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- Joshi, Niranjan
(författare)
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Bias-adjusted analysis of global natural disaster records and an assessment of seismic hazard in Sweden
- 2024
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Natural disasters pose significant challenges today and demand efficient allocation of society’s limited resources for disaster risk reduction. This relies on analysis of natural disaster records, which are prone to (reporting) biases that can affect the inferences drawn from their analysis. Data incompleteness is common in earthquake seismology, and, with a starting point in the Gutenberg-Richter law, this thesis studies the power-law behaviour between fatalities and the frequency of global natural disasters recorded by EM-DAT from 1900 to 2020. Analysing, first, the power-law behaviour of global earthquake frequency and magnitude reveals overlapping patterns, and apparent improved reporting completeness over the studied periods, implying any increase in recorded earthquakes is a reporting artefact. Similarly, “frequency-magnitude” analysis of earthquake disasters shows power-law behaviour and an almost-unchanged distribution of fatalities over time, which also implies improved reporting. Similar analysis of hydro-meteorological disasters in EM-DAT shows their substantially increased number with time to be a reporting artefact driven by improved reporting of low-fatality disasters. The increasing gradient of the power-law part of the frequency-magnitude graphs implies that the worst events have become less fatal on average. Climate-related disaster risk reduction actions have thus been very successful, resulting in a continuous decline in aggregate associated fatalities, to the extent that earthquakes have been the most fatal natural disasters over the last few decades. Notably, while the earthquake magnitude distribution is dominated by earthquakes in high-seismicity zones, fatalities predominantly occur in low-seismicity intraplate regions. This implies that seismic hazard in Sweden, an intraplate area, may be significant, especially as earthquakes as large as M8 have occurred in its recent deglaciation phase. Hazard analysis based on data from the recently expanded Swedish seismic network clearly unveils significant hazard posed by Post-Glacial faults in the North. This research advances our understanding of natural disaster dynamics, emphasizing the necessity for non-conventional methodologies to analyze historical data. It offers important insights to help form effective disaster risk reduction strategies, particularly in the context of seismic hazard assessment for specific regions like Sweden.
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| 2. |
- Dehghannejad, Mahdieh, 1979-
(författare)
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Reflection seismic investigation in the Skellefte ore district : A basis for 3D/4D geological modeling
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The Skellefte ore district in northern Sweden is a Palaeoproterozoic volcanic arc and one of the most important ones hosting volcanogenic massive sulfide (VMS) deposits, producing mainly base metals and orogenic gold deposits. Due to high metal prices and increased difficulties in finding shallow deposits, the exploration for and exploitation of mineral resources is quickly being moved to greater depths. For this reason, a better understanding of the geological structures in 3D down to a few kilometers depth is required as a tool for ore targeting. As exploration and mining go deeper, it becomes more and more evident why a good understanding of geology in 3D at exploration depths, and even greater, is important to optimize both exploration and mining.Following a successful pilot 3D geological modeling project in the western part of the district, the Kristineberg mining area, a new project "VINNOVA 4D modeling of the Skellefte district" was launched in 2008, with the aim of improving the existing models, especially at shallow depth and extending the models to the central district. More than 100 km of reflection seismic (crooked) profiles were acquired, processed and interpreted in conjunction with geological observations and potential field data. Results were used to constrain the 3D geological model of the study area and provided new insights about the geology and mineral potential at depth.Results along the seismic profiles in the Kristineberg mining area proved the capability of the method for imaging reflections associated with mineralization zones in the area, and we could suggest that the Kristineberg mineralization and associated structures dip to the south down to at least a depth of about 2 km. In the central Skellefte area, we were able to correlate main reflections and diffractions with the major faults and shear zones. Cross-dip analysis, reflection modeling, pre-stack time migration, swath 3D processing and finite-difference seismic modeling allowed insights about the origin of some of the observed reflections and in defining the imaging challenges in the associated geological environments.
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| 3. |
- Jeddi, Zeinab, 1986-
(författare)
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Seismological Investigation of Katla Volcanic System (Iceland) : 3D Velocity Structure and Overall Seismicity Pattern
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The work in this thesis concentrates on Katla volcano in southern Iceland. This is one of Europe’s most active volcanoes and its history tells us that it poses many threats to society, both locally (Iceland) and on a broader scale (Europe). Its geological setting is complex, where the effects of a melting anomaly in the mantle and a changing rift geometry, perturb the classical setting of volcanism in a rifting setting.The work has focused on two aspects. The first is the varying distribution of physical properties in the subsurface around the volcano. The second is the distribution of microearthquakes around the volcano. The physical properties that we study are the speeds of seismic waves that reflect variations of temperature, composition and fracturing of the rocks. These can, therefore, help us learn about long-term processes in the volcano. The seismicity gives shorter-term information about deformation associated with current processes.I have applied two tomographic techniques to study Katla’s subsurface to a depth of 5-10 km, namely local-earthquake and ambient-noise tomography. The former makes use of the timing of waves generated by local earthquakes to constrain the earthquakes’ locations and the distribution of wave speed. Here I have concentrated on compressional waves or P waves with a typical frequency content around 10 Hz. With the latter, surface waves are extracted from microseismic noise that is generated far away at sea and their timing is measured to constrain their wave-speed distribution, which then is used to map shear-wave velocity variations. This is done at a typical frequency of 0.3 Hz. I find that the volcano contains rocks of higher velocity than its surroundings, that Katla’s caldera is underlain by low velocities at shallow depth that may be explained by hot or partially molten rocks and that beneath the caldera lies a volume of particularly high velocities that may constitute differentiated cumulates. But, I also find that it is not simple to compare results from such different wave types and discuss a number of complications in that regard.In addition to the well-known microearthquake distribution in the caldera region of Katla and to its west, we have discovered two additional areas of microearthquake activity on the volcano’s flanks, south and east of the caldera. These point to current activity and are, therefore, of interest from a hazard point of view. However, it is difficult to pinpoint their underlying process. Speculation about possible interpretation leads me to hydrothermal processes or small pockets of melt ascending due to their buoyancy or locally enhancing fluid pressure, thereby lowering the effective stress.
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