SKS splitting measurements in the Apenninic-Tyrrhenian domain (Italy) and their relation with lithospheric subduction and mantle convection

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. B4, 2218, doi: /2002jb001793, 2003 SKS splitting measurements in the Apenninic-Tyrrhenian domain (Italy) and their relation with lithospheric subduction and mantle convection L. Margheriti, F. P. Lucente, and S. Pondrelli Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy Received 28 January 2002; revised 27 September 2002; accepted 13 December 2002; published 29 April [1] We present a compilation of singular shear wave splitting measurements in the Italian region. We used the method of Silver and Chan [1991] to determine the splitting parameters of fast direction and delay time from broadband SKS waveforms. The stations analyzed were mainly deployed in temporary experiments; a few are permanent MedNet broadband stations. The splitting parameters reveal the presence of strong seismic anisotropy in the mantle beneath Italy. The fast polarization directions define two anisotropic domains: a Tyrrhenian domain with E-W prevalent fast directions and an Apennines-Adriatic domain with NW-SE to NNW-SSE prevalent fast directions. The stations along the boundary between these two main domains show fast polarization directions that vary for earthquakes coming from different back azimuths. This variability is a sign of a complex upper mantle structure (also shown by seismic tomography) but allows us to constrain the depth of the anisotropic layer in the km range, at least in the boundary zone between the two domains. Data at station VSL (Sardinia) suggest the presence of depth-dependent anisotropic structure, where the shallower signature is consistent with the Tyrrhenian domain results and the deeper one is possibly related to the presence of a subhorizontal slab laying on the 670-km discontinuity. We argue that the complex pattern of fast directions is strictly related to the upper mantle structure and is caused by asthenospheric flows induced by slab rollback and contemporary deformation of the overriding Tyrrhenian plate. INDEX TERMS: 7203 Seismology: Body wave propagation; 7218 Seismology: Lithosphere and upper mantle; 8120 Tectonophysics: Dynamics of lithosphere and mantle general; 8150 Tectonophysics: Plate boundary general (3040); 9335 Information Related to Geographic Region: Europe; KEYWORDS: seismic anisotropy, lithospheric subduction, mantle convection Citation: Margheriti, L., F. P. Lucente, and S. Pondrelli, SKS splitting measurements in the Apenninic-Tyrrhenian domain (Italy) and their relation with lithospheric subduction and mantle convection, J. Geophys. Res., 108(B4), 2218, doi: /2002jb001793, Introduction [2] The Apenninic belt-tyrrhenian basin system, in the central Mediterranean region, is part of the tectonic boundary between two slowly converging macroplates, Eurasia and Africa [McKenzie, 1972; Dewey et al., 1989; Rebaï et al., 1992]. The Mediterranean region is unique in that here, as like nowhere else in the world, the complex quasi-final stages of a continental collision, which started during the Late Cretaceous, are still going on [Faccenna et al., 2001a, and references therein]. The protracted collision between these two continental macroplates led to the existence of a wide belt of crustal deformation that masks the actual plate margins, favoring the fragmentation of the lithosphere into a poorly known number of microplates trapped between the large plates [Westaway, 1990, and references therein; Gvirtzman and Nur, 2001]. The complex interactions among these microplates result in a puzzling pattern of extensional basins Copyright 2003 by the American Geophysical Union /03/2002JB001793$09.00 of newly formed oceanic crust, compressional arcuate mountain belts, and subduction zones of remnant oceanic lithosphere [Dercourt et al., 1986; Séranne, 1999; Jolivet and Faccenna, 2000]. Therefore, in the general context of the N-S convergence of Europe and Africa, which acts as primary force shaping the Mediterranean area, the existence of differently oriented tectonic and physiographic features is still enigmatic [Durand et al., 1999]. Moreover, the timescale of formation of some of these tectonic features, like the Tyrrhenian basin and the Apennines, is by far faster than the rate of convergence between Africa and Eurasia [Malinverno and Ryan, 1986; DeMets et al., 1990; Faccenna et al., 2001b], implying that driving forces independent of Africa- Europe collision are important. [3] The formation of the Apennines and the opening of the Tyrrhenian back arc basin (Figure 1) result from the incomplete subduction of the Adriatic and Ionian microplates, mainly during the last 10 to 20 Myr [Malinverno and Ryan, 1986; Gueguen et al., 1998; Faccenna et al., 2001b]. At present, the subduction process from the northern to the southern Apennines has probably ended or is close to the ESE 12-1

2 ESE 12-2 MARGHERITI ET AL.: SKS SPLITTING IN APENNINES SUBDUCTION SYSTEM Figure 1. Tectonic setting of Apenninic-Tyrrhenian domain. ending because of the exhaustion of the oceanic or thinned continental crust available for subduction along the Adriatic plate margin and because of the introduction of buoyant continental lithosphere into the mantle [Lucente et al., 1999]. The availability of old oceanic lithosphere in the Ionian domain [De Voogd et al., 1992], in front of the Calabrian Arc, might allow further subduction and arc migration, even if the lateral span of the Calabrian Arc is now so limited that it is the narrowest active trench-arc system known. [4] In such a complex tectonic framework, shear wave splitting measurements from teleseismic SKS phases can provide important information about strain in the mantle, helping to clarify the dynamics of the lithospheric processes. In fact, seismic anisotropy and shear wave splitting have been documented in subduction zones and have been used to investigate mantle flow on both the back arc and seaward sides of subducting slabs [e.g., Russo and Silver, 1994; Fouch and Fisher, 1996]. Moreover, in our study area, both the forearc and the back arc regions of the past and present subduction zones are partially exposed, and it is therefore potentially possible to study the anisotropic structure of the mantle beneath and above the subducted slab using land-based seismographs, while in many other subduction zones these regions lie under water and are therefore less easy to study. [5] The SKS phase travels as an S wave in the crust and mantle and as a P wave through the liquid outer core. At the

