| 1. |
- Koči, Love, et al.
(författare)
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Molecular dynamics calculation of liquid iron properties and adiabatic temperature gradient in the Earth's outer core
- 2007
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Ingår i: Geophysical Journal International. - 0956-540X .- 1365-246X. ; 168:2, s. 890-894
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Tidskriftsartikel (refereegranskat)abstract
- The knowledge of the temperature radial distribution in the Earth's core is important to understand the heat balance and conditions in the Earth's interior. Molecular dynamics (MD) simulations were applied to study the properties of liquid iron under the pressure-temperature conditions of the Earth's outer core. It is shown that the model used for the MD simulations can reproduce recent experimentally determined structure factor calculations to the highest pressure of 58 GPa. Applying this model for higher pressures, the calculated densities and diffusion parameters agree well with the results of first-principles. The MD calculations indicate that a reasonable estimate of the adiabatic temperature profile in the Earth's outer core could be evaluated.
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| 2. |
- Juhlin, C., et al.
(författare)
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Crustal reflectivity near the Archean-Proterozoic boundary in northern Sweden and implications for the tectonic evolution of the area
- 2002
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Ingår i: Geophysical Journal International. - 0956-540X .- 1365-246X. ; 150:1, s. 180-197
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Tidskriftsartikel (refereegranskat)abstract
- Sm–Nd isotope ratios of 1.9–1.8 Ga granitoids delineate the Archaean–Proterozoic boundary in northern Sweden, an important feature in the Fennoscandian Shield. The boundary strikes approximately WNW–ESE and is defined as a c. 20 km wide zone with juvenile Palaeoproterozoic rocks to the SSW and Archaean and Proterozoic rocks, derived to a large extent from Archaean sources, to the NNE. It therefore constitutes the strongly reworked margin of the old Archaean craton. Extrapolation of the boundary offshore into the Bothnian Bay and correlation with the marine reflection seismic BABEL Lines 2 and 3/4 indicates that the boundary dips to the south-southwest, consistent with interpretation of the Sm–Nd data. In order to tie the BABEL results with onshore surface geology and obtain detailed images of the uppermost crust a short (30 km of subsurface coverage) pilot profile was acquired in the Luleå area of northern Sweden during August 1999. The profile consisted of a high-resolution shallow component (1 kg shots) and a lower-resolution deep component (12 kg shots). Both components image most of the reflective crust, with the deep component providing a better image below 10 s. Comparison of signal penetration curves with data acquired over the Trans-Scandinavian Igneous Belt (a large batholith) indicate the transparent nature of the crust there to be caused by geological factors, not acquisition parameters. Lower crustal reflectivity patterns on the Luleå test profile are similar to those observed on the BABEL lines, suggesting the same lower crust onshore as offshore. Interpreted Archaean reflective upper crust in the NE extends below more transparent Proterozoic crust in the SW. This transparent crust contains a number of high-amplitude reflectors that may represent shear zones and/or mafic rock within granite intrusions. A marked boundary in the magnetic field in the SW has been interpreted as being the result of a gently west-dipping contact zone between meta-sediments and felsic volcanic rocks, however, the seismic data indicate a near-vertical structure in this area. By correlating the onshore and offshore seismic data we have better defined the location of the Archaean–Proterozoic boundary on the BABEL profiles. Our new interpretation of the crustal structure along the northern part of the BABEL Line 2 shows a more bi-vergent geometry than previous interpretations. Comparison of the re-interpreted crustal structure in northern Sweden with that found in the Middle Urals shows several similarities, in particular the accretion of a series of arcs to a stable craton. Based on this similarity and geological data, we deduce that a continental arc accreted to the southwestern margin of the Archaean craton at c. 1.87 Ga. Shortly thereafter, the Skellefte island arc underthrust the continental arc owing to a collision further to the southwest resulting in the bi-vergent crustal structure observed today.
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| 3. |
- Mouyen, Maxime, 1982, et al.
(författare)
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Erosion effects assessed by repeated gravity measurements in southern Taiwan
- 2013
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Ingår i: Geophysical Journal International. - : Oxford University Press (OUP). - 0956-540X .- 1365-246X. ; 192:1, s. 113-136
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Tidskriftsartikel (refereegranskat)abstract
- We analyse temporal variations of gravity measured in southern Taiwan since November 2006 at 10 sites using absolute gravimeters and, since November 2008, at 70 sites using a relative gravimeter. We describe and apply methods to interpret the gravity changes in terms of local hydrological processes and vertical ground motions. The effect of land water is computed from local rainfall data and a model of rain accumulation and discharge in the ground. The effect of the vertical motions of the ground is estimated using time-series of permanent Global Positioning System (GPS) stations and the theoretical gravity to height ratio of -2 μGal cm-1. Unexpectedly, Morakot typhoon (2009 August), the strongest typhoon in Taiwan in 50 yr, was responsible for the highest gravity changes. Morakot triggered numerous large landslides and debris flow deposits. Their thicknesses are estimated from the gravity changes (up to 280 μGal) along with field observations and satellite images.
