Refine
Document Type
- Doctoral Thesis (8)
Has Fulltext
- yes (8)
Is part of the Bibliography
- no (8)
Keywords
- Anisotropie (1)
- GRSN (1)
- Nördlicher Oberrheingraben (1)
- SKS (1)
- Schwarmbeben (1)
- Seismische Gefährdung (1)
- Seismizität (1)
Institute
This research was conducted in the Rwenzori Region of the Western Branch, East African Rift System (EARS). The EARS is a tectonic structure extending over a length of more than 3000 km from the Afar Triple Junction, in Ethiopia, to Lake Malawi in the south. The Western Rift System is a roughly NE to ENE trending sector of the EARS, which runs along the western boundary of Uganda and the neighboring Democratic Republic of Congo (D.R.C). It stretches 2100 km from Nimule, NW on Uganda-Sudan border, extending to Lake Malawi in the SE of Africa. The unusual uplift of the Rwenzori Mountains within an extensional regime and the mechanisms associated with the high frequency of seismic activity in the region was hardly understood and therefore, had remained a subject of contention that needed to be critically addressed in detail. To my knowledge, this was probably the first study to be performed and documented in great depth within the domains of seismic noise variation, seismic anisotropy and b value analyses beneath the Rwenzori Region. After about six years of operation (2006-2012), the seismology group of the RIFTLINK Research Project (www.riftlink.org) acquired a vast amount of high-quality, digital data that were collected using a seismic network of well calibrated seismic equipment. The project was divided into two phases. Phase I, that operated between February 2006 - September 2007, consisted of thirty-two temporary seismic stations, which were selectively spread out in the Rwenzori Region on the Ugandan side, to detect and record extremely weak as well as strong naturally occurring earthquakes. The seismic equipment used included EDL and REFTEK digitizers, which were coupled with Güralp and MARK sensors respectively (REFTEKS: only short-period MARK sensors, EDLs: short-period MARK plus few broadband Güralp Sensors). Exactly 22375 earthquakes were recorded. The data were processed using the SEISAN software package. About 14413 earthquakes were carefully localized using the velocity model of Bram (1975) that implements a Vp=Vs ratio fixed at 1.74. Phase II, that extended between 2009-2012 consisted of thirty-two seismic stations, which were spread out around the Rwenzori Mountains, both on the Ugandan side and the neighboring D.R.C. Only Taurus digitizers that were coupled with Trillium sensors were used in the D.R.C. On the Ugandan side however, both EDL and Taurus digitizers, which were coupled with Trillium and Güralp sensors were used. ...
In this study, I investigate the crustal and upper mantle velocity structure beneath the Rwenzori Mountains in western Uganda. This mountain range is situated within the western branch of the East African Rift and reaches altitudes of more than 5000 m. I use four different approaches that belong to the travel-time tomography method. The first approach is based on the isotropic tomographic inversion of local data, which contain information about 2053 earthquakes recorded by a network of up to 35 stations covering an area of 140×90 km2. The LOTOS-09 algorithm described here is used to realize this approach. The second approach is based on the anisotropic tomographic inversion of the same local dataset. This method employs the tomographic code ANITA, developed with my participation, which provides 3D anisotropic P and isotropic S velocity distributions based on P and S travel-times from local seismicity. For the P anisotropic model, four parameters for each parameterization cell are determined. This represents an orthorhombic anisotropy with one vertically-oriented predefined direction. Three of the parameters describe slowness variations along three horizontal orientations with azimuths of 0°, 60° and 120°, and one is a perturbation along the vertical axis. The third approach is based on tomographic inversion of the teleseismic data, which contain information about the traveltimes of P-waves coming from 284 teleseismic events recorded by the seismic network stations. The TELELOTOS code, which is my own modification of the LOTOS-09 algorithm, is used in this approach. The TELELOTOS code is designed to iteratively invert the local and/or teleseismic datasets. Finally, I present the results of the new tomographic approach, which is based on the simultaneous inversion of the joint local and teleseismic data. The simultaneous use of these datasets for the tomographic inversion has several advantages. In this case, the velocity structure in the study area can be resolved as deep as in the teleseismic approach. At the same time, in the upper part of the study volume, the resolution of the obtained models is as good as in the local tomography. The TELELOTOS algorithm is used to perform the joint tomographic inversion. Special attention is paid in this work to synthetic testing. A number of different synthetic and real data tests are performed to estimate the resolution ability and robustness of the obtained models. In particular, synthetic tests have shown that the results of the anisotropic tomographic inversion of the local data have to be considered as unsatisfactory. For all approaches used in this study, I present synthetic models that reproduce the same pattern of anomalies as that obtained by inverting the real data. These models are used to interpret the results and estimate the real amplitudes of the obtained anomalies. The obtained models exhibit a relatively strong negative P anomaly (up to -10%) beneath the Rwenzori Mountains. Low velocities are found in the northeastern part of the array at shallower depths and are most likely related to sedimentary deposits, while higher velocities are found beneath the eastern rift shoulder and are thought to be related to old cratonic crust. The presence of low velocities in the northwestern part of the array may be caused by a magmatic intrusion beneath the Buranga hot springs. Relatively low velocities were observed within the lower crust and upper mantle in the western and southern parts of the study area (beneath the rift valley and the entire length of the Rwenzori range). The higher amplitude of the low-velocity anomaly in the south can be related to the thinner lithosphere in the southern part of the Albertine rift. In the center of the study area, a small negative anomaly is observed, with the intensity increasing with depth. This anomaly is presumably related to a fluids rising up from a plume branch in the deeper part of the mantle. According to the interpretation of the local earthquake distribution, the Rwenzori Mountains are located between two rift valleys with flanks marked by normal faults. The Rwenzori block is bounded by thrust faults that are probably due to compression.