3 MARGHERITI ET AL.: SKS SPLITTING IN APENNINES SUBDUCTION SYSTEM ESE 12-3 core-mantle boundary on the receiver side, the P wave is converted to an S wave polarized in the vertical plane of propagation for a spherical symmetric, isotropic Earth, and follows a nearly vertical mantle path. If this shear wave encounters anisotropic material on its path to the receiver, it may split into two quasi-shear waves with polarization directions orthogonal to each other and propagating at different velocities, denoted as fast and slow. It is generally believed that the main cause of SKS splitting is the latticepreferred orientation (LPO) of crystallographic axes of elastically anisotropic minerals in the upper mantle, mainly olivine [e.g., Vinnik et al., 1989; Silver, 1996; Savage, 1999, and references therein]. The shear wave is split by an anisotropic medium when the wave s propagation direction is not parallel to a symmetry axis of the medium and when its polarization is not parallel to the fast or slow direction [Babuska and Cara, 1991]. The parameters used to characterize the anisotropic medium are the polarization direction of the first arriving quasi-shear wave, i.e., fast direction, f, measured clockwise from the north, and the travel time difference between the fast and slow quasi-shear waves, dt. Because of the systematic relationship of olivine LPO to strain [Nicolas and Christensen, 1987; Ismaïl and Mainprice, 1998; Ribe and Yu, 1991; Tommasi, 1998], observational constraints on the mantle anisotropy have a unique potential to map patterns of mantle flow. [6] Here we present SKS splitting measurements for the Apenninic-Tyrrhenian domains, compare the results with tomographic images of the area, and discuss models for the possible mantle flow in the region. 2. Data Set [7] Our initial data set consists of about 500 three components earthquake recordings. The earthquakes were recorded at three permanent MedNet stations and at about 45 temporary stations deployed during various experiments by staff of the Istituto Nazionale di Geofisica e Vulcanologia (INGV; see Figure 2 and Table 1). In the GeoModAp project, three linear arrays were deployed across the northern (NAP?), central (CA??), and southern Apennines (SAP?), respectively [Amato et al., 1998]. The stations (triangles in Figure 2) were equipped with 24-bit RefTek digitizers, in continuous recording mode at 40 or 20 Hz, and with Lennartz 5-s or broadband Guralp sensors (CMG3, CMG4, and CMG40). Each experiment lasted for 3 4 months. Preliminary results from these deployments have been published [Margheriti et al., 1996; Amato et al., 1998]. In the BroadVes experiment [De Gori et al., 1998], the array consisted of five continuously recording RefTek seismic stations (The four stations that gave results in this study are circles in Figure 2), equipped with three-component broadband sensors (CMG40T, 20 s; CMG3T, 100 s or 360 s) deployed in the Vesuvian area in the summer The Catania downtown site response experiment [Azzara et al., 2000] lasted 2 months in 1999; stations (inverted triangles in Figure 2) were equipped with RefTek data loggers and Guralp CMG40T sensors (20 s). These three projects were short deployments, and the single stations lack a complete azimuthal coverage of recorded teleseisms, but the close spacing of the stations allowed us to investigate the variation of anisotropic parameters in some detail. The MidSea Figure 2. Map distribution of the seismic stations used in this study. project lasted more than a year [van der Lee et al., 2001], from spring 1999 to summer 2000, and part of the data collected were available for use in this study; the INGV stations (diamonds in Figure 2) consisted of Guralp CMG3 and STS2 seismometers and RefTek data loggers installed at four stations in the Tyrrhenian region. Three permanent very broadband MedNet stations (Stations AQU, VSL, and CII; equipped with STS1-VBB sensors and 24-bit Quanterra data loggers) are located in the area of interest (squares in Figure 2); data analyzed include events from 1990 to MedNet stations AQU (L Aquila) and VSL (Villasalto, Sardinia) [Mazza et al., 1998] provide a larger data set and a very good azimuthal coverage to check an azimuthal dependence of the anisotropic parameters at these sites. 3. Data Analysis [8] We use the method of Silver and Chan [1991] to retrieve shear wave splitting parameters f and dt, assuming that shear waves traverse a single homogeneous anisotropic layer. The method is based on a grid search over the possible splitting parameters space to find the pair of f and dt that, when used to correct for the anisotropy, most successfully removes its effect [Silver and Chan, 1988]. Because SKS is initially radially polarized, this is done by minimizing the energy on the reconstructed transverse component. The error bounds for the estimated splitting parameters are obtained through F test analysis as described by Silver and Chan [1991]. We analyzed only SKS and SKKS phases of earthquakes with distances greater than 85 and magnitude mainly greater than 6. Most of the measurements shown are made from unfiltered records. In a few cases we kept the measures obtained by filtered waveforms (between 0.05 and 0.5 Hz). Generally, low-pass-filtered seismograms yielded approximately the

4 ESE 12-4 MARGHERITI ET AL.: SKS SPLITTING IN APENNINES SUBDUCTION SYSTEM same splitting parameters as unfiltered seismograms, indicating that the scattering of high frequencies did not significantly affect our results. Delay times do not seem to be frequency-dependent as observed by Marson-Pidgeon and Savage [1997], but generally higher delay times have larger errors. [9] Owing to near-vertical incidence, SKS (and SKKS) phases have excellent lateral resolution beneath the stations. However, they fail to constrain the depth of the anisotropic region because the splitting parameters represent a vertically integrated effect from core-mantle boundary to the recording station. The depth of the anisotropy source can be estimated by analyzing both the lateral variation in splitting parameters at contiguous stations of a relatively dense array and the variation in splitting parameters at single stations for events coming from different back azimuths [Alsina and Snieder, 1995]. In our case, the relatively dense deployments of the GeoModAp, BroadVes, and Catania experiments (Figure 2), allowed us to make some inferences concerning the depth of the anisotropic source. [10] Knowing that the data quality is crucial to obtain a reliable estimate of f and dt, we only retained measurements from seismograms with low-noise transverse components, a good match between fast and slow components, and welldefined splitting parameters (e.g., Figure 3). The analyses yielded out from an initial data set of about 500 three components earthquake recordings 232 shear wave splitting measurements, reported in Table 2; including around 70 null measurements. A null measurement does not necessarily rule out the presence of anisotropy; in fact a null result from a single polarization direction will also occur for shear wave that is initially polarized parallel to either the fast direction or the slow direction of an anisotropic medium [Huang et al., 2000]. 4. Splitting Measurements [11] The complete compilation of 232 shear wave splitting measurements calculated in this study is mapped in Figures 4 and 5, where measures are superimposed on the topographic map of the area. We choose a shaded relief base to enhance the relationship between the land and submarine topographic structures and the fast directions. We present single measurements determined for individual station-earthquake pairs and do not calculate station averages because of the very complicated tectonic setting of the area; in fact station averages may be quite misleading in cases where the anisotropic parameters vary with the earthquake back azimuth, especially when different sets of events are used to estimate splitting at different stations [Levin et al., 1999]. All measurements are plotted at the 150 km depth piercing point of the SKS ray. The major effect of this plotting method is to separate the measurements from different back azimuths and incidence angles [Savage and Sheehan, 2000]. The choice of this particular depth for the measurements projection is based on considerations involving both the amplitudes of the Fresnel zones at various depths for the analyzed SKS waves periods (further exploited in the following) and the complexity of the uppermost mantle beneath the Apennines, as revealed by tomography [e.g., Lucente et al., 1999; Piromallo and Morelli, 2003]. Figure 3. Examples of shear wave splitting analysis at two stations, SARN and VSL. In both examples the top panel shows radial and transverse components before and after removing the delay time on the slow component; in the four small boxes on the left are shown normalized fast and slow components and their particle motions, before and after removing the delay time on the slow component. The right panel plots the grid search of f and dt and error estimate associated (double line include the 95% probability area).

