Browsing by study line "Fasta jordens fysik"
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(2023)In Estonia and Gotland, Sweden Ordovician and Silurian rocks can be found in exposed outcrops, they have been studied for the past years to create a better understanding of the biological and geological events that transpire during the Ordovician and Silurian. Finland is not known to have sedimentary fossiliferous limestones, but the Åland Islands is one of the places where Ordovician and Silurian erratic limestones can be found. The limestones were glacially transported from the Bothnia Sea area. The erratic limestones found in the Åland Islands have not been studied and have not been dated. The goal of this research is to reconstruct the depositional age of the erratic crinoidal limestone from the Åland Islands through 13C, 87Sr/86Sr, and identification of microfossil proxies. We used in situ strontium isotope analysis by MC-ICP-MS and 13C analysis in different materials of the sample, the materials analyzed were cement, sediment matrix, and skeletal material (crinoid stem). The samples were sent to experts for microfossil identification. The Åland samples were compared with known samples from Estonia and Gotland. The samples from Niibi Estonia showed the most petrographical similarities and were also analyzed for strontium and carbon isotopes. The strontium analysis showed that the material had a different strontium signal, which then contributed to diagenetic alteration due to rubidium-87. It was observed that the sediment matrix was the most susceptible to any diagenetic alteration followed by the skeletal material and then the cement. There were conodonts found in the Åland samples that indicated that the erratic limestone is from the lower Haljala Regional Stage, Sandbian, Late Ordovician.
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(2023)On the hotter Paleoproterozoic Earth, the regime of plate tectonics was likely in transition, including features from the early Earth but having e.g. subduction already established. The different mode of tectonics affected also on the deformation on plate boundaries, on orogenesis, for example. Due to the increased lithospheric temperatures, the Paleoproterozoic orogens were hotter and lower. Such conditions made also HP-LT metamorphism rare. In addition to the different temperature conditions, the past tectonics pose another challenge. Largely the Paleoproterozoic rocks that can be accessed today, are the remnants of ancient middle or lower crustal layers, which have exhumed due to deep levels of erosion. Hence, a great amount of evidence of past orogens is erased. To overcome this issue, geodynamic modeling is used to build set of 19 continent-continent collision models, with the temperature conditions ranging from the Paleoproterozoic to the Phanerozoic. Additionally, P-T-t paths are recorded in the models for comparison of pressure-temperature conditions using pseudosection diagrams. Another significant quantity governing the deformation on collisional continental margins, is the angle of convergence obliquity. With roughly 60° obliquity, full strain partitioning should be triggered in a orogen, forming a strike-slip fault. In the models, different temperature cases with the obliquity angle are varied. The geodynamic modeling software to produce the models was DOUAR, a 3D thermo-mechanical code coupled with erosion model FastScape. The initial models with 35 km thick crust were not able to produce proper strain partitioning due to low resolution, so another set of models with 45 km thick crust was run. On top of that, the lower crustal strength was varied. Outcome of the thicker crust was wider orogens, and hence more space for strain partitioning to develop. However, strain partitioning was not able to be preserved that well for the whole 40 Ma, which was the runtime of the models. Though in terms of strain partitioning, the results were not ideal, the angle of obliquity affected on the crustal shear zones as well, in a interesting way.
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(2024)The need caused by climate change and the pursuit to use natural resources more efficiently has accelerated the search for new alternatives to fossil fuels. This has increased the popularity of geothermal energy, of which the most common and versatile energy system is the Enhanced Geothermal System (EGS). An unavoidable byproduct of EGS power plants is seismicity, which occurs particularly during hydraulic stimulation to enhance the flow of heat transfer fluid in the bedrock. Stimulation significantly increases the seismic risk in the area to a level well above the natural state, thus increasing the probability of significant earthquakes. To minimize the risks and damages caused by significant seismic events, it is important to determine the maximum credible earthquake magnitude (Mmax) for EGS projects. Mmax serves as a crucial parameter in seismic risk assessment, defining the largest possible magnitude event that can be reached during the injections. This work investigates the ST1 Deep Heat project that operated in Otaniemi, Espoo, during which two stimulations were conducted in 2018 and 2020. The combined seismic risk from these stimulations has not yet been evaluated in terms of the Mmax and the probability distribution of magnitudes. In this study, we aim to determine the Mmax value for Otaniemi based on a probabilistic approach, using the method presented by Shapiro et al. (2010), where the Gutenberg-Richter relation is modified by incorporating the seismogenic index and the volume of injected fluid. This work suggests that the value of Mmax produced by the Otaniemi stimulations is M2.33. Therefore, the probabilities for significant earthquakes are low, indicating a low seismic risk. When previous studies are complemented with the effects of the 2020 stimulation, this study shows that the 2018 stimulation is the main contributor to the seismic risk in Otaniemi and that the increase in seismic risk from the 2018 stimulation to the end of the 2020 stimulation is small. The results of this work show that the seismogenic index can be used to estimate Mmax and probabilities of significant magnitudes from statistical injection-based data and provide structural geological justifications for these results. The results support the idea that injection-based Mmax is the result of the interaction between injection strategy and natural seismotectonic conditions. The lower seismicity of the 2020 stimulation can be explained by these factors. In EGS operations, the injection strategy is crucial for managing the seismic risk. The seismogenic index determined in this study describes the seismotectonic state of the Otaniemi reservoir. Once determined, it can be used in future studies or projects focusing on the Otaniemi reservoir and boreholes. It is also possible that another geologically similar reservoir would have a similar seismogenic index, which would allow the findings of this study to be applied to the assessment and management of similar risks elsewhere. Continued and high-quality research is a requirement for the development of policies and methods to make geothermal energy production safer in the future. This research provides information on the relationship between EGS operations and induced seismicity and will increase the understanding of induced seismicity and its hazards in general.
