SKS splitting measurements beneath Northern Apennines region: A case of oblique trench-retreat

Tectonophysics 462 (2008) 68–82

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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

SKS splitting measurements beneath Northern Apennines region: A case of oblique trench-retreat S. Salimbeni a,b,⁎, S. Pondrelli a, L. Margheriti b, J. Park c, V. Levin d a

Istituto Nazionale di Geofisica e Vulcanologia, Sez. Bologna, Italy Istituto Nazionale di Geofisica e Vulcanologia, Sez. CNT, Rome, Italy Yale University, Department of Geology and Geophysics, New Haven, USA d Rutgers University, Department of Geological Sciences, Piscataway, NJ, USA b c

a r t i c l e

i n f o

Article history: Received 31 January 2007 Accepted 15 November 2007 Available online 22 August 2008 Keywords: Seismic anisotropy Trench retreat Apennines

a b s t r a c t We present here the new observations of seismic anisotropy obtained from SKS birefringence analysis. We studied 27 teleseismic earthquakes recorded by the temporary seismic network of RETREAT project in the Northern Apennines region. For each station–event couple we calculate the anisotropic parameters (delay time and fast-polarization direction) by minimizing the energy in the transverse component. Our measurements confirm the existence of two domains. The Tuscany domain, on the south-west with respect to the Apennines, shows mostly NW–SE fast axes directions, with a rotation toward E–W direction moving toward the Tyrrhenian Sea. The Adria domain, north-east of the Apennines orogen, shows more scattered measurements, with prevailing N–S to NNE–SSW directions, also with back-azimuthal dependence. The transition between the two domains is abrupt in the northern part of the study region but more gradual in the southern part. Measured delay times (1.8 s on average) suggest that the detected anisotropy is located principally in the asthenosphere. Beneath the Adria domain, where the presence of a double-layer structure seems consistent, a lithospheric contribution is plausible. An interpretation in terms of ongoing mantle deformation suggests a differential evolution of the trenchretreat process along the Northern Apennines orogen. The orogen-parallel anisotropy in the study region is beneath the inner part of the belt instead of beneath its crest and no orogen-normal measurements are found in the Tuscany side. Compared to the anisotropy pattern of the typical slab retreat seen in southern part of the Northern Apennines, in the northernmost one the anisotropy suggests that an oblique trench-retreat has occurred, possibly linked to Northern Apennines retreat since 5 Ma. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Seismic anisotropy measurements are an important constraint to understand conditions and evolution of different geodynamic environments (Savage, 1999). Studies on birefringence data are well known for several types of tectonic conditions, as continental collisions (i.e. Lev et al., 2006), strike-slip faults (i.e. Ryberg et al., 2005; Levin et al., 2006), rift zones (Kendall et al., 2005), stable cratonic regions (Fouch et al., 2004) and subduction zones. For this last geodynamic environment shear-wave splitting associated with upper mantle anisotropy has been found everywhere, even if with different patterns, as to demonstrate that the interpretation is usually not easy. In the Tonga back-arc, in the Izu Bonin region and in the Mariana subduction zone, at least in part, the anisotropy pattern is sub-parallel to the absolute plate motion of the subducting plate (Fouch and Fisher, 1998; Volti et al., 2006; Pozgay et al., 2007). On the other hand, more ⁎ Corresponding author. Via D. Creti 12, 40128, Bologna, Italy. Tel.: +39 0514151460; fax: +39 0514151499. E-mail address: [email protected] (S. Salimbeni). 0040-1951/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2007.11.075

complex patterns have been found elsewhere. Fast directions trenchperpendicular in the mantle wedge and trench-parallel beyond the trench, beneath the orogen or the arc, are found in the Lau back-arc (Smith et al., 2001) and in Kamchatka (Peyton et al., 2001). Subparallel-trench shear-wave polarization directions in the mantle wedge are found in New Zealand (Audoine et al., 2000), in Ryukyu arc and in Japan (i.e. Long and van der Hilst, 2005, 2006). Somewhere the fast-polarization distribution is very complex, e.g. South America (Polet et al., 2000), Central Alaska (Wiemer et al., 1999), and often subduction zones display abrupt changes in shear-wave polarization directions, as in Japan (Nakajima and Hasegawa, 2004). The different patterns of seismic anisotropy found in subduction zones have been traditionally related to flow-induced strain in the upper mantle, on the basis of the relation between lattice preferred orientation (LPO) of intrinsically anisotropic mantle minerals and strain induced in the mantle by tectonic processes (i.e. Silver, 1996; Savage, 1999). Mantle flow regimes that potentially affect the anisotropic pattern in subduction zones include (1) return flow in the mantle wedge induced by the presence of windows or tears in the slab (Peyton et al.,

S. Salimbeni et al. / Tectonophysics 462 (2008) 68–82

2001; Civello and Margheriti, 2004), (2) trench-parallel flow beneath the slab produced by its rollback (Buttles and Olson, 1998), or (3) trench-parallel flow above the slab, related to transpression or changes in dip of the subducting plate (Hall et al., 2000). Trenchparallel anisotropy can also be due to the presence of melt (Holtzman et al., 2003) or hydrated mantle minerals (Jung and Karato, 2001), that may cause a rotation of the a-axis of olivine with respect to the direction of shear, even for a regular trench-normal corner flow. Any other source of anisotropy, i.e. the lithospheric frozen anisotropy, may overlap the described pattern, increasing the complexities in the interpretation. Birefringence of shear-wave happens when it traverses an anisotropic medium, and splits in two phases with different velocities. The direction of the faster wave and the delay time with respect to the slower one are the two parameters determined in the shear-wave splitting analysis. Analyzing a teleseismic signal, as with the SKS phases used in the present work, mantle anisotropy is detected, due to the characteristic long period of the processed signal, while to identify crustal anisotropy high-frequency recordings are needed. The Northern Apennines orogen lies above a high-velocity perturbation in the upper mantle that has been identified as a subducting slab (Wortel and Spakman, 1992; Lucente et al., 1999; Wortel and Spakman, 2000; Piromallo and Morelli, 2003). Geologic reconstructions of the western Mediterranean argue that for 35 Ma this slab has been retreating as it subducted, separating from Corsica and Sardinia as early as 10 Ma to form the Tyrrhenian Sea (Inset in Fig. 1; Rosenbaum and Lister, 2004). Anisotropy is prominent in the Northern Apennines region, showing the presence of domains characterized by different patterns. An orogen-parallel fast polarization along the trench, and an orogen-normal one in the back-arc basin are found in the edge of the Northern Apennines, south of the area studied in this work (Margheriti et al., 1996). Beneath the northern part of Northern Apennines, on the contrary, the anisotropy trend is different, showing an orogen-parallel pattern in the retro plate region, in Tuscany and an orogen-normal pattern in the outer part of the Apennines and in the Po-Plain foreland (Plomerová et al., 2006). Moreover the transition between these domains is sharp, occurring within about 30–40 km (Salimbeni et al., 2007).

