Vol49_2_2006 739 ANNALS OF GEOPHYSICS, VOL. 49, N. 2/3, April/June 2006 Key words earthquake location – ocean bottom seismometers and hydrophones – seismic networks – synthetic tests – Sicily 1. Introduction The Southern Tyrrhenian region (fig. 1) is characterised by a fairly rapid transition from a nearly oceanic crustal structure beneath the abyssal plain to a continental-like structure be- neath Sicily and Calabria. Fault discontinuities have been mapped on grounds of geological and geophysical investigations: i) major NW- trending fault systems potentially capable to generate magnitude ≥ 6 earthquakes cross the lithosphere underneath the southernmost Tyr- Seismic location improvements from an OBS/H temporary network in Southern Tyrrhenian Sea Graziella Barberi (1), Laura Beranzoli (2), Paolo Favali (2) (3), Giancarlo Neri (4) and Tiziana Sgroi (2) (1) Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Catania, Italy (2) Istituto Nazionale di Geofisica e Vulcanologia, Roma, Italy (3) Università degli Studi di Chieti «G. D’Annunzio», Chieti Scalo, Italy (4) Dipartimento di Scienze della Terra, Università di Messina, Italy Abstract We present the first investigation performed on the seismicity of Southern Tyrrhenian Sea, off-shore Sicily with the contribution of data from broad-band ocean bottom seismometers and hydrophones (OBS/H). Off-shore da- ta were recorded during the TYrrhenian Deep-sea Experiment (TYDE) from December 2000 to May 2001 in the Southern Tyrrhenian Sea. Hypocenter locations of a cluster of 53 seismic events which occurred in March 2001 in north-eastern Sicily were estimated by the integration of land (permanent network) and off-shore (temporary network) data and compared with locations estimated from land data only. The scatter of the cluster was evalu- ated by dispersion parameters. The off-shore data significantly reduced the scatter of the swarm hypocenters al- so restricting the depth range of the cluster. Moreover, space trends of the event distribution originally shown by the land data were only partially confirmed by the land-sea joint data. In order to assess the efficiency of a land- sea integrated network with respect to a land-based network in terms of hypocenter mislocations in the subject area, we performed simulations by assuming a grid distribution of earthquakes and a recent local 3D velocity model, computing synthetic arrival times of body waves to the stations of both network configurations (integrat- ed and land-based) perturbing the computed times and relocating earthquakes by inversion. The results of the synthetic tests demonstrated that the presence of sea bottom stations in the Tyrrhenian Basin can reduce the mis- locations of large magnitude and/or superficial earthquakes in the southernmost Calabria and Messina Strait and of low magnitude and/or deep earthquakes in north-eastern Sicily. The accuracy of synthetic earthquake loca- tions obtained including OBS/H data provides additional support to the interpretation of the cluster occurred in March 2001 and the opportunity of long-term installation of an off-shore network like TYDE in the study region. Mailing address: Dr. Graziella Barberi, Istituto Nazio- nale di Geofisica e Vulcanologia, Sezione di Catania, Piaz- za Roma 2, 95125 Catania, Italy; e-mail: barberi@ct.ingv.it 740 Graziella Barberi, Laura Beranzoli, Paolo Favali, Giancarlo Neri and Tiziana Sgroi rhenian Sea (Finetti and Del Ben, 1986; Neri et al., 2003); ii) NNE- to N-trending normal faults are responsible for earthquakes of magni- tude up to 7 in the Messina Strait and Southern Calabria (Boschi et al., 1997); iii) E-trending re- verse faults produce events of magnitude up to 6 seismic in Central and Western Sicily (Ben-Avra- ham and Grasso, 1990, 1991; Neri et al., 2003). These fault systems cross the crust underneath off-shore areas, and this emphasises the impor- tance of off-shore seismometry in the region. An Ionian lithospheric slab subducting to WNW beneath the Southern Tyrrhenian Sea (fig. 1) has been suggested on the basis of distribution of intermediate and deep seismicity (e.g., Gas- parini et al., 1982; Anderson and