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| 4. |
- Mouyen, Maxime, 1982, et al.
(författare)
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Investigating possible gravity change rates expected from long-term deep crustal processes in Taiwan
- 2014
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Ingår i: Geophysical Journal International. - : Oxford University Press (OUP). - 0956-540X .- 1365-246X. ; 198:1, s. 187-197
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Tidskriftsartikel (refereegranskat)abstract
- We propose to test if gravimetry can prove useful in discriminating different models of long-term deep crustal processes in the case of the Taiwan mountain belt. We discuss two existing tectonic models that differ in the deep processes proposed to sustain the long-term growth of the orogen. One model assumes underplating of the uppermost Eurasian crust with subduction of the deeper part of the crust into the mantle. The other one suggests the accretion of the whole Eurasian crust above crustal-scale ramps, the lower crust being accreted into the collisional orogen. We compute the temporal gravity changes caused only by long-term rock mass transfers at depth for each of them. We show that the underplating model implies a rate of gravity change of -6 × 10-2 μGal yr-1, a value that increases to 2 × 10-2 μGal yr-1 if crustal subduction is neglected. If the accretion of the whole Eurasian crust occurs, a rate of 7 × 10-2 μGal yr-1 is obtained. The two models tested differ both in signal amplitude and spatial distribution. The yearly gravity changes expected by long-term deep crustal mass processes in Taiwan are two orders of magnitude below the present-day uncertainty of land-based gravity measurements. Assuming that these annually averaged long-term gravity changes will linearly accumulate with ongoing mountain building, multidecadal time-series are needed to identify comparable rates of gravity change. However, as gravity is sensitive to any mass redistribution, effects of short-term processes such as seismicity and surface mass transfers (erosion, sedimentation, ground-water) may prevent from detecting any long-term deep signal. This study indicates that temporal gravity is not appropriate for deciphering the long-term deep crustal processes involved in the Taiwan mountain belt.
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| 5. |
- Hieronymus, Christoph, 1967-, et al.
(författare)
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Complex cratonic seismic structure from thermal models of the lithosphere : effects of variations in deep radiogenic heating
- 2010
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Ingår i: Geophysical Journal International. - 0956-540X .- 1365-246X. ; 180:3, s. 999-1012
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Tidskriftsartikel (refereegranskat)abstract
- Cratons are the long-term tectonically stable cores of the continents. Despite their thermal stability they display substantial seismic complexity with lateral and vertical lithospheric anomalies of up to several percent in both VS and VP . Although some of these anomalies have been correlated with compositional variations, others are too large to be explained with any common mantle lithosphere compositions ranging from fertile peridotites to highly melt-depleted dunites, under the assumption that thermal perturbations are negligible. To test whether temperature anomalies could contribute to seismic complexity, we performed a set of 2-D thermal calculations for a range of cratonic tectonic models and converted them into seismic structure, accounting for variations in phase and elastic and anelastic response to pressure and temperature. With the long thermal equilibration time in cratonic settings, even relatively mild variations in concentrations of radioactive elements can leave long-lasting lithospheric thermal anomalies of 100–300 °C. Concentrations of radioactive elements decrease with increasing melt depletion (or decreasing metasomatic refertilization), resulting in lower temperatures and increased seismic velocities. This thermal seismic effect enhances the intrinsic velocity-increasing compositional seismic signature of melt depletion. The joint thermochemical effects can leave cratonic seismic anomalies of up to 3–4.5 per cent in VS and up to 2.5–4 per cent in VP , with gradients sometimes as sharp as a few kilometre in width. Thus the variations in major and minor element mantle lithosphere composition commonly seen in mantle samples can account for much of the variability in imaged seismic structure of cratonic lithosphere.
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| 6. |
- Lindman, Mattias, et al.
