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| Titel |
Strong lateral variations of S-wave velocity in the upper mantle across the western Alps |
| VerfasserIn |
Chao Lyu, Helle Pedersen, Anne Paul, Liang Zhao |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250131767
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| Publikation (Nr.) |
 EGU/EGU2016-12207.pdf |
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| Zusammenfassung |
| Absolute S-wave velocity gives more insight into temperature and mineralogy than relative
P-wave velocity variations (ΔV p/ V p) imaged by teleseismic traveltime tomography.
Moreover, teleseismic P-wave tomography has poor vertical but good horizontal resolution.
By contrast, the inversion of surface waves dispersion data gives absolute S-wave velocity
with a good vertical but relatively poor horizontal resolution. However, the horizontal
resolution of surface wave imaging can be improved by using closely spaced stations in
mini-arrays. In this work, we use Rayleigh wave phase velocity dispersion data to measure
absolute S-wave velocities beneath the CIFALPS profile across the French-Italian western
Alps. We apply the array processing technique proposed by Pedersen et al. (2003) to derive
Rayleigh wave phase dispersion curves between 20 s and 100 s period in 15 mini-arrays
along the CIFALPS line. We estimate a 1-D S-wave velocity model at depth 50-150 km
beneath each mini-array by inverting the dispersion curves jointly with receiver functions.
The joint inversion helps separating the crustal and mantle contributions in the
inversion of dispersion curves. Distinct lithospheric structures and marked lateral
variations are revealed beneath the study region, correlating well with regional
geological and tectonic features. The average S-wave velocity from 50 to 150 km depth
beneath the CIFALPS area is ∼4.48km/s, almost the same as in model AK135,
indicating a normal upper mantle structure in average. Lateral variations are dominated
by relatively low velocities (∼4.4km/s) in the mantle of the European plate, very
low velocities (4.0km/s, i.e. approximately 12% lower than AK135) beneath the
Dora Maira internal crystalline massif and high velocities (∼ 5.0km/s, i.e. 12%
higher than AK135) beneath the Po plain. The lateral variations of S-wave velocity
perturbation show the same features as the P wave tomography (Zhao et al., submitted),
but with different amplitudes. In summary, this study reveals a strong S-wave low
velocity anomaly in the upper mantle beneath the Dora Maira massif and confirms
this unexpected and intriguing result of the P traveltime tomography. To further
strengthen our results, we measure P and S wave arrival times for 15 events located
approximately in the same azimuth as the CIFALPS profile (2-D cross-section). We expect
these complementary observations from P and S waves to be a valuable input to
further understand the lithosphere-asthenosphere system beneath the western Alps. |
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