Vs ratio beneath the Italian peninsula from local earthquake tomography

Tectonophysics 465 (2009) 1–23

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Tectonophysics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t e c t o

P wave seismic velocity and Vp/Vs ratio beneath the Italian peninsula from local earthquake tomography Davide Scafidi a,⁎, Stefano Solarino b, Claudio Eva a a b

DipTeRis Dipartimento per lo studio del Territorio e delle sue Risorse, Università di Genova, Viale Benedetto XV, 5, 16132 Genova, Italy Istituto Nazionale di Geofisica e Vulcanologia, CNT (Centro Nazionale Terremoti), c/o DipTeRis, Viale Benedetto XV, 5, 16132 Genova, Italy

a r t i c l e

i n f o

Article history: Received 17 September 2007 Received in revised form 31 March 2008 Accepted 11 July 2008 Available online 23 July 2008 Keywords: Seismic tomography Lithospheric structure Vp/Vs ratio Italian peninsula

a b s t r a c t We investigate the P wave velocity structure and the Vp/Vs ratio beneath the Italian peninsula down to 100 and 60 km depth respectively by seismic travel time tomography. We invert data provided by the International Seismological Centre (ISC) (1997–2005), making use of some alternative strategies for the travel time approach in a well constrained and worldwide adopted code (SIMULPS). Resolution for the different layers is discussed and sensitivity analyses are performed through test inversions to explore the resolution characteristics of the model at different spatial scales. The resulting tomographic images provide a detailed sketch of the P wave anomalies, clearly showing, among the other features, the shape of the Ivrea body in the Western Alps, the upwelling of the oceanic crust in the Ligurian Sea and the slab under the Calabrian arc. They are less informative for the Vp/Vs ratio. Nevertheless, some features are very interesting and deserve further investigation like the anomalous decrease of the Vp/Vs ratio under the Ligurian Sea or the variations of the Vp/Vs ratio calculated in the first 10 km depth of the Apenninic region with respect to the lower values of the Alpine region at the same depth. The tomographic cross sections reveal a continuous superposition of two kinds of crusts (transitional over Adriatic) all along the peninsula but do not show any slab, intended as a clear, vertical downgoing high velocity material in either the northern or central Apennines. © 2008 Elsevier B.V. All rights reserved.

1. Introduction In this paper we present updated images of the deep velocity structure of the Italian peninsula (down to 100 km depth for Vp and to 60 km depth for Vp/Vs) as obtained from a tomographic experiment on a large dataset of local earthquakes. In the Italian peninsula, the convergence between the African and Eurasian plates, active since at least 65 Ma, has been accommodated by the subduction of oceanic and partly continental lithosphere (De Jonge et al., 1994) and has produced a belt of subducted material beneath the Alps, the Apennines and the Calabrian arc dominated by great complexity (Fig. 1, Bigi et al., 1992). It is evident that the description of this complexity plays a key role in understanding the geodynamic evolution of the Mediterranean area. In fact, in the Alpine region an European Moho that reaches more than 55 km depth can be distinguished from an Adriatic shallower Moho that reaches only 25–30 km depth. The Western Alps are also characterized by a strong N–S elongated positive gravity (100 mgal) (Klingele et al., 1992), high Vp anomaly (Scafidi et al., 2006) due to the presence of the so called “Ivrea body”. There are yet different structural interpretations about this area that has been explained or as a ⁎ Corresponding author. Tel.: +39 010 3538098. E-mail address: scafi[email protected] (D. Scafidi). 0040-1951/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2008.07.013

mantle wedge inserted into the European lithosphere (Nicolas et al., 1990), or as a thickening of the European lower crust (Schmid and Kissling, 2000). In the Apenninic region there are even three different types of Moho: the Adriatic plate, a new Moho forming with the opening of the Tyrrhenian basin, and the Moho inherited in the hangingwall European plate, that is now stretched and abandoned westward underneath Sardinia (Locardi and Nicolich, 1988; Nicolich, 2001). Although many geological, geophysical and geodynamical studies have been conducted on the Italian area, there are still open questions, and different models have been proposed to explain the present-day structural setting. Some of the most debated questions are the presence or not of continuous subduction under the Apennines, and the presence or not of a slab detachment in the northern or in the central part of the Apenninic chain. Different geodynamical models have also been proposed for the Tyrrhenian area considering it as an active (Faccenna et al., 1996) or as a passive margin (Lavecchia et al., 2003; Scalera, 2005). Unfortunately, subduction — zone processes and their control on surface deformation are only poorly understood, largely because few direct methods are available for observation of such processes (Royden et al., 1987). Seismic tomography has developed in the last two decades to map the three-dimensional (3-D) heterogeneous velocity structure of the Earth's interior. Through tomography on a regional and global

