Fakultät für Geowissenschaften - Digitale Hochschulschriften der LMU

High-silica volcanic systems are considered to be the most devastating. Their highly viscous properties create a high pressurised non-

fluent system which consequently relaxes the stress mostly by exploding through the brittle regime. Even if an explosion is avoided and the magma fl

ows, it often generates lava domes at the top of the volcano; which, patiently, accumulate magmas that will rush down

the slopes once the yield stress is crossed. Thus, such volcanoes have an explosive nature and often generate catastrophic pyroclastic

flows.

The modelling of magma ascent inside eruptive conduits is commonly based on fl

uid mechanic principles. The difficulty of the approach is however not as much driven by the physical equations of the numerical model as the variability of the parameters of the magma itself. It is well established that the rheology of magma strongly depends on the temperature, the stress, the strain, the chemical composition, the crystal and the bubble contents. In other words, magmatic modelling involves a set of movement equations which call for a comportmental/rheological law. The movement equations give roughly equivalent results through the different

models; however magma rheology is poorly controlled. The deformation of highly crystallised dome lavas is key to understanding their rheology and to fixing their failure onset. It is thus essential to adequately understand magma

rheology before performing complex numerical models.

Here we focus on the well studied Unzen volcano, in Japan, which had a recent period of activity between 1990 and 1995. The dome building eruptions in Unzen generate repeated dome failure and pyroclastic

ows. They vary in

character and behaviour from effusive domes to brittle pyroclastic events. Since then, the Unzen Scientic Drilling Project, initiated in April 1999, drilled through the volcano and sampled the eruptive conduit. This provides us with rare original samples for study and characterisation. The physico-chemical properties of these valuable samples were determined with an array of devices. Of these a large

uniaxial deformation press, which can operate at high load (0 up to 300KN), and

temperature (25-1200°C), will be of utmost importance. This press deforms the samples under known parameters and allows us to determine the viscosity of the melt.

In this study we investigate the stress and strain-rate dependence on several glasses and Mt Unzen dome lavas. Their rheology has been determined for temperatures from 900 to 1010°C and stresses from 2 to 120 Mpa (60 MPa for

crystal bearing melt) in uniaxial compression . This survey aims to distinguish the Non-Newtonian effects perturbing magmatic melts, also known as indicators of the brittle field. Towards our experiments we observed three majors viscosity decrease types: Two were dependant and typical of the solid fraction (Shear thinning & Time weakening effect). The first is instantaneous and on the whole recovered during stress release. The second is time dependent and non-recoverable.

The third and last effect observed is attributed to the melt fraction and its self heating under stress (Viscous Heating Effect).

We extensively investigated this last eect on pure silicate liquids and crystal bearing melts. Our findings suggest that most of the Non-Newtonians effects observed in silicate melts are linked to a self heating of the sample and can subsequently be corrected with the temperature without involving other laws than a pure viscous material. Moreover we observed that this self heating reorganise

the energy distribution within the sample and by localising the strain may favours the formation of shear banding and the apparition of 'hot cracks'.

Crystal bearing melts exhibit two more Non-Newtonian eects. The first one,

the shear thinning, is typical of that observed in previous experiments on crystal-bearing melts. On crystal free melts, this viscosity decrease is observed at much lower magnitude. We infer that the crystal

A long-standing question in the study of Earth’s deep interior is the origin of seismic mantle heterogeneity. The challenge is to efficiently mine the wealth of information available in complex seismic waveforms and to separate the potential contributions of thermal anomalies and compositional variations. High expectations to gain new insight currently lie within the application of high-performance computing to geophysical problems. Modern supercomputers allow, for example, the simulation of global mantle flow at Earth-like convective vigor or seismic wave propagation through complex three-dimensional structures. The sophisticated computational tools incorporate a variety of physical phenomena and result in synthetic datasets that show a complexity comparable to real observations. However, it is so far not clear how to combine the results from the various disciplines in a consistent manner to obtain a better understanding of deep Earth structure from the expensive large-scale numerical simulations. In particular, it is important to understand how to build conceptual models of Earth’s mantle based on geodynamic considerations that can be quantitatively assessed and used to test specific hypotheses. One specific goal is to generate seismic heterogeneity from dynamic flow calculations that can be used in global wave propagation simulations so that synthetic seismograms can be directly compared to seismic data without the need to perform inversions.