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Here we present the complete dataset of SKS splitting analyses performed within the RETREAT project (Margheriti et al., 2006). The large number of measurements allows mapping with high definition the variations of the anisotropy in the entire Northern Apennines, helping us understand the source of these changes. Moreover, this dataset allows some reconstruction of the anisotropy distribution at depth, using Fresnel-zone assumptions and the ray-paths of analyzed SKS phases. On the basis of our results we can hypothesize how the retreat process worked in the Northern Apennines. 2. Geodynamic setting The Apenninic chain–Tyrrhenian Sea system (Fig. 1) is located in the central western Mediterranean region. This system developed since the Late Cretaceous via the slow convergence between African and Eurasian plates and the fast slab-rollback and opening of back arc basin. 35 My ago, a continuous trench oriented NE–SW, connecting the southern Iberia to the Ligurian region was active (Dewey et al., 1989) and a NW dipping subduction of the Jurassic oceanic basin started (Faccenna et al., 2004). The rifting and spreading of the LiguroProvencal basin (from 30 Ma) allowed the migration of the trench in south-east to east direction. In the former Italian region from 5 Ma onward the continuity of the trench was broken and the differential retreating directions induced the separation of the Apenninic front into two main arcs, the Calabrian Arc to the South and the Northern Apennines arc to the north (Patacca et al., 1990). This latter is our area of study (Fig. 1). Here we can identify three regions with different crustal deformation styles (Faccenna et al., 2004). In the retro plate domain (Tyrrhenian and Tuscany region), the regional processes of extension prevail, explaining the presence of Quaternary basins in the area. Moving eastward the extension persists until just before the Apenninic crest, in the orogenic wedge. Outward the crest, on the Adriatic Sea side, we find the external foreland thrust belt formed by units offscraping from the African-Adria margin stacked in a NE vergence (Fig. 1). The end of this zone culminates in the north with the buried arcs of Emilia, Ferrara-Romagna and Adriatic folds (AE, AF-R and AF in Fig. 1) where the deformation is described as compressional (Pieri and

Fig. 1. Tectonic map of the Northern Apennines. 1) Main thrust, 2) normal and vertical faults, 3) buried thrust arcs, 4) fold axes, 5) volcanic centers and 6) African and Adriatic foreland. EI: Elba Island, AE: Emilia arc, AF-R: Ferrara-Romagna arc, AF: Adriatic fold. Upper right inset: sketch of the geodynamic evolution of the Apennines front. Grey dashed lines approximately indicate the position of the trench at A (20 Ma), B (10 Ma), C (1 Ma) and actual position (D) according to the paleotectonic reconstruction by Gueguen et al. (1998) and Faccenna et al. (2001). Line D has a hatches pattern with direction corresponding to subduction dip and represents the present-day location of the convergent boundary. Dotted line draws the boundary of Adria plate. Arrows indicate the present-day convergence direction between Africa and Eurasia plate.

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Table 1 Coordinates of seismographic stations for which data have been analyzed (see also Fig. 2) Station

Latitude

Longitude

Station

Latitude

Longitude

ANZR ARCI BARR BOB CAIR CING CORR CRER CSNR CSTR CUTR ELBR ERBM FIRR FIU FNVD FOSR GABR GERF GROG GUSR MAON MASR MCUR

44.58 42.85 44.28 44.77 44.29 43.38 44.47 43.62 43.47 44.44 44.10 42.75 44.42 44.19 44.64 44.17 44.14 43.50 43.15 43.43 44.35 42.43 43.86 44.01

11.15 11.46 12.08 9.45 11.00 13.20 10.09 11.95 11.29 11.03 10.76 10.21 10.41 11.43 11.49 11.12 10.02 10.41 10.98 9.89 10.59 11.13 11.38 11.18

MNGR MSTR MTVR MURB PDCR PESA PESR PIZR PNTR POPR PRUR PTCR RAVR RONR RSMR SACS SASR SCUR SFIR USOR VLC VOLR VRGR ZOCR

44.51 43.91 44.47 43.26 43.78 43.94 43.94 44.13 44.01 43.02 44.11 44.24 44.76 44.22 43.93 42.85 43.26 44.42 43.90 44.01 44.16 43.55 43.64 44.35

10.79 10.49 11.09 12.52 10.58 12.84 12.83 10.86 10.82 10.53 10.31 10.97 11.12 10.92 12.45 11.91 10.69 9.54 11.85 10.70 10.39 10.86 10.47 10.98

Groppi, 1981; Pieri, 1983). Present day seismicity (Frepoli and Amato, 1996; Pondrelli et al., 2006), geodetic strain (Serpelloni et al., 2005), seismic strain pattern (Vannucci et al., 2004) and stress field studies (Mariucci et al., 1999; Montone et al., 2004) confirm this zonation, underlining the prevailing extension in the inner part of the orogen and the compression in the outer part. This geologic and tectonic emplacement was caused by retreat of the west-dipping Adriatic slab beneath the European plate (Doglioni, 1991). The arcuate shape of the belt, the low topography of the relief

and the contemporaneous extension and compression in the crust are the main clues to a retreat process (Royden, 1993). At depth, several tomographic models (Lucente et al., 1999; Piromallo and Morelli, 2003) showed the presence of a high-velocity body striking along the Apennines that is identified as a westward subducting Adriatic slab. In tomographic sections the slab beneath the Northern Apennines has a near-vertical dip. The slab-rollback may be an ongoing process (Doglioni, 1991) or it may be stalled (Faccenna et al., 2001). Detachment and sinking of the subducting lithosphere have also been proposed beneath the northernmost part of the Northern Apennines, as a consequence of the termination of the subduction process (Wortel and Spakman, 1992; Gvirtzman and Nur, 1999). 3. The RETREAT seismological deployment and data The seismological part of the RETREAT project (Margheriti et al., 2006) comprised 50 temporary and permanent sites in the Northern Apennines (Table 1 and Fig. 2) from international sources. 10 sets of GAIA instruments were lent by the Geophysical Institute of Prague recording for nearly three years from October 2003 to September 2006. Each GAIA station consisted of an STS-2 sensor and a VISTEC digitizer. 25 stations were lent by the PASSCAL program of the Incorporated Research Institutions in Seismology (IRIS), recording for about 2 years, between October 2004 and August 2006. These sites were all equipped with a Reftek130 digitizer; 10 sites had a STS-2 and 15 a CMG-40T motion sensor. These 35 instrument sets were used to cover 39 sites. Many PASSCAL stations were aligned along a transect cutting the Northern Apennines in a NE–SW direction, from the PoPlain to the Tyrrhenian Sea, named the northern transect (Fig. 2). Other stations were installed NW and SE as a cloud around this transect. After 1 year of recordings, in 2005, four stations belonging to the northern transect were moved to another less dense transect southward, from station ELBR, located on Elba island, to station PESR, along the Adriatic coast (southern transect in Fig. 2). Moreover, to

Fig. 2. Map of the seismic network used in this work. Temporary stations of the RETREAT project are mapped with circles for PASSCAL instruments (with different colors corresponding to different recording time) and with squares for GFU instruments. The triangles represent the permanent stations of the MedNet and INGV networks. The two black lines sketch the northern and southern transects.