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Fig. 1. Tectonic sketch of the Italian peninsula (Bigi et al., 1992).

scale, seismology is able to provide useful information about geodynamics and tectonics. Indeed, tomography yields more realistic estimates of the amount of subducted lithosphere than seismicity alone can do, it detects possible upwellings of hot (low velocity) material from different depths in the mantle, images roots of orogens and allows one to argue about the style of convection. In recent years, images of the subducting plate in the upper mantle of Italy have been provided by numerous tomographic studies (Spakman et al., 1993; Solarino et al., 1997, Di Stefano et al., 1999; Piromallo and Morelli, 1997, 2003; Waldhauser et al., 2002). Each of these papers was able to provide details of the structure of portions of the Italian peninsula, with variable resolution from fair to good. By merging all the information a very large part of the properties of the structure is now known, at least for what concerns the P seismic velocities and the upper layers. Unfortunately, since the experiments have been conducted under different conditions and using different data, the relationship between the various “pieces” and a comprehensive interpretation are not easy to achieve. In this paper we try to take a further step by introducing overall Vp and Vp/Vs models and by imaging the subcrustal structure using local

earthquake tomography. As known, lithospheric structures are the goal of teleseismic and surface-wave tomography (Kissling, 1988); however these techniques have a resolving power limited to rather large structures and in most cases do not allow the introduction of S-wave model. On the other hand, local earthquake tomography is indeed able to provide P and S detailed models but only on limited volumes, the thickness of which is limited to the deeper earthquakes comprised in the area under investigation. In our study, the use of an improved methodology for the direct problem within the tomographic process allows us to enlarge the area under study to the whole peninsula, to investigate deeper layers and to provide the details that local tomography can offer, all in one single experiment. We first describe the data and the method, we then estimate the ability of our experiment to image structures with tests, and we finally illustrate the tomographic model with horizontal maps at different depths and vertical cross sections sliced through selected areas. We provide a discussion of our results and a comparative analysis both on large and small scales, however, a detailed interpretation of the tomographic model in terms of tectonics and geodynamics is beyond the scope of this paper and will be given elsewhere.

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2. Data and tomography setup The inverse problem for both Vp and Vp/Vs ratio is approached via the widely used code “SIMULPS” (Thurber, 1993), a software designed to linearize and solve the coupled velocity-hypocentre problem in a three-dimensional model subdivided into layers and nodes. The direct problem is solved through appropriate ray tracing in the study volume. In particular, the implementation (Haslinger, 1998; Haslinger et al., 1999) of the accurate shooting ray-tracer algorithm proposed by Virieux and Farra (1991) permits one to account for source–receiver distances much greater than the classical bending and pseudo-bending methodologies without introducing large errors. As a consequence, longer ray paths provide information for deeper layers and ensure a better cross-firing throughout the inverted area. This study was conducted in a manner analogous to Thurber's (1983, 1993) and Eberhart-Phillips's (1990) original studies, so we refer to their papers for details on the tomographic method and the reasoning behind the choice of input parameters. Table 1 lists the most relevant variables used to rule the inversion. Regional and local seismic networks monitor the seismic activity of the Italian peninsula. Many of them exchange data on a regular basis. In some cases they share seismic waveforms, and later do their own pickings, in certain others they directly exchange phase pickings. Not always is this cooperation safe from blunders in data treatment: as an example, it is very difficult for a data user to always be up to date with the station book or the instrument parameters (Solarino et al., 1997). The amount of data collected per day and the interest for limited regions only makes merging on a national scale rather difficult. Moreover, data collected from temporary networks are seldom exchanged. Fortunately, most of the seismic networks operating in Italy contribute to the ISC (International Seismological Centre) (2001) database. The work done by seismologists at ISC is fundamental for the homogenization of seismic data. ISC indeed regularly collects, merges and processes data from networks spread all over the world. Data are revised, merged and assigned a location (or relocated): this is sufficient to clean the database of the most obvious errors and to ensure a fairly complete data merging, with of course some delay with respect to the occurrence of the earthquakes (on average, the database is two years behind current time). When using pre-compiled databases, the user assumes that all data have similar quality and reliability, the cleaning of “bad data” already having been carried out. Nevertheless, the quality of the dataset is unknown and some kind of evaluation is necessary. Spakman et al. (1993) based several tomographic experiments on the ISC database. Roehm et al. (2000) quantified not only the amount of error that biases