The East African Rift System (EARS) was initiated in the Eocene epoch between 50 and 21 Ma probably due to the influence of mantle plumes that caused volcanism, flood basalts and rifting extensions in Ethiopa and the Afar region. As a result of magmatic intrusions and adiabatic decompression melting within the lithosphere caused by the impact of the Kenya plume, there was a southward propagation of the EARS of about 30 – 15 Ma from Ethiopia to Kenya, which coincide with the occurrence of volcanism. The EARS developed towards the south along the margins of the Tanzania Craton between 15 and 8 Ma. Previous findings of low-velocity anomalies within the upper mantle and the mantle transition zone indicate an upwelling of hot mantle material in the vicinity of the Afar region and the East African Rift. This study includes the analysis of P- and S-receiver functions in order to determine further impacts on the lithosphere from below. The aim was to determine the topographic undulations of further boundary layers and to identify their variability owing to the rifting processes and the formation of the EARS. The study area included the Tanzania Craton and the surrounding rift branches of the East African Rift System.
The region of the Rwenzori Mountains can be analysed in detail because of the large dataset of the RiftLink project. The use of the P-receiver function technique and the H-K stacking method enabled to determine different vP /vS ratios depending on the tectonic setting in the Rwenzori region: Rift shoulders (vP /vS =1.74), Albert Rift segment (vP /vS =1.80), Edward Rift segment (vP /vS =1.87) and Rwenzori Mountains (vP /vS =1.86). To determine the topography of the Moho, it is necessary to take into account the thickness of the sedimentary layer, the surface topography, the azimuthal variations in crustal thickness and the impact of local anomalies. After correcting these effects on the Moho depths, significant variations in Moho topography could be determined. The Moho depths range from 29 to 39 km beneath the rift shoulders of the Albertine Rift. Within the rift valley, the crustal thickness varies between 25 – 31 km in the Edward Rift segment and 22 – 30 km in the Albert Rift segment. An averaged crustal thickness of about 26 km within the rift valley indicates the lack of the crustal root beneath the Rwenzoris. Similar variations in crustal thickness were determined by using an automatic procedure for analysing S-receiver functions that was developed in this study.
The S-receiver functions are created by applying a rotation criterion in order to rotate the Z, N and E components into the L, Q and T components. It is necessary to perform trial rotations using different incident and azimuth angles to determine the correct rotation angles. The latter are identified by the use of the rotation criterion, including the amplitude ratio of the converted Moho signal to the direct S/SKS-wave signal. The L component is rotated correctly in the direction of the incident shear wave in the case of the maximum amplitude ratio. After analysing the frequency content of the receiver functions in order to sort out harmonic and long-periodic traces, the individual Moho signals are checked for consistency in order to remove atypic signals. To increase the signal-to-noise ratios on the traces, the S-receiver functions are stacked. For this purpose, the signals of the direct shear waves must originate from similar epicenters. On the basis of similar ray paths, the receiver functions show comparable waveforms and converted signals. To perform the stacking procedure, it is necessary to merge the datasets of the adjacent stations in order to obtain a sufficient number of receiver functions. This analysis is based on the assumption that the incident seismic waves arriving at the adjacent stations penetrate to some extent the same underground structures in the case of similar wave propagation paths. This approach accounts for the fact that the converted signals do not result exclusively from the piercing points at the boundary layers. Further signals originate from the conversions at the boundary layer within the Fresnel Zone. The piercing points are derived from the significant signals in the receiver functions. Depending on the order of arrival of the converted phases on the traces, the signals are attributed to the theoretical discontinuities DIS1, DIS2, DIS3 and DIS4. However, partly due to the low signal-to-noise ratios on the traces, it is difficult to identify the real conversions on the traces and to ensure that the converted signals are attributed to the correct boundary layers. For this reason, it is necessary to check the consistency of the conversion depths among each other. In the case of inconsistent conversion depths, the corresponding signals are either adjusted to another seismic boundary layer or removed from the dataset. To verify the functionality of the automatic procedure and to determine the resolvability with respect to two boundary layers, several models are tested including horizontal and dipping discontinuities. To resolve distinct discontinuities, their depths must differ by at least 60 km, otherwise, due to similar depth ranges of the different boundary layers, the converted signals cannot be separated from each other. As a consequence, the converted signals that originate from different discontinuities are attributed to a single one. Further tests including break-off edges of seismic discontinuities are performed to check the attributions of the converted signals to the discontinuities. Owing to the varying number of boundary layers, the converted signals cannot be attributed to the discontinuities according to the order of their arrivals on the traces. It is necessary to correct their attributions to the seismic discontinuities in order to resolve the boundary layers.