Jackson, 1987; Giardini and Velonà, 1991; Selvaggi and Chia- rabba, 1995), high velocity revealed in the upper mantle by tomographic images (Piromallo and Morelli, 1997; Lucente et al., 1999; Cimini, 1999) and calk-alkaline volcanism of the Aeolian Is- lands (Barberi et al., 1973; Argnani and Savelli, 1999). According to many investigators (e.g., Ma- linverno and Ryan, 1986; Faccenna et al., 1996, 2001a; Hirn et al., 1997; Argnani, 2000; Nicolich et al., 2000; Doglioni et al., 2001; Gvirtzman and Nur, 2001) geological and geophysical evidence in this region can be interpreted in the framework of a geodynamic model assuming an approxi- mately NW-SE slow convergence between the African and European plates (the average motion direction of the African plate respect to the Euro- pean plate is shown in fig. 1 by black arrows) and gravity-induced south-eastward roll-back of the Fig. 1. Structural map of Southern Italy, from Meletti et al. (2000) and Neri et al. (2005). In agreement with stan- dard symbols, barbs indicate the downthrown blocks in normal faults, fault arrows show the strike-slip component of the fault mechanism, triangles indicate the sense of dipping of thrust structures. The shaded belt indicates the lo- cation of the Apennine chain in the Calabrian Arc. The arrows indicate the motion direction of Africa with respect to Europe as predicted by large-scale crustal motion models (Sella et al., 2002; Calais et al., 2003; Nocquet and Calais, 2004). NW-SE plate convergence is considered the main tectonic engine in the study region acting togeth- er with gravity-induced southeastward roll-back of the Ionian lithosphere subducting beneath the Tyrrhenian Sea (e.g., Faccenna et al., 2001b). The inset displays a map of the Italian region showing the main tectonic units and microplates identified between the Africa and Europe stable margins (Scandone and Stucchi, 1999). 741 Seismic location improvements from an OBS/H temporary network in Southern Tyrrhenian Sea Ionian subducting slab. Roll-back is widely be- lieved to have been the primary tectonic source for: i) Tyrrhenian Basin opening; ii) southeast- ward kinematics of the Southern Tyrrhenian lith- osphere, and iii) its thinning and overthrusting onto the Ionian lithosphere. The space-time evo- lution of the roll-back process would have pro- duced a clear structural differentiation between the Calabrian Arc and the marginal tectonic unit of Sicily. North-eastern Sicily experienced disas- trous earthquakes in the last few centuries, often accompanied by tsuna-mis, producing wide de- struction and hundreds of thousands of casualties (Boschi et al., 1997; Tinti et al., 2004). Active volcanoes (e.g., Mt. Etna, Aeolian Island Arc) represent additional sources of risk. Although the regional seismometric network, solely land based, has been largely improved in the last two decades both in type of sensors and distribution, the seis- micity of several sectors in the study region is still hard to locate accurately because of the presence of large marine areas. A good opportunity to obtain hypocentre lo- cations with an accuracy higher than usual was given by the deployment of a temporary network of fourteen OBS/H stations for around six months in the area between the Tyrrhenian abys- sal plain and the northern coast of Sicily (fig. 2) in the framework of the TYrrhenian Deep-sea Experiment (TYDE; Dahm et al., 2002) partially supported by the European Commission, coordi- nated by the Istituto Nazionale di Geofisica e Vul- canologia (INGV) with the cooperation of IFM- GEOMAR and the University of Hamburg. TYDE has been the most important OBS/H ex- periment, in terms of number of sensors installed and duration, ever performed in the Italian region. Two previous shorter experiments were carried out with the installation of OBS in the south-east- ern Tyrrhenian Sea off-shore Calabria (Soloviev et al. 1990) and around the Aeolian Islands (Be- ranzoli et al., 1997; Favali et al., 2004). Data recorded in the framework of TYDE allowed an unprecedented joint analysis of on-shore and off- shore seismic data in Italy (see also Sgroi et al., 2006) exploring the