Table 1 Details of data input and values of inversion parameters used in the tomographic experiment Earthquakes Seismic station P phases S phases Inversion grid nodes P waves damping S waves damping Eigtol Distance full weighting Distance zero weighting Number of inversion iterations Improvement of weighted- RMS (from 1st to 8th run)

1507 251 48,053 17,011 28 × 28 × 12 100 200 0.080 0–200 km 700 km 8 77%

P and S dampings are the values used in the damped least square routine for velocity computation. Eigtol is the cut-off for the SVD of the earthquake location routine. Weightings (full and zero) are the limits of the linear decrease for data weighting. In particular, up to receiver–source distance the data are fully used, while from 200 to the input data are progressively down-weighted to zero. Finally, 77% is the overall improvement in the earthquake location in terms of RMS.

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the data published in the ISC bulletin, but also their effects on global tomographic models. They found that the errors that affect the ISC data blur the tomographic model to some degree but do not change the overall amplitude or shape of seismic anomalies. Although their tests were conducted on a different subset of data, it is plausible to assume that similar conditions affect the whole ISC database. However, a further selection of the available data has been done, mainly for quality enhancement, but not only. In fact, in a tomography experiment, the number of data to be used must be restricted to avoid computational burden and to reduce computing time (Michelini and Lomax, unpublished manuscript). Not only must the data be as far as possible representative of the entire volume but also, according to Kissling (1988), it must be ensured that data are spread homogeneously in the volume to be investigated in order to avoid preferential directions of ray path, which would overload the tomographic problem without adding any new information. Finally, since the deeper volume that can be investigated in a local tomography experiment is strictly dependent on the greatest depth of the events, as far as possible deep earthquakes must be included in the dataset. To comply with these necessities, we processed the data through a composite selection on the basis of the following criteria: – Quality: data have been subjected to a pre-selection to isolate outliers. The recognition of bad data has been conducted by plotting phase readings in a Wadati-like diagram (Fig. 2, bottom panel) and flagging the data that significantly deviated from the main trend. Then all events having at least 12 P + S pickings (for the stations comprised in the study area) have been pulled apart. Among these data, only those having a gap less than 180° have been considered for the following selection steps. – Depth: all events included in the quality selection with a depth greater than 25 km are moved to the final dataset. The choice of the threshold value 25 km has been made by visual inspection of the distribution of depth of all earthquakes: it represents the value above which the curve shows an abrupt decrease in the number of events. – Spatial homogeneity: the remaining data (events with depth in the range 0–25 km) have been selected by superimposing a grid, attributing events to each cell of the grid and selecting only a limited number of events (10) for each cell. In this way, the distribution of data is less odd, and the dependence of the ray path on a particular direction is less likely. The final dataset consists of 1507 earthquakes registered by 251 seismic stations with 48,053 P phase readings and 17,011 S phase readings. The number of “deep” events (N25 km) is about 1/3 of the total. The distribution of events (circles) after the selection is shown in Fig. 2, upper panel. The figure also shows the position of the recording stations (triangles) and the inversion nodes (crosses), the geometry of which is discussed further in the paper. A few areas (in particular the marine ones) do not host any seismic event while limited event clustering (partly enhanced by the 2D nature of the figure) characterizes some sectors. Fig. 3 shows ray coverage for P (left panel) and S (right panel) waves, the latter being coincident with the coverage that will be used for Vp/Vs ratio computation. Each line represents the surface projection of the ray path connecting the hypocentre to the station. It clearly appears that, partly due to the use of longer ray paths, it is possible to have information even in sectors where there is no seismicity and/or seismic stations: this also applies to the marine areas surrounding the peninsula. The comparison between the two panels shows that the coverage for the P rays is much more complete than for S, especially in the southern sector of the peninsula. This feature will be reflected in the final tomo images. The volume has been subdivided into 28 × 28 × 12 nodes, for an approximate spatial resolution of 40 km horizontally and of 5 to 20 km vertically. In each layer, the nodes have been assigned an initial velocity derived from a preliminary 1-D inversion running VELEST (Ellsworth,

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Fig. 2. Upper panel: distribution of earthquakes (circles), recording stations (triangles) and inversion grid nodes (crosses) used in the tomographic study. Lower panel: Wadati diagram for the selected data.