The crust-mantle boundary and further discontinuities within the lithospheric mantle are investigated by applying this automatic procedure. Depending on the tectonic setting, the conversion depths of the Moho range from about 30 – 45 km beneath the western rift shoulder to 20 – 35 km within the rift valley up to 30 – 40 km beneath the eastern rift shoulder. The long wavelengths of the shear waves hamper the correct identification of the converted phases in the S-receiver functions. With respect to the relative differences in conversion depth, the topographic undulations of the crust-mantle boundary are consistent with the Moho depths derived from P-receiver functions. In contrast to the Rwenzori region, it is difficult to resolve completely the trend of the Moho in the remaining area of the East African Rift due to the small dataset provided by IRIS. The results exibit an increase in crustal thickness to up to 45 km in the region of the Cenozoic volcanics such as Virunga, Kivu, Rungwe and Kenya. The greatest Moho depths of more than 50 km are located near Mount Kilimanjaro. In addition to the Moho, the analysis of the S-receiver functions revealed two further boundary layers at depths of 60 – 140 km and 110 – 260 km, which are associated with a mid-lithospheric discontinuity and the lithosphere-asthenosphere boundary, respectively. The shallowest conversion depths of the LAB are focussed to small-scale regions within the rift branches, namely the northern Albertine Rift, the Chyulu Hills and the Mozambique Belt, which are located around the Tanzania Craton. The larger thickness of the lithosphere beneath the cratonic terrain indicates that the Tanzania Craton is not significantly eroded. However, there are indications that the lithosphere beneath the craton and the rift branches is penetrated by ascending asthenospheric melts to depths of up to 140 and 60 km, respectively. The top of the ascending melts is associated with the occurrence of the mid-lithospheric discontinuity. The shallowest conversion depths of this boundary layer (60 – 90 km) are related to the rifted areas of the EARS and the Cenozoic volcanic provinces, which are located along the Albertine Rift, the Kenya Rift and the Rukwa-Malawi rift zones. The deepest conversion depths of up to 140 km are related to the Rwenzori Belt, the Ugandan Basement Complex and the interior of the Tanzania Craton.
Die Seismizität des nördlichen Oberrheingrabens (ORG) ist aufgrund seines Potentials für die geothermische Nutzung und der damit möglicherweise verbundenen seismischen Risiken von allgemeinem Interesse. Detaillierte Kenntnisse der natürlichen Seismizität erlauben Rückschlüsse auf aktive Störungszonen und Spannungsverhältnisse im Untergrund. Sie liefert außerdem wichtige Hintergrundinformationen für die Abschätzung einer möglichen induzierten Seismizität. Untersuchungen zur Charakterisierung der natürlichen Seismizität, des Spannungsfeldes und der seismischen Gefährdung des nördlichen ORG sind Hauptbestandteil dieser Arbeit, die innerhalb des BMU/BMWi-Projektes SiMoN (Seismisches Monitoring im Zusammenhang mit der geothermischen Nutzung des Nördlichen Oberrheingrabens) entstanden ist. Aufzeichnungen eines Netzwerkes aus 13 seismischen Stationen dienen als Datengrundlage zur Charakterisierung der Seismizität innerhalb eines etwa 50 x 60 km2 großen Areals im dichtbesiedelten Rhein-Main Gebiet. Untersuchungen der Rauschbedingungen zur Bewertung der Eignung der Stationsorte für das Aufzeichnen der natürlichen Seismizität lieferten bei den Stationen auf felsigem Untergrund sehr gute spektrale Eigenschaften, während alle Stationen im Sediment des ORG deutlich höhere Rauschanteile aufzeigten. Anhand systematischer Messungen in flachen Bohrlöchern konnten laterale und vertikale Variationen des seismischen Rauschens beschrieben werden und dadurch eine Verbesserung der Detektionsschwelle beobachtet werden.
