Isotropic and anisotropic seismic structures across Northern Apennines (Italy)

7 Isotropic and anisotropic seismic structures across Northern Apennines (Italy) 8 subduction zone are imaged using a new method for the analysis of teleseismc 9 receiver functions (RFs). More than 13,000 P-wave coda of teleseismic records 10 from the 2003-2007 RETREAT experiment are able to reveal new insights 11 into a peculiar subduction zone which develops at the convergent margin of 12 two continental plates and is considered a focal point inside the Mediterranean 13 evolution. A new methodology for the analysis of receiver function is devel-14 oped, which combines both migration and harmonic decomposition of the 15 receiver function data-set. While migration technique follows a classical " Com-16 mon Conversion Point " scheme and helps to focus on a crucial depth range 17 (20-70 km) where mantle wedge develops, harmonic decomposition of a re-18 cevier function data-set is a novel and less explored approach to the anal-19 ysis of P-to-S converted phases. The separation of the back-azimuth harmon-20 ics is achieved through a numerical regression of the stacked radial and trans-21 verse receiver function from which we obtain independent constraints on both 22 isotropic and anisotropic seismic structures. The application of our new method 23 to RETREAT data-set succeeds both to confirm previous knowledge about 24 seismic structure in the area and to highlight new structures beneath the North-25 ern Apennines chain, where previous studies failed to clearly retrieve the ge-26 ometry of the subducted interfaces. We present our results in closely spaced 27 profiles across and along the Northern Apennines chain to highlight the pres-28 ence of the Tyrrhenian and the Adriatic side which differ in the thickness of the crust (about 25 km the first one, and about 40 km the second one). 30 These two plates converge in the central zone where the Adriatic plate subducts 31 beneath the Tyrrhenian. A signature of the dipping Adriatic Moho is clearly 32 observed beneath the Tyrrhenian Moho in a large portion of the forearc re-33 gion. In the area where the two Mohos overlap, our new analysis reveals the 34 presence of an anisotropic body above the subducted Moho. There is a strong 35 Ps converted phase with anisotropic characteristics from the top of the Adri-36 atic plate to a depth of at least 80 km. Because the Ps conversion occurs much 37 deeper than similar Ps phases in Cascadia and Japan, dehydration of oceanic 38 crust seems …