In the multi-disciplinary study presented here, a new method is developed to theoretically predict and assess seismic mantle heterogeneity. Forward modeling of global mantle flow is combined with information from mineral physics and seismology. Temperatures inside the mantle are obtained by generating a new class of mantle circulation models at very high numerical resolution. The global average grid spacing of ~25 km (around 80 million finite elements) allows for the simulation of flow at Rayleigh numbers on the order of 10^9 and to resolve a thermal boundary layer thickness of around 100 km. To assess the predicted present day temperature fields, the geodynamic flow calculations are post-processed with published thermodynamically self-consistent models of mantle mineralogy for a pyrolite composition to convert thermal structure into elastic parameters. Quantitative predictions of the magnitudes of seismic velocity and density variations are thereby possible due to the appropriately high numerical resolution necessary to obtain temperature variations that are consistent with the mineralogical conversion. The resulting structures are compared to tomographic models based on a variety of statistical measures taking into account the limited resolving power of the seismic data. In a final step, the geodynamic models are investigated with respect to the influence of strong convective mass transport on the stability of Earth’s rotation axis. This additional and independent analysis provides information on whether strongly bottom heated isochemical mantle circulation can be reconciled with paleomagnetic estimates of true polar wander. One specific question that can be addressed with this approach is the origin of two large regions of strongly reduced seismic velocities in the lowermost mantle. Several seismological observations are interpreted as being caused by compositional variations. However, a large number of recent geodynamical, mineralogical and also seismological studies argue for a strong thermal gradient across the core-mantle boundary that might provide an alternative explanation through the resulting large temperature variations. Here, the forward modeling approach is used to test the assumption whether the presence of a strong thermal gradient in isochemical whole mantle flow is compatible with a variety of geophysical observations.

The results show that the temperature variations deduced from the new high-resolution mantle circulation models are capable of explaining gross statis

Many plate boundaries appear to be broad deformation zones, composed of several smaller microplates (e.g. California, Alaska, Mediterranean sea). To accurately address plate boundary deformation, as needed for seismic hazard assessment, it is important to study the motion of these microplates. Furthermore the dynamics of microplates, such as their driving forces and the implication of their motion on the surrounding plate boundary region are not well understood. Following suggestions from previous studies that Baja California is a microplate located within the North America – Pacific plate boundary region, a kinematic study using horizontal velocity data from Global Positioning System was performed. Building on the kinematic results and using numerical modelling techniques, a 2D mechanical model that simulates the transport of the Baja California microplate by lithospheric coupling with the Pacific plate was created. This model addresses the transport forces, and necessary preconditions in the North America – Pacific plate boundary region for rigid microplate transport. In addition, this model provides insights on the deformational response of western North America to Baja California motion, in particular the kinematic response of neighbouring microplates and the activation of passive fault systems. This latter was examined in more detail by separating the contribution of deformation from Baja California microplate motion and Sierra Nevada microplate motion to the formation of the Eastern California Shear Zone. Overall, it was found, that microplates play an important role for plate boundary kinematics and that their motion strongly influences the dynamics and fault evolution in plate boundary regions.

In the present work three topics are outlined which are connected via biomineralisation. The investigation of the mechanical response of brachiopod shell material, the macroscopic calcite crystal growth, and the microscopic calcite crystal growth are discussed.

Brachiopod shells have been investigated with nanoindentation and scanning electron microscopy to determine the material properties and the effects of the microstructure on the material properties. Brachiopod shells are highly optimised fibre composite materials. The material properties are adapted to the local material demands. The calcitic brachiopod \emph{Megerlia truncata} exhibits a hard primary layer and a softer secondary layer with a tendency to higher hardness at the inner shell margin. At the hinge region the hardness is considerably higher and reflects the possibly high abrasive wear. The phosphatic brachiopod shells of \emph{Lingula anatina} and \emph{Discradisca stella} have laminated architecture with alternating hard and soft layers. They are harder in the center and have softer, more flexible regions at the margins.

