrees discontinuity, or as a mid-lithospheric discontinuity (MLD), similar to observations in North America. Seismic tomography places the LAB near 200 km in Scandinavia, which is close to the top of our deeper low-velocity zone. We also observed the bottom of the asthenosphere (the Lehmann discontinuity) deepening from 180 km in Germany to 260 km below Sweden. Remnants of old subduction in the upper about 100 km below Scandinavia and Finland are known from controlled source seismic experiments and local earthquake studies. Recent tomographic studies indicate delamination of the lithosphere below southern Scandinavia and northern Germany. We are suggesting that the large-scale layered structure in the Scandinavian upper mantle may be caused by processes similar to the ongoing lithospheric stacking in Tibet.
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| 10. |
- Maleki, V., et al.
(författare)
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Earthquake relocation in the Central Alborz region of Iran using a non-linear probabilistic method
- 2013
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Ingår i: Journal of Seismology. - : Springer Science and Business Media LLC. - 1383-4649 .- 1573-157X. ; 17:2, s. 615-628
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Tidskriftsartikel (refereegranskat)abstract
- In this study, we calculate accurate absolutelocations for nearly 3,000 shallow earthquakes(≤20 km depth) that occurred from 1996 to 2010 inthe Central Alborz region of northern Iran using a nonlinearprobabilistic relocation algorithm on a localscale. We aim to produce a consistent dataset with arealistic assessment of location errors using probabilistichypocenter probability density functions. Ourresults indicate significant improvement in hypocenterlocations and far less scattering than in the routineearthquake catalog. According to our results, 816earthquakes have horizontal uncertainties in the 0.5–3.0 km range, and 981 earthquakes are relocated withfocal-depth errors less than 3.0 km, even with a suboptimalnetwork geometry. Earthquake relocated aretightly clustered in the eastern Tehran region and aremainly associated with active faults in the study area(the Mosha and Garmsar faults). Strong historicalearthquakes have occurred along the Mosha andGarmsar faults, and the relocated earthquakes alongthese faults show clear north-dipping structures andalign along east–west lineations, consistent with thepredominant trend of faults within the study region.After event relocation, all seismicity lies in the upper20 km of the crust, and no deep seismicity (>20 kmdepth) has been observed. In many circumstances, theseismicity at depth does not correlate with surfacefaulting, suggesting that the faulting at depth doesnot directly offset overlying sediments.
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