advantages of land-sea data integration with respect to local seismicity loca- tion. 2. Temporary and permanent networks and data The TYDE temporary OBS/H stations were installed at depths ranging from 1500 m to 3500 m on the sea bottom around the Aeolian Islands from the northern coast of Sicily to Marsili Basin. The stations were equip-ped with broad- band seismometers (PMD sensor: flat response in the range 0.025-32 Hz; SpharWebb-SCRIPPS sensors: flat response 0.02-100 Hz) and hy- drophones (E-2PD: bandwidth 0.5-50 Hz) and 21bit digitisers with sampling rate of 50 Hz. Fur- ther technical details and the description of in- stallation and recovery procedures can be found in the papers by Dahm et al. (2002) and Sgroi et al. (2006). The network operated in continuous mode and local recording from December 2000 to May 2001. We selected ten OBS/H stations, out of the fourteen deployed, the closest ones to the area under investigation. The locations of the ten temporary stations are reported in fig. 2 (full Fig. 2. Map of on-shore (empty triangles) and off- shore stations (full triangles) used in the present study. The on-shore stations belong to the local and national networks of INGV for the Mt. Etna and Ital- ian territory surveillance. The square shows the north-eastern Sicily area of the March 2001 swarm. 742 Graziella Barberi, Laura Beranzoli, Paolo Favali, Giancarlo Neri and Tiziana Sgroi triangles) together with those of the thirty-one permanent on-shore stations (empty triangles) which also provided data for this study. Four sta- tions were also temporarily installed by INGV on land (Sicily and Aeolian islands) to make the coverage of the area denser. The permanent land stations are managed by INGV in the frame of a regional network addressed to monitoring East- ern Sicily, where the Mt. Etna volcanic edifice is located, and in the frame of the Italian national seismological network. The land stations select- ed for this study are equipped with short-period three-component seismometers (eleven stations) and short-period vertical component seismome- ters (twenty stations). The signals are acquired with a sampling rate of 50 Hz (national network) and 160 Hz (regional network), and are transmit- ted via radio or phone cables from the remote stations to the acquisition centres – Catania (Si- cily) for the regional network, Rome for the na- tional one – where they are stored and analysed. 3. The seismic swarm of March 2001 During the TYDE experiment, a low-energy seismic swarm occurred from 26 to 30 March, 2001 in north-eastern Sicily (boxed area in fig. 2). This swarm comprised 53 shocks with dura- tion magnitude 1.4 ≤Md ≤2.9, and was original- ly located by means of the land network data at depths ranging approximately from 25 to 40 km. The characteristics of the events are report- ed in fig. 3; the upper panel reports the number of events per day with the indication of the maximum magnitude, and the bottom panel shows the number of events per magnitude range. The swarm started close to the locality of Santa Lucia on 26 March with six shocks and maximum magnitude 2.3. Most of the earth- quakes are concentrated on 27 and 28 March, with more than twenty shocks per day and max- imum magnitude 2.4 and 2.9 respectively. In the last two days of the swarm occurrence, the number of events abruptly decreases to a single shock per day with magnitude 1.7 (29 March) and 2.4 (30 March) to cease completely. The first step of our analysis concerns the in- tegration of the data belonging to the land-based network and the sea-bottom stations. The integra- tion requires, as usual, the correction of the data time tag as the clock signal of each OBS/H mod- ule progressively delays over time. The internal clock of each OBS/H is indeed synchronised be- fore the deployment and after the recovery and, given a linear trend of the clock drift, data can be corrected before processing. After the correction, the P and S arrival times of the swarm events are picked on the OBS/H waveforms and integrated with the readings of the land stations in a single database to be used for further steps. The final database resulted in 724 P-phase and 522 S-phase arrival times, 242 and 120 of which from sea-bot- tom stations for the two types of phases. In order to compare the location results obtainable by the ‘integrated’ data set and the ones obtainable by the land-based network, the «1D minimum veloc- ity model» and the 3D local crustal model of the study area, both estimated by Neri et al. (2002), were used to locate the events of the swarm. In Fig. 3. Upper: histogram of the number of events per day: each histogram column reports the maxi- mum magnitude of the day. Bottom: histogram of the number of events per magnitude range. 