5 MARGHERITI ET AL.: SKS SPLITTING IN APENNINES SUBDUCTION SYSTEM ESE 12-5 Table 1. Stations Coordinates Station Latitude Longitude AQU VSL NAP NAP NAP NAP NAP NAP NAP NAP NAP NAP CA CA CA CA CA CA CA CA CA CA CA CA CA SAP SAP SAP SAP SAP SAP CII MNTV RAIN SARN BKER VENT CALT SALI USTI CAVO CITT FERL POLI [12] In Figures 4 and 5 a positive (i.e., nonnull) measurement is represented by a single solid line oriented parallel to the fast polarization direction, f, with length proportional to the delay time, dt. Null measurements are plotted with two small lines with directions equal to the two allowed fast directions. The delay times ranges between 0.5 and 3 s, the higher values (mainly found at VSL, Sardinia) being among the highest in the world [Savage, 1999]; the mean delay time is about 1.6 s. Assuming an average anisotropy of 5%, a delay time of 1.6 s corresponds to an anisotropic layer 180 km thick [Mainprice and Silver, 1993], but again, given the very complicated velocity structure revealed by tomography, it is unlikely that such a single anisotropic layer actually represents the upper mantle anisotropy adequately. We note an interesting consistency between the topographic structures and some fast directions, i.e., the arcuate structure of the northern Apennines, the E-W structures in the southern Tyrrhenian (at USTI), and the roughly N-S scarp on the eastern offshore of Sicily. [13] The most striking result derived from the observation of Figures 4 and 5 is that the pattern of the fast polarization directions, as a whole, defines the existence of two distinct anisotropic domains: a Tyrrhenian domain, to the west, with prevalent fast directions (f) about E-W and an Apennines- Adriatic domain, to the east, with prevalent f ranging from NW-SE to N-S. The existence of two domains with distinct anisotropic parameters was already identified by Margheriti et al. [1996] but was limited to the northern Tyrrhenian- Apennines area. [14] The Tyrrhenian domain, defined by prevalently E-W oriented f measurements, ranges from Corsica (NAP0) to Elba Island (NAP1) to the very inner part of the northern Apenninic arc (between NAP2 and NAP3) in the north (see Figure 2 for station locations) and is limited by the measurements at southern Sardinia (VSL) and Ustica Island (USTI) sites in the west and south, respectively, but it does not extend to the Eolian Islands volcanic arc (SALI). The boundary between the two domains runs along the Apenninic range, and is very complicated in the central Apenninic area, where we observe E-W directed f reaching the core of the range (see measurements from the west at AQU and CII), while, slightly to the south, E-W measurement are limited to the offshore site of VENT (Ventotene). [15] The NW-SE f measurements defining the Apennines-Adriatic domain extend from the coast of Tuscany (NAP3) to the highest elevations of the chain (NAP7), in the northern area, crossing the whole northern Apenninic arc. In the central Apennines, NW-SE f measurements appear at points to the east of those in the northern line of stations, in the center of the Apenninic range (see measurements from the east at AQU and CII), and NW-SE f are mainly found in the areas corresponding to the highest elevations of the mountains. In the southern Apennines this domain occupies the whole peninsular area, from the Campania Coast (BKER) to the Puglia region (SAP7), with the easternmost E-W f measurements in the offshore area (VENT). The fast direction seems to rotate somewhat on the Adriatic coast all along the peninsula to a more N-S direction (see NAP8, CA14 and SAP7). This N-S direction is enhanced also by the few nonnull measurements of the Sicilian stations (CALT, CAVO and FERL) and of the Eolian Islands (SALI). [16] Our results reveal a complicated variation of splitting parameters, indicating that any attempt to relate the upper mantle anisotropy to an average anisotropic layer is not justified. Also, to average the splitting parameters at the stations would be both misleading and mask an interpretable signal in the data variations. In some areas the anisotropic parameters vary such that a single earthquake recorded at nearby stations yields different splitting parameters, and earthquakes from different back azimuths recorded at a given station are also different. These variations could be explained by the presence of laterally inhomogeneous anisotropy. Also, back azimuthal variations are sometimes related to vertical variations in anisotropy (i.e., double-layer anisotropy [Silver and Savage, 1994]) or to complicated anisotropic symmetry systems (i.e., nonhorizontal anisotropic layers [Babuška et al., 1993] and inclined olivine a axis [Hartog and Schwartz, 2000]). However, the relatively small number of measurements at each station precludes a station-by-station analysis of such possibilities. Thus, to constrain the depth and geometry of the source of splitting, we performed a simple first-order analysis in terms of lateral variation in anisotropy and, for the two sites with a more

6 ESE 12-6 MARGHERITI ET AL.: SKS SPLITTING IN APENNINES SUBDUCTION SYSTEM Table 2. SKS Splitting Measurements Station Date a deg Latitude, Longitude, deg Depth, km f f dt dt AQU null null null null null null null null null null null null null null null null null VSL null null null null null null null NAP null Table 2. (continued) Station Date a deg Latitude, Longitude, deg Depth, km f f dt dt NAP NAP null null NAP null NAP null NAP NAP NAP NAP NAP null CA CA CA CA CA CA CA CA CA CA null null CA null null null null CA CA SAP

7 MARGHERITI ET AL.: SKS SPLITTING IN APENNINES SUBDUCTION SYSTEM ESE 12-7 Table 2. (continued) Station Date a deg Latitude, Longitude, deg Depth, km f f dt dt null SAP null null null SAP null null SAP null null SAP null SAP null null CII null null null null null null MNTV RAIN null null SARN null null BKER null VENT CALT null null null null null SALI null Table 2. (continued) Station Date a deg Latitude, Longitude, deg complete azimuthal coverage (VSL and AQU), in terms of vertical variation in anisotropy Lateral Variation in Anisotropy [17] Variation in splitting parameters for a single teleseismic earthquake recorded on a relatively tight array can only be explained by anisotropy if lateral variations in anisotropy exist across the array [Savage and Sheehan, 2000]. This is the case of two of the three linear arrays deployed across the Apennines in the GeoModAp project [Amato et al., 1998], the northern and central Apennines transect, stations NAP? and CA??, respectively (see Figure 2). Variations also occur between earthquakes from different back azimuths recorded at the same station. [18] The depth of seismic anisotropy can be estimated by analyzing the lateral variation in SKS splitting parameters both for single events recorded at different stations of a relatively dense array and for events with different directions of approach at a given station [Alsina and Snieder, 1995]. We have attempted to constrain the depth of the anisotropy in the Italian region by calculating the Fresnel zones for these phases. For single layer splitting, the depth of the anisotropy beneath adjacent stations exhibiting different splitting parameters is constrained by the condition that Fresnel zones corresponding to the observations do not overlap significantly. The size of the Fresnel zone in a uniform medium is approximately given by rffiffiffiffiffiffiffiffiffiffiffiffiffi Tnh Rf ¼ ; 2 cos J Depth, km f f dt dt null USTI CAVO null null CITT null null null FERL null POLI null null null a Dates are in year, month, day, i.e, is 23 September where Rf is the radius of the first Fresnel zone (in the horizontal plane), T is the dominant period, n is the wave velocity, q is the angle of incidence, and h is the depth. In Figures 6a and 6b, Fresnel zones (circles) at 100, 150, and 200 km depth are shown for different stations. The dominant period at the stations NAP1, NAP2, CA01, and CA02 is 5 s, while at AQU it is 10 s. Different results from opposite back azimuths at AQU and NAP2 suggest that anisotropy occurs below a certain depth, Z1 (Figure 6a), since above that depth, Fresnel zones of events arriving from opposite directions overlap. We estimate Z1 to be shallower than 100 km below both the stations (Figure 6a,

8 ESE 12-8 MARGHERITI ET AL.: SKS SPLITTING IN APENNINES SUBDUCTION SYSTEM Figure 4. Compilation of SKS wave splitting measurements superimposed on a shaded relief map of Italy. A positive measurement is represented by a solid line oriented parallel to the fast polarization, with length proportional to the delay time. Null measurements are plotted with two small lines with directions equal to the two allowed fast directions. Each measurement is plotted at the surface projection of the 150 km depth SKS ray piercing point. Boxes indicate enlargements shown in Figures 5a and 5b. See color version of this figure at back of this issue.