The macroscopic crystal growth of calcite was performed in gels. The obtained crystals were investigated with scanning electron microscopy. With inert pore solutions the $\left\{104\right\}$ rhombohedron is the dominating habit. Succinic acid and aspartic acid inhibit the acute steps of the calcite $\left\{104\right\}$ surface. In combination with $Mg^{2+}$ and $Sr^{2+}$ one side divergant forms are obtained. Pore solutions containing silicic acids produce catastrophic nucleation. Crystal aggregates from calcite, aragonite, and vaterite develop. The aggregates have morphological similarities with calcites from travertines. $Mg^{2+}$ and $Sr^{2+}$ counteract the effect of silicic acids.

The microscopic crystal growth of the calcite $\left\{104\right\}$ surface was investigated with the atomic force microscope in the presence of silicic acids. The two dimensional nucleation is enhanced. The form of the hillocks change from the additive free rhomb to lenses with increasing silicic acids concentrations. The step velocities increase from 0 ppm to 20 ppm silicic acids and decrease after 20 ppm. Therefore silicic acids possess a promoter and a inhibitor effect on calcite crystal growth. The inhibitory effect can be fitted by a Langmuir isotherm. Both effects can be fitted together with a polynom of rank three. For supersaturation $\beta = 30$ the step velocities are constant. This means that the process follows the step pinning model.

Die vorliegende Arbeit behandelt zwei unterschiedliche Anwendungen aus dem Bereich der numerischen Seismologie: Das erste Thema umfasst die Entwicklung und Anwendung eines Programms zur Berechnung der lokalen Wellenausbreitung

in seismischen Störungszonen (Fault Zones) mit spezieller Fokussierung auf geführte Wellen (Trapped Waves). Dieser Wellentyp wird an vielen Störungszonen beobachtet und aus seinen Eigenschaften können Informationen über die jeweilige Tiefenstruktur abgeleitet werden.

Das zweite Thema dieser Arbeit behandelt die Entwicklung und Anwendung zweier Verfahren zur Berechnung der globalen Wellenausbreitung, also der Ausbreitung seismischer Wellen durch die gesamte Erde einschließlich des äußeren und inneren Erdkerns. Die verwendeten Methoden ermöglichen es, kleinräumige Strukturen in großen Tiefen wie zum Beispiel die Streueigenschaften des Erdmantels oder die kleinskalige Geschwindigkeitsstruktur an der Kern-Mantelgrenze in knapp 2900 km Tiefe zu untersuchen.

Wellenausbreitung in seismischen Störungszonen:

Seismische Störungszonen, wie zum Beispiel der San Andreas Fault in Kalifornien, zeigen auf beeindruckende Weise, wie die Gestalt der Erdoberfläche durch seismische Aktivität als Folge tektonischer Prozesse geprägt wird. Die genaue Kenntnis der Tiefenstruktur einer Störungszone hingegen bietet zusätzlich einen Einblick in die vergangene Seismizität, die die Struktur der jeweiligen Störung geprägt hat. Neben den tektonischen Eigenschaften einer Region lassen sich aus der Tiefenstruktur auch Voraussagen über Häufigkeit und zu erwartende Stärke zukünftiger Erdbeben ableiten. Da Erdbeben vorzugsweise in solchen

Störungszonen auftreten, ist eine möglichst genaue Kenntnis der Geometrie einer Schwächezone wichtig, um Regionen mit erhöhtem Gefährdungspotenzial zu erkennen.

Für die Untersuchung der Tiefenstruktur einer Störungszone stehen in vielen Fällen ausschließlich Messungen von der Erdoberfläche zur Verfügung, etwa von seismischen Netzen, die in unmittelbarer Umgebung oder direkt auf einer Störung

platziert wurden. Ereignet sich nun ein Erdbeben in einigen Kilometern Tiefe innerhalb der Störungszone, breitet sich ein Teil der angeregten seismischen Wellen durch die gesamte Störungszone bis zur Erdoberfläche aus, wo sie

registriert werden. Die aufgezeichneten Signale werden somit entlang ihres gesamten Laufweges durch die Struktur der Störungszone beeinflusst, was die Ableitung der tiefenabhängigen Struktur aus den Messdaten erschwert.

