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| Titel |
From local earthquakes tomography to Moho topography beneath the Western Alps |
| VerfasserIn |
Bertrand Potin, Bernard Valette, François Thouvenot, Vadim Monteiller |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250103283
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| Publikation (Nr.) |
 EGU/EGU2015-2690.pdf |
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| Zusammenfassung |
| The Western Alps are the result of the collision between the European margin and the
Apulian margin, which started about 35ÂMa ago. Nowadays, the lithosphere beneath the
Western Alps consists of the subduction of the European plate underneath the Apulian plate.
Such geometry implies a complex Moho discontinuity.
Over the past 25Âyears, several dense seismic networks settled in France, Italy and
Switzerland have permitted to locate more than 45,000 local earthquakes. In this study, we
used 335 stations spread over a 200,000 km2 area, and about 35,000 events that have been
located with at least 5 stations and 7 P and S–waves picks. The resulting dataset is formed of
more than 820,000 data. Most of the Western Alps earthquakes occurred within the first
10 km beneath surface, nevertheless a large part of P and S–waves are refracted waves on
the Moho discontinuity.
In order to build up the Moho topography of the western Alps, we used at first this data
set to perform a crust and upper mantle tomography based on travel-times analysis.
Our model consists of a set of V P and V P/V S values given at each node of a
three-dimensional, regularly spaced grid, which constitutes the inversion grid. Transition
between crust and mantle is modeled by a continuous change in velocity, as we do not
introduce any a priori information on the Moho interface. Earthquake locations and
site-effect residuals at each station are also determined in the process. The forward
computation of travel times in the 3D model is performed by integrating slowness
along the rays, which are determined by the Podvin-Lecomte algorithm (basically a
finite difference resolution of eikonal equation). Inversion is carried out using a
non-linear least-squares approach based on a stochastic description of data and
model. The smoothing and damping parameters are adjusted by means of L–curves
analysis.
The Moho discontinuity is obtained by matching an isovelocity surface of this
tomography model with information on the edge of the area (where our model is poorly
resolved) coming from previews studies. We chose a value of about V P = 7.5 km.s-1 for
this isovelocity surface, which corresponds to an intermediate value between the crust and the
upper mantle. This new Moho interface is then used as an a priori discontinuity in a new
tomography process in which the parameters within the crust and the upper mantle are now
decorrelated. Thus, refracted waves are modeled more correctly and the resolution within the
crust can be improved.
Finally, we constituted a set of local earthquakes which were accurately relocated by
studies involving temporary networks, due to their relatively high magnitude or to the
damages they caused. To evaluate the quality of our models, we compared these published
locations to the ones we could obtain. |
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