743 Seismic location improvements from an OBS/H temporary network in Southern Tyrrhenian Sea particular, VELEST (Kissling et al., 1994) and SIMULPS12 (Evans et al., 1994) algorithms were applied to carry out the locations in 1D and 3D respectively. As expected, the locations achieved with the 3D model produced a clearly better fitting of the experimental arrival times (rms=0.16s) compared to the 1D locations (rms= = 0.30s). We therefore decided to use only 3D lo- cations in the subsequent steps of the analysis and to compare the locations of the swarm events ob- tained using two different network configura- tions: configuration LS – integrated land-sea net- work; configuration L – land network only. The epicentral map and hypocenter distribu- tion with depth resulting from the location proce- dure in the 3D model are displayed in fig. 4 for both LS and L configurations. Hypocenter distri- bution obtained with configuration L shows trends, like for instance a roughly E-W distribu- tion of epicentres, which are not found when us- ing configuration LS; on the other hand, the west- ward deepening trend of the hypocenters of the swarm is confirmed in both cases. Using config- uration LS, the hypocenter volume appears main- ly restricted to depth ranging from 27 to 37 km. These values of hypocentral depths are quite un- usual in general for the Italian Peninsular region, characterised by a seismicity hardly exceeding 15-20 km. As the events are clustered both in time and space, we assumed that they were generated by a single structure or by different structures very closely linked to each other. According to this assumption, the space dispersion of the swarm can be used as an indicator of the overall quali- ty of locations. Starting from these considera- tions, we defined the dispersion factors Dxy and Dxyz of the earthquake locations around the swarm barycentre of a given dataset in the hor- izontal plane and in the 3D space (3.1) == -(LAT Dxy ( ) ( ) ) ( ) LAT LON LON D i i i N xy i N 2 2 1 2 1 + - = = = N N i / / Fig. 4. Epicentre locations and hypocentre depth distribution of the Santa Lucia swarm obtained using the 3D model by Neri et al. (2002) and the data from the integrated network (configuration LS, left panel) and the land- based network (configuration L, right panel). Triangles and crosses indicate the stations lying in the swarm area and the barycentres used for estimates of the Dxy and Dxyz dispersion factors (see eqs. (3.1) and (3.2) in Section 3). 744 Graziella Barberi, Laura Beranzoli, Paolo Favali, Giancarlo Neri and Tiziana Sgroi (3.2) with (3.3) (3.4) where i identifies the generic event in the dataset including N earthquakes, , and are the average values of epicentre latitudes LATi, epicentre longitudes LONi and focal depths DEPi. The terms LAT and LON represent linear coordinates. Figure 5a-d shows the histograms of the distances and : in the case of configuration LS (panels a and b) most of the events have distances and generally smaller than the ones estimated for configuration L (panels c and d). The estimates of Dxy and Dxyz are 3.19 km and 3.65 km in the case of network LS and 4.63 km and 6.04 km for L. This result can be consid- ered an element supporting the conclusion that off-shore stations, even far and with an asym- metric distribution around the swarm area, have reduced the uncertainty in the locations of the D xy iD xy i D xyz iD xy i DEP LONLAT ( ) ( ) ( ) D LAT LAT LON LON DEP DEP xyz i i i i 2 2 2 = - + - + + + ( ) ( )D LAT LAT LON LONxy i i 2 2= - + -i = (( ) ( ) ( ) ) ( ) D LAT LAT LON LON N DEP DEP N D xyz i i i N i xyz i N 2 2 1 2 2 1 - + - + + = + = = i / / swarm events. It is worth noting that the rms of the swarm locations is higher for network LS (0.16 s) than for network L (0.13 s). This can be explained by the fact that the inversion is better constrained by the experimental data of the net- work LS. In addition it should be remarked that the sea-bottom stations were sited in a zone of the 3D model where the spread function become larger than 4 (Neri et al., 2002), suggesting a re- duction of model reliability in the marine sector. 