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| Titel |
The Influence of Crustal Heterogeneity on Translational and Rotational Motions using Data from Local and Teleseismic Events |
| VerfasserIn |
Peter Gaebler, Christoph Sens-Schönfelder, Michael Korn |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250095457
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| Publikation (Nr.) |
 EGU/EGU2014-10911.pdf |
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| Zusammenfassung |
| In this study we use Monte Carlo (MC) solutions to the Radiative Transfer Equations (RTE)
to model translational and rotational motion seismogram envelopes in random elastic media
with deterministic background structure. The observation and modeling of the three
additional components of rotational motions can provide independent information about wave
propagation in the Earth’s structure. Rotational motions around the vertical axis
observed in the P-wave coda are of particular interest as they can only be excited by
horizontally polarized shear waves and therefore indicate the conversion from P to
SH energy by multiple scattering of the high-frequency seismic wave field at 3D
heterogeneities.
Radiative Transfer Theory (RTT) is used to model the propagation of seismic energy in a
deterministic structure described by macroscopic medium properties with statistically
distributed small scale heterogeneities. It describes the spatial and temporal distribution of
seismic energy emitted from a seismic source. The central quantity of the RTT, the specific
intensity I(n,r,t), is modeled by a number density of particles N(n,r,t) located at position
r and moving into direction n at time t. Particles can experience scattering processes at
medium heterogeneities that are described by the Born scattering coefficients. This processes
include mode conversion and a change of propagation direction. When no scattering
events occur particles move through the medium according to ray theory including
the interaction with interfaces (reflection, transmission, mode conversions). Using
projections of I(n,r,t) onto specific directions we can simulate the three rotational
components of the wave-field in a random elastic medium additional to the translational
components.
The MC-RTT simulation results are verified by comparisons with 3D finite difference
simulations. Six-component envelopes from the two approaches are compared and a
reasonable agreement for translational and rotational energy is obtained.
To investigate crustal scattering and attenuation parameters in South-East Germany beneath
the Gräfenberg array, multi component seismogram envelopes from Monte Carlo Radiative
Transfer Theory simulations are compared to reference traces from seismic data of local
swarm-earthquakes and of deep teleseismic events. In the local case a nonlinear genetic
inversion process is used to estimate scattering and attenuation parameters at high frequencies
(4-8Hz) that result in energy density traces that fit the measured local reference seismogram
envelopes. Our preferred model includes crustal heterogeneities with velocity fluctuations ε in
the range of 3%, autocorrelation lengths a in the order of a few hundred meters and
an intrinsic quality factor for S-waves sQi of 625. In a second step simulations
using this estimated set of scattering and attenuation parameters are compared to
envelopes of P-wave Coda from deep teleseismic events. Results from the local and
teleseismic simulations with consistent parameters both show a good agreement with
data.
We therefore conclude that scattering in this region also the scattering that generates the
teleseismic P-wave coda is mainly confined to the crustal part of the lithosphere beneath the
sensor. Our observations do not require scattering in the upper mantle, but weak scattering in
the lithospheric mantle cannot be ruled out. |
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