S. Salimbeni et al. / Tectonophysics 462 (2008) 68–82 Table 2 Date, location, bulletin magnitude (from NEIC), epicentral distance and back-azimuth for earthquakes used in the analysis Date Time Latitude Longitude Depth Magnitude Δ (yyyy/mm/dd) (hh:mm:ss) (°) (°) km M (°)

BAZ (°)

2003/10/31 2004/ 01/11 2004/02/05 2004/02/07 2004/02/21 2004/04/23 2004/04/29 2004/05/03 2004/07/25 2004/09/05-A 2004/09/05-B 2004/09/06 2004/10/09 2004/11/11 2004/11/15 2004/11/26 2005/02/05 2005/03/02 2005/04/10 2005/05/14 2005/06/13 2005/06/15 2005/08/07 2005/08/16 2005/09/26 2005/10/29 2006/01/02

36.3 147.2 67.1 67.9 193.8 81.5 282.6 238.0 89.7 43.1 42.7 202.3 283.5 79.0 272.3 67.3 70.4 73.9 92.1 91.3 249.0 329.0 164.7 36.5 264.1 127.7 196.0

01:06:28 04:32:47 21:05:02 02:42:35 02:34:42 01:50:30 00:57:21 04:36:50 14:35:19 10:07:07 14:57:18 12:42:59 21:26:53 21:26:41 09:06:56 02:25:03 12:23:18 10:42:12 10:29:11 05:05:18 22:44:33 02:50:53 02:17:46 02:46:28 01:55:37 04:05:56 06:10:49

37.81 −36.70 − 3.62 − 4.00 −58.42 − 9.36 10.81 − 37.69 − 2.43 33.07 33.18 −55.37 11.42 − 8.15 4.70 − 3.61 5.29 − 6.53 − 1.64 0.59 −19.99 41.30 −47.09 38.28 − 5.68 −45.21 −60.92

142.62 53.35 135.54 135.02 −14.96 122.84 −86.00 −73.41 103.98 136.62 137.07 −28.98 −86.67 124.87 −77.51 135.40 123.34 129.93 99.61 98.46 −69.20 −125.97 33.62 142.04 −76.40 96.90 −21.58

10 5 16 10 10 65 10 21 582 14 10 10 35 10 15 10 525 201 19 34 115 10 10 36 115 8 10

7.0 6.2 7.1 7.5 6.6 6.7 6.2 6.6 7.3 7.2 7.4 6.9 7.0 7.5 7.2 7.2 7.1 7.1 6.7 6.8 7.8 7.2 6.2 7.2 7.5 6.5 7.4

87.5 89.0 116.9 116.8 104.3 112.1 87.6 111.4 93.8 88.7 88.8 104.6 87.6 112.7 85.7 116.8 102.1 115.2 90.1 87.8 96.9 86.7 92.9 86.9 92.0 116.9 107.9

Distance and back-azimuth are computed with respect to a point in the middle of the network (see also Fig. 3).

make the RETREAT network denser, data recorded by temporary stations are integrated with those of INGV permanent network and one station, VLC, of the MedNet network. All data recorded on RETREAT sites are archived at the Data Management Center of IRIS. We selected all available recordings for 27 teleseismic earthquakes between October 2003 and January 2006, with M ≥ 6.2 and with an epicentral distance between 85° and 120° (Table 2 and Fig. 3). These events give us a good azimuthal coverage, at least for sites recording

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for longer time intervals; however, from the north and south-west events that can be analyzed are few (Fig. 3b). To better isolate the SKS phase in analysing window, most of data were bandpassed with a Butterworth filter between 2 to 100 s. 4. Measurements: Tuscany and Adria domains We analyze SKS phases using the method described by Sileny and Plomerová (1996) and we determine the anisotropy parameters (fast axes ϕ and delay time dt) minimizing the energy in the transverse component, assuming that the anisotropy has a horizontal symmetry axis. To verify the stability to noise, the two parameters obtained are tested with a bootstrap procedure (Sandvol and Hearn, 1994). Beside measurements already published for sites along the northern transect (Salimbeni et al., 2007) the 289 new results obtained by SKS splitting analysis are reported in Table 3 and mapped in Fig. 4a and b. In both maps each symbol is related to a single event– station couple. On the basis of our knowledge of heterogeneities distribution at depth from tomographic studies (Lucente et al., 1999; Piromallo and Morelli, 2003), we choose to map the measurements at a piercing point of 150 km of depth. This choice gives a better distinction between measurements done at the same station and makes azimuthal differences more readable. We use also different colors for NE back-azimuth events (−45° to 135°) and SW backazimuth events (135° to −45°) to further clarify azimuthal variations. This NE–SW distinction is chosen as a boundary to follow the morphological shape of the region where the NW–SE striking direction of the Apennine chain dominates. Several null birefringence measurements were obtained (Fig. 4b). We considered a measurement null when there is no energy in the transverse component. The presence of nulls is linked in theory to an absence of anisotropy, but all other measurements assure the presence of anisotropy at depth. Consequently, what is more probable is that the ray-paths of studied events are parallel or perpendicular to the anisotropy symmetry axis. For this reason we have mapped null measurements as two crossing-lines oriented parallel and perpendicular to back-azimuth. These crossing-lines (Fig. 4b) frequently align with single SKS splitting measurements nearby (Insets A and B in Fig. 4b), suggesting often a consistency between them.

Fig. 3. a) Map of teleseismic earthquakes (dots) used for seismic anisotropy analysis. The RETREAT area is the star. Grey lines are event–station great circle paths. b) Azimuthal coverage pattern of analysed teleseismic earthquakes (black stars). Along the radius is reported the event–station distance (degrees).

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Table 3 Shear-wave splitting parameters for each station-event measurement, labelled with 2) in reference column when determined for this work Station

Event yy/mm/dd

ϕ (°)

dϕ (°)

δt s

dδt s±

Source

ANZR ANZR ANZR ANZR ANZR ANZR ANZR ANZR ARCI ARCI BARR BARR BARR BARR BARR BARR BARR BARR BARR BARR BARR BARR BARR BARR BOB BOB BOB BOB BOB BOB BOB BOB CAIR CAIR CAIR CAIR CAIR CAIR CAIR CAIR CAIR CAIR CING CING CING CING CING CING CING CING CING CORR CORR CORR CRER CRER CSNR CSNR CSNR CSNR CSNR CSNR CSNR CSNR CSNR CSNR CSNR CSNR CSNR CSTR CSTR CSTR CSTR CSTR

04/11/11 04/11/26 05/02/05 05/03/02 05/04/10 05/06/15 05/05/14 05/06/13 05/06/13 05/09/26 04/07/25 04/09/05-B 04/09/06 04/10/09 05/02/05 05/03/02 05/06/15 05/08/07 05/08/16 05/10/29 04/09/05-A 05/05/14 05/06/13 05/09/26 04/09/05-B 04/10/09 05/06/13 05/09/26 04/02/05 04/09/05-A 04/09/06 05/02/05 04/11/26 05/02/05 05/03/02 05/06/15 05/08/07 05/04/10 05/05/14 05/06/13 05/08/16 05/09/26 04/09/06 05/02/05 05/03/02 04/04/29 04/09/05-A 04/09/05-B 05/06/13 05/08/16 05/09/26 05/08/16 05/09/26 05/10/29 04/11/15 04/11/26 04/01/11 04/02/21 04/07/25 04/09/06 04/11/15 05/08/07 05/09/26 04/02/05 04/09/05-A 04/09/05-B 04/11/11 05/02/05 05/08/16 04/11/11 04/11/15 04/11/26 05/02/05 05/03/02