4. Synthetic tests for hypocenter location accuracy We performed a series of synthetic tests to assess the accuracy of the location of events with different energy release occurring in a crustal volume wider than the one involved in the March 2001 swarm both LS and L configu- rations. The area selected for the synthetic tests is shown in fig. 6 (grey rectangle) together with the sets of stations chosen to form LS and L net- work configurations. We consider a hypo-cen- ter distribution coincident with the nodes of a 3D grid extending from the Earth surface down to 35 km of depth with a horizontal and vertical node spacing of 1.5 and 5.0 km respectively. The theoretical arrival times of P and S-waves from the grid nodes to the stations of the two networks were computed with the 3D-model of Neri et al. (2002). The synthetic arrival times were then perturbed to simulate random reading errors. Based on a critical evaluation of the Fig. 5a-d. Histograms of and (see equations in Section 3) of the events of the swarm from the inte- grated network LS (panels a and b) and the land-based network L (panels c and d). Arrows indicate the average values of D. D xyz iD xy i a b c d 745 Seismic location improvements from an OBS/H temporary network in Southern Tyrrhenian Sea reading uncertainties during the investigation of March 2001 earthquakes, we assumed a stan- dard deviation of perturbations of 0.3 s. The perturbed arrival times were inverted using the same 3D velocity model (Neri et al., 2002). The locations were performed by placing the start- ing hypocenter at a distance (∆LAT = ∆LON = 6 km; ∆DEPTH = 8 km) from the true hypocenter (grid node) larger than the grid spacing. The dislocation of the initial hypocentres is compa- rable to the largest errors that we expect from routine hypocenter locations in this area. We performed different trials by changing the start- ing hypo-center locations toward N, S, E, W, NE, NW, SE, SW with ∆LAT = ∆LON = ±6 km and increasing depth of ∆DEPTH = +8 km. We found that these dislocations determine mislocations smaller than the grid spacing. Two parameters were estimated for each synthetic event (grid node): the epicentral dis- tance D and the focal depth difference H be- tween the synthetic and relocated event. Figure 7 shows the horizontal distribution at different depths of the D values obtained using the net- work configurations LS and L. The comparison between the D values computed from the two different network configurations reveals that, in spite of the uneven geometry of the sea network with respect to the area, the presence of seismo- logical stations in the Tyrrhenian Basin reduces the horizontal mislocations at all depths, not only in north-eastern Sicily but also in the southernmost Ionian Calabria, south-east of Messina Strait. This reduction appears to be significant in the depth range 25-35 km also for the latter zone where the red areas (D > 6 km) appear to be less extended. The horizontal dis- tribution of the H values for the LS and L con- figurations are displayed in fig. 8. It is worth noting that the vertical mislocation of events can be reduced in north-eastern Sicily more than in Calabria by the presence of sea-bottom stations in the Tyrrhenian Sea. This provides an additional element in support of the evaluation of the depth range of the swarm of March 2001 and interpretation. An additional synthetic test is performed with a standard deviation 0.05 s of the perturba- tion introduced into the arrival times. This value accounts for energetic and/or