Abruzzi region (central Italy) producing vast damage in the L'Aquila town and surroundings. In this paper we present the location and geometry of the fault system as obtained by the analysis of main shock and aftershocks recorded by permanent and temporary networks. The distribution of aftershocks, 712 selected events with M L ! 2.3 and 20 with M L ! 4.0, defines a complex, 40 km long, NW trending extensional structure. The main shock fault segment extends for 15-18 km and dips at 45°to the SW, between 10 and 2 km depth. The extent of aftershocks coincides with the surface trace of the Paganica fault, a poorly known normal fault that, after the event, has been quoted to accommodate the extension of the area. We observe a migration of seismicity to the north on an echelon fault that can rupture in future large earthquakes.

The accurate definition of 3-D crustal structures and, in primis, the Moho depth, are the most important requirement for seismological, geophysical and geodynamic modelling in complex tectonic regions. In such areas, like the Mediterranean region, various active and passive seismic experiments are performed, locally reveal information on Moho depth, average and gradient crustal V p velocity and average V p /V s velocity ratios. Until now, the most reliable information on crustal structures stems from controlled-source seismology experiments. In most parts of the Alpine region, a relatively large number of controlled-source seismology information are available though the overall coverage in the central Mediterranean area is still sparse due to high costs of such experiments. Thus, results from other seismic methodologies, such as local earthquake tomography, receiver functions and ambient noise tomography can be used to complement the controlled-source seismology information to increase coverage and thus the quality of 3-D crustal models. In this paper, we introduce a methodology to directly combine controlled-source seismology and receiver functions information relying on the strengths of each method and in relation to quantitative uncertainty estimates for all data to derive a well resolved Moho map for Italy. To obtain a homogeneous elaboration of controlledsource seismology and receiver functions results, we introduce a new classification/weighting scheme based on uncertainty assessment for receiver functions data. In order to tune the receiver functions information quality, we compare local receiver functions Moho depths and uncertainties with a recently derived well-resolved local earthquake tomography-derived Moho map and with controlled-source seismology information. We find an excellent correlation in the Moho information obtained by these three methodologies in Italy. In the final step, we interpolate the controlled-source seismology and receiver functions information to derive the map of Moho topography in Italy and surrounding regions. Our results show high-frequency undulation in the Moho topography of three different Moho interfaces, the European, the Adriatic-Ionian, and the Liguria-Corsica-Sardinia-Tyrrhenia, reflecting the complexity of geodynamical evolution.

[1] Isotropic and anisotropic seismic structures across the Northern Apennines (Italy) subduction zone are imaged using a new method for the analysis of teleseismic receiver functions (RFs). More than 13,000 P-wave coda of teleseismic records from the [2003][2004][2005][2006][2007] Retreating-Trench, Extension, and Accretion Tectonics (RETREAT) experiment are used to provide new insights into a peculiar subduction zone between two continental plates that is considered a focal point of Mediterranean evolution. A new methodology for the analysis of receiver functions is developed, which combines both migration and harmonic decomposition of the receiver function data set. The migration technique follows a classical "Common Conversion Point" scheme and helps to focus on a crucial depth range (20-70 km) where the mantle wedge develops. Harmonic decomposition of a receiver function data set is a novel and less explored approach to the analysis of P-to-S converted phases. The separation of the back-azimuth harmonics is achieved through a numerical regression of the stacked radial and transverse receiver functions from which we obtain independent constraints on both isotropic and anisotropic seismic structures. The application of our method to the RETREAT data set succeeds both in confirming previous knowledge about seismic structure in the area and in highlighting new structures beneath the Northern Apennines chain, where previous studies failed to clearly retrieve the geometry of the subducted interfaces. We present our results in closely spaced profiles across and along the Northern Apennines chain to highlight the convergence of the Tyrrhenian and the Adriatic microplates which differ in their crustal structure where the Adriatic microplate subducts beneath Tuscany and the Tyrrhenian sea. A signature of the dipping Adriatic Moho is clearly observed beneath the Tyrrhenian Moho in a large portion of the forearc region. In the area where the two Mohos overlap, our new analysis reveals the presence of an anisotropic body above the subducted Moho. There is a strong Ps converted phase with anisotropic characteristics from the top of the Adriatic plate to a depth of at least 80 km. Because the Ps conversion occurs much deeper than similar Ps phases in Cascadia and Japan, dehydration of oceanic crust seems unlikely as a causative factor. Rather, the existence of this body trapped between the two interfaces supports the hypothesis of lower crustal delamination in a postsubduction tectonic setting.

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[1] We present a new data set of Moho depth estimates for peninsular Italy, obtained from receiver functions analysis and a grid search approach. Teleseismic records from 174 broadband seismic stations were used to compute the receiver function data set. In general, our results agree well with previous knowledge, adding original information in many regions where previous data were lacking or uncertain. We find broad regions with consistent Moho depth, namely, (1) the peri-Tyrrhenian areas bordering the homonymous sea have a 20 to 25 km deep Moho; (2) the foreland regions (Adriatic, Apulia, Iblean) with 30-35 km deep Moho; (3) the Apennines belt, where we find westward deepening of the Adriatic Moho down to 50 km depth, that well delineate the westward Adria subduction, overlayed by the shallow Tyrrhenian Moho; (4) Calabria shows a flat, 40 km deep Moho suddenly dipping at high angle from the Tyrrhenian coast westward, defining the Ionian subduction; and (5) a clear separation between northern and southern Apennines is marked by a region of almost flat Moho, possible evidence of a detached slab in between. We also find that in the northern Apennines the shallow Tyrrhenian Moho is continuous for more than 200 km from the sea to under the northern Apennines, with a generally flat geometry and a few undulations suggesting either the ''boudinage'' or episodic pulses of extension of the Tyrrhenian crust during its Pliocene-Quaternary stretching. The eastward extent of the Tyrrhenian Moho below the Apenninic belt approximately corresponds to the eastward extent of the observed extension in the upper crust as mapped by the normal faulting earthquakes. In this portion of the Apennines, we interpret an intracrustal discontinuity, SW dipping below the belt parallel to the Adria Moho, as the top of the lower crust being subducted beneath the Apennines. The ''subcrustal'' earthquakes of this region (down to 80 km depth) seem to be located within this subducted crustal layer. The southern Apennines show the same general geometry of subduction as the northern arc, although the former have a less defined on-land extended region and no earthquakes in the lower crust-uppermost mantle. In between, the central Apennines present a different Moho geometry: no clear doubling of the two plates but rather a constant bilateral deepening possibly associated to a crustal root, with no evidence of a subduction setting. This reinforces the hypothesis of a detached slab in this area. We also present the first large-scale mapping of average crustal V p /V s ratios for peninsular Italy. The results, although representing averages over the whole crust, enhance some interesting features. The most evident are belts of high V p /V s in the thinned crust of the Apennines where extension has long been active in the Quaternary, probably as an effect of highly fractured rocks with abundant fluid circulation. Strong correlation exists between areas of high V p /V s and Quaternary volcanoes, suggesting the presence of fluids at depth.Citation: Piana Agos...