216 209 210 227 335 213 – – 102 – 322 199 328 171 193 195 168 315 170 16 – – – – 292 161 136 136 – – – – 193 176 177 205 273 – – – – – 307 174 172 – – – – – – 190 152 359 130 273 290 310 331 309 135 310 153 – – – – – – 222 166 229 194 177

5 6 5 3 6 4 – – 15 – 5 5 15 2 2 2 6 2 8 8 – – – – 5 7 12 15 – – – – 2 2 2 5 12 – – – – – 2 3 2 – – – – – – 7 4 4 3 5 4 5 5 2 1 13 8 – – – – – – 14 8 4 6 4

1.93 1.60 1.42 1.03 0.91 2.00 Null Null 1.20 Null 1.78 0.91 1.55 2.96 2.02 1.63 2.40 2.30 0.81 1.17 Null Null Null Null 1.40 1.90 1.40 0.80 Null Null Null Null 2.40 2.30 2.40 2.80 1.44 Null Null Null Null Null 2.40 1.60 1.30 Null Null Null Null Null Null 0.96 1.57 2.27 2.42 1.09 1.68 0.95 1.57 1.80 2.69 2.42 2.40 Null Null Null Null Null Null 1.20 2.00 1.70 1.50 1.80

0.17 0.28 0.10 0.08 0.15 0.18 – – 0.30 – 0.26 0.15 0.30 0.13 0.20 0.07 0.43 0.19 0.10 0.22 – – – – 0.40 0.40 0.40 0.30 – – – – 0.24 0.19 0.21 0.32 0.35 – – – – – 0.40 0.40 0.30 – – – – – – 0.23 0.23 0.24 0.29 0.23 0.23 0.12 0.26 0.19 0.06 0.43 0.42 – – – – – – 0.24 0.60 0.26 0.20 0.27

1) 1) 1) 1) 2) 1) 2) 1) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 1) 1) 1) 1) 2) 2) 2) 1) 1) 1) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 1) 1) 1) 1) 1)

Table 3 (continued ) Station

Event yy/mm/dd

ϕ (°)

dϕ (°)

δt s

dδt s±

Source

CSTR CSTR CSTR CSTR CSTR CUTR CUTR CUTR CUTR CUTR CUTR CUTR CUTR CUTR CUTR ELBR ELBR ELBR ELBR ELBR ELBR ELBR ELBR ELBR ELBR ELBR FIRR FIRR FIRR FIRR FIRR FIRR FIRR FIRR FIRR FIRR FIRR FIRR FIRR FNVD FNVD FNVD FOSR FOSR FOSR FOSR FOSR FOSR FOSR FOSR FOSR FOSR FOSR FSSR GABR GABR GABR GABR GABR GABR GABR GABR GABR GABR GROG GROG GUSR GUSR GUSR GUSR GUSR GUSR GUSR GUSR GUSR

05/08/07 05/04/10 05/05/14 05/06/13 05/09/26 04/11/15 05/05/14 05/08/07 06/01/02 04/11/11 04/11/26 05/02/05 05/03/02 05/04/10 05/08/16 04/11/26 05/08/16 05/10/29 04/11/11 04/11/15 05/02/05 05/03/02 05/04/10 05/05/14 05/09/26 06/01/02 04/11/11 04/11/15 05/02/05 05/06/15 05/08/07 06/01/02 04/11/26 05/03/02 05/04/10 05/05/14 05/08/16 05/09/26 05/10/29 05/02/05 05/03/02 05/06/13 04/11/15 05/03/02 05/05/14 05/09/26 06/01/02 04/11/11 05/02/05 05/04/10 05/08/07 05/08/16 05/10/29 06/01/02 05/03/02 05/04/10 05/05/14 05/06/13 05/06/15 05/09/26 04/11/11 05/02/05 05/08/16 06/01/02 05/08/16 05/09/26 05/02/05 05/03/02 05/05/14 05/08/07 05/09/26 05/04/10 05/08/16 05/10/29 06/01/02

277 – – – – 126 351 291 302 – – – – – – 301 270 275 – – – – – – – – 296 17 173 156 283 303 – – – – – – – 176 264 145 138 280 312 140 315 – – – – – – 4 321 290 286 100 116 108 – – – – 266 132 175 172 190 288 144 – – – –

9 – – – – 2 4 8 19 – – – – – – 8 2 4 – – – – – – – – 20 2 2 3 5 2 – – – – – – – 3 11 3 3 5 15 4 8 – – – – – – 4 5 7 2 4 8 2 – – – – 12 10 3 2 1 3 7 – – – –

1.64 Null Null Null Null 2.48 2.12 2.39 1.61 Null Null Null Null Null Null 1.50 2.02 2.74 Null Null Null Null Null Null Null Null 1.79 2.29 1.86 3.36 1.49 2.00 Null Null Null Null Null Null Null 1.10 0.70 2.60 2.01 0.74 1.44 1.90 2.22 Null Null Null Null Null Null 1.92 1.00 3.23 2.33 2.00 1.30 2.52 Null Null Null Null 0.60 1.50 1.71 1.55 2.09 1.41 1.26 Null Null Null Null

0.41 – – – – 0.16 0.51 0.43 0.36 – – – – – – 0.15 0.14 0.29 – – – – – – – – 0.21 0.17 0.21 0.60 0.19 0.26 – – – – – – – 0.20 0.50 0.50 0.10 0.12 0.23 0.10 0.29 – – – – – – 0.55 0.19 0.45 0.31 0.16 0.22 0.19 – – – – 0.10 0.20 0.23 0.19 0.28 0.14 0.13 – – – –

2) 2) 2) 1) 1) 1) 2) 2) 2) 1) 1) 1) 1) 2) 1) 1) 1) 2) 1) 1) 1) 1) 2) 2) 1) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 1) 2) 2) 1) 1) 1) 1) 1) 1) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2)

MAON

04/09/05-A

293

8

1.50

0.50

2)

S. Salimbeni et al. / Tectonophysics 462 (2008) 68–82 Table 3 (continued )

73

Table 3 (continued )

Station

Event yy/mm/dd

ϕ (°)

dϕ (°)

δt s

dδt s±

Source

Station

Event yy/mm/dd

ϕ (°)

dϕ (°)

δt s

dδt s±

Source

MAON MAON MAON MAON MAON MAON MAON MASR MASR MASR MASR MASR MASR MASR MASR MASR MASR MASR MASR MASR MASR MASR MASR MNGR MNGR MNGR MNGR MNGR MNGR MNGR MNGR MNGR MNGR MSTR MSTR MSTR MSTR MSTR MSTR MSTR MSTR MSTR MSTR MSTR MSTR MSTR MSTR MTVR MTVR MTVR MTVR MTVR MTVR MTVR MTVR MTVR MTVR MTVR MTVR MTVR MURB MURB MURB MURB PDCR PDCR PDCR PDCR PDCR PDCR PDCR PDCR PDCR PESR PIIR PIIR

