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Magnitude-frequency distribution of the seismic activity preceding the Fagradalsfjall 2021 eruption
Severini, Bianca Georgina ; Fischer, Tomáš (advisor) ; Hrubcová, Pavla (referee)
iv Abstract This bachelor's thesis focuses on the estimation of the parameter b derived from the Gutenberg- Richter law talking about the dependency of the events' frequencies on their magnitude during seismic activity preceding the effusive eruption on 19th March 2021 Fagradalsfjall, Iceland. The data were selected in space and time from seismic catalogue comprising records from 10th January 2021 until 10th August 2022. The maximum likelihood method was used when computing the b value and the goodness of fit method for estimating the magnitude of completeness MC. In order to give a comparison to resulting b values, newly introduced method called b-positive was added to this thesis. B-positive was designed to withstand temporal incompleteness of the seismic catalogue and to be fully independent on the MC. All values were computed with computational script made in MATLAB in cooperation with consultant of this thesis. It was searched for the space and time dependency among the selected seismic data clusters in Fagradalsfjall and Krýsuvík system. Results at the Fagradalsfjall system are proving the activity was connected to tectonic movements and to the intrusion of magmatic dyke. Divergent plate boundary in this area shows lower values in comparison with the magmatic dyke which has higher values. For the...
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Analysis of volcanic tremor during the 2021 Fagradalsfjall eruption
Sova, František ; Fischer, Tomáš (advisor) ; Burjánek, Jan (referee)
Volcanic tremors are low frequency seismic events, occurring prior to or during eruptions and are believed to be caused by the movement of magma. The study of those events can reveal subsurface processes during volcanic activity, furthermore it can help to reconstruct eruptions, which cannot be observed directly. The presented study describes the volcanic tremor during the 2021 Fagradals- fjall eruption in Iceland. Seismic data from several stations were processed to produce a half-year record of volcanic activity. Additionally, the tremor ampli- tude attenuation was investigated. The seismic record was marked by a reoccurring tremor signal in the fre- quency band 1-10 Hz, which temporally matched with the different phases of effusive eruption. Its amplitudes varied in time, showing three distinct pat- terns. The weak continuous tremor in the first weeks of eruption transitioned into regular pulses, lasting several minutes, and later into a different pulsation with a 1.5-day period. Those variations might reflect the changes in geometry and magma flow of the underground magmatic system. 1
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Source Parameters of Microearthquakes and their Uncertainties
Michálek, Jan ; Fischer, Tomáš (advisor) ; Gallovič, František (referee) ; Kwiatek, Grzegorz (referee)
Title: Source Parameters of Microearthquakes and their Uncertainty Author: Jan Michálek Department: Department of Geophysics Supervisor: Doc. RNDr. Tomáš Fischer, PhD., Institute of Hydrogeology, Engineering Geology and Applied Geophysics, Charles University in Prague Abstract: The aim of this thesis is to analyze the spectral methods used for the determination of the earth- quake source parameters like the seismic moment M0 and the corner frequency fc and to apply these meth- ods to seismic data from the West Bohemian region. Considering some assumptions about the source the other important parameters like the source radius r or the stress drop in the source ∆σ can be evaluated. Determination of the parameters is performed in the spectral domain by comparing a simple Brune's source model (non-causal slip on a circular rupture; spectral slope ω−2) with the displacement of the P or S wave. The methods were applied to 56 selected earthquakes of the West Bohemian swarms from 2000 and 2008 by the absolute and relative approach in several modifications. The absolute method allows to determine not only the source parameters but also the quality factor Q (attenuation), which significantly affects the deter- mination of fc. Therefore, the absolute method was applied also as the joint inversion when Q is stabilized....