1977; Kissling, 1988) after proper adaptation (Scafidi et al., 2006). This is necessary since VELEST provides a layered model with constant layer velocities but SIMULPS requires a gradient model as input. The rearranged velocities are reported in Fig. 4. 1.75 has been chosen as the initial Vp/Vs ratio on the basis of the Wadati diagram compiled with the input selected data (Fig. 2, bottom panel). 3. Reliability and resolution tests Tomographic results are always biased by non-optimal distribution of data, uneven ray coverage and mathematical artifacts, therefore the

reliability and accuracy of inversion outcomes must be estimated before their interpretation. There are several aspects that must be taken into account to attribute validity to a result. In particular, since the success of a seismic tomography is a compromise between geometrical (ray paths), mathematical and geophysical variables (final velocities must be meaningful with respect to a real earth) they should be validated together. The reliability varies on the investigated area, so it is also essential to find a way to distinguish and flag the areas where the established requisite for reliability, or better a set of them, is achieved. A basic discussion on the reliability of tomographic results can be found in Kissling (1988) or Menke (1989). Basically, depending on the

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Fig. 3. Ray coverage for P (left) and S (right) waves. Each line is the planar projection of the ray connecting the hypocentre and the recording station.

kind of resolving scheme, any inversion problem may be accompanied by mathematical tools to estimate the quality of resolution, which is the case of SIMULPS, where a variety of a posteriori direct or indirect solution indicators allow the user to estimate quality of resolution. In fact, since the problem is solved via the damped least square method, a resolution matrix is available. The representation of the whole matrix would be very difficult (it would require one 3-D image per model

Fig. 4. Initial mono-dimensional models for Vp and Vs introduced in the tomographic inversion. The Vp/Vs ratio has a constant value of 1.75.

parameter) and a more compact way to display the information is to consider the Resolution Diagonal Elements only (RDE hereinafter). Resolution matrix, diagonal and non-diagonal elements only depend on the grid spacing, the damping and the number of model parameters: they are therefore independent of the actual values of the data. They provide quality of the tomographic inversion for a given design of the problem. They may then be used to test several initial parameter settings on a given data set. The effect of the model grid spacing and dimensioning and the distribution of the data on the imaging of structures can instead be tested with synthetic data. They can be designed on a priori geometries or on actual results: in both cases the aim is to check which parts and how many of the imposed anomalies can be restored given a 3-D grid and a station distribution. A very common test consists of the checkerboard resolution test. To make a checkerboard, positive and negative perturbations are assigned to the 3-D grid nodes which will be used for the tomographic experiment. This simple alternation of positive and negative values becomes an image which is straightforward and easy to remember. Therefore, from the simple visual analysis of results and the comparison with the original distribution one can easily understand where the resolution is poor and where it is good. The results of the checkerboard test give insights not only into which part of the model can be solved but also into the minimum and maximum size of the anomaly that can be resolved, upon repetition of the tests with different sizes of grid and nodes. In particular cases it also gives information about smearing and it helps in designing the colour scale for the definitive outcomes. Figs. 5 and 6 show the results for some layers of such tests, for Vp and Vp/Vs respectively. The main difference between the two figures is the maximum depth that can be reached: indeed, while for Vp it extends to 100 km, for Vp/Vs it is only limited to 60 km. In both cases the resolution for the upper layers is limited to the land parts only, while it interests part of the marine areas in the intermediate layers. More appropriate tests have been conducted by either recovering anomalies resulting from a preliminary tomo run (RRT, Restoring Resolution Test, Zhao et al., 1992) or anomalies a-priori imposed. The results of both tests are not reported, but their findings have been used to discriminate well solved areas. On the basis of our tests, we can state that the capacity of our data and method to distinguish between low and high-velocity anomalies is fair; the presence of only few “merged” nodes is especially noteworthy.

6 D. Scafidi et al. / Tectonophysics 465 (2009) 1–23 Fig. 5. Checkerboard test for Vp. In the left panel the synthetic imposed model is reported; in the right panels the models restored by the tomography for some layers at different depth are displayed. The dotted white lines contour the areas where RDE values are greater than 0.2, which almost coincides with the areas where the reconstruction of the synthetic model is acceptable.