Es werden die Ergebnisse des seismischen Monitorings für den Zeitraum November 2010 bis Dezember 2014 dargestellt. Die Detektionsschwelle für das Netzwerk liegt bei einer Lokalmagnitude von etwa 0,5, die Vollständigkeitsmagnitude beträgt Mc = 1,2. Seit Beginn der Datenaufzeichnung konnten 243 Erdbeben im unmittelbaren Bereich des Stations-netzwerkes mit Magnituden im Bereich zwischen ML = -0,5 und ML = 4,2 lokalisiert werden. Die Epizentren liegen hauptsächlich entlang der östlichen Grabenschulter und im Graben; entlang der westlichen Grabenschulter ist die seismische Aktivität deutlich geringer. Eine weitere aktive Region konnte entlang der südlichen Ausläufer des Taunus im Nordwesten des Untersuchungsgebietes identifiziert werden. Die Seismizität erstreckt sich bis in eine Tiefe von 24 km mit einem Maximum der hypozentralen Tiefenverteilung im Bereich von 12-18 km. Im Graben ist die Seismizität dabei auf die tiefere Kruste im Bereich von 9-24 km beschränkt. Das Fehlen von seismischer Aktivität in der oberen Kruste bis ca. 9 km Tiefe im Graben könnte auf eine aseismische Deformation in diesem Tiefenbereich hindeuten. Seit Mai 2014 konnte südöstlich von Darmstadt bei der Ortschaft Ober-Ramstadt zum ersten Mal seit fast 150 Jahren eine Schwarmbebenaktivität im Bereich des nördlichen ORG registriert werden. Die Hypozentren sind in zwei Cluster unterteilt, die räumlich voneinander getrennt sind und unterschiedliche Aktivitätsraten aufweisen. Die Herdtiefen liegen im Bereich von 1-8 km.
Zusätzlich zu den Daten des SiMoN Netzwerkes wurden Aufzeichnungen der regionalen Erdbebendienste in Herdflächenanalysen für insgesamt 58 Erdbeben einbezogen. Die Herdflächenlösungen weisen überwiegend Blattverschiebungen (Strike-slip-Mechanismen) auf. Auf- und Abschiebungen spielen nur eine untergeordnete Rolle. Die berechneten Herdmechanismen bestätigen, dass sich das Spannungsfeld des nördlichen ORG transtensional verhält, im Vergleich zu früheren Studien konnte jedoch eine deutlich ausgeprägte Blattverschiebungskomponente identifiziert werden. Zur Bestimmung der Hauptspannungsachsen wurde eine Inversion der Herdflächenlösungen durchgeführt und die Richtung der maximalen horizontalen Spannung, welche hauptsächlich in N135°E orientiert ist, bestimmt.
Aufbauend auf den neu gewonnen Erkenntnissen zur natürlichen Seismizität und zum Spannungsfeld des nördlichen ORG wurde eine probabilistische seismische Gefährdungsanalyse durchgeführt. Um Unsicherheiten in den seismischen Quellregion-modellen zu berücksichtigen, wurden sechs unterschiedliche Modelle entwickelt. Für jede Quellregion wurden spezifische Parameter bestimmt. Ihre Unsicherheiten werden in einem logischen Baum behandelt. Auf der Grundlage eines neu zusammengestellten Momentmagnituden-basierten Erdbebenkatalogs wurden die Magnitudenhäufigkeits-parameter bestimmt. Unter Berücksichtigung des tektonischen Regimes in jeder Quelle wurden unterschiedliche Dämpfungsrelationen der Bodenbeschleunigung verwendet. Zur Quantifizierung der maximal zu erwartenden Magnitude in jeder Quelle wurden Wahrscheinlichkeitsdichtefunktionen berechnet. Die Resultate der Gefährdungsanalyse werden in Form von Karten der Spektralbodenbeschleunigungen und Spitzenboden-beschleunigungen für Wiederkehrperioden von 475 und 2475 Jahren und Antwortbeschleunigungsspektren dargestellt. Im Vergleich zu früheren Studien konnte eine erhöhte seismische Gefährdung für den nördlichen ORG festgestellt werden.
Mantle convection is the process by which heat from the Earth’s core is transferred upwards to the surface and it is accepted to explain the dynamics of the Earth’s interior. On geological time-scales, mantle material flows like a viscous fluid as a consequence of the buoyancy forces arising from thermal expansion. Indeed, mantel convection provides a framework which links together the major disciplines, such as seismology, mineral physics, geochemistry tectonic and geology. The numerical model has been applied to understand the dynamic, structure and evaluation of the Earth, and other terrestrial planets and the investigations continue to explore, different aspects of the mantle convection.
In fact, to model this phenomenon, two complementary approaches are possible. On the one hand, one can solve self-consistently the equations of thermal convection, including parameters and employing physical relationships derived from mineral physics. Our understanding of mantle convection depends ultimately upon the success of such fully self-consistent dynamic models in explaining observable features of the flow. Although, these models presently unable to predict the actual convection pattern of the Earth, they are extremely useful to investigate general characteristics of given physical systems. On the other hand, to permit comparison with specific observables associated with the flow, one can consider a more restricted problem. Instead of focusing on the time evolution of mantle flow, if we know a priori the temperature - and hence presumably the density - anomalies that drive the convection, we can try to build a snapshot of the present-day flow pattern, consistent with those anomalies, that can successfully predict the observables. As matter of fact, the aim of this study is to investigate both approaches in comparison with the main geophysical constraints on mantle structure. These constraints include the geoid anomalies, the dynamic surface and core-mantle boundary topography and tectonic plate motions.