9 MARGHERITI ET AL.: SKS SPLITTING IN APENNINES SUBDUCTION SYSTEM ESE 12-9 Figure 5. (a and b) Enlargements around AQU and VSL stations of boxes in Figure 4 to better separate the large number of measurements at these stations. See color version of this figure at back of this issue.

10 ESE MARGHERITI ET AL.: SKS SPLITTING IN APENNINES SUBDUCTION SYSTEM Figure 6. Fresnel zones (circles) at 100, 150, and 200 km depth at the stations NAP1, NAP2, CA01, CA02, and AQU. The larger radius of the AQU Fresnel zone is due to a dominant period of about 10 s while at the other stations it is about 5 s. (a) Different results from opposite back azimuths suggest that anisotropy occurs below a certain depth, Z1 (shaded circles). (b) At the same time, differences in splitting parameters of the same event between nearby stations suggest that anisotropy occurs above a depth, Z2 (shaded circles). shaded circles). At the same time (Figure 6b), differences in splitting parameters for the same event between nearby stations suggest that anisotropy occurs above a certain depth, Z2, and below this depth the Fresnel zones overlap. Z2 is a function of the Fresnel zones amplitude and of the distance between the nearby stations. Between stations NAP1 and NAP2 we observe a significant change in fast direction for the same event; in this case, given the station separation and the Fresnel zone widths we estimate Z2 is about 200 km depth (Figure 6b shaded circles). At CA01 and CA02 we again observe a strong change in fast direction for the same event, but due to the small station separation, Z2 appears to be about 100 km (Figure 6b shaded circles). This last observation is in contrast with the Z1 estimate at AQU, although AQU lies only few km away from CA01 and CA02. These results delineate a quite complicated situation where vertical boundaries between different anisotropic sources with a horizontal symmetry axis do not fully explain the data in central Apennines Vertical Variation in Anisotropy [19] The presence of depth-dependent anisotropy produces azimuthal variations of the f and dt parameters at a single station [Silver and Savage, 1994; Rumpker et al., 1999]. Most importantly, the apparent splitting parameters are different from those expected for a homogeneous medium with constant mean anisotropy, so that some information on the depth dependence of the anisotropy is preserved [Levin et al., 2000]. We have only two stations with azimuthal coverage good enough to check for depthdependent anisotropy: AQU (L Aquila, central Apennines) and VSL (Villasalto, Sardinia), with 43 and 27 measurements, respectively. Rumpker et al. [1999] have shown that in the presence of depth-dependent anisotropy, splitting parameters measured under the assumption of a single anisotropic layer will be apparent parameters and will display a p/2 periodicity in their variations. [20] At AQU (Figure 7a) the fast directions are different for NE and SW back azimuths, excluding the presence of vertically varying anisotropy (opposite back azimuths should give the same fast direction in a p/2 periodicity scheme). This observation may imply the presence, beneath AQU, of an anisotropic medium with a symmetry that is more general than hexagonal with horizontal symmetry axis. Hartog and Schwartz [2000] for similar behaviors suggest the presence of an anisotropic source

11 MARGHERITI ET AL.: SKS SPLITTING IN APENNINES SUBDUCTION SYSTEM ESE Figure 7. We checked for azimuthal dependence of anisotropic parameters f and dt to verify the possibility of a two-layer anisotropic structure. (a) Fast directions and delay times at AQU. The f are different for NE (black squares) and SW (gray squares) back azimuths excluding the presence of two simple horizontal anisotropic layers. (b) VSL measurements (f and dt) show a p/2 periodicity that can be interpreted as two anisotropic layers; one of the possible two-layer model is plotted with the data. (c) Cumulative c 2 curve for the 32,000 best models. (d) Among all the possible two-layer models we selected 2500 models with the lowest c 2 ; we constrained an upper anisotropic layer with about E-W f and a lower anisotropic layer with f in the NE-SW quadrants.