04/09/05-B 04/09/06 05/06/13 05/08/16 05/09/26 04/05/03 04/11/15 04/07/25 04/11/15 05/03/02 05/10/29 04/09/05-A 04/09/05-B 04/09/06 04/11/11 04/11/26 05/02/05 05/04/10 05/06/13 05/06/15 05/08/07 05/08/16 05/09/26 04/11/15 05/02/05 05/03/02 05/06/15 05/10/29 06/01/02 04/11/11 04/11/26 05/04/10 05/05/14 04/11/11 04/11/15 04/11/26 05/02/05 05/03/02 05/04/10 05/05/14 05/06/13 05/06/15 05/08/07 05/09/26 05/08/16 05/10/29 06/01/02 04/11/11 04/11/26 05/02/05 05/03/02 05/04/10 05/06/15 05/08/07 05/09/26 06/01/02 04/11/15 05/05/14 05/06/13 05/08/16 04/09/05-B 04/09/06 05/02/05 05/06/13 04/11/15 05/02/05 05/03/02 05/08/07 05/08/16 05/09/26 05/10/29 04/11/11 04/11/26 06/01/02 04/01/11 04/02/05

263 52 99 269 104 288 – 320 142 304 346 – – – – – – – – – – – – 152 177 175 117 347 344 – – – – 310 124 172 272 287 312 289 111 178 294 122 – – – 303 177 304 179 332 134 1 164 49 – – – – – – – – 128 274 285 282 244 108 282 – – 335 286 295

13 9 4 12 7 0 – 3 3 4 5 – – – – – – – – – – – – 6 5 1 3 1 3 – – – – 5 3 9 2 2 4 5 6 11 8 3 – – – 3 4 5 2 21 9 6 2 6 – – – – – – – – 4 4 13 10 9 3 3 – – 9 2 4

2.00 1.50 1.60 2.20 1.10 2.44 Null 2.45 3.00 2.21 2.13 Null Null Null Null Null Null Null Null Null Null Null Null 1.51 2.99 2.04 2.36 2.80 1.53 Null Null Null Null 2.40 2.00 2.30 0.93 0.81 2.83 1.03 2.24 1.37 2.18 2.92 Null Null Null 2.40 2.00 1.90 2.14 1.48 2.20 2.46 1.90 1.78 Null Null Null Null Null Null Null Null 1.75 1.10 0.60 1.51 0.59 2.95 2.64 Null Null 1.70 2.50 2.24

0.50 0.30 0.20 0.10 0.30 0.00 – 0.15 0.19 0.12 0.30 – – – – – – – – – – – – 0.13 0.49 0.09 0.20 0.10 0.19 – – – – 0.32 0.24 0.26 0.06 0.05 0.40 0.31 0.16 0.38 0.42 0.10 – – – 0.28 0.30 0.22 0.19 0.30 0.37 0.64 0.48 0.15 – – – – – – – – 0.10 0.17 0.14 0.33 0.14 0.23 0.37 – – 0.34 0.07 0.12

2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 1) 1) 1) 1) 1) 2) 2) 1) 2) 2) 1) 1) 2) 2) 1) 1) 1) 1) 2) 1) 2) 1) 2) 1) 2) 1) 1) 2) 2) 2) 2) 1) 1) 1) 2) 1) 1) 2) 1) 1) 2) 2) 1)

PIIR PIIR PIIR PIIR PIIR PIIR PIIR PIIR PIIR PIIR PIIR PIIR PIIR PIIR PIIR PIIR PIZR PIZR PIZR PIZR PIZR PIZR PIZR PIZR PIZR PIZR PIZR PNTR PNTR PNTR PNTR PNTR PNTR PNTR PNTR POPR POPR POPR POPR POPR POPR POPR POPR POPR PRUR PRUR PRUR PRUR PRUR PRUR PRUR PRUR PRUR PRUR PRUR PTCR PTCR PTCR PTCR PTCR PTCR PTCR PTCR PTCR PTCR PTCR RAVR RAVR RAVR RAVR RAVR RAVR RAVR RAVR RAVR

04/07/25 04/09/05-A 04/11/11 04/11/15 05/03/02 05/05/14 05/06/13 05/08/07 05/09/26 05/10/29 03/10/31 04/09/05-B 04/11/26 05/04/10 05/06/15 05/08/16 04/11/11 04/11/15 05/04/10 05/06/13 05/08/07 05/08/16 05/09/26 05/10/29 04/11/26 05/02/05 05/03/02 04/11/15 05/03/02 05/04/10 05/05/14 05/02/05 05/06/13 05/06/15 05/08/16 05/02/05 05/03/02 05/05/14 05/06/13 05/10/29 04/11/26 05/04/10 05/06/15 05/08/07 04/11/15 04/11/26 05/05/14 05/06/13 05/08/07 05/09/26 04/11/11 05/04/10 05/06/15 05/08/16 05/10/29 04/11/15 05/02/05 05/06/13 05/08/07 05/10/29 06/01/02 05/03/02 05/04/10 05/05/14 05/06/15 05/09/26 04/01/11 04/02/05 04/02/07 04/02/21 04/07/25 04/09/05-A 04/09/05-B 04/09/06 04/10/09

343 287 308 122 311 295 117 298 114 268 – – – – – – 178 142 323 133 311 136 166 6 – – – 160 317 309 306 – – – – 280 302 284 94 324 – – – – 124 298 286 125 292 148 – – – – – 130 170 149 303 298 312 – – – – – 356 184 229 345 203 156 164 322 152

27 8 15 1 5 7 2 5 8 4 – – – – – – 4 5 6 17 9 2 1 3 – – – 4 2 17 4 – – – – 6 2 4 1 9 – – – – 1 8 3 6 12 3 – – – – – 6 2 2 6 3 3 – – – – – 2 6 8 4 1 7 13 5 5

1.22 1.34 1.64 1.80 0.91 1.63 1.89 1.72 2.29 1.57 Null Null Null Null Null Null 1.90 1.94 1.98 1.00 1.98 1.32 2.20 2.90 Null Null Null 1.63 1.32 1.52 1.43 Null Null Null Null 0.83 1.01 1.43 1.99 1.83 Null Null Null Null 2.62 2.16 2.59 2.63 2.28 2.73 Null Null Null Null Null 1.60 2.00 2.80 1.77 1.34 2.50 Null Null Null Null Null 1.37 2.70 1.70 1.64 1.81 2.22 1.50 1.75 1.60

0.51 0.16 0.33 0.10 0.09 0.27 0.05 0.09 0.26 0.12 – – – – – – 0.60 0.26 0.35 0.31 0.42 0.22 0.32 0.27 – – – 0.20 0.07 0.26 0.19 – – – – 0.13 0.10 0.29 0.06 0.48 – – – – 0.10 0.27 0.33 0.28 0.53 0.24 – – – – – 0.18 0.20 0.47 0.16 0.28 0.17 – – – – – 0.14 0.43 0.57 0.20 0.06 0.43 0.52 0.22 0.23

2) 1) 2) 1) 1) 2) 2) 2) 2) 2) 1) 1) 1) 2) 2) 2) 1) 1) 2) 1) 2) 1) 1) 2) 1) 1) 1) 1) 1) 2) 2) 1) 1) 1) 1) 1) 1) 2) 1) 2) 1) 2) 1) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 1) 1) 1) 2) 2) 2) 1) 2) 2) 1) 1) 2) 1) 1) 1) 1) 1) 1) 1) 1)