superficial events. Figures 9 and 10 show the D and H horizontal distributions at different depths obtained with this smaller value of perturbation. The results of this additional test show a significant reduction of both D and H values in configuration LS in Calabria and Messina Strait in the depth range 5-25 km. 5. Conclusions P and S arrival times of local seismicity rec- orded by a temporary seafloor network of OBS/H and INGV land-base network, for a total amount of 1246 data, were used to analyse a moderate en- ergy swarm in March 2001 in north-eastern Sici- ly. Moreover, the sea-bottom station distribution was used jointly with the land station distribution Fig. 6. Network configurations used for tests per- formed with synthetic earthquakes in order to assess the hypocentral location quality. Triangles and cir- cles mark the land and sea-bottom stations. The inte- grated network (configuration LS) included both sets of stations. The land stations constitute the network configuration L. Hypocenters of synthetic earth- quakes were positioned on a 3D grid, with horizon- tal spacing of 1.5 km and vertical spacing 5 km from the Earth surface down to 35 km. The gray rectangle indicates the area selected for the synthetic tests. 746 Graziella Barberi, Laura Beranzoli, Paolo Favali, Giancarlo Neri and Tiziana Sgroi Fig. 7. Comparison of D-parameter horizontal distributions at different depths obtained by synthetic tests with the LS integrated network (left panels) and the L land-based network (right panels). The standard deviation of the random noise perturbation of synthetic arrival times is 0.3 s. Fig. 8. Comparison of H-parameter horizontal distributions at different depths like in fig. 7 with the same stan- dard deviation (0.3 s). 747 Seismic location improvements from an OBS/H temporary network in Southern Tyrrhenian Sea Fig. 9. Comparison of D-parameter horizontal distributions at different depths like in fig. 7. The standard de- viation of the random noise perturbation of synthetic arrival times is reduced to 0.05 s. Fig. 10. Comparison of H-parameter horizontal distributions at different depths like in fig. 7. The standard de- viation of the random noise perturbation of synthetic arrival times is 0.05 s. 748 Graziella Barberi, Laura Beranzoli, Paolo Favali, Giancarlo Neri and Tiziana Sgroi to assess the efficiency of a sea-land integrated monitoring network in terms of hypocenters loca- tion accuracy for events eventually occurring in north-eastern Sicily and Southern Calabria. The swarm of 53 events, with maximum magnitude 2.9, was relocated using a 3D veloc- ity model of the region (Neri et al., 2002). The locations were performed twice, the first time using land and sea stations jointly (integrated network) and the second time using only land stations. In the case of integrated network con- figuration the relocated swarm has dispersion factors Dxy and Dxyz smaller than those found with the land-based network relocation. That could be attributed to a lower parameter vari- ance due to a smaller scatter of the hypocenters. The westward dipping of swarm hypocenters seems the only feature confirmed by both net- work configurations. The depth range of the swarm turned out between 27 and 37 km, a quite unusual depth for seismogenesis in the Italian peninsula. The depth range of the swarm could be interpreted in the frame of the transi- tion nature of the area from the geological and geodynamic point of view. North-eastern Sicily belongs to the southernmost sector of the Cal- abrian Arc unit and is adjacent to the Hyblean foreland. The southern border of the Ionian lithospheric slab has been roughly positioned just beneath north-eastern Sicily by previous studies (e.g., Argnani, 2000). Moreover, Sel- vaggi and Chiarabba (1995) estimated the depth of the shallower part of the subducting slab in the area to decrease from 50 to 25 km in the NW-SE direction. In order to assess the improvement of hypocenter location accuracy achievable by the integration of sea-bottom seismological observa- tions and land-based data, synthetic tests for a larger area, including the swarm, and extended up to