12 ESE MARGHERITI ET AL.: SKS SPLITTING IN APENNINES SUBDUCTION SYSTEM with inclined olivine a axis (downward or upward flow). However, measurements at close stations in central Apennines do not show splitting parameters consistent with AQU azimuthal trend (e.g., station CA01 for waves coming from NE), indicating, in any case, the presence of laterally different anisotropic domains beneath this region. It should be noted that at AQU probably both the effects of inclined olivine a axes and of laterally varying anisotropy are present in the data. [21] At station VSL (Figure 7b), measurements show a p/2 periodicity that can be interpreted as depth-dependent anisotropy and that may also explain the high delay times, up to 3 s, found at this station. Silver and Savage [1994] have derived analytical equations for the variation of apparent parameters, f and dt, in the presence of two anisotropic layer, as a function of back azimuth, dominant period of the incoming wave, and splitting parameters of the lower and the upper layers. We use these equations to compute the theoretical variation curves of splitting parameters in a simple two-layer model for a comparison with the observed data (i.e., Figure 7b). We have generated all the possible twolayer models with steps of 1 in fast directions (from 0 to 180 ) and of 0.1 s in delay time (from 0 to 4 s) for a total of 51,840,000 models. A quite large number of two-layer models fit the measured apparent splitting parameters at VSL, in fact a depth-dependent anisotropic model can never be uniquely determined from SKS measurements without independent constrains [Hartog and Schwartz, 2001]. To quantify the goodness of the fit for each of them, we have computed a chi-square merit function c 2 in relation to the VSL data (both f and dt): c 2 f ¼ f 2 o f p s 2 f ; c 2 dt ¼ dt 2 o dt p s 2 dt where f o and dt o are the observed values, s 2 f and s 2 dt are their errors and f p and dt p are the predicted value for the two-layer model. Null measurements are not included. We find our preferred models minimizing c 2 =(c 2 f + c 2 dt ). In Figure 7c the cumulative curve of normalized chi-square (ratio (c 2 f + c 2 dt )/(c 2 f + c 2 dt ) min ) for the 32,000 models with lowest c 2 is shown. We search for change points in the cumulative c 2 curve through an approach based on Kolmogorov-Smirnov two-sample nonparametric statistics [Mulargia and Tinti, 1985]. It identifies two main change points (Figure 7c) that separate models in three families; we selected the 2500 models with c 2 lower than the first change point and excluded the rest. The selected c 2 identifies a family of acceptable models (Figure 7d) with fast direction of the lower layer in the NE-SW quadrants and the fast direction in the upper layer rotated clockwise from the lower layer f; generally delay time in the lower layer increase as the delay in the upper layer decrease. If we allow only delay times below 3 s for each single layer (none of the other stations show such a large dt) more than 75% of the selected models have upper layer parameters 78 f2 110 and 0.8 s dt2 < 3 s and lower layer parameters 0 f1 63 and 0.7 s dt1 < 3 s. Thus data at station VSL (Sardinia) suggest the presence of depth-dependent anisotropic structure: an upper layer with anisotropic fast symmetry axis trending close to E-W and a delay time between 0.8 and 3 s and a lower layer with NNE-SSW f and dt ranging between 0.7 and 3 s. 5. Discussions and Comparison With Other Studies 5.1. Relation Between Tomographic Images and Anisotropy [22] Beneath the Apenninic-Tyrrhenian domain, the main feature in the tomographic images [e.g., Lucente et al., 1999; Piromallo and Morelli, 2003] is a continuous highvelocity body located between 250 and 670 km depth running along the entire Apenninic belt, dipping toward the Tyrrhenian area (Figure 8b). It continues upward segmented into two main anomalies in the northern Apenninic and the Calabrian Arcs (Figure 8a). This high-velocity feature is interpreted as the subducted oceanic lithosphere (thickness of km) between the Eurasian and African plates. [23] In Figure 8a the SKS splitting, plotted at the pierce depth of 150 km (see the preceding paragraphs), is overlain on the tomographic image at depth between 100 and 170 km. We see a good correspondence between the image of the slab at these depths and the change in fast SKS directions, especially in the northern Apennines, where the slab signature is quite clear and the number of SKS measurements is significant. Here the boundary between what we call the Tyrrhenian domain, characterized by E-W fast directions, and the Apennines-Adriatic domain, with NW- SE fast directions corresponds to the subducted lithosphere signature. In the Calabrian arc the boundary zone between Tyrrhenian and Apennines-Adriatic domains seems to be shifted toward the northwest with respect to the slab trace at km depth, but the low number of SKS splitting and the spacing of the stations in that area is probably insufficient to warrant further speculation. In the central southern Apennines, where the tomographic image at km depths shows a zone of almost unperturbed mantle, the change between Tyrrhenian and Apenninic fast directions is abrupt and seems to happen just at the Tyrrhenian coast, shifted toward the west with respect to the transition to the north. This shift to the west of the boundary could be related to lateral heterogeneities of the subducted lithosphere in a region where different hypotheses on the subduction setting were made: tear migration as part of a slab detachment process [Wortel and Spakman, 1992; Spakman et al., 1993; Wortel and Spakman, 2000], existence of a slab window or slab breakoff at km depths [Amato et al., 1993; Lucente et al., 1999; Lucente and Speranza, 2001] or 150 km depth [Cimini and De Gori, 2001], or subduction of a promontory of continental or mixed lithosphere in the last few million years [Lucente et al., 1999]. [24] Tomographic images indicate that the subducted slab reaches the upper/lower mantle boundary along all its lateral extent, from the western Alps to Calabria and Sicily, and flattens in the km depth range beneath the Tyrrhenian basin [Lucente et al., 1999; Piromallo and Morelli, 2003; Faccenna et al., 2001b]. Cross sections across both the Calabrian and the central Apennines show the flattening of the steeply subducting slab toward a horizontal posture over the whole Tyrrhenian basin and beyond Corsica

13 MARGHERITI ET AL.: SKS SPLITTING IN APENNINES SUBDUCTION SYSTEM ESE Figure 8. (a) Teleseismic P wave tomography at km depth interval (white circles are the earthquakes foci at these depths, modified from Lucente et al. [1999]), with splitting measurements at 150 km depth rays piercing point superimposed. The white square dots region enhances the boundary between the Tyrrhenian and the Apenninic-Adriatic anisotropic domains. (b) Sections across the tomographic model showing the steeply dipping slab and its subhorizontal portion laying on the upper/ lower mantle boundary. Intermediate and deep earthquakes within 30 km of each cross section are also displayed (white circles). See color version of this figure at back of this issue.

14 ESE MARGHERITI ET AL.: SKS SPLITTING IN APENNINES SUBDUCTION SYSTEM Figure 9. Pn anisotropy (gray pattern, modified after Mele et al. [1998]) versus SKS splitting measurements (black solid bars, this study). (Figure 8b). Similar slab geometry with a steep-dipping Wadati-Benioff zone and a subhorizontal remnant of slab in the transition zone of the mantle is found in other subduction zones (e.g., see Chen and Brudzinski [2001] for the Tonga back arc). Brudzinski and Chen [2001] claim an unequivocal case of seismic anisotropy in the transition zone (delay times up to 3 s) Comparison With Pn Anisotropy in the Area [25] The SKS splitting results are not uniformly correlated with Pn anisotropy results [Mele et al., 1998] (Figure 9). The NW-SE fast directions in northern and in the eastern part of central Apennines and the E-W fast directions below the southern Tyrrhenian Sea are consistent with Pn results. However, the fast Pn directions in the southern and western part of the central Apennines are at high angle to the fast SKS. Moreover, no relevant anisotropy is required by Pn propagation under Sardinia-Corsica and the northern Tyrrhenian Sea, while SKS splitting there includes some of the higher delays we found. These discrepancies suggest vertical variations in anisotropy within the upper mantle. In fact, Pn samples the mantle above 50 km, while we show that the probable source of SKS splitting is below 100 km, at least in the boundary region between the Tyrrhenian and the Apenninic domains. It is interesting to note that the consistency of splitting and Pn is good in the area where the evidence of a continuous subducting slab is stronger, e.g., the North Apennines and South Tyrrhenian Sea, while a greater discrepancy in fast directions is found in the central southern Apennines where no slab is tomographically resolved in the upper few hundred kilometers depth range Depth Distribution of the Anisotropy in the Mantle [26] Recent reviews on the presence of anisotropy in different layers of the Earth [Savage, 1999; Montagner and Guillot, 2001] demonstrated that anisotropy is a very general feature. However, it is not present in all depth ranges nor at all scales. Even if debate continues about whether anisotropy due to olivine lattice preferred orientation is confined to the upper 200 km beneath the mantle or continues down to the transition zone, most observations indicate that the main sources of anisotropy are confined to the upper few hundred kilometers [see Savage, 1999, and references therein]. In a few cases, in collisional or subduction plate boundary [e.g., Fouch and Fisher, 1996], anisotropy is found at much greater depths. This may be due to subductive flow or to the difference in temperature between a cold slab and the surrounding mantle. This difference allows for the possibility that anisotropic metastable olivine, with a preexisting anisotropic fabric [Sandvol and Ni, 1997], and/or ilmenite (MgSiO 3 )[Da Silva et al., 1999], and/or hydrous minerals and free fluids might be present down to mantle transition zone depths. In this last case, the shear wave speed may be slow enough to provide a large contrast against coexisting minerals, which is needed to develop a fine-scale, laminated structure [Brudzinski and Chen, 2001]. Anisotropy in the upper/lower mantle transition zone (typically 1 2% up to 5%, [Savage, 1999; Montagner and Guillot, 2001]) may exist in some regions and relate to a variety of phenomena (see Mainprice et al. [2000] for a review). Brudzinski and Chen [2001] indicate two leading candidates for the petrologic anomaly in the transition zone: the presence of metastable olivine (alpha phase) or volatiles brought down by subduction. This last hypothesis may also make the entire slab buoyant, prohibiting further penetration in the lower mantle. [27] The deeper anisotropic layer at VSL in Sardinia, not seen by the other Tyrrhenian stations probably due to their scarce azimuthal coverage, can be explained by the existence of an accumulation of cold slab in the transition zone. The delay times that were found suggest a possible thickness of this anisotropic layer larger than 100 km. [28] The complex pattern of SKS fast directions, found in this study, is not easily interpretable but, at least in the boundary region between the Tyrrhenian and the Apennines-Adriatic domains, the anisotropic layer is constrained to be deeper than 100 km and 150 km thick, suggesting a large contribution to anisotropy from the asthenosphere. The available Pn anisotropy inversion [Mele et al., 1998] shows the anisotropic behavior of the uppermost mantle; Pn velocity is anisotropic especially (up to 5% of anisotropy) along the major arcs evidenced by tomography (northern Apennines and Calabrian arc) with a fast direction that follows the arcs signatures. [29] Overall, the anisotropic structures at different depths beneath Italy show some consistency. They are related to the presence of the Apennines mountain belt and to the subducted lithosphere involved in its orogenesis. These observations suggest that the orogenesis of the Apennines is a process that involved the whole upper mantle and lithosphere characterizing the entire deformation pattern of the investigated volume Models of Subduction Zone Anisotropy [30] Seismic anisotropy is usually attributed to the alignment of olivine crystal orientations (LPO), which in turn can be related to the strain history and to some extent to the mantle flow. Modeling of strain resulting from flow coupled to a subducting plate predicts a fairly uniform pattern of anisotropy, with a fast direction parallel to the absolute