(continued on next page)

(continued on next page)

74

S. Salimbeni et al. / Tectonophysics 462 (2008) 68–82

Table 3 (continued )

Table 3 (continued )

Station

Event yy/mm/dd

ϕ (°)

dϕ (°)

δt s

dδt s±

Source

Station

Event yy/mm/dd

ϕ (°)

dϕ (°)

δt s

dδt s±

Source

RAVR RAVR RAVR RAVR RAVR RAVR RAVR RAVR RAVR RAVR RAVR RAVR RAVR RAVR RONR RONR RONR RONR RONR RONR RONR RONR RONR RONR RONR RONR RONR RSMR RSMR RSMR RSMR RSMR RSMR RSMR RSMR RSMR RSMR RSMR RSMR RSMR RSMR RSMR SACS SACS SACS SASR SASR SCUR SCUR SCUR SCUR SCUR SCUR SCUR SCUR SCUR SCUR SCUR SCUR SFIR SFIR SFIR SFIR SFIR SFIR SFIR SFIR SFIR SFIR SFIR SFIR SFIR SFIR SFIR SFIR SFIR

04/11/11 04/11/26 05/02/05 05/03/02 05/06/15 05/08/07 05/08/16 05/09/26 05/10/29 03/10/31 04/11/15 05/04/10 05/05/14 05/06/13 04/11/11 04/11/15 05/02/05 05/06/13 05/08/07 05/08/16 05/10/29 06/01/02 04/11/26 05/03/02 05/05/14 05/06/15 05/09/26 03/10/31 04/02/05 04/02/07 04/02/21 04/07/25 04/09/05-A 04/09/06 04/10/09 04/11/11 04/11/15 04/11/26 05/04/10 05/06/15 04/09/05-B 05/06/13 05/03/02 05/06/13 05/09/26 05/03/02 05/02/05 04/02/21 04/09/05-A 04/09/06 04/10/09 05/02/05 05/03/02 05/05/14 04/01/11 04/07/25 04/09/05-B 04/11/11 04/11/15 04/01/11 04/07/25 04/10/09 04/11/11 04/11/15 05/02/05 05/08/07 05/09/26 05/10/29 04/04/23 03/10/31 04/02/05 04/02/07 04/02/21 04/09/05-A 04/09/05-B 04/09/06

190 198 196 199 214 319 286 154 12 – – – – – 232 124 180 133 296 136 333 310 – – – – – 292 207 199 343 193 288 319 166 306 167 181 233 205 – – 323 90 123 316 – 333 159 310 139 327 324 308 – – – – – 344 294 163 335 148 189 288 161 264 288 – – – – – – –

4 3 5 2 6 4 4 2 2 – – – – – 5 4 2 3 4 4 6 4 – – – – – 6 2 8 2 1 2 3 7 8 1 6 4 18 – – 15 13 10 3 – 6 4 4 26 13 6 10 – – – – – 1 13 6 6 2 3 5 10 1 0 – – – – – – –

1.60 2.65 1.60 1.61 1.97 1.00 2.88 1.32 2.80 Null Null Null Null Null 2.34 2.23 1.33 1.30 1.24 2.20 0.98 2.66 Null Null Null Null Null 1.50 1.19 1.52 1.63 2.36 1.51 1.73 2.09 1.34 2.17 2.15 1.87 1.19 Null Null 0.80 1.30 1.30 1.00 Null 2.04 1.84 2.61 1.17 0.55 1.10 2.46 Null Null Null Null Null 2.54 0.66 2.36 2.88 1.70 1.12 2.01 2.24 1.57 0.80 Null Null Null Null Null Null Null

0.09 0.13 0.11 0.07 0.39 0.16 0.37 0.07 0.14 – – – – – 0.38 0.18 0.16 0.08 0.17 0.33 0.23 0.25 – – – – – 0.35 0.11 0.22 0.06 0.19 0.17 0.16 0.24 0.14 0.15 0.26 0.18 0.42 – – 0.40 0.40 0.20 0.11 – 0.14 0.28 0.30 0.27 0.23 0.22 0.53 – – – – – 0.30 0.19 0.46 0.68 0.08 0.11 0.17 0.59 0.10 0.00 – – – – – – –

1) 1) 1) 1) 2) 2) 2) 2) 2) 1) 1) 2) 2) 2) 1) 1) 1) 1) 2) 1) 2) 2) 1) 1) 2) 1) 1) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 1) 1) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2)

SFIR SFIR SFIR SFIR SFIR SFIR SFIR USOR USOR USOR USOR USOR USOR USOR USOR USOR USOR USOR USOR USOR VOLR VOLR VOLR VOLR VOLR VOLR VOLR VOLR VOLR VOLR VOLR VOLR VOLR VOLR VOLR VOLR VOLR VRGR VRGR VRGR VRGR VRGR VRGR VRGR ZOCR ZOCR ZOCR ZOCR ZOCR ZOCR ZOCR ZOCR ZOCR ZOCR ZOCR ZOCR ZOCR ZOCR ZOCR

04/11/26 05/03/02 05/04/10 05/05/14 05/06/13 05/06/15 05/08/16 05/03/02 05/05/14 05/08/07 05/09/26 04/11/11 04/11/15 04/11/26 05/02/05 05/06/13 05/06/15 05/08/16 05/10/29 06/01/02 03/10/31 04/07/25 05/03/02 05/05/14 05/06/15 05/08/07 05/09/26 05/10/29 04/01/11 04/02/05 04/09/05-A 04/09/05-B 04/09/06 04/11/26 05/02/05 05/04/10 05/08/16 05/03/02 05/04/10 05/05/14 05/08/07 05/08/16 04/11/26 05/02/05 04/02/21 04/07/25 04/09/06 04/11/11 04/11/15 05/02/05 05/03/02 05/04/10 05/10/29 04/02/07 04/09/05-A 04/09/05-B 04/11/26 05/06/13 05/06/15

– – – – – – – 321 336 306 136 – – – – – – – – – 197 330 307 334 132 303 150 342 – – – – – – – – – 297 309 308 272 290 – – 72 201 350 313 160 194 185 348 337 – – – – – –

– – – – – – – 2 8 12 6 – – – – – – – – – 4 11 5 5 4 9 6 6 – – – – – – – – – 3 15 12 2 2 – – 5 4 7 6 3 4 5 2 6 – – – – – –

Null Null Null Null Null Null Null 1.51 1.43 1.58 2.10 Null Null Null Null Null Null Null Null Null 2.02 0.82 1.07 2.83 2.95 2.88 2.44 1.53 Null Null Null Null Null Null Null Null Null 1.13 1.14 2.36 1.43 1.94 Null Null 2.79 2.61 2.2 1.98 1.90 1.09 1.14 2.96 1.69 Null Null Null Null Null Null

– – – – – – – 0.10 0.31 0.32 0.30 – – – – – – – – – 0.29 0.23 0.06 0.28 0.29 0.49 0.41 0.20 – – – – – – – – – 0.08 0.29 0.41 0.12 0.30 – – 0.15 0.27 0.31 0.21 0.19 0.13 0.13 0.16 0.19 – – – – – –

2) 2) 2) 2) 2) 2) 2) 1) 2) 2) 1) 1) 1) 1) 1) 1) 1) 1) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 2) 1) 2) 2) 2) 1) 1) 1) 1) 1) 1) 1) 1) 1) 1) 2) 2) 1) 1) 1) 1) 1) 1)

Also measurements from Salimbeni et al. (2007), labelled with 1) in reference column, are reported. For each measurement, the station name, the event name, fast axis direction, delay time and associated errors are listed.