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Fig. 6. Checkerboard test for Vp/Vs. The significance of the white lines is the same as in Fig. 5. 7

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Finally, it is also to be remarked that the limit of acceptable recognition of anomalies almost coincides with the RDE value of 0.2 (lines with dots in Figs. 5 and 6). In fact within this contouring lay the areas where most of the imposed velocity anomalies can be recalled. Since only the areas where quality thresholds are all satisfied may be interpreted, in the final images of the results (slices and cross sections) gray sectors show very poor ray sampling and green or white lines contour areas where all synthetic tests proved fair to good reconstruction of the imposed anomalies and the RDE value exceeds 0.2. Only these areas will be considered reliable and then taken into account in the following discussion. 4. Description and discussion of the results Tomographic images of the three-dimensional model of the Italian peninsula are reported in Figs. 7–13 in both horizontal plane views and cross sections. For convenience, the discussion of the tomographic results is subdivided in three sections for each geographical sector: Northern Italy, that comprehends the Alpine region, the Po Plain and the Ligurian Sea area; Central Italy, with the northern and central part of the Apennines, and Southern Italy with the southern part of the Apennines and the Calabrian Arc. Fig. 7 displays the plane views of Vp tomographic results for all the layers of the model. All images are in velocity percentage with respect to the average for each layer where bluish colours indicate higher velocities and red areas indicate lower velocities. Fig. 8 shows the Vp/Vs ratio distribution. It is evident that the overall resolution of the images of the Vp/Vs is poorer, being limited to the first 60 km depth, than what concerning Vp only. A significant difference between the resolution of the northern and the southern parts of the peninsula has also to be remarked. Nevertheless, at any level of detail, the knowledge of the distribution of the Vp/Vs ratio in an area may contribute to understand the rheology, the tectonic setting and the thermal status since these features are related to the velocity ratio itself. The relationship with the Poisson's ratio and with the elastic parameters of the rocks (compressibility and shear moduli) makes in fact any change in the Vp/Vs value an indirect way to assess variations of pressure, presence of water, strength and temperature. Tomographic images are often best interpretable in cross sections. For this reason we show (Fig. 9) some cross sections of the results that we believe most interesting. We accompany each Vp cross section with the equivalent plot for Vp/Vs ratio, both in absolute value. The five cross sections cut the most interesting anomalous areas evidenced in the horizontal slices to infer their characteristics with depth. Cross sections in absolute velocities give the exact value of the anomalies, therefore they are very useful for petrological investigations. Nevertheless they are less informative from the geodynamical point of view if compared to the representation in velocity percentage adopted in the plane views. For this reason we also present and analyze in the discussion Vp cross sections in velocity percentage for some of the most intriguing areas of the Italian peninsula, like the Western and the Eastern Alps (Fig. 10) and the Calabrian Arc in the Southern Apennines (Fig. 13). The same graphical representation has also been used for a series of cross sections parallel (Fig. 11) and perpendicular (Fig 12) to the peninsula to get a view of the structural setting of the whole area. In particular, Fig. 11 shows comprehensive sections crossing the peninsula from north to south with the advantage of being contemporary almost perpendicular to both the Western Alps and the Calabrian arc and parallel to the Apenninic chain. The panel on the upper right side of the figure shows the map of the

crustal types as proposed by Cassinis et al. (2005). In this inlay, the European Moho is blue, the Adriatic-African Moho is yellow, the transition crust is green and the oceanic crust is pink. The thick line with dashes represents the thrust fronts. The overall comment is that on the single section the alternation of the different types of crust is very clear. Moreover, the subduction of the European Moho under the Adriatic in the upper part of the sections is clearly evident, especially in the central ones, and has a vergence opposite with respect to the subduction of the Calabrian arc. Earthquakes are deepening under the thrust and of course along the Calabrian slab. Fig. 12 shows cross sections with anti-Apenninic direction. Because of their orientation they are particularly suitable for imaging the deep structure of the northern–central Apennines, as discussed in the following. 4.1. Northern Italy An overview of the plane view images for this area (Figs. 7 and 8) reveals that some anomalies of relevant size extending to depth and visible across several layers are present. The most extended anomaly is the red, low velocity area corresponding to the roots of the Alps. It is visible from 25 to 40 km depth and accounts for up to 10–12% negative values. The second anomaly which we focus on is the high velocity anomaly corresponding to the Ivrea body. The anomaly extends circa N–S down to 40 km depth, and it seems to split into two parts at 35 km depth. More details can be inferred from cross section 1 of Fig. 9, that cuts the western Alpine chain and the Ivrea body and reaches in its eastern limit the Po Plain. From west to east, the section shows the subduction of the European lower crust and the upwelling under the Ivrea area. The absolute velocities reveal that the Ivrea body is characterized by lower crust material or impoverished mantle (it has a velocity of 7.5 to 7.7 km/s), as also confirmed by a local study (Scafidi et al., 2006), instead of mantle material as sometimes suggested. It is also possible that the body was originally made of mantle material that lost its characteristics due to the emplacement. The Ivrea anomaly vanishes in the eastern side of the cross section where the extended and deep (down to 20 km depth) low velocity corresponding to the sedimentary materials of the Po Plain is imaged. For what concerns the Vp/Vs ratio, the Ivrea body is characterized by low values, especially in the first 5 km depth and around 30 km depth where it is as low as 1.68, conversely to what happens in the surrounding regions where values higher than 1.75 are observed. Medium with high velocity and low Vp/Vs is relatively hard, it has strong shear resistance and it is favorable to stress accumulation (Liu et al., 2003). In our interpretation, the Ivrea body is a “hard, compact and dense” piece of lower crust or impoverished mantle. These characteristics would be then confirmed by its velocity, ratio values and gravity. Conversely, the surrounding materials (which have low Vp and high Vp/Vs ratio) are partly involved in the plate collision, and they can be referred to as relatively “soft” medium, which implies rocks being prone to deformation, rupture, multi fissure and local melting. In fact these are characteristics expected for rocks involved in a collision to partly accommodate the stress. To make a comparison between our results and already existing models, tomographic cross-sections coincident with the ECORS-CROP (Roure et al., 1996) and TRANSALP (Transalp Working Group, 2001, 2002) profiles are drawn (Fig. 10) and aligned with the most recent tectonic sketch for each profile (Schmid et al., 2004). Although the features shown in the tectonic sketches have been obtained with other