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(2022)Space weathering can be defined as the combination of physical and chemical changes that occur in material exposed to an interplanetary environment on the surface of airless bodies. This process produces amorphous surface layers often containing small opaque particles such as nanophase metallic iron (npFe0). This darkens the topmost layer resulting in alterations in material spectroscopic features.Eventually it can lead to misinterpretation of remotely sensed data in the visible- near-infrared (VIS-NIR) spectrum. The goal of this research is to simulate solar wind effects on asteroid spectra through low energy 1 keV hydrogen ion irradiation of meteorite powder samples and measure the changes in their reflectance spectra. This allows to understand how space weathering depends on the mineralogy of the material. We used Bjurböle (L/LL4), Avanhandava (H6) and Luotolax (Howardite) meteorites. H+ ion irradiation was carried out on powdered samples compressed into pellets. The pellets were placed into a vacuum chamber with pressure between 1.2 x 10 -7-2.4 x 10 -7 mbar for the whole experiment. To simulate solar wind irradiation, H+ ions were used with 1 keV under three fluences; 1 x 1017, 2 x 1017 and 5 x 1017 ions/cm2. Subsequently reflectance spectra of the samples were measured and processes using Modified Gaussian Model (MGM) to derive key spectral parameters. Both chondrites show significant reddening in the VIS region. Bjurböle being an LL, it is more oxidized than Avanhandava. The reddening in the NIR region is more significant in Avanhandava than in Bjurböle. My work indicates that even for low-energy solar wind conditions, the chondritic materials (Q/S-type asteroids) with high olivine content and/or higher fayalite (Fa) compositions are more susceptible to silicate absorption bands reduction. Luotolax meteorite being howardite rich in orthopyroxene and clinopyroxene, shows VIS reddening but not observable band depth changes with increasing exposure to H+ ion irradiation. The smaller change in Luotolax may be due to higher pyroxene resistance to low-energy ion irradiation. Overall, at short timescales and typical solar wind energies, VIS slope reddening is the most dominant factor in all three material compositions.
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(2023)A new three-dimensional crustal and upper mantle P-wave velocity model and a Moho depth map of Finland and the surrounding area were constructed using kriging interpolation. The models are based on the latest wide-angle reflection and refraction (WARR) data from the Fennoscandian shield. The Moho depth map agrees with previous Moho maps but also shows new details in the large Moho depression in central Finland compared to the previous Moho maps. The new Moho features include a new Moho depth local maximum near the center of the depression and increased Moho depths extending to the northwest and south of the depression. The three-dimensional crustal and upper mantle P-wave velocity model differs from previous models by showing a non-existent high-velocity lower crust beneath the Wiborg Rapakivi Batholith. Both the Moho map and the velocity model exhibit distinct features within the tectonic provinces in the Fennoscandian Shield. The uppermost velocity model layer is shown to roughly correlate with general features of surface geology. Statistical analysis of Moho depth and P-wave velocity data was performed, and variogram models were fitted to capture spatial autocorrelation. Ordinary kriging was used to generate a Moho model with a 25 × 25 km grid cell size. The three-dimensional P-wave velocity model was constructed in two parts, with separate universal kriging schemes for the crustal and upper mantle velocities. The velocity model has a grid cell size of 50 × 50 × 1 km in the uppermost 4 km and a lower resolution of 50 × 50 × 2 km below 4 km depth. The presented models can be utilized for a variety of applications, including seismic source location, crustal effect correction for seismic tomography and teleseismic studies, and general modeling of large-scale tectonic processes.
Now showing items 1-5 of 5