Fig. 4a and b shows that, moving from the south-west to the northeast across the Apennines, the orientation of the fast shear-wave polarization direction changes from a dominant NW–SE (orogenparallel) trend to a NNE–SSW (orogen-normal) main direction. Using the Apennines crest as a boundary we can describe our data starting from the south-western part, i.e., the Tuscany side (Fig. 1), where the NW–SE directions are homogeneously distributed from the orogen toward the Tyrrhenian Sea, while a rotation to an E–W direction occurs toward Elba island (Fig. 1) and also south of it. These directions are corroborated by null measurements (Inset A in Fig. 4b). Delay

S. Salimbeni et al. / Tectonophysics 462 (2008) 68–82

times have values ranging from 0.6 to 3.3 s, with a 1.8 s on average. Fast velocity directions and delay times do not show a clear dependence on back-azimuths (Fig. 5a). In the eastern and north-eastern area, that we call the Adria region (Fig. 1), the pattern of fast axes is less uniform. A NNE–SSW direction prevails, but NW–SE and N–S fast-polarization directions are also detected. Several null measurements support these trends (Inset B in Fig. 4b). Moreover, a dependency on the back-azimuths, already detected for the stations belonging to the northern transect (Salimbeni et al., 2007) exists for all sites on the Adria side: blue symbols in Fig. 4a strike more frequently in a NW–SE direction and red symbols show mainly N–S and NNE–SSW directions. This behaviour would mean that easterly ray-paths sample mainly a N–S to NNE–SSW anisotropy, while westerly arrivals cross a medium characterized by NW–SE seismic anisotropy. In the Adria side delay times are 0.8 b dt b 3.2 s (1.8 s on average), without any noticeable trend. It's important to note that the same average delay time (1.8 s) is found in both sectors, as to indicate that the cumulative thickness of the anisotropic structures beneath the Tuscany and Adria domains is the same. RAVR and BARR are the two stations for which we have the richest dataset among those located further NE in the Adria domain. The variability of their anisotropy directions with back-azimuth shows a pattern close to π/2 periodicity (Fig. 5b). This suggests that the presence of double-layer anisotropy beneath Po-Plain should be taken into account. In Fig. 5 is represented, together with measurements, the expected behaviour of fast axis directions and delay times (computed using analytical equations by Silver and Savage, 1994 and Özalaybey and Savage, 1994) for three different models of double-layer anisotropy. Every model assumes horizontal symmetry axes, a cumulative thickness of about 200 km and ~ 5% of anisotropy (Mainprice and Silver, 1993). Among possible models we chose to plot only three patterns which are more similar to our measurements distribution. In Fig. 5b, solid line is for an upper and lower layer with respectively fast axis directions and delay times of 15° and 0.5 s, 135° and 1.0; dashed line is for an upper and lower layer with respectively fast axis directions and delay times of 190° and 1.0 s, 150° and 0.5 s; dotted line is for an upper and lower layer with respectively fast axis directions and delay times of 170° and 1.0 s, 135° and 0.4 s. This latter model shows the best similarity with the periodicity in the data. The number of events coming from different back-azimuths is however not sufficient to characterize the structure beneath each single station (not enough measurements at any of the stations), but the cumulative approach used here gives some more light to some hypotheses. The geographical variations observed for single measurements are confirmed by the orientations of averaged splitting directions computed as 1/2 arctan (∑sin2ϕ/∑cos2ϕ) for each station (Fig. 6). Fast-axis directions rotate gradually from E–W to NW–SE in the Tuscany region, then across the Apennines change to N–S and NNE– SSW in the Adria side. The clockwise rotation is gradual in the southern part of the study region, while more abrupt in the northern part, where the network extends toward the Po-Plain. The transition between the two domains is abrupt, about 30 km (Salimbeni et al., 2007) when crossing the orogen along the northern transect. This feature is confirmed along the northernmost part of the study region where, even with larger distances between stations, we see sharp variations in the anisotropy directions between sites separated by 50– 60 km (e.g., between PRUR and GUSR or FOSR and CORR, Figs. 2 and 6). Moving southward, along the southern transect, the rotation from one domain to the other occurs between stations 80–100 km away (i.e. compare CRER and FSSR). This suggests that the transition in the southern portion of the RETREAT field area, along the southern transect, could be more gradual than in the north, even if this characteristic could be related to the different distance between stations. Another feature underlined in the station-averaged birefringence is the non-homogeneity of the anisotropy of the Adria region compared to the Tuscany side. Comparing, for individual stations, average fast-

75

polarization directions (red arrows on rose diagrams of Fig. 6) with fast direction frequency distributions (rose diagrams in Fig. 6), we often find a difference between the most frequent direction and the average one. This occurs at sites located in the transition between the two domains (for instance PNTR, GUSR, RONR, FIRR; Fig. 6) or located toward the PoPlain (e.g., CSTR, MNGR and RAVR). This discrepancy is more evident when the dispersion of rose diagram is greater, adding more constraints on the hypothesis of structure heterogeneity of this region. In the Tuscany sector, the average and most frequent directions are often nearly the same (e.g., from FOSR, to MAON along the Tyrrhenian coast; Fig. 6), supporting the idea that measurements here reflect a more homogeneous structure at depth. What seems more interesting in the averaged-birefringence pattern is the regular rotation we observe along the eastern part of the study region, from nearly NNE–SSW trend in central Apennines (e.g., stations PESR, RSMR and SFIR, but see also data from the NAP 8 measured by Margheriti et al., 1996) to a more NE–SW trend in the Po-Plain (i.e., ANZR and RAVR). This is a characteristic feature of sites located on the Adria plate region. 5. The anisotropy distribution at depth The regional patterns found for the direction of fast polarization have several possible interpretations that we could better discuss having some knowledge of the depth distribution of the anisotropy. As a first approximation, we should take into account that the thickness of the anisotropic medium crossed by our seismic signals should be nearly 200 km, assuming that it is influenced by ~ 5% anisotropy (Mainprice and Silver, 1993) with horizontal symmetry axis and considering that the average delay time over all the network is 1.8 s. To estimate the depth of seismic anisotropy we study lateral variations of single measurements, having determined them for a dense array. We calculated the Fresnel zones for some of the stations following the approach of Pearce and Mittleman (2002), a method which assumes a horizontal fast axis and a lateral variation of the anisotropic properties of the medium. In the Northern Apennines we likely have an almost vertical subducting slab (Lucente et al., 1999), thus it is plausible to assume lateral variations in the anisotropic properties of the mantle; moreover this theory is supported by the existence of geographical domains with different but internally homogeneous anisotropic parameters. The lateral variation of the anisotropy below a single station, as expressed in variable anisotropic parameters with horizontal symmetry axis, can be resolved by analyzing two teleseismic earthquakes with opposite back-azimuths (Fig. 7a and b). The depth defined in this case (Z1) corresponds to the depth above which the ray-paths sample the same medium and below which paths are different. If we use observations of only one teleseismic earthquake recorded at two or more nearby stations (Fig. 7c and d) we can evaluate the depth (Z2) where the two ray-paths start to separate. Differences in birefringence parameters would be due to an anisotropy located at a depth above the point where paths separate. The Fresnel zones at depth h are calculated following (Pearce and Mittleman, 2002): Rf=

