Download Anisotropy and deformation beneath the Eastern Alps

Exposé for the dissertation proposal
Anisotropy and deformation beneath the Eastern Alps
Ehsan Qorbani Chegeni
Thesis advisor: Univ.-Prof. Dr. Götz Bokelmann
Department of Meteorology and Geophysics (IMGW)
University of Vienna
September 2012
Abstract
The Alpine belt is divided into E-trending Eastern Alps and the arc of the Western Alps. At the
surface, the plate tectonic activity is mirrored by the different geological structures in the Eastern and
Western Alps. Even though surface geology of the Alps is relatively well known and several studies,
proposing geodynamical hypotheses, have been accomplished the dynamic processes that occur down
to the upper mantle are still not clear. This research project is aimed to investigate the mechanisms
and the nature of geodynamic processes focusing on the upper mantle beneath the Eastern Alps. By
the characterization of the seismic anisotropy of upper mantle rocks, the strain field due to geodynamic
forces within the Earth’s upper mantle is evaluated. Seismic anisotropy is the velocity variation of the
seismic waves with respect to the direction of wave propagation, which is mainly created by crystal
alignment of the upper mantles materials. Measuring the anisotropic parameters allows us to map the
present-day mantle flow direction that brings hints to interpret past geodynamic activities and gain
insights on the geodynamic evolution of the Earth. By using the seismic records of teleseismic earthquakes, the anisotropic parameters will be measured applying both shear-wave splitting technique and
P-wave polarization as complementary methods to cover the imperfections of each fashion. Modeling
approaches will be utilized using synthetic data to find the best possible anisotropic structures fitting
the data. The overall interpretation of the results from two applied methods and modeling approaches
is aimed to propose a geodynamic model and deformation patterns for the Eastern Alps.
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Introduction
Based on seismic observations, our planet is separated into three major zones; cores, mantle, and crust.
According to rock rheology and thermal divisions the outer shell of the Earth is defined as the lithosphere.
This body, consisting of the rigid upper mantle and crust, is considered to be cold and shows an inflexible
behavior in response to geodynamic processes during geological time intervals that we know as plate
tectonic phenomena. Plate tectonics is believed to be a model in which the lithosphere is divided into
a number of rigid plates that move at relative velocities with respect to one another [28]. Below the
lithosphere lies the asthenosphere; considered as a soft and hot layer, it constitute the thickest part of
the upper mantle. The lithospheric blocks slowly creep over the asthenosphere with different velocities
causing plates motion and consequent deformations mainly at the plate boundaries. Hence, investigating
the interaction between plate motion and upper mantle flow beneath the lithosphere is a beneficial key
to evaluate the upper mantle’s geodynamics [19, 10]. Consequently, studying the dynamic processes in
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the upper mantle is an important task to evaluate the Earths dynamic evolution. Although the effects of
geodynamic processes on the lithosphere are observable at the Earths surface as the orogenic processes,
faulting, earthquakes, volcanoes, and natural hazards, their mechanisms and nature in the upper mantle
are still poorly understood.
The selected region in this study mainly comprises the Eastern Alps (Figure 1). The Alpine belt,
according to geomorphological features, orogenic zones and geological structures [23, 22], is divided into
E-trending Eastern Alps and the arc-belt of the Western Alps. Because of the uncertainty and ambiguity
of geodynamic mechanisms in the Alps, the understanding and interpretation of tectonics and deformation
patterns are still a matter of some debate. Therefore, the aim of this study is to achieve more detailed
information on deeper lithosphere and upper mantle structures beneath the Eastern part of the Alps.
Furthermore the outputs of this study are valuable to be considered as important inputs to further seismic
studies in the smaller scales with respect to lithosphere scale.
One of the best possible ways to understanding the upper mantle structures and dynamic mechanisms is
to carry out a study on seismic anisotropy. Seismic anisotropy is known as one of the major characteristic
of the lithospheric mantle and it is thought to be the best approaches to image the deformation in the
Earth’s interior [10]. It gives many information on its geodynamic history and insights on tectonic
processes, which drive the shaping of the continents. In principle, seismic anisotropy is defined as the
dependence of seismic velocity on the direction of wave propagation. Consequently, the seismic waves
traveling in anisotropic structures move faster in one direction than in the other. One of the most
important anisotropy sources in the Earth is the upper mantle anisotropy caused by aligned olivine
crystals in the asthenosphere, i.e. the most ductile and viscous layer in the upper mantle. The cause of
anisotropy is the so-called lattice-preferred-orientation (LPO), generated by the structural alignment of
the olivine crystals, which constitute the upper most part of mantle. The anisotropic property of these
materials can be explained as a function of the strain field, which is aligned from the mantle flow pattern.
