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| Titel |
Thin-layer resolution from multiscale analysis of teleseismic P-to-S converted waves: the oceanic nature of the African slab subducted under Peloponnesus. |
| VerfasserIn |
Alexandrine Gesret, Mireille Laigle, Jordi Diaz, Maria Sachpazi, Alfred Hirn |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250040752
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| Zusammenfassung |
| In Greece, receiver-function processing of teleseismic P-waves in the standard frequency
band results in the image of a low-velocity layer at the top of the Hellenic slab twice thicker
than expected for an oceanic crust. The objective of this work is to assess if this result could
be due to a lack of resolution of the standard processing.
A multiscale approach with the receiver-functions is developed in order to quantify the
resolution. It is based on the wavelet-response of the medium, akin to the wavelet-transform
of the velocity-depth function. The synthetic response in conversion of multiscale
singularities, like that formed by two opposite velocity-steps at the boundaries of a crust
embedded in mantle material, leads to quantify the domain of non-interaction between the
signal wavelength and the heterogeneity thickness. Only at the shortest periods can the
low-velocity layer (LVL) between upper plate and slab mantle be simply characterized in
term of thickness and velocity contrast at its boundaries. For a 7 km thick oceanic crust, only
wavelet periods shorter than about 0.8 s will allow to identify clearly this thin LVL. Going
to longer periods will first lead to underestimate the time-thickness of the LVL,
then overestimate it increasingly such that for a band-pass commonly used in data
processing, it will thus appear twice thicker than it is. The amplitudes also vary,
increasing from the short period non-interaction domain where their values only
depend on the velocity contrast, to the resonance period, then decreasing in the
thin-layer domain. The analysis of the response in conversion from full waveform
synthetic seismograms in a dipping slab model allows to validate a multiresolution
approach to real observations. This approach involves building the wavelet response
of the slab top as the scalogram of receiver-functions for P-wavelets of different
scales extracted from observations through application of band-pass filters to the
seismograms.
We present the first results of the application of this analysis to examples of a real dataset
recently collected in the frame of the European Union THALES WAS RIGHT project. A
tight-array teleseismic receiver-function network was dedicated to resolve at depth the
heterogeneity at the top of the African slab under the Hellenic region and its variations
along-dip and along-strike. With earthquakes of broad-enough spectrum towards the short
periods yielding energy to provide wavelet periods significantly shorter than 1s, the P to S
conversions obtained beneath the eastern coast of Peloponnesus allow resolving for the first
time a standard oceanic crust at the slab top. This result is consistent with the fast trench
retreat model, attested by the southwestward fast motion of the edge of the Aegean upper
plate domain documented from GPS. Indeed, if this occurred since 4-5 million
years ago, the slab underlying now eastern Peloponnesus to about 180 km from the
backstop tip should be the most reduced buoyancy oceanic slab of the Ionian sea basin. |
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