Southern Calabria were performed. The as- sessment was accomplished through the evalua- tion of parameters D and H, defined as the hori- zontal distance and depth difference between the ‘synthetic’ (grid node) and the relocated hypo- centers. In order to simulate the occurrence of low and high energy and of deep and shallow earthquakes, synthetic arrival times were per- turbed with white noise characterised by two dif- ferent values of standard deviations (0.3 and 0.05 s). The results of the synthetic tests confirm that the presence of sea-bottom stations in the Tyr- rhenian Basin reduce the mislocations of large and/or shallow earthquakes in Southern Calabria and Messina Strait and of low intensity and/or deep earthquakes in north-eastern Sicily. The definition of proper strategies for the ex- tension of seismometric networks toward marine basins is evidently a basic step in the set of initia- tives addressed to the accurate characterisation and knowledge of seismogenic areas and conse- quently to the mitigation of the seismic risk. REFERENCES ANDERSON, H. and J. JACKSON (1987): Active tectonics of the Adriatic region, Geophys. J. R. Astr. Soc., 91, 937-983. ARGNANI, A. (2000): The Southern Tyrrhenian subduction system; recent evolution and neotectonic implications, Ann. Geofis., 43 (3), 585-607. ARGNANI, A. and C. SAVELLI (1999): Cenozoic volcanism and tectonics in the Southern Tyrrhenian Sea; space- time distribution and geodynamic significance, J. Geo- dynamics, 27, 409-432. BARBERI, F., P. GASPARINI, F. INNOCENTI and L. VILLARI (1973): Volcanism of the Southern Tyrrhenian Sea and its geodynamical implications, J. Geophys. Res., 78, 5221-5232. BEN-AVRAHAM, Z. and M. GRASSO (1990): Collisional zone segmentation in Sicily and surrounding areas in the Cen- tral Mediterranean, Ann. Tectonicae, 4 (2), 131-139. BEN-AVRAHAM, Z. and M. GRASSO (1991): Crustal structure variations and transcurrent faulting at the eastern and western margins of the Eastern Mediterranean, Tectonophysics, 196, 269-277. BERANZOLI, L., G. ETIOPE, P. FAVALI, F. FRUGONI, G. SM- RIGLIO, Y. AOYAGI, T. MURAYAMA, M. SHINOHARA and K. SUYEHIRO (1997): Microsismicità nel Mar Tirreno Meridionale registrata da una rete di Ocean Bottom Seismometers nel periodo 15 Ottobre-3 Novembre 1996, in Riassunti del XVI Convegno GNGTS, 11-13 Novembre, Roma , 66. BOSCHI, E., E. GUIDOBONI, G. FERRARI, G. VALENSISE and P. GASPERINI (1997): Catalogue of the Strong Earthquakes in Italy from 461 b.C. to 1990 (ING, Roma-SGA, Bolo- gna), pp. 973. CALAIS, E., C. DE METS and J.-M. NOCQUET (2003): Evi- dence for a post-3.16-Ma change in Nubia-Eurasia- North America plate motions?, Earth Planet. Sci. Lett., 216, 81-92. CIMINI, G.B. (1999): P-wave deep velocity structure of the Southern Tyrrhenian Subduction Zone from nonlinear traveltime tomography, Geophys. Res. Lett., 26, 3709- 3712. DAHM, T., M. THORWART, E. R. FLUEH, T. BRAUN, R. HERBER, P. FAVALI, L. BERANZOLI, G. D’ANNA, F. FRUGONI and G. SMRIGLIO (2002): First deployment of new ocean bottom broadband seismological instruments in the Tyrrhenian 749 Seismic location improvements from an OBS/H temporary network in Southern Tyrrhenian Sea Sea reveals unknown centers of submarine activity, Eos, Trans. Am. Geophys. Un., 83 (29), 309-320. DOGLIONI, C., F. INNOCENTI and G. MARIOTTI (2001): Why Mt. Etna?, Terra Nova, 13, 25-31. EVANS, J.R., D. EBERHART-PHILLIPS and C.H. THURBER (1994): User’s manual for simulps12 for imaging VP and VP/VS: a derivative of the «Thurber» tomographic inversion simul3 for local earthquakes and explosions, USGS Open-file Rep., 94-431. FACCENNA, C., P. DAVY, J.P. BRUN, R. FUNICIELLO, D. GIARDI- NI, M. MATTEI and T. NALPAS (1996): The dynamics of back-arc extension: an experimental approach to the opening of the Tyrrhenian Sea, Geophys. J. Int., 126, 781-795. FACCENNA, C., T.W. BECKER, F.P. LUCENTE, L. JOLIVET and F. ROSSETTI (2001a): History of Subduction and Back- arc Extension in the Central Mediterranean, Geophys. J. Int., 145, 809-820. FACCENNA, C., F. FUNICIELLO, D. GIARDINI and F.P. LUCENTE (2001b): Episodic back-arc extension during restricted mantle convection in the