Therefore, the results of anisotropy preferential directions can be translated into observations of nature
of geodynamic processes in upper mantle.
Despite the high number of studies done on the Alps, many questions still remain unanswered in
relation with their geodynamics mechanism and deformation patterns, particularly for the Eastern Alps.
One of the most useful investigations to address these questions is the study of seismic anisotropy. This
research project will be carried out for imaging the deformation patterns and mantle flow direction from
the observed seismic data for the Eastern Alpine orogenic belt. Therefore, the aim of this study is to
achieve more detailed information on deeper lithosphere and upper mantle structures beneath this area
of the Alps.
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State of the Art
The Eastern portion of the Alps is still an unexplored territory in terms of seismic investigations of the
lithosphere. The main reason for this was the lack in the past of a good coverage of seismic stations
recording for long time (i.e. several years). The investigation of the lithosphere is the key to understand
the dynamic processes, which shape the continents, the interaction between the plates at their boundary,
and consequently the deformation patterns along and across the lithosphere. In the Alpine region, several
studies have been performed including geophysical and 3D tomographic models [32, 13, 22, 6, 18]. In
particular, tomographic images show velocity anomalies linked to suture zones, and subduction. The
tectonic history of the Alps is complex, including besides the two major plates, namely the European and
the Adriatic plate [12, 6, among others], also some small microplates (i.e. Meliata plate and Pannonian
fragment), which involvement has driven the complex evolution of the area. The above-cited studies
did not clarify yet the dynamic of the collision, but brought different hypotheses on the polarity of the
subduction in the Eastern Alps [13, 18], leaving unsolved questions. Moreover, the Eastern Alps evolve
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Figure 1: The Satellite image of Alpine belt. Our study region, the Eastern part of the Alps, includes Austria,
Slovenia and the most northern part of Italy. (NASA: http://eoimages.gsfc.nasa.gov/images)
in their easternmost portion towards the Carpathians, thus giving to the belt an asymmetrical shape
elongated towards ENE. A lateral eastward escape was proposed in Meissner [17] to explain the passage
between the Alps and Pannonian basin, but further investigation into this area is needed to provide more
reliable evidence and to prove this hypothesis.
Even though several arguments have been raised regarding the geodynamic processes and deformation
models in the Alps, very little can be said about the tectonic mechanisms and past-present deformation
patterns, particularly in case of deep lithosphere and upper mantle structures. This problem is especially
significant concerning the Eastern Alps geodynamics. In the Western Alps, where denser deployments of
temporary and permanent seismic stations were installed in the past, interesting results came from the
analysis of the core shear wave (SKS) phases. Barruol [1] demonstrated that the fast axis directions show
a rotating pattern in accordance with the arc shape in the Western Alpine belt. For the central part of the
Alpine chain, the fast anisotropy directions were achieved only for the TRANSALP profile (12° E) [12],
showing a NE-ward trend. The results of these two studies for the Western Alps and TRANSALP profile
are shown in Figure 2. Although, the above-mentioned studies and some other anisotropic investigations
have been accomplished in the Alps and central Europe (e.g. [5, 30, 20]), the fast axis direction and its
relation with orogen trends in the Eastern Alps is still unknown. Despite the recent growing interest in
anisotropic investigations, no anisotropy study has yet been attempted in the Eastern Alps (Figure 2).
Therefore, performing a new anisotropic study in order to facilitate the geodynamic interpretation is
crucial for this area of the Alpine belt.
Preliminary results, which ground this study, come from the SKS wave analysis of the data recorded
at the seismic stations located in Austria. A good agreement with former studies in the overlap area (red
bars in Figure 2) is clear. As Figure 2 shows, any information for the Eastern Alps is lacking, and the
goal of this research project is to fill that gap. The importance of the results lies in the observed change
in the fast axes pattern between Eastern and Western Alps. It is a prominent feature and its meaning
can be answered by completing the measurement of anisotropic parameters in the area this is what this
research project is aimed to.