The most appropriate mathematical basis functions for describing a bounded and continuous function on a spherical surface are spherical harmonics. We may therefore expand the geodynamic observables in terms of spherical harmonics. We have investigated two methods of the global spherical harmonic analysis by specific attention to the dynamic geoid computation of the geodynamic models. The first method is the quadrature method in which the loss of the orthogonality of the Legendre functions in transition from continues to discrete case is the major drawback to the method. Particularly, we showed that in the absence of the tesseral harmonics, quadrature formulation leads to obtain inaccurate results. The second method is the least-squares which can be considered as the best linear unbiased estimator that provides the exact results. We showed that even with a low resolution grid data it is possible to reconstruct the data and achieve an accurate result by using this method, which is extremely remarkable in three-dimensional global convection studies. However, special care has to be taken since there is some source of errors that might influence the efficiency of this method.
In general, to better understanding of the properties of the mantle, it is useful to assess observable characteristics of plumes in the mantle, including geoid, topography and heat flow anomalies. However, only few studies exist on geoid and topography for axi-symmetric convection and their models were restricted to isoviscous (or stratified) mantle and low Rayleigh numbers. We studied fully coupled depth and temperature dependent Arrhenius type of viscosity in axi-symmetric spherical shell geometry in order to investigate the shape of geoid anomalies and dynamic topography above a plume. Indeed, the topography and geoid anomalies produced from plumes are sensitive to rheology of the mantle and rheology of the plume; both have effects on shape and amplitude of the geoid anomalies. As results we are able to define different classes of plumes by their geoid signals.
Mainly depth-dependent viscosity models show a geoid with negative sign above the plume which can turn to the positive sign by decrease the viscosity contrast. This can be considered as a transition between the strongly depth dependent and the constant viscosity case. Our results basically support the idea by Morgan [1965] and McKenzie [1977]. They have shown the magnitude and even the sign of the total gravity anomaly depend on the spatial variation in effective viscosity. In addition, Hager [1984] has concluded that the total gravity field is depend on the radial distribution of effective viscosity, and a small change in viscosity contrast leads to varying sign of the response function.
In the case of temperature-dependent viscosity, the formation of an immobile lithosphere is a natural result, and the flow as well as the total geoid becomes strongly time dependent. When we increase the activation energy, all geoids associated with the first arriving plumes look like bell shaped whereas for typical plumes, after reaching a statistical steady state, bell-shaped geoids with decreasing amplitude as well as linear flank shaped geoids are observed. It is surprising that in spite of large differences in lateral and depth varying viscosities, the shapes of the geoid anomalies remained rather similar. We also identified different behaviors in the combined model with temperature-and pressure-dependent viscosity. In fact, in spite of the strongly different rheology, the geoid anomalies in all cases were surprisingly similar. Furthermore, we proposed a scaling law for the geoid which makes our results directly applicable to other planets. Moreover, we can apply the results of our calculation to find relations between different rheology and sub-lid temperature, since we know that the mantle temperature can change significantly with variation in pressure-temperature dependent viscosity. It is also possible to define a range of stagnant lid thickness related to the amplitude of the geoid which can be reasonable for study of the lid thickness in Venus or Mars.
Nevertheless, in these series of models, we simplified a number of complexities within the Earth. One of the most important of such simplification is the Boussinesq approximation. This approximation is valid if the temperature scale height (i.e. the depth over which temperature increases by a factor of “ ” due to adiabatic compression) is much greater than the convection depth. However, a temperature scale height in the Earth’s mantle is at best only slightly greater than the mantle depth. Hence, the Boussinesq approximation could mask some very important stratification and compressibility effects that influence both the spatial and temporal structure of the convection. Therefore, in more advance models we considered compressibility in our mantle convection models, assuming that density vary both radially and laterally, being determined as a function of pressure and temperature through an appropriate equation of the state. Moreover, thermodynamic properties assumed to be a function of depth.
We examined the details of the structure of the spherical axi-symmetric Anelastic Liquid Approximation model (ALA) with special attention to the Arrhenius rheology, and compare it to the cases of compressible convection without depth dependent thermodynamical properties, and to cases of the extended Boussinesq approximation. At the same time, the effects of the interaction between temperature and pressure-dependent viscosity and thermodynamic parameters in the compressible mantle convection on the geoid and topography have been studied. We showed that assuming compressible convection with depth-dependent thermodynamic properties strongly influence the geoid undulations. Using compressible convection with constant thermodynamic properties is physically inconsistent and may lead to spurious results for the geoid and convection pattern. Indeed, by a systematic study of different approaches of compressibility in the spherical shell convection for different Arrhenius viscosity laws we proved that only in the unrealistic case of zero activation energy the different compressibility modes result in comparable convection and geoid patterns. In all other rheological cases, large differences have been obtained, that stressing the important role of consistent compressible thermodynamic properties for mantle convection.