Fig. 7. Results of 3-D tomography for P wave velocities reported in horizontal slices at different depth. Values are in percentage velocity with respect to the average value of each slice. Grey sectors show not sampled areas, the hit count being lower than 20 hit rays. Green contouring is based on the combination of synthetic tests and RDE values (N 0.2) and it indicates areas of reliable tomographic results.

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Fig. 8. Results as in Fig. 7 for Vp/Vs ratio.

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Fig. 9. Tomographic cross-sections along the Italian peninsula. For each section the topographic profile, the Vp values in absolute velocities (Km/sec) and the Vp/Vs ratio are reported. Grey areas delimit areas with insufficient hit count. White lines contour well resolved areas. Some structural elements are also indicated.

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Fig. 10. Tomographic cross sections along the ECORS-CROP and TRANSALP directions (reprinted from Schmid et al., 2004). The P velocities are expressed as a percentage with respect to the average, the Vp/Vs are in absolute values.

geophysical techniques, several similarities with our tomographic results are clearly evident. For example, the shape, thickness and geometry of the Ivrea body or of the crusts involved in the subduction are very much alike.

The comparison between the western and eastern section reveals that to the east of Trento (TR in figure) the vergence of the subduction changes and the Adriatic Moho is subducting under the European one. Our tomographic results thus confirm the findings of Lippitsch et al.

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Fig. 11. A series of cross sections along the Italian peninsula, from west to east. All sections are NW–SE oriented. The inlay on the right side show the position of the cross-sections with respect to the map of crustal types proposed by Cassinis et al. (2005). 1) European crust; 2) Adriatic crust; 3) Styrian and Pannonian basins; 4) Ligurian, Tuscan–Perityrrhenian transitional crust; 5) Oceanic–suboceanic crust; 6) Over-thrusting fronts of the Moho boundary: the Adriatic over the European plate (Alpine range); Ligurian, Tuscan, Perityrrhenian transitional crust over the Adriatic–African plate (Apennines range); of the Ligurian–Tuscan over the European (Corsica); 7) Fragmentation lines in the upper mantle; 8) Moho depth contour lines (km); 9) Moho depth contour lines (subducted). Blue small squares in the map indicate Italian cities with the relative abbreviated names reported also in cross sections.

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Fig. 12. A series of cross sections of anti-Apenninic direction. For the significance of the inlay see Fig. 11.

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Fig. 13. Tomographic cross-sections in velocity percentage underneath the Calabrian arc.