Central Mediterranean, Earth Planet. Sci. Lett., 187, 105-116. FAVALI, P., L. BERANZOLI and A. MARAMAI (2004): Review of the Tyrrhenian Sea seismicity: how much is still to be unknown?, in From Seafloor to Deep Mantle: Archi- tecture of the Tyrrhenian Back-arc Basin, edited by M.P. MARANI, F. GAMBERI and E. BONATTI, Mem. De- scr. C. Geol. Ital., LXIV, 57-70. FINETTI, I. and A. DEL BEN (1986): Geophysical study of the Tyrrhenian opening, Boll. Geof. Teor. Appl., 110, 75-156. GASPARINI, C., G. IANNACCONE, P. SCANDONE and R. SCARPA (1982): Seismotectonics of the Calabrian Arc, Tectono- physics, 82, 267-286. GIARDINI, D. and M. VELONÀ (1991): Deep seismicity of the Tyrrhenian Sea, Terra Nova, 3, 57-64. GVIRTZMAN, Z. and A. NUR (2001): Residual topography, lithospheric structure and sunken slabs in the Central Mediterranean, Earth Planet. Sci. Lett., 187, 117-130. HIRN, A., R. NICOLICH, J. GALLART, M. LAIGLE, L. CERNOBORI, B. ACCAINO, F. AVEDIK, M. BIANCA, J. DIAZ, J.-C. LEPINE, G. METTON, C. MONACO, A. NERCESSIAN, L. PETRONIO, S. POU, M. ROMANELLI, V. GALLARES, M. SAPIN, L. TOR- TORICI, I. MARSON, M. SACHPAZI and B. DELLA VEDOVA (1997): Roots of Etna Volcano in faults of great earth- quakes, Earth Planet. Sci. Lett., 148, 171-191. KISSLING, E., W.L. ELLSWORTH, D. EBERHART-PHILLIPS and U. KRADOLFER (1994): Initial reference models in local earthquake tomography, J. Geophys. Res., 99, 19635- 19646. LUCENTE, F.P., C. CHIARABBA and G.B. CIMINI (1999): To- mographic constraints on the geodynamic evolution of the Italian region, J. Geophys. Res., 104 (B9), 20307- 20327. MALINVERNO, A. and W.B.F. RYAN (1986): Extension in the Tyrrhenian Sea and shortening in the Apennines as re- sult of arc migration driven by sinking of the litho- sphere, Tectonics, 5, 227-245. MELETTI, C., E. PATACCA and P. SCANDONE (2000): Con- struction of a seismotectonic model: the case of Italy, Pure Appl. Geophys., 157 (1-2), 11-35. NERI, G., G. BARBERI, B. ORECCHIO and M. ALOISI (2002): Seismotomography of the crust in the transition zone be- tween the Southern Tyrrhenian and Sicilian tectonic do- mains, Geophys. Res. Lett., 29 (23), 2135, doi:10.1029/ 2002GL015562. NERI, G., G. BARBERI, B. ORECCHIO and A. MOSTACCIO (2003): Seismic strain and seismogenic stress regimes in the crust of the Southern Tyrrhenian region, Earth Planet. Sci. Lett., 213, 97-112. NERI, G., G. BARBERI, G. OLIVA and B. ORECCHIO (2005): Spatial variations of seismogenic stress orientations in Sicily, South Italy, Phys. Earth and Planet. Int., 148, 175-191. NICOLICH, R., M. LAIGLE, A. HIRN, L. CERNOBORI and J. GALLART (2000): Crustal structure of the Ionian margin of Sicily: Etna volcano in the frame of regional evolu- tion, Tectonophysics, 329, 121-139. NOCQUET, J.-M. and E. CALAIS (2004): Geodetic measure- ments of crustal deformation in the Western Mediter- ranean and Europe, Pure Appl. Geophys., 161 (3), 661- 682. PIROMALLO, C. and A. MORELLI (1997): Imaging the Mediterranean upper mantle by P-wave travel time to- mography, Ann. Geofis., XL (4), 963-979. SCANDONE, P. and M. STUCCHI (1999): La zonazione sismo- genetica ZS4 come strumento per la valutazione della pericolosità sismica, in Le Ricerche del GNDT nel Campo della Pericolosità Sismica (1996-1999), edited by F. GALADINI, C. MELETTI and A. REBEZ (GNDT). SELLA, G.F., T.H. DIXON and A. MAO (2002): REVEL: a model for recent plate velocities from space geodesy, J. Geophys. Res., 107, doi: 10.1029/2000JB000033. SELVAGGI, G. and C. CHIARABBA (1995): Seismicity and P- wave velocity image of the Southern Tyrrhenian sub- duction zone, Geophys. J. Int., 121, 818-826. SGROI, T., T. BRAUN, T. DAHM and F. FRUGONI (2006): An improved seismicity picture of the Southern Tyrrhenian area by the use of OBS and land-based networks: the TYDE experiment, Ann. Geophysics, 49 (2/3), 801-817 (this volume). SOLOVIEV, S.L., I.P. KUZIN, S.A. KOVACHEV, M. FERRI, I. GUERRA and G. LUONGO (1990): Micro-earthquakes in the Tyrrhenian Sea as revealed by joint land and sea- bottom seismographs, Mar. Geol., 94, 131-146. TINTI, S., A. MARAMAI and L. GRAZIANI (2004): The new catalogue of Italian tsunamis, Natural Hazards, 33, 439-465.