In addition, we examine the impact of compressibility as well as different rheologies on the power law relation that connects the Nusselt number to the Rayleigh number. We have discovered that the power law index of the relationship is controlled by the rheology, independent of which approximation is used. Instead, the bound of this relation is controlled by a combination of different approximation and rheology.
Next, instead of focusing on the time evolution of mantle flow, we have carried out three-dimensional spherical shell models of mantle circulation to investigate the effects of joint radial and lateral viscosity variations on the Earth’s non-hydrostatic geoid, surface and core-mantle boundary topographies. These models include realistic lateral viscosity variations (LVV) in the lithosphere, upper mantle and lower mantle in combination with different stratified viscosity structures. We have demonstrated that the contradictory results concerning the effects of LVV can be clarified by the most straight-forward problem in geoid modeling; namely, rather poorly known stratified viscosity structure. We explored three classes of dynamic geoid models due to lateral viscosity variations. In the first class, the LVV strongly improved the fit to the observed geoid. Indeed, when the viscosity contrast between lower and upper mantles is not large enough to produce a good fit to geoid the LVVs are able to perform this action by adjusting amplitudes, so that it becomes comparable with observation. In the second class, inducing the LVV moderately improved the fit. Actually, when the geoid induced by a stratified viscosity structure already has a good correlation with observation, then the LVV causes its amplitude to further improve. In the last class, if the viscosity contrast between upper and lower mantle would be high enough, inducing LVV deteriorate the fit to the observed geoid.. Indeed, depending on the stratified viscosity, inducing the LVV may take place in one of these categories.
We also quantified the effects of LVV in the mantle and lithosphere individually. We found that the presence of LVV in the mantle (upper and lower) improves the fit to the observed geoid regardless of stratified viscosity. While LVV in the lithosphere is a crucial parameter, and dependent of the stratified viscosity, may increase or decrease the geoid fit. In fact, when the lower mantle considers being viscous enough, it would support the negative buoyancy of subducting slabs. Thus, it transmits some of the stress back to the top boundary and causes a weak coupling between slab and surface. Therefore, by including the low viscous plate boundaries in this model, the slabs and overriding plates decouples and the fit to the observed geoid degrades. In contrast, when the lower mantle viscosity is not sufficiently stiff, the presence of the low viscous plate boundaries assists to weaken the strong mechanical coupling between slab and surface. Hence, a better fit achieved.
This thesis presents the experimental and numerical analysis of seismic waves that are produced by wind farms. With the aim to develop renewable energies rapidly, the number of wind turbines has been increased in recent years. Ground motions induced by their operation can be observed by seismometers several kilometers away. Hence, the seismic noise level can be significantly increased at the seismic station. Therefore, this study combines long-term experiments and numerical simulations to improve the understanding of the seismic wavefields emitted by complete wind farms and to advance the prediction of signal amplitudes.
Firstly, wind-turbine induced signals that are measured at a small wind farm close to Würzburg (Germany) are correlated with the operational data of the turbines. The frequency-dependent decay of signal amplitudes with distance from the wind farm is modeled using an analytical method including the complex effects of interferences of the wavefields produced by the multiple wind turbines. Specific interference patterns significantly affect the wave propagation and therefore the signal amplitude in the far field of a wind farm. Since measurements inside the wind turbines show that the assumption of in-phase vibrating wind turbines is inappropriate, an approach to calculate representative seismic radiation patterns from multiple wind turbines, which allows the prediction of amplitudes in the far field of a complete wind farm, is proposed.
In a second study, signals with a frequency of 1.15 Hz, produced by the Weilrod wind farm (north of Frankfurt, Germany) are observed at the seismological observatory TNS (Taunus), which is located at a distance of 11 km from the wind farm. The propagation of the wavefield emitted by the wind farm is numerically modeled in 3D, using the spectral element method. It is shown that topographic effects can cause local signal amplitude reductions, but also signal amplification along the travel path of the seismic wave. The comparison of simulations with and without topography reveals that the reduction and amplification are spatially linked to the shape of the topography, which could be an explanation for the relatively high signal amplitude observed at TNS.
Finally, the reduction of the impact of wind turbines on seismic measurements using borehole installations is studied using 2D numerical models. Possible effects of the seismic velocity, attenuation, and layering of the subsurface are demonstrated. Results show that a borehole can be very effective in reducing the observed high-frequency signals emitted by wind turbines. However, a borehole might not be beneficial if signals with frequencies of about 1 Hz (or lower) are of interest, due significant wavelength-dependent effects. The estimations of depth-dependent amplitudes with a layered subsurface are validated with existing data from wind-turbine-induced signals measured at the top and bottom of two boreholes.
The experimental analysis of measurements conducted at wind farms and the advances of modeling such signals improve the understanding of the propagation of wind-farm induced seismic wave fields. Furthermore, the methods developed in this work have a high potential of universal application to the prediction of signal amplitudes at seismometers close to wind farms with arbitrary layout and geographic location.