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TRANSALP
48˚
MOA
CONA
RETA
DAVA
WTTA
ARSA
KBA
FETA
ABTA
MYKA
SOKA
OBKA
46˚
44˚
IV, NI, SI (Italian Netwoks)
SL (Slovenian Network)
OE (Austrian Seismic Network)
1.5 sec
42˚
4˚
6˚
8˚
10˚
12˚
14˚
16˚
18˚
Figure 2: Distribution of fast axis directions from the former studies (e.g. [1, 12]). is demonstrated. The vectors
show the azimuth of fast direction (φ) and the lengths vector refers to the value of delay time (δt). The red
vectors represent preliminary measurements of fast azimuths for the stations of OE network. Note the changing
fast directions pattern from west to east around the TRANSALP profile. The location of stations of OE, SL, IV,
NI, and SI networks are indicated by different symbols. The lack of any investigation for the eastern part of Alps
is noticeable.
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Main research questions
According to the aim of this study, the following research questions will be addressed throughout this
research project:
1. What is the distribution and dominant fast direction azimuth for the central and Eastern Alps?
2. How does the fast direction pattern change in comparison to previous results in the Western Alps?
Do the fast azimuth orientations in the central and Eastern Alps follow the same directions as the
Western arc-belt, or do they show a different pattern? Is there, despite the complex structure, a
coherent pattern of seismic anisotropy under the Alps?
3. What is the major anisotropic structure in upper mantle beneath Eastern Alps? Can it be explained
as a single horizontal anisotropic layer, or is there a possibility of more complexity with the existence
of two different anisotropic layers and/or dipping axis anisotropic layer?
4. What causes the observed anisotropic anomalies in the Eastern Alps: The crustal structures or the
aligned olivine crystals in the upper mantle? Which depth does the anisotropic layer(s) correspond?
5. How do fast orientation azimuths relate with upper mantle flow in the Eastern Alps?
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6. Can the two suggested subduction mechanisms in the Alps be clarified in relation to the obtained
mantle flow direction in this study?
7. With regard to the observation of both trench-parallel and trench-perpendicular fast orientations in
the subduction zone setting [31], which possible mechanism will be dominant beneath the Eastern
Alps?
8. What is the relationship between the suggested eastward lateral escaping in the Eastern Alps and
measured fast azimuths in this study?
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Methodology
Several developmental seismological methods have been applied to assess anisotropic structures in the
upper mantle. The main goal of these approaches is to develop the relationship between mantle flow and
past and present day deformation patterns. The methods that will be used in this study are shear-wave
splitting and P wave polarization. By comparing the results of these two different methods, we will able
to make a reliable conclusion about upper mantle anisotropy and deformation pattern in the Eastern
Alps.
4.1
Shear wave splitting method
Analysis of displacements due to seismic wave energy is an important part of seismological study. The
seismic displacement is analyzed as a scalar potential and a vector potential corresponding to P wave
(primary wave) and S wave (secondary) respectively. In the 3-components seismometers, three orthogonal components (north-south, east-west, and vertical) record the displacements in three directions.
The records of horizontal components are often rotated to source-receiver direction, called the radial
component, and the perpendicular direction with respect to radial, called transverse component. The
displacement of S wave (shear wave), can be considered as two orthogonal phases, namely SV which is observed on the radial component, and SH that is observed on the transverse component. These two phases
travel at the same speed in isotropic medium but they propagate with different velocity in anisotropic
media. Because of the difference in velocities they arrive at different times at the seismic station. One
travels faster, is the fast wave and defines the fast polarization. The other has a delay time, is the slow
wave, and defines the slow polarization direction. This phenomena caused by anisotropic structures is
referred to as shear-wave splitting.
The most useful method for constraining upper mantle anisotropy is shear-wave splitting that uses
the splitting of the teleseismic shear waves like SKS/SKKS core phases [26]. Because of the fluid nature
of the outer core, there is no shear wave traveling through it. Therefore, the shear wave passing from
the mantle to the outer core would convert to P phase and would be converted back to shear wave
(called SKS) passing to lower mantle. For this reason, during the conversion of the P wave to SKS core
phase at the core-mantle-boundary (CMB), SKS phase is entirely polarized in the radial direction (i.e.
direction of propagation of the wave). Hence, it is theoretically expected that SKS does not contain any
energy on the transverse component and the amplitudes of this component should be zero. However,
the anisotropic structures in the mantle result as a combination of fast and slow polarization in the two
radial and transverse components [27]; therefore we often observe a significant energy on both radial and
transverse components.