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(2003), obtained with high resolution teleseismic tomography. According to these authors, the origin of the different vergence must be sought in the primitive existence of a different ocean subducted towards the northeast in the eastern Alps, forcing the Adriatic continental lithosphere to subduct north–eastwards beneath the Austro-Alpine one. In correspondence with the Ligurian Sea another high P waves velocity anomaly extending down to 60 km depth, the size of which dramatically reduces below 35 km depth, is visible. The western part of the top two sections of Fig. 12 shows the upwelling of the mantle in the Ligurian Sea (as foreseen from the Moho map) in correspondence with the oceanic crust (pink sectors of the inlay of Fig. 12). The anomalous area is characterized by small Vp/Vs values (4–6% under the average) in the first 35 km depth. The Ligurian Sea experiences heat flux (of the order of 80 to 100 mW/m2, (Cermak et al., 1992)) above the average of the peninsula, therefore there must be some other conditions than the thermal status to justify such an anomaly. According to Zhang et al. (2004), a Vp/Vs ratio of the same order and at a similar depth for Japan may be a consequence of petrological– petrophysical changes. In particular, the presence of water and dehydrated minerals could justify the low values found in this 35 km thick oceanic crust. Nevertheless, a more complete discussion of this topic requires knowledge of the P, T conditions and is beyond the scope of this paper. Finally, a minor high-velocity zone situated at the north-eastern border of Italy and confined within the Adriatic Sea below 20 km depth has to be remarked. 4.2. Central Italy The main general feature of this area can already be inferred from the analysis of the Vp/Vs plane views. In fact the shallower layers (0–15 km depth) show high values (blue areas, reaching up to 7–8% positive values) for the Apenninic chain in opposition to the smaller Vp/Vs values of the Alpine chain. The divergence highlights the different natures of the two chains. In particular, in a grossly analysis, such differences in the Vp/Vs ratio could derive from the different “hardness” and stability of an older chain (the Alps, 65 Ma) with respect to the very recent Apenninic chain (20 Ma). Another very clear feature is the presence of a low velocity anomaly, extending all over the peninsula, corresponding to the roots of the Apennines. As for the Alps, the anomaly is distinguishable from 25 km depth, but its geometry is clearly evident in the layer at 35 km depth. It is worth noting that, although located at the eastern border of the inversion grid and thus at the limit of the well resolved area, a similar anomaly coincident with the Dinarides is visible. This chain has Vp/Vs values very close to those computed for the Apennines in the shallower layers (down to 15 km depth). Many features can be inferred from the analysis of tomographic cross-sections. Sections 2 and 3 in Fig. 9 look very intriguing. Section 2 crosses the Northern Apennines and it clearly shows in its western part the switch from oceanic to continental crust (where blue colours deepen from 20 to 40 km depth). Section 3 shows the upwelling of high velocity material under the Tyrrhenian Sea, consequence of the recent ocean spreading. The central-eastern part of the section shows a gentle deepening of high velocity material towards the west overlapped by relatively low velocity material in the shallower layers. The deepening block is cut by a thin, vertical upwelling of mantle material the eastern border of which is interested by earthquakes. No earthquakes are instead observed in the western flank. The reconstruction of the shape of both the asthenospherical upwelling and the deepening body is limited by the resolution at about 80 km depth. However, in the hypothesis of a body split by the intervention of mantle material, we should assume that the uprising material is more recent than the descending one. The cross sections displayed in Fig. 12 are useful for comprehensive considerations on the Apenninic area as well. The shallow part of the

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first three sections is characterized by the transitional crust (green sectors in the inlay, red areas in the sections) overlying the faster Adriatic crust (yellow areas in the inlay, blue sectors in the sections). The downgoing part of the Adriatic crust under the transitional crust shows a very different character to what expected for a slab. First, the dip of the anomaly has a very low angle: in fact no real vertical anomalies are seen. Second, it is not bordered by seismicity, which is limited to the upper 40 to 60 km in any case. This clearly confirms that the subduction style in this part of the peninsula is very different from the southern part of it, where a subvertical slab is evident as described in the following. The absence of a continuous, high velocity body beneath the Apennines has been interpreted by some researchers (Wortel and Spakman, 2000) as an evidence of the detachment of the Apenninic slab. According to this view the Apenninic slab is expected to be inactive whether the Ionian lithosphere subducting underneath Calabria is considered to be on the verge of detaching or just detached. Other researchers (Guegen et al., 1998), however, suggest that a fairly continuous and fast slab exists beneath the Apennines and the Calabrian arc. The hypothesis based on tomography studies of a lateral interruption (slab window) of the subducting slab beneath the southern Apennines and the existence of two main arcs of subduction (northern Apenninic and Calabrian arc) (Lucente et al., 1999) is not supported by alternative tomographic experiments (De Gori et al., 2001), which show the existence of a slab beneath the southern Apennines, favoring the continuous slab model. Both continuous and slab-window scenarios, however, imply the existence of a slab beneath the northern Apennines, in apparent contrast with the absence of subcrustal seismicity. Carminati et al. (2002) showed that different rheological behaviours of the continental versus oceanic lithosphere can account for the shallower and minor subcrustal seismicity below the northern Apennines with respect to deeper and more frequent and intense seismicity below the Calabrian arc. In particular, the low seismicity or aseismic behaviour of orogenic roots or slabs may in some cases be ascribed to a ductile deformation of quartz-feldspar rich subducting continental lithosphere rather than to the absence of active subduction. In practice the kind of seismicity may depend on reasons different from the subduction, and should not be considered evidence for the absence of it. 4.3. Southern Italy The major structural feature of this sector is the presence, under the Calabrian arc, of a well pronounced subducting slab. It appears (in particular in Fig. 13) as a high velocity steep anomaly extending from 30 to 150 km depth. Similar images obtained from teleseimic tomography (Wortel and Spakman, 2000) or seismicity studies not only confirm the geometry and dip (about 65°–70°) of the slab but also add information on the continuation of the anomaly below the area of reliability of our tomography down to 600 km depth. In our images, the slab is bordered by deep seismicity especially concentrated where the Vp/Vs ratio is greater, that is in the Tyrrhenian side. This is the area where the surface heat flux pattern shows maxima (Pasquale et al., 1999). Unfortunately, the depth of the resolution for the Vp/Vs images is limited only to the upper crust, but it is plausible to believe that a similar trend characterizes the entire subducting slab. Two more areas, visible in Section 4 of Fig. 9, deserve a mention. They both are depressions of low velocity material (green areas in figure) underneath the Gargano promontory and the Gulf of Naples (NA in figure). The latter anomaly seems to be split into two parts, the easternmost of which extends underneath Campobasso (CB in figure). The deep structure under this cross-section recalls that of Section 3, the differences between the two being partly explained by their different orientations.