In dieser Arbeit wird die Richtungsabhängigkeit seismischer Geschwindigkeiten im Erdmantel unterhalb Deutschlands und angrenzender Gebiete durch die Analyse der teleseismischen Kernphase SKS auf Doppelbrechung untersucht (Scherwellen-Splitting). Die Anisotropie wird durch die Splittingparameter Φ und δt beschrieben und erlaubt Rückschlüsse auf geodynamische Prozesse.
Untersucht werden Aufzeichnungen des Deutschen Seismologischen Regionalnetzes (GRSN) und assoziierter Stationen aus dem Zeitraum von 1993 bis 2009. Für drei Stationen des Gräfenberg-Arrays (GRF-Array) sind Wellenformen ab 1976 verfügbar, welche damit einen weltweit einmaligen Datensatz liefern.
Auf Grund des stetigen Ausbaus der seismologischen Netze und des langen Beobachtungszeitraumes können über 3.000 Seismogramme ausgewertet werden. Der Hauptteil dieser Arbeit besteht daher in der Entwicklung einer automatischen Methodik zur Analyse von SKS-Splitting: ADORE ("Automatische Bestimmung von DOppelbrechnungsparametern in REgionalseismischen Netzwerken"). Für regionale Netze wie das GRSN gewährleistet ADORE eine objektive Bestimmung der Splittingparameter. Zunächst wird das seismologische Netzwerk als seismisches Array aufgefasst, um durch eine Frequenz-Wellenzahl-Analyse den Einsatz der SKS-Phase ohne manuellen Eingriff zu bestimmen. Die Berechnung der Splittingparameter erfolgt durch eine Inversion nach der Methode der Minimierung des transversalen Energieanteils. Automatisch wird das optimale Fenster um den SKS-Einsatz positioniert, für jede Beben-Stations-Kombination werden dazu 3.600 Einzelinversionen durchgeführt.
Um diese Vielzahl von Auswertungen in akzeptabler Zeit zu berechnen, nutzt ADORE moderne Rechnerarchitekturen aus, verteilt die Berechnungen auf mehrere Computer im lokalen Netzwerk und erzielt damit eine Beschleunigung um einen Faktor 60.
Die Analyse des gesamten Datensatzes ergibt folgende Ergebnisse: An allen analysierten Stationen wurde ein Scherwellen-Splitting festgestellt, der Stationsuntergrund weist somit überall Anisotropie auf. Für 240 Erdbeben können insgesamt 494 Wertepaare mit höchster Qualität bestimmt werden.
Unter der Annahme einer homogenen ungeneigten anisotropen Schicht unterhalb der jeweiligen Station können die Einzelmessungen pro Station gemittelt werden. Damit sind Regionen mit ähnlichen Merkmalen gut zu identifizieren: Im Norden Deutschlands herrschen NW-SO-, in der Mitte W-O-Richtungen und im Süden SW-NO-Richtungen vor.
Die Verzögerungszeiten liegen im Bereich zwischen 1.0 (Station Taunus) und 2.2 Sekunden (Tannenbergsthal, TANN). Auf Grund des hohen Wertes sind die Ursachen für die hier beobachteten Zeiten dem Erdmantel und nicht der Kruste zuzuordnen. Die bevorzugte Ausrichtung von anisotropen Kristallen auf Grund von Fließprozessen von Mantelmaterial ist Quelle der beobachteten Anisotropie. Rezente Fließprozesse von Mantelmaterial sind vor allem an der Unterkante der Lithosphäre wahrscheinlich. Durch Gebirgsbildungsprozesse, vorhandene Gebirgswurzeln oder regionale Veränderungen in der Mächtigkeit der Lithosphäre entstehen Barrieren für viskoses Mantelmaterial.
Als tektonische Ursachen für die hier gemessenen Orientierungen ist im Norden die Tornquist-Teisseyre-Linie (TTZ), in der Mitte die Variszische Gebirgsbildung und im Süden Einflüsse des Alpenbogens anzusehen. Ausnahmen bilden die Stationen Clausthal-Zellerfeld (CLZ), Rügen und Black-Forest-Observatory (BFO). Während bei letzterer ein Einfluss der Spreizungszone des Oberrheingrabens zu vermuten ist, scheint die Intrusion des Brockengranits die Beobachtungen an CLZ zu prägen. Rügen liegt in einer Übergangszone zwischen Sorgenfrei-Tornquist-Zone und TTZ.