To characterize the nature of anisotropic structures using the shear wave splitting method, two splitting parameters are defined as: the fast direction azimuth (φ, angle between fast axis and radial direction)
and arrival delay time between the fast and slow polarizations (δt). To carry out the measurements of
anisotropic parameters based on shear-wave splitting techniques, the SplitLab package [34] will be used
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as the software. The initial measurements of splitting parameters have been done using the SplitLab for
the stations of Austrian seismological network (OE) (see Figure 2). In addition, as an important part of
this study, the software development process will be undertaken to improve the abilities of the SplitLab
package by adding more features.
4.2
P-wave polarization method
The P-wave polarization method will be used as a complementary technique to investigate anisotropic
structures in upper mantle [9]. Some advantages of using the P-wave polarization method include the
following: 1) Using teleseismic events (at distances larger than 90°) in the shear-wave splitting method, the
SKS core phases have near vertical incidence angles. Therefore the SKS/SKKS phases offer an excellent
lateral resolution although their vertical resolution is poor [26], whereas the P waves are affected by upper
mantle anisotropy with some lateral offset because their incidence angles are larger than SKS phases [9].
2) The azimuthal coverage will be increased using the P wave polarization technique. 3) The P wave
polarization method is able to vertically locate the anisotropy. 4) To identify eventual mis-orientation of
horizontal components of the seismograph.
The definition of polarization varies in diverse applications. In the seismic anisotropy field, the
polarization of the P wave is defined as the direction of particle motion of P waves [24, 9]. In general, the
particle motion of P wave consists of alternating compression and dilation (extension) which is parallel
to the direction of propagation (longitudinal) when propagating through the isotropic medium. The
propagation direction of the P wave can be defined by two angles: the incidence angle that can be
calculated from the normal to the horizontal plane; back azimuth that measured from the station to the
surface projection of seismic source (epicenter) [3]. However, in the heterogeneous and anisotropic media
the estimated polarization azimuth differs from the direction of source-receiver on the great circle and
the observed incidence angle does not correspond to the predicted one from Earth models as well [3].
Therefore, measurement of the polarization direction of P wave allows us to achieve useful information
about anisotropic structures.
The sensitivity of the P-wave particle motion to anisotropic structures depends on the period of the
P phase. At high frequency (i.e. 1 Hz) the P wave particle motion is influenced by the structure of the
upper crust, whereas it is affected by the upper mantle structures at longer periods (up to 30 s) [24].
Based on the major aim of this study, which is focusing on upper mantle structures, the direction of
particle motion of the long period P wave will be measured in order to obtain a better understanding on
anisotropy in the upper mantle. The following processes will be performed in the P wave polarization
method: Selecting teleseismic events with high signal-to-noise ratio; filtering data to obtain long period
P phases; measuring the angles of particle motion; calculation of fast axis direction. Through the particle
motion we can constrain the azimuth direction with respect to radial direction (δθ), and the vertical
polarization (), meaning the vertical inclination in the vertical-radial plane [24, 9].
4.3
Modeling
Based on the theory of anisotropy measurement techniques, we start by assessing a single anisotropic
medium with a horizontal symmetric axis to characterize the anisotropic structure from the seismic
waveforms. In this study, to provide a reliable interpretation of the anisotropic structure, the modeling
fashion by synthetic data will be utilized. In this process, synthetic waveforms and synthetic anisotropic
parameters will be created based on the assumed anisotropic structures to assess the possibility and
the effect of two remarkable features: the presence of two anisotropic layers with different natures and
thicknesses, and/or the presence of dipping anisotropic structures. In the case of modeling using the
results of shear-wave splitting method, the splitting parameters (fast axis direction and delay time) for
the two assumed layers are created as a function of polarization angle [25]. The best possible anisotropic
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features are obtained by fitting the variation of measured splitting parameters versus the polarization
angles by the same variation in the synthetic data. In addition, to evaluate the presence of dipping
structures and the effect of sensor misorientation, the modeling process by the results of the P wave
polarization method will be utilized.
4.4
Data
For this study, the recorded seismic waveforms from the three-component seismic stations will be used.