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5. Conclusions

References

More than 1500 seismic events, recorded by the seismic stations contributing to the ISC database in the period 1997–2005, have been used for tomographic inversion beneath the Italian peninsula. According to the authors who have previously made use of similar data compilations, the errors associated to the data do not compromise the overall amplitude or shape of seismic anomalies potentially revealed by tomography. The data set used is the result of a selection aimed at reducing the problems of ray coverage (especially in marine areas) possibly arising from the uneven distribution of seismic events and recording stations and from the absence of OBS (Ocean Bottom Seismometers). Unfortunately, neither an improved procedure designed to avoid data clustering nor the use of an improved ray tracer could completely take care of the initial bias of the data distribution. Nevertheless, most sectors out of the large inverting area proved to be of a quality high enough to make a preliminary comment on the distribution of anomalies for the P wave velocity and the Vp/Vs ratio. The outcomes of the tomographic experiment summarized in this paper have confirmed the complexity of the structure under the Italian peninsula. Our tomographic images provide reconstructions of the crust and upper mantle that are more detailed than the tomographies quoted in the text. The results of these experiments, being based on teleseismic data, indeed able to investigate much thicker volumes with a limited lateral resolution (Kissling, 1988), cannot be directly compared. Nevertheless, it is necessary to underline the capability of our inversion problem to well define a slab down to the inferior limit of the model (150 km) even if with a very variable level of resolution. In fact the geometry of the slab underneath the Calabrian arc is described in details and the reconstruction is in agreement with what proposed in other studies. Since a similar feature is not visible in any other part of the peninsula, and provided that the method and data used have the potential to reveal such a structure, either there is no similar slab continuous through depth all along the peninsula except under the Calabrian Arc or it begins at a depth greater than what our investigation spans. However, we must point out that the resolution power of our images is in theory limited by the input data to 150 km depth, and this limit is shallower in some places (on average it does not exceed 100 km). This may prevent our results from picturing any vertical trend that starts under the resolved depth. To summarize, we favour the hypothesis that the subduction under the Apennines is not continuous. We do not see any slab in the northern–central Apennines in the first 100 km depth. The downgoing material (Adriatic plate) of this area has a rather low dip angle, as also partly shown by the distribution of the (few) deep seismic events. Along the Central and also the northern part of the Apennines there are more overlapping geometries than subducting geometries. It is confirmed that the Calabrian arc has instead a very pronounced slab, visible even at shallow depth, and that the subduction in the Alps changes vergence from the western to the eastern part of the chain, around 11° longitude. From the analysis of the velocity ratio distribution, it is pointed out that there are marked differences between the Alpine and the Apenninic areas especially in the first 15 km depth, where the Apennines have higher Vp/Vs values. Finally, anomalous low Vp/Vs values have been calculated under the Ligurian Sea in the first 35 km depth.

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Acknowledgments We like to thank Tom Parsons and an anonymous reviewer for precious comments that helped to improve the paper. Developments of the current tomographic studies will be available on the web page http://www.dipteris.unige.it/geofisica/tomo.html, where interested scientists may download cross-sections and plane views. Quotation of this paper upon use of the electronic material is welcome. Many plots are based on GMT (Wessel and Smith, 1998).

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