15 MARGHERITI ET AL.: SKS SPLITTING IN APENNINES SUBDUCTION SYSTEM ESE mantle might result in a fast direction almost perpendicular to the one expected in water-poor conditions (the addition of water changes f from parallel to perpendicular to the shear plane). Variable water content may be appropriate for the mantle wedge in subduction zones, especially near the trench but may explain variations in fast directions only in the case of large stress and large amount of water [Kaminski, 2002]. The alternative is to invoke complicated localscale flow geometries. [31] The Apenninic belt-tyrrhenian basin system seems to fit the case illustrated in Figure 10c, where the main force acting in the system is the rollback process and the overriding plate is stretched leading to asthenospheric flow and to the formation of new oceanic crust (see Figure 1). The western shifting of the boundary of the two anisotropic domains with respect to the Apenninic chain axis, going from north to south, could be explained by two classes of arguments: (1) in the central southern Apennines, where the subduction seems to have stopped about five million years ago [Lucente and Speranza, 2001], the westernmost NW-SE f (i.e., at MNTV, SAP0) could possibly fix the loci of the last rollback effects acting in this region of the trench; and (2) the presence, in central southern Apennines, of waterrich condition associated with more recent and active volcanism (Mount Vesuvio, Eolian Islands) could be consistent with the recent finding of Jung and Karato [2001], resulting in NW-SE f where a perpendicular direction was expected. Figure 10. Cartoon representations of three models of anisotropic fast directions induced by different mechanisms in subduction environments: (a) model taking into account a down dip motion of the plate and the coupling between lithosphere and asthenosphere; (b) model representing a slab with prevalent rollback motion; and (c) model with stretching of the overriding plate added to the rollback motion. motion of the down-going plate [Alvarez, 1982] (Figure 10a); this pattern is found in several subduction zones [Fisher and Wiens, 1996; Sandvol and Ni, 1997]. A strong deviation from this fast direction pattern is obtained considering not only the longitudinal down-dip motion of the slab but also a prevalent retrograde rollback motion. The rollback component of motion induces strong trench-parallel alignments of the fast directions beneath a rolling back plate (Figure 10b). Fast directions in the wedge vary upon the slab dip angle and are clearly trench perpendicular only when the dip of the slab is shallow. For steeply dipping slabs, LPO develops only near the slab (in the dip direction) and is weak elsewhere [Buttles and Olson, 1998]. This model does not match the observations in back arc regions (as our Tyrrhenian domain) probably because it does not take into account the deformation of the overriding plate (e.g., the presence of spreading centers) which may directly influence the flow of the underlying mantle [Fouch and Fisher, 1996] (Figure 10c). Furthermore, recently Jung and Karato [2001] demonstrated that the deformation fabric (LPO) of olivine under water-rich condition in the upper 6. Conclusions [32] SKS splitting measurements reveal the presence of strong seismic anisotropy in the upper mantle below Italy. The fast polarization directions define the existence of two main anisotropic domains: a Tyrrhenian domain with a prevalent fast direction of about E-W, and an Apennines- Adriatic domain with a prevalent fast direction of about NW-SE to NNW-SSE. The stations near the boundary between these two main domains show fast polarization directions that vary for earthquakes coming from different back azimuths. This variability is a sign of a complex upper mantle structure and confines the anisotropic layer along the boundary zone between the two domains in the km depth range. [33] The E-W direction of fast polarization in the Tyrrhenian domain could possibly be the expression of the asthenospheric flow related to the opening of the Tyrrhenian basin induced by slab retreat occurrence (Figure 10). The Apennines-Adriatic domain shows fast polarization direction parallel to the NW-SE strike of the Apenninic mountain belt, suggesting the presence of strained material in the mantle inside and below the slab as a consequence of pressure induced by the retrograde motion of the slab. This trench-parallel fast direction is interpretable as flow due to retrograde motion of the slab, decoupled from the underlying mantle, and acting as partial barrier to mantle flow at depth. The lower anisotropic source beneath Sardinia can be related to the presence of a sub horizontal slab laying on the 670-km discontinuity. [34] Acknowledgments. We would like to thank all the people involved in the deployments of the analyzed stations especially the INGV