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Seismic noise analysis at selected stations of the WEBNET network
Bachura, Martin ; Fischer, Tomáš (advisor) ; Růžek, Bohuslav (referee)
Seismic signal is affected by noise of non-geological origin. The main source of seismic noise is in human activities. Seismic noise of human origin occurs in the frequency range from one to the first tens of Hz. The noise intensity limits the lower level of the earthquake size that can be recorded by a seismic network. The aim of this thesis is to analyze the seismic noise on selected stations of the West Bohemian seismic network WEBNET which operates in the area since the early nineties. The earthquakes in West Bohemia/Vogtland are manifested in the form of long-lasting earthquake swarms with the magnitudes ranging up to ML = 3.8. The way how the seismic noise influences the seismic stations and their capability of recording small earthquakes can be classified by a statistical and time-frequency analysis. We applied this method to the selected stations NKC, KRC and SKC. The results show that the effect of noise does not deteriorate the seinsitivity of the three stations for recording small earthquakes. Despite of quite good results, some differences occur among the three stations, which are most probably caused by the distance of the stations to the populated areas. The smallest noise influence is observed at the stations located far from settlement (NKC and KRC) compared to the station located in a small...
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Analysis of coda waves of the West-Bohemian earthquakes
Bachura, Martin ; Fischer, Tomáš (advisor) ; Vavryčuk, Václav (referee)
Coda of high frequency earthquakes contains information about attenuation propetries of the environment between earthquake hypocentre and seismic station. By application of different methods of coda anlysis one can derive these properties. I applied three methods to seismograms from autumn 2011 from WEBNET network monitoring seismic activity in West Bohemia/Vogtland area: basic method to derive Qc parameter of the environment, and two methods that allow for separation of intrinsic attenuation (Qi) and attenuation due the scattering (Qsc): method MLTWA (Multiple Lapse Time Window Analysis) and method based on energy flux model. Values of Qc are relatively small (from 22 to 600, which depends on the length of the window in which the analysis is carried out and on the frequency and station used), which is in good agrement with the high seismic activity in the area. Qc shows frequency, depth and lateral dependances. It increases with frequency and depth and in the southwest direction. The applicability of methods for separation of Qi and Qsc are limited. With short hypocentral distancies as we have coda is not affected by scattering. Because of this, I was unable to use MLTWA method and the method of energy flux model was usable to derive only the values of Qi. These values are from 120 (1,5 Hz at...
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Relocation of the 1962 earthquake swarm in West Bohemia
Konrádová, Diana ; Fischer, Tomáš (advisor) ; Doubravová, Jana (referee)
Since the swarm at the turn of 1985/86 the current activity of seismic swarms in western Bohemia takes place mainly in the area of Nový Kostel at the eastern edge of the Cheb basin. However, the location of previous activities, where only more remote seismic stations were in operation, is not exactly known. This also applies to the swarm of 1962, which this bachelor thesis is focused on. Seismic bulletin data are available from eight seismic stations at a suitable distance for sufficient localization quality for this swarm. The theoretical part of the thesis presents research results of fundamental existing localization methods and their properties. In the practical part, the location of the earthquake epicenter of swarm 1962 is presented.
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Seismic monitoring of fluid contacts from horizontal wells
Janská, Eva ; Vilhelm, Jan (advisor) ; Fischer, Tomáš (referee)
When producing oil from a thin layer with a gas cap, gas is generally unwanted since the reservoir looses pressure. The pressure loss might cause the fluid vertical migration and therefore a misplacement of the horizontal production well. This is the main reason why the oil production is held back when a gas breakthrough occurs. The well is chocked, or run on lower production rates, until the oil column is recovered, but it is not known how long this will take. If the gas-oil contact could be monitored over time, then the gas breakthrough could be better prevented by reducing production from a particular well branch or even by blinding a specific part of the branch. The aim of this research is to test if monitoring of the gas-oil contact by a seismic acquisition inside the horizontal well is feasible. This could be done much cheaper, more often and with better repeatability than classical 4D seismic. The feasibility was investigated on 2D Troll subsurface models, using finitedifference acoustic modelling. Receivers are planned in the horizontal part of the production well and suggesting the best source position was one of objectives. The influence of variations in geological settings was also investigated. The most accurate information about the gas-oil contact shape and its distance from the well is...
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