Um trotzdem ein genaues seismisches Abbild einer Störungszone zu bekommen, analysiert man unterschiedliche Wellentypen im Seismogramm, wodurch ein Maximum an Strukturinformation abgeleitet werden kann. Einer dieser

Wellentypen, der sich durch besondere Eigenschaften auszeichnet, ist die geführte Welle (Trapped Wave). Diese entsteht, wenn eine Störungszone einen ausgeprägten vertikal ausgedehnten Bereich drastisch reduzierter seismischer Ausbreitungsgeschwindigkeit (Low Velocity Layer) und nicht zu komplexer Geometrie besitzt, der als seismischer Wellenleiter wirkt. In einem solchen Wellenleiter kann sich eine geführte Welle ausbreiten, die als mit Abstand stärkstes Signal an der Erdoberfläche registriert wird, also deutlich stärkere Bodenbewegungen hervorruft als etwa die direkte Welle. Dieser Verstärkungseffekt hat unter anderem Konsequenzen für die Abschätzung der seismischen Gefährdung in der Nähe einer Störungszone, zum Beispiel wenn die Störungszone durch dicht besiedeltes Gebiet verläuft. Geführte Wellen beinhalten aufgrund ihrer hohen Sensitivität bezüglich der

Eigenschaften von Niedergeschwindigkeitszonen Strukturinformationen, die aus anderen Wellentypen nicht abgeleitet werden können. Daher leistet das Verständnis dieses Wellentyps einen wichtigen Beitrag für die Ableitung

möglichst vollständiger Modelle von Störungszonen.

Ausbreitung von SH- und P-SV Wellen in Erdmantel und der ganzen Erde:

Das allgemeine Verständnis der Struktur und Dynamik des tiefen Erdinneren basier

A general feature of tectonic faults is the juxtaposition of materials with dissimilar elastic properties

in a variety of contexts and scales. Normal and reverse faults offset vertical stratifications,

large strike-slip faults displace different crustal blocks, oceanic and continental crusts at subduction

interfaces, and oceanic transforms juxtapose rocks of different ages. Bimaterial interfaces

associated with rock damage are present with various degrees of sharpness in typical fault zone

structures, and failure along a bimaterial interface can be effective even on microscopic scale of

grain boundaries.

A first order representation of a geological fault for seismic events is a frictional interface

embedded in an elastic body. This study focusses on dynamic effects in the presence of material

discontinuities altering dynamics of failure and dynamic rupture propagation on frictional interfaces.

When the medium surrounding a fault is heterogeneous, the symmetry of stress is broken

up and perturbations of normal stress introduces additional instability potentially generating

additional propagation modes of rupture.

This study presents three specific numerical investigations of the aforementioned rupture

phenomena associated with material contrasts at the fault. A first numerical study (a) investigates

2-D in-plane ruptures in a model consisting of two different half-spaces separated by

a low-velocity layer and possible simultaneous slip along multiple faults. This study shows

that bimaterial frictional interfaces are attractive trajectories of rupture propagation, and ruptures

tend to migrate to material interfaces and becoming self-sustained slip pulses for wide

ranges of conditions. In a second numerical study (b), the propagation of a purely material

contrast driven rupture mode, that is associated with the so-calledWeertman or Adams-instable

pulse, is shown to exist also in the general 3-D case, where there is a mixing of in-plane and

anti-plane modes, the bimaterial mechanism acting in the in-plane direction only. Finally, in

a further numerical investigation (c) it is demonstrated, that the rupture dynamics and ground

motion can be significantly influenced by bimaterial mechanisms of rupture propagation for

ranges of parameters. The model studied here comprises heterogeneous initial shear stress on a

slip-weakening frictional interfaces separating two dissimilar elastic bodies, a free surface. The

discussion focusses on the diversity of existing rupture propagation modes and ground motion.

The investigated models and obtained results are motivated and discussed in the context of

complementary numerical investigations, theoretical studies of stability analysis, seismological

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observations of earthquakes and aftershock sequences, geological observations of fault zone

structures, tomographic studies, and geodetic observations.