Anisotropy and Attenuation in a Retreating Subduction Zone: Southern Italy
Abstract
We present a collection of high quality S-wave splitting measurements in the Southern Italy subduction system. We analyzed deep earthquakes that occurred within the descending slab to determine the splitting parameters φ and δt. The local deep earthquakes allowed us to analyze ray-paths primarily sampling the slab and the wedge above it. Mainland Calabria is a forearc high, enabling us to sample rays that propagate up the slab. S splitting parameters show a complex pattern of anisotropy with variable fast directions across the subduction zone and delay times ranging from 0.1 sec to 2.2 sec. Measurements at single stations are quite variable excluding the overriding plate as main source of anisotropy. The S wave splitting parameters also show frequency-dependent behaviour that we attribute to the presence of small-scale anisotropic heterogeneities. Comparison of the S splitting measurements to the P-wave velocity anomaly at 100-200 km depth shows that where the rays primarily sample the slab the delay times are small. In contrast, where the S rays sample the mantle wedge, the delay times are quite high. This δt pattern depicts the slab as a weakly anisotropic region and suggests that the main source of anisotropy in the subduction zone is the surrounding asthenosphere. We also determined the attenuation structure of the slab and of the surrounding regions by the inversion of high quality S-waves t* from slab earthquakes. We obtained high-resolution Qs model down to 300 km depth. The results indicate low values of Qs (values down to 200) corresponding to crustal layers (down to 25 km depth), while the slab is characterized by higher but heterogeneous Qs structure (Qs values up to 1100). At 100 km depth the high Qs body is well reconstructed beneath the Calabrian Arc and at 200 km depth it is extended offshore the Southern Tyrrhenian Basin beneath the Aeolian Islands. These preliminary attenuation results allowed us to better define the geometry and the boundary of the Ionian slab and distinguish between anisotropy in the slab and the wedge.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFMDI13A1855B
- Keywords:
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- 7203 SEISMOLOGY / Body waves;
- 7240 SEISMOLOGY / Subduction zones;
- 8170 TECTONOPHYSICS / Subduction zone processes