(författare)
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Spatiotemporal characteristics of aftershock sequences in the south Iceland seismic zone : Interpretation in terms of pore pressure diffusion and poroelasticity
- 2010
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Ingår i: Geophysical Journal International. - 0956-540X .- 1365-246X. ; 183:3, s. 1104-1118
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Tidskriftsartikel (refereegranskat)abstract
- In seismology numerous observations indicate a relationship between pore pressure in the Earth's crust and the occurrence of earthquakes. In this paper we study aftershock sequences in the South Iceland Seismic Zone (SISZ), where poroelastic rebound has been observed in the post-seismic period of two M 6.5 earthquakes in 2000 June. We analyse characteristic features in the spatiotemporal distribution of aftershocks following the two M 6.5 2000 June 17 and 21 earthquakes and a M 4.5 earthquake on 1999 September 27. These features include an initial pre-power-law decay period characterized by an initially finite aftershock rate, a subsequent power-law decay interrupted by distinct and temporary rate increases and decreases as well as increased clustering of aftershocks with time in the main shock fault zones. Extending the analysis to a M 3.2 aftershock sequence in the same region confirms an increase in the duration of the initial pre-power-law decay period with increasing main shock magnitude. We find, from the return time of aftershock magnitudes to the long-term completeness level, that the initial pre-power-law decay period and its durational dependence on main shock magnitude may not only represent incompleteness artefacts but may also reflect the physics of the aftershock process in the SISZ. Based on pore pressure diffusion modelling, we interpret the origin of the observed SISZ aftershock features in terms of a spatially non-linear coseismic influence of the main shock on stresses in the surrounding crust and poroelastic adjustment of stresses and pore pressures during main shock initiated diffusion processes. In a discussion of alternative interpretations, we find that rate and state friction and dynamically propagating crack models, the statistical ETAS model, afterslip models, viscoelastic relaxation of the lower crust and upper mantle and a recently proposed dependence on the crustal state of stress all appear inconsistent with at least one of the characteristic spatiotemporal features of the studied SISZ aftershock sequences. We conclude that these features constitute strong evidence for pore pressure effects in aftershock triggering within the SISZ and recommend that poroelastic adjustment of stresses is taken into account in modelling of main shock initiated pore pressure diffusion.
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| 7. |
- Lund, Björn, et al.
(författare)
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Calculating horizontal stress orientations with full or partial knowledge of the tectonic stress tensor
- 2007
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Ingår i: Geophysical Journal International. - 0956-540X .- 1365-246X. ; 170:3, s. 1328-1335
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Tidskriftsartikel (refereegranskat)abstract
- Earthquakes potentially serve as abundant and cost-effective gauges of tectonic stress provided that reliable means exist of extracting robust stress parameters. Several algorithms have been developed for this task, each of which typically provides information on the orientations of the three principal stresses and a single stress magnitude parameter. A convenient way of displaying tectonic stress results is to map the azimuth of maximum horizontal compressive stress, which is usually approximated using the azimuth of the larger subhorizontal principal stress. This approximation introduces avoidable errors that depend not only on the principal stress axes' plunges but also on the value of the stress magnitude parameter. Here we outline a method of computing the true direction of maximum horizontal compressive stress (SH) and show that this computation can be performed using only the four stress parameters obtained in routine focal mechanism stress estimation. Using theoretical examples and new stress inversion results obtained with focal mechanism data from the central Grímsey lineament, northern Iceland, we show that the SH axis may differ by tens of degrees from its commonly adopted proxy. In order to most appropriately compare tectonic stress estimates with other geophysical parameters, such as seismic fast directions or geodetically measured strain rate tensors, or to investigate spatiotemporal variations in stress, we recommend that full use be made of the routinely estimated stress parameters and that a formal axis of maximum horizontal compression be calculated.
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| 8. |
- Scarponi, M., et al.