Durch die Vielzahl von vorhandenen Einzelmessungen lassen sich an manchen Stationen komplexe Modelle untersuchen. Dazu zählen neben Gradientmodellen auch die geneigte Schicht und Zwei-Schicht-Modelle. Für sechs Stationen kann ein Zwei-Schicht-Modell erstellt werden: BFO, Gräfenberg A1, Fürstenfeldbruck (FUR), Rüdersdorf (RUE), TANN und Unterbreitzbach (UBBA). Die Interpretation der Richtungen von oberer und unterer Schicht gelingt für einen Teil der genannten Stationen: An BFO liegt die Orientierung der unteren Schicht parallel zur Vorzugsrichtung der variszischen Gebirgsbildung, jene der obere Schicht antiparallel zur Spreizungsrichtung des Rheingrabens. Für die Station FUR ist eine Überlagerung mit der Streichrichtung des Alpenmassivs zu beobachten. An GRA1 wird die untere Schicht offenbar durch rezente oder eingefrorene Anisotropie des Böhmischen Massivs bzw. des Eger-Riftsystems beeinflusst. Eine vergleichbare Wirkung ist durch die TTZ an der Station RUE zu erkennen.
ADORE wurde weiterhin auf einen Datensatz des temporären RIFTLINK-Projektes angewandt.
Indian Ocean came into existence with the breakup of Gondwana in the Mesozoic era. The presence of complex aseismic ridges and plateaus in the Indian Ocean makes it the least-understood of all the oceans. Mascarene Plateau, apart from Central Indian Ridge (CIR) running north-south between 2◦N and 25◦S in the Indian Ocean, is one such complex feature in the Indian Ocean that consists of Seychelles microcontinent in the north and the volcanic islands of Mauritius, La Réunion and Rodrigues in the south.
Most of the previous seismological studies on the islands of Mauritius, Rodrigues and Seychelles are restricted as each of them has only one operational permanent station. In the current study, I present the results obtained from the investigations of the seismological data obtained from the deployment of temporary seismic network on Mauritius (November, 2012–August, 2014) and Seychelles (March, 2013–March, 2015) under Réunion Hotspot and Upper Mantle–Réunions Unterer Mantel (RHUM–RUM) project and later in Rodrigues (September, 2014–June, 2016) under a collaborative project between Goethe-Universität, Frankfurt, Germany and Mauritius Oceanography Institute (MOI), Mauritius. Additional data from the permanent stations were also used in this study. The investigations and results are presented under three themes, namely: (1) crustal structure beneath Mauritius, (2) upper mantle anisotropy below Mauritius, Rodrigues and Seychelles and (3) intraplate seismicity in the Rodrigues–CIR region.
Upper mantle anisotropy in south-west Indian Ocean region are very limited, especially from the islands of Mauritius and Rodrigues. With the new data from the seismic stations deployed in Mauritius and Seychelles, under RHUM–RUM, and permanent stations in Rodrigues, I constrain the upper mantle flow pattern beneath these islands. From the joint-splitting analysis, I obtain fast-polarisation direction (φ) dominant in N80◦E and delay time (δt) of ≈0.85 s for Mauritius and φ tending east–west in Rodrigues with δt of ≈1.1 s. Parabolic asthenospheric flow model explains the orientation of the fast-polarisation direction beneath Mauritius, whereas deep mantle circulation patterns best explain the horizontal alignment of the fast-polarisation direction in Rodrigues. From Seychelles data, the results show φ trending NE and δt ≈0.74 s, even for the island close to Amirante Ridge, suggesting an asthenospheric deformation induced by relative motion between the plate and the deep mantle flow.
It has recently been suggested that the volcanic island of Mauritius may be underlain by a remnant of continental origin termed “Mauritia.” To constrain the crustal thickness beneathMauritius, I analysed data from 11 land stations, 10 of which were deployed recently under the RHUM–RUM project. From the recordings, I obtained 382 P-receiver functions. On the obtained receiver functions, I applied the H–κ stacking technique and derived the crustal thickness of ≈10–15 km. I observe a considerable variation in the VP/VS ratio caused by a lack of clear multiples. Using forward modelling of receiver functions, I show that the lack of clear multiples can be explained by a transitional Moho, where the velocity increases gradually. The modelling further indicates that the thickness of this gradient zone is estimated to be ≈10 km. I argue that my findings suggest oceanic crust thickened by crustal underplating due to the mantle plume currently located beneath La Réunion.
Seismicity around Rodrigues Island is generally associated with events recorded by the global networks along the CIR. Using seismological array techniques on the data collected by the temporary deployment of seismic array on Rodrigues Island for a period of 22 months (September, 2014–June 2016), 62 new events were located, which were not reported by any global network. Determination of backazimuth and apparent velocity were performed by applying array methods in the time-domain instead of the more conventional frequency-domain analysis. Event distances were calculated using a 1-D velocity model and the measured travel-time differences between S- and P-wave arrivals. Local magnitudes of the events were obtained by removing the velocity response from the seismographs and then convolving with Wood–Anderson transfer function to obtain ground motion in nanometers. Most of the newly-detected events are located off the ridge axis and can be classified as intraplate events. Three different seismic clusters were observed around the island. Most of the events were localised in the north-east of Rodrigues at a distance of ≈138 km from the reference station. A distinguishable swarm of earthquakes was observed on the west of the spreading segment from March to April 2015. The local magnitudes (ML) of the events varied between 1.6 and 3.7.