The data will be selected from the following networks:
1. Austrian broadband seismological network (OE)
2. Seismic network of republic of Slovenia (SL)
3. Italian seismic network (IV)
4. NE-Italy broadband network (NI)
5. Sudtirol network (SI)
Data from OE and SL networks are available at Orfeus data center (http://www.orfeus-eu.org/) and
WebDC-Integrated Seismological Data Portal (http://www.webdc.eu/). Data from SI, NI and IV are
provided directly from ZAMG, OGS and INGV data centers.
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Expected Outcome
With respect to the described problem statement, the expected outcomes of this thesis will now be
outlined:
• Measurements and analysis of fast axis azimuths from SKS phases splitting in order to find the
anisotropy fast directions distribution for the Eastern Alps.
• To find an anisotropic model that describes the fast directions pattern in relation with presence of
single or double anisotropic layers in the upper mantle based on the SKS splitting measurements.
• Developing the SplitLab package by adding codes to obtain more abilities to measure the splitting
parameters.
• Develop a code for P-wave polarization method to carry out the calculation processes of anisotropic
parameters (particle motions angles).
• Calculation and analysis of fast axis azimuths from the polarization of P-waves to obtain the fast
direction pattern in the study region.
• To find an anisotropic model describing the existence of dipping anisotropic structures based on
the obtained distribution of fast azimuths from the P-wave polarization method.
• Create the best anisotropic model combining the obtained anisotropic models from both SKS splitting and P-wave polarization measurements.
• Interpret the results of both methods in terms of geodynamic processes and mantle flow direction
in the asthenosphere.
• Proposing a geodynamic model describing the deformation patterns of lithosphere beneath the
Eastern Alpine belt using the all results of this study and the related outputs of former investigations
in the Alps.
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Timetable
The time plan of this research project is described below by a Gantt chart. Tasks are grouped into seven
phases and each phase is divided into subsections to explain the processes and the objectives.
Task
2012
3
4
5
6
7
8
2013
9
10
11
12
1
2
3
4
5
6
7
2014
8
9
10
11
12
1
2
3
4
5
6
7
2015
8
9
10
11
12
1
2
Phase A (Collecting and preparing the data)
A1- Austrian seismological network (OE)
A2- Seismic network of Slovenia (SL)
A3- Italian seismic network (IV)
A4- NE-Italy broadband network (NI)
A5- Sudtirol network (SI)
Phase B (Shear wave splitting)
B1-Reading and research on theory and methodology
B2- Being familiar with SplitLab package
B3- Preliminary experience in measurement of splitting parameters
B4- MSP for OE network stations
B5- MSP for SL network stations
B6- MSP for IV, NI, and SI network stations
B7- MSP for all networks using new developed package (D2)
Phase C (P-wave polarization)
C1- Reading and research on the theory and methodology
C2- Develop a code to measure the anisotropic parameters
C3- MPP for OE network stations
C4- MPP for SL network stations
C5- MPP for IV, NI, and SI network stations
Phase D (softwares Developing)
D1- Additional reading into MATLAB graphical user interfaces
D2- Developing the SplitLab package by adding the new codes
Phase E (Modeling)
E1- Modeling using shear wave splitting results
E2- Modeling using P-wave polarization results
Phase F (Interpretation)
F1- Integrating and interpreting the results of B and C phases
F2- Combination the results of two modeling strategies (E1, E2)
F3- Interpreting the results of F1 and F2
Phase G (final outputs and deliverable)
G1- Presenting the anisotropy model from B phase results
G2- Presenting the anisotropy model from C phase results
G3- Create the possible Geodynamic model and final conclusion
G4- Writing the thesis
MSP: Measurement of Splitting Parameters
MPP: Measurement of Particle motion Parameters
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Deliverables
The proposed disseminations of results for this research project can be outlined as follows:
1) Publishing the results as papers. According to the arranged work plan, it is expected that the first
paper will be submitted during the first year including the interpretation and results of SKS phases
splitting using the Austrian network data (referring the research question number.
2) Attending the international conferences and meeting such as: PANGEO AUSTRIA (Salzburg, 2012
and 2014); American Geophysical Union (AGU) fall meeting (December 2012 and 2013, San Francisco,
USA); European Geosciences Union (EGU) General Assembly (Vienna, Austria, April 2013).
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