(författare)
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New gravity data and 3-D density model constraints on the Ivrea Geophysical Body (Western Alps)
- 2020
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Ingår i: Geophysical Journal International. - : Oxford University Press (OUP). - 0956-540X .- 1365-246X. ; 222:3, s. 1977-1991
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Tidskriftsartikel (refereegranskat)abstract
- We provide a high-resolution image of the Ivrea Geophysical Body (IGB) in the Western Alps with new gravity data and 3-D density modelling, integrated with surface geological observations and laboratory analyses of rock properties. The IGB is a sliver of Adriatic lower lithosphere that is located at shallow depths along the inner arc of the Western Alps, and associated with dense rocks that are exposed in the Ivrea-Verbano Zone (IVZ). The IGB is known for its high seismic velocity anomaly at shallow crustal depths and a pronounced positive gravity anomaly. Here, we investigate the IGB at a finer spatial scale, merging geophysical and geological observations. We compile existing gravity data and we add 207 new relative gravity measurements, approaching an optimal spatial coverage of 1 data point per 4–9 km2 across the IVZ. A compilation of tectonic maps and rock laboratory analyses together with a mineral properties database is used to produce a novel surface rock-density map of the IVZ. The density map is incorporated into the gravity anomaly computation routine, from which we defined the Niggli gravity anomaly. This accounts for Bouguer Plate and terrain correction, both considering the in situ surface rock densities, deviating from the 2670 kg m–3 value commonly used in such computations. We then develop a 3-D single-interface crustal density model, which represents the density distribution of the IGB, including the above Niggli-correction. We retrieve an optimal fit to the observations by using a 400 kg m–3 density contrast across the model interface, which reaches as shallow as 1 km depth below sea level. The model sensitivity tests suggest that the ∼300–500 kg m–3 density contrast range is still plausible, and consequently locates the shallowest parts of the interface at 0 km and at 2 km depth below sea level, for the lowest and the highest density contrast, respectively. The former model requires a sharp density discontinuity, the latter may feature a vertical transition of densities on the order of few kilometres. Compared with previous studies, the model geometry reaches shallower depths and suggests that the width of the anomaly is larger, ∼20 km in west–east direction and steeply E–SE dipping. Regarding the possible rock types composing the IGB, both regional geology and standard background crustal structure considerations are taken into account. These exclude both felsic rocks and high-pressure metamorphic rocks as suitable candidates, and point towards ultramafic or mantle peridotite type rocks composing the bulk of the IGB.
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| 9. |
- Spada, G., et al.
(författare)
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A benchmark study for glacial isostatic adjustment codes
- 2011
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Ingår i: Geophysical Journal International. - 0956-540X .- 1365-246X. ; 185:1, s. 106-132
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Tidskriftsartikel (refereegranskat)abstract
- The study of glacial isostatic adjustment (GIA) is gaining an increasingly important role within the geophysical community. Understanding the response of the Earth to loading is crucial in various contexts, ranging from the interpretation of modern satellite geodetic measurements (e. g. GRACE and GOCE) to the projections of future sea level trends in response to climate change. Modern modelling approaches to GIA are based on various techniques that range from purely analytical formulations to fully numerical methods. Despite various teams independently investigating GIA, we do not have a suitably large set of agreed numerical results through which the methods may be validated; a community benchmark data set would clearly be valuable. Following the example of the mantle convection community, here we present, for the first time, the results of a benchmark study of codes designed to model GIA. This has taken place within a collaboration facilitated through European Cooperation in Science and Technology (COST) Action ES0701. The approaches benchmarked are based on significantly different codes and different techniques. The test computations are based on models with spherical symmetry and Maxwell rheology and include inputs from different methods and solution techniques: viscoelastic normal modes, spectral-finite elements and finite elements. The tests involve the loading and tidal Love numbers and their relaxation spectra, the deformation and gravity variations driven by surface loads characterized by simple geometry and time history and the rotational fluctuations in response to glacial unloading. In spite of the significant differences in the numerical methods employed, the test computations show a satisfactory agreement between the results provided by the participants.
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| 10. |
- Adamczyk, A., et al.
(författare)
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High-resolution near-surface velocity model building using full-waveform inversion-a case study from southwest Sweden
- 2014
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Ingår i: Geophysical Journal International. - : Oxford University Press (OUP). - 0956-540X .- 1365-246X. ; 197:3, s. 1693-1704
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Tidskriftsartikel (refereegranskat)abstract
- Full-waveform inversion (FWI) is an iterative optimization technique that provides high-resolution models of subsurface properties. Frequency-domain, acoustic FWI was applied to seismic data acquired over a known quick-clay landslide scar in southwest Sweden. We inverted data from three 2-D seismic profiles, 261-572 m long, two of them shot with small charges of dynamite and one with a sledgehammer. To our best knowledge this is the first published application of FWI to sledgehammer data. Both sources provided data suitable for waveform inversion, the sledgehammer data containing even wider frequency spectrum. Inversion was performed for frequency groups between 27.5 and 43.1 Hz for the explosive data and 27.5-51.0 Hz for the sledgehammer. The lowest inverted frequency was limited by the resonance frequency of the standard 28-Hz geophones used in the survey. High-velocity granitic bedrock in the area is undulated and very shallow (15-100 m below the surface), and exhibits a large P-wave velocity contrast to the overlying normally consolidated sediments. In order to mitigate the non-linearity of the inverse problem we designed a multiscale layer-stripping inversion strategy. Obtained P-wave velocity models allowed to delineate the top of the bedrock and revealed distinct layers within the overlying sediments of clays and coarse-grained materials. Models were verified in an extensive set of validating procedures and used for pre-stack depth migration, which confirmed their robustness.
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