16 ESE MARGHERITI ET AL.: SKS SPLITTING IN APENNINES SUBDUCTION SYSTEM staff and the other technicians not mentioned in the references. MedNet Data Center provided the data from permanent stations. Some stations were deployed in the project GeoModAp (EEC contract EV5V-CT ). We thank Claudio Chiarabba, Alessandro Amato, Antonio Piersanti, Warner Marzocchi, Vadim Levin, and Mark Brandon for useful discussions. Raimond Russo and Renate Hartog improved the original manuscript with their reviews. We use GMT software [Wessel and Smith, 1998] to prepare the maps. References Alsina, D., and R. Snieder, Small-scale sublithospheric continental mantle deformation; constraints from SKS splitting observations, Geophys. J. Int., 123, , Alvarez, W., Geological evidence for the geographical pattern of mantle return flow and driving mechanism of plate tectonics, J. Geophys. Res., 87, , Amato, A., B. Alessandrini, and G. B. Cimini, Teleseismic wave tomography of Italy, in Seismic Tomography: Theory and Practice, edited by H. M. Iyer and K. Hirahara, pp , Chapman and Hall, New York, Amato, A., et al., Passive seismology and deep structure in central Italy, Pure Appl. Geophys., 151, , Azzara, R., G. Coco, M. Corrao, S. Imposa, G. Lombardo, and A. Rovelli, Preliminary identification of different nearsurface geology effects in the area of Catania (Italy), paper presented at EGS XXV General Assembly, Eur. Geophys. Soc., Nice, France, April Babuška, V., and M. Cara, Seismic Anisotropy in the Earth, Mod. Approaches Geophys., vol. 10, 217 pp., Kluwer Acad., Norwell, Mass., Babuška, V., J. Plomerovà, and J. Šileny, Models of seismic anisotropy in the deep continental lithosphere, Phys. Earth Planet. Inter., 78, , Brudzinski, M. R., and W. P. Chen, Seismic anisotropy in the transition zone of the mantle: Implications for mantle dynamics and deep earthquakes, Eos Trans. AGU, 82(47), Fall Meet. Suppl., Abstract S42B-0625, Buttles, J., and P. Olson, A laboratory model of subduction zone anisotropy, Earth Planet. Sci. Lett., 164, , Chen, W. P., and M. R. Brudzinski, Evidence for a large-scale remnant of subducted lithosphere beneath Fiji, Science, 292, , Cimini, G. B., and P. De Gori, Nonlinear P-wave tomography of subducted lithosphere beneath central-southern Apennines (Italy), Geophys. Res. Lett., 28, , Da Silva, C. R. S., B. B. Karki, L. Stixrude, and R. M. Wentzcovich, Ab initio study of the elastic behavior of MgSiO 3 ilmenite at high pressure, Geophys. Res. Lett., 26, , De Gori, P., et al., The BROADVES seismic experiment: First results on the lithospheric structure beneath the Campanian region around the Vesuvius volcano, paper presented at EGS XXIII General Assembly, Eur. Geophys. Soc., Nice, France, April DeMets, C., R. G. Gordon, D. F. Argus, and S. Stein, Current plate motions, Geophys. J. Int., 101, , Dercourt, J., et al., Geological evolution of the Tethys belt from the Atlantic to the Pamir since the Lias, Tectonophysics, 123, , De Voogd, B., C. Truffert, N. Chamot-Rooke, P. Huchon, S. Lallemant, and X. Le Pichon, Two-ship deep seismic soundings in the basins of the eastern Mediterranean Sea (Pasiphae cruise), Geophys. J. Int., 109, , Dewey, J. F., M. L. Helman, E. Turco, D. W. H. Hutton, and S. D. Knott, Kinematics of the western Mediterranean, in Alpine Tectonics, edited by M. P. Coward, D. Dietrich, and R. G. Park, Geol. Soc. Spec. Publ., 45, , Durand, B., L. Jolivet, F. Horvàth, and M. Séranne (Eds.), The Mediterranean Basins: Tertiary Extension Within the Alpine Orogen, Geol. Soc. Spec. Publ., 156, 570 pp., Faccenna, C., T. W. Becker, F. P. Lucente, L. Jolivet, and F. Rossetti, History of subduction and back-arc extension in the central Mediterranean, Geophys. J. Int., 145, , 2001a. Faccenna, C., F. Funiciello, D. Giardini, and F. P. Lucente, Episodic backarc extension during restricted mantle convection in the central Mediterranean, Earth Planet. Sci. Lett., 187, , 2001b. Fisher, K. M., and D. A. Wiens, The depth distribution of mantle anisotropy beneath the Tonga subduction zones, Earth Planet. Sci. Lett., 142, , Fouch, M. J., and K. M. Fisher, Mantle anisotropy beneath northwest Pacific subduction zones, J. Geophys. Res., 101, 15,987 16,002, Gueguen, E., C. Doglioni, and M. Fernandez, On the post-25 Ma geodynamic evolution of the western Mediterranean, Tectonophysics, 298, , Gvirtzman, Z., and A. Nur, Residual topography, lithospheric structure and sunken slabs in the central Mediterranean, Earth Planet. Sci. Lett., 187, , Hartog, R., and S. Y. Schwartz, Subduction-induced strain in the upper mantle east of the Mendocino triple junction, California, J. Geophys. Res., 105, , Hartog, R., and S. Y. Schwartz, Depth-dependent mantle anisotropy below the San Andreas Fault system; apparent splitting parameters and waveforms, J. Geophys. Res., 106, , Huang, W. C., et al., Seismic polarization anisotropy beneath the central Tibetan Plateau, J. Geophys. Res., 105, 27,979 27,989, Ismaïl, W. B., and D. Mainprice, An olivine fabric database; an overview of upper mantle fabrics and seismic anisotropy, Techtonophysics, 296, , Jolivet, L., and C. Faccenna, Mediterranean extension and the Africa- Eurasia collision, Tectonics, 19, , Jung, H., and S. Karato, Water induced fabric transitions in olivine, Science, 293, , Kaminski, É., The influence of water on the development of lattice preferred orientation in olivine aggregates, Geophys.Res.Lett., 29(12), 1576, doi: /2002gl014710, Levin, V., W. Menke, and J. Park, Shear-wave splitting in Appalachians and Urals: A case for multilayered anisotropy, J. Geophys. Res., 104, 17,975 17,994, Levin, V., W. Menke, and J. Park, No regional anisotropic domains in the northeastern U.S. Appalachians, J. Geophys. Res., 105, 19,029 19,042, Lucente, F. P., and F. Speranza, Belt bending driven by lateral bending of subducting lithospheric slab: Geophysical evidences from the northern Apennines (Italy), Tectonophysics, 337, 53 64, Lucente, F. P., C. Chiarabba, G. B. Cimini, and D. Giardini, Tomographic constraints on the geodynamic evolution of the Italian region, J. Geophys. Res., 104, 20,307 20,327, Mainprice, D., and P. G. Silver, Interpretation of SKS-waves using samples from the subcontinental lithosphere, Phys. Earth Planet. Inter., 78, , Mainprice, D., G. Barruol, and W. B. Ismaïl, The seismic anisotropy of the Earth s mantle: From single crystal to polycrystal, in Earth s Deep Interior: Mineral Physics and Tomography From the Atomic to the Global Scale, Geophys. Monogr. Ser., vol. 117, edited by S. Karato et al., pp , AGU, Washington, D. C., Malinverno, A., and W. B. F. Ryan, Extension in the Tyrrhenian sea and shortening in the Apennines as results of arc migration driven by sinking of the lithosphere, Tectonics, 5, , Margheriti, L., C. Nostro, M. Cocco, and A. Amato, Seismic anisotropy beneath the Northern Apennines (Italy) and its tectonic implications, Geophys. Res. Lett., 23, , Marson-Pidgeon, K., and M. K. Savage, Frequency dependent anisotropy in Wellington, New Zealand, Geophys. Res. Lett., 24, , Mazza, S., A. Morelli, and E. Boschi, Near real-time data collection and processing at MedNet, Eos Trans. AGU, 79(45), Fall Meet. Suppl., Abstract S72B-15, McKenzie, D., Active tectonics of the Mediterranean region, Geophys. J. R. Astron. Soc., 30, , Mele, G., A. Rovelli, D. Seber, T. M. Hearn, and M. Barazangi, Compressional velocity structure and anisotropy in the uppermost mantle beneath Italy and surrounding regions, J. Geophys. Res., 103, 12,529 12,543, Montagner, J. P., and L. Guillot, Seismic anisotropy in the Earth s mantle, in Problems in Geophysics for the New Millennium, edited by E. Boschi, G. Ekström, and A. Morelli, pp , Compositori, Bologna, Italy, Mulargia, F., and S. Tinti, Seismic sample areas defined from incomplete catalogues: An application to the Italian territory, Phys. Earth Planet. Inter., 40, , Nicolas, A., and N. I. Christensen, Formation of anisotropy in upper mantle peridotites A review, in Composition, Structure and Dynamics of the Lithosphere-Asthenosphere System, Geodyn. Ser., vol. 16, edited by K. Fuchs and C. Froidevaux, pp , AGU, Washington, D. C., Piromallo, C., and A. Morelli, P wave tomography of the mantle under the Alpine-Mediterranean area, J. Geophys. Res., 108(B2), 2099, doi: / 2002JB001757, Rebaï, S., H. Philip, and A. Taboada, Modern tectonic stress field in the Mediterranean region: Evidence for variations in stress directions at different scales, Geophys. J. Int., 110, , Ribe, N. M., and Y. Yu, A theory for plastic deformation and textural evolution of olivine polycrystals, J. Geophys. Res., 96, , 1991.

17 MARGHERITI ET AL.: SKS SPLITTING IN APENNINES SUBDUCTION SYSTEM ESE Rumpker, G., A. Tommasi, and J. M. Kendall, Numerical simulations of depth dependent anisotropy and frequency-dependent wave propagation effects, J. Geophys. Res., 104, 23,141 23,153, Russo, R. M., and P. G. Silver, Trench-parallel flow beneath the Nazca Plate from seismic anisotropy, Science, 263, , Sandvol, E. A., and J. F. Ni, Deep azimuthal seismic anisotropy in the southern Kurile and Japan subduction zones, J. Geophys. Res., 102, , Savage, M. K., Seismic anisotropy and mantle deformation: What have we learned from shear wave splitting?, Rev. Geophys., 37, , Savage, M. K., and A. F. Sheehan, Seismic anisotropy and mantle flow from the Great Basin to the Great Plains, western United States, J. Geophys. Res., 105, 13,715 13,734, Séranne, M., The Gulf of Lion continental margin (NW Mediterranean) revisited by IBS: An overview, in The Mediterranean Basins: Tertiary Extension Within the Alpine Orogen, edited by B. Durand et al., Geol. Soc. Spec. Publ., 156, 15 36, Silver, P. G., Seismic anisotropy beneath the continents: Probing the depths of geology, Annu. Rev. Earth Planet. Sci., 24, , Silver, P. G., and W. W. Chan, Implications for continental structure and evolution from seismic anisotropy, Nature, 335, 34 39, Silver, P. G., and W. W. Chan, Shear wave splitting and subcontinental mantle deformation, J. Geophys. Res., 96, 16,429 16,454, Silver, P. G., and M. K. Savage, The interpretation of shear-wave splitting parameters in the presence of two anisotropic layers, Geophys. J. Int., 119, , Spakman, W., S. Van der Lee, and R. Van der Hilst, Travel-time tomography of the European-Mediterranean mantle down to 1400 km, Phys. Earth Planet. Inter., 79, 3 74, Tommasi, A., Forward modeling of the development of seismic anisotropy in the upper mantle, Earth Planet. Sci., 160, 1 13, van der Lee, S., et al., New seismographic data from the Eurasia-Africa plate boundary region, Eos Trans. AGU, 82(51), 637, , Vinnik, L. P., V. Farra, and B. Romanowicz, Azimuthal anisotropy in the Earth from observations of SKS at Geoscope and NARS broadband stations, Bull. Seismol. Soc. Am., 79, , Wessel, P., and W. H. F. Smith, New version of the Generic Mapping Tools released, Eos Trans. AGU, 79, 579, Westaway, R., Present-day kinematics of the plate boundary zone between Africa and Europe, from the Azores to the Aegean, Earth Planet. Sci. Lett., 96, , Wortel, M. J. R., and W. Spakman, Structure and dynamics of subducted lithosphere in the Mediterranean region, Proc. K. Ned. Akad. Wet, 95(3), , Wortel, M. J. R., and W. Spakman, Subduction and slab detachment in the Mediterranean-Carpathian region, Science, 290, , F. P. Lucente, L. Margheriti, and S. Pondrelli, Istituto Nazionale di Geofisica e Vulcanologia, via di Vigna Murata 605, Rome, Italy. (margheriti@ingv.it)

18 MARGHERITI ET AL.: SKS SPLITTING IN APENNINES SUBDUCTION SYSTEM Figure 4. Compilation of SKS wave splitting measurements superimposed on a shaded relief map of Italy. A positive measurement is represented by a solid line oriented parallel to the fast polarization, with length proportional to the delay time. Null measurements are plotted with two small lines with directions equal to the two allowed fast directions. Each measurement is plotted at the surface projection of the 150 km depth SKS ray piercing point. Boxes indicate enlargements shown in Figures 5a and 5b. ESE 12-8

19 MARGHERITI ET AL.: SKS SPLITTING IN APENNINES SUBDUCTION SYSTEM Figure 5. (a and b) Enlargements around AQU and VSL stations of boxes in Figure 4 to better separate the large number of measurements at these stations. ESE 12-9

20 MARGHERITI ET AL.: SKS SPLITTING IN APENNINES SUBDUCTION SYSTEM Figure 8. (a) Teleseismic P wave tomography at km depth interval (white circles are the earthquakes foci at these depths, modified from Lucente et al. [1999]), with splitting measurements at 150 km depth rays piercing point superimposed. The white square dots region enhances the boundary between the Tyrrhenian and the Apenninic-Adriatic anisotropic domains. (b) Sections across the tomographic model showing the steeply dipping slab and its subhorizontal portion laying on the upper/ lower mantle boundary. Intermediate and deep earthquakes within 30 km of each cross section are also displayed (white circles). ESE 12-13