1 pffiffiffiffiffiffiffiffi Tvh 2

where Rf is the radius of the Fresnel zone expressed in km, T is the dominant period of the wave, ν is the wave velocity. We choose 10 s for the dominant period from waveform observations and the shearwave velocity of the S phases from IASP91 model (4.476 km/s at 50 km depth, 4.49 km/s at 100 km depth, 4.45 km/s at 150 km depth and 4.51 km/s at 200 km depth). We applied this method for some stations located within the transition between Tuscany and Adria domains (Fig. 7). For station RONR (Fig. 7a), located close to the Apennines crest, we obtained two

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S. Salimbeni et al. / Tectonophysics 462 (2008) 68–82

77

Fig. 5. Back-azimuth vs splitting parameters pattern. Black symbols are used for SW back-azimuth events, gray for NE ones. Null measurements are plotted with empty symbols and with back-azimuth as fast axis. No null measurements are plotted in delay time vs back-azimuth map. a) Measurements for PIIR, CSNR and ELBR, that are Tuscany stations, b) measurements for stations RAVR and BARR, located in the Po-Plain. On the plot at right are reported the expected behaviour of fast-polarization azimuths and delay times for three different models of doublelayer anisotropy determined for anisotropy parameter values reported in the label (see text).

different measurements analysing two events with opposite backazimuth. The Fresnel zones reveal that this difference may be due to a different anisotropy sampled by the ray-paths below a depth of 150 km, at which rays separate (and circles in Fig. 7a and b do not cross anymore). In the case of station PIZR (Fig. 7b), located close to RONR, we can observe that beneath the crest of the chain the anisotropy has a lateral variation below a depth between 100 and 150 km: in the west (Tuscany domain) the fast polarization is NW–SE and in the east (Adria domain) it is nearly N–S. We consider also the ray-paths for two different events recorded at nearby stations to constrain the lower boundary of the region where anisotropy changes laterally: at the transition between Tuscany domain and Adria domain no resolving examples were found. On the other hand, in the Adria side, at stations RAVR and MTVR measured anisotropic parameters for the same event are different (Fig. 7c and d). This difference is due to changing anisotropy sampled above 50 km of depth, where the paths of the two rays separate significantly (and circles of Fig. 7c and d do not cross anymore). This behaviour occurs for two events coming from different back-azimuths, reinforcing the idea that another lateral variation occurs in the upper 50 km in the Adria domain between the chain and the Po-Plain. These considerations about the distribution of the anisotropy at depth can also be compared to mantle velocity structure depicted by seismic tomography. Plotting possible SKS ray-paths over a tomographic section we can define which structures are sampled. The two tomographic sections (Fig. 8; Piromallo and Morelli, 2003) cross the Northern Apennines in two different directions. In the green section,

the E–W directions measured at station ELBR can be certainly attributed to the mantle wedge that is mainly crossed by the raypaths (black lines in Fig. 8). This interpretation can be extended also to measurements done for stations south of 43.5N, from GROG to MAON (Fig. 2). The NW–SE directions found at station PIIR instead can be related mainly to the slab and mantle immediately below the slab, taking into account that mantle wedge is too thin beneath this station compared to the 200 km thickness needed by birefringence delay times. Beneath station RONR, the Fresnel zones analysis demonstrates the presence of a lateral variation between two anisotropy domains (NW–SE and N–S) below 150 km. Following SKS paths beneath this station we can suppose that events from the west sample the anisotropy located in the slab and in the mantle just beneath it (NW–SE) and those from the eastern back-azimuths sample the Adria mantle (N–S to NNE–SSW). We hypothesize that the mantle below Adria just beneath the slab and at some distance from it underwent different deformations. The mantle just beneath the slab was deformed directly by the retreating process (Buttles and Olson, 1998) showing a fast direction parallel to the strike of the slab while farther in the foreland the dominant N–S fast direction cannot be ascribed to the same process. For station RAVR, all SKS rays sample the mantle beneath Adria, in spite of their variability. In the red section (Fig. 8) we observe that some rays reaching the station ELBR from the east sample slab anisotropy; very few measurements (Fig. 4a and b) show a NW–SE rather than an E–W direction. For station CSNR the SKS splitting data sample the slab and sub-slab mantle deformed by retreat, and show homogeneity of NW–SE fast axes. Station RSMR is

Fig. 4. a) Map of the splitting measurements reported in Table 3. Red color is used for NE back-azimuth events (from −45° to 135°) while blue ones for SW events (from 135° to −45°). Single measurements are represented with a line oriented with fast-polarization azimuth and scaled with the delay time, mapped at the piercing point of 150 km of depth to give a better distinction between measurements done at the same station and to make azimuthal differences more readable. Shaded zones are Tuscany and Adria domains. b) Map of the null measurements reported in Table 3. Single null measurement is mapped with a cross oriented to the back-azimuth of events. Diamonds represent seismic stations. Insets A and B contain splitting and null measurements for stations inside the two black boxes in the main map.

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Fig. 6. Red symbols represent the average fast-polarization direction. Dark red is for sites for which the average direction has been computed over less than 4 measurements. Black symbols in the background represent single shear-wave splitting measurements from previous papers (Margheriti et al., 1996; Plomerová et al., 2006; Lucente et al., 2006 and reference therein). Boxes include normalized rose diagrams of fast directions for single stations: in blue is the rose diagram, in yellow is the most frequent slice (pointed by two yellow triangles) and the overlapped red arrow is the average direction. Upper left box: rose diagrams for the most representative sites of the transition zone. Lower left box: rose diagrams for the most representative sites of the Tuscany domain. Right box: rose diagrams for the most representative sites of the Adria domain.

above a less perturbed portion of the Adria domain where the only source of anisotropy has roughly N–S fast direction. 6. Discussion In this study, we have increased the number of splitting measurements in the Northern Apennines region. Previous studies were localized along the northern transect (Salimbeni et al., 2007) or had only sparse measurements (Plomerová et al., 2006). New results confirm the presence of two domains with different anisotropic pattern: in the Tuscany and Adria regions. The crest of the Apennines marks the boundary between these two domains, along the whole RETREAT deployment, as found in Salimbeni et al. (2007), at the northern transect. In the southern part of the study region, our results are similar to those obtained in previous studies (Margheriti et al., 1996; Barruol and Granet, 2002; Barruol et al., 2004), along which the typical slab-rollback anisotropy directions were found. A denser dataset than the one reported in Plomerová et al. (2006), allowed us to see how the anisotropic parameters vary along the whole region and to interpret this variation in the general frame of the subduction system. We assumed that most of the detected anisotropy reflects mantle deformation due to the evolution of the trench-retreat/slab-rollback system which produced the Northern Apennines, excluding the possibility that detected anisotropy is related to absolute plate motion as hypothesized by Smith et al. (2001). The pattern of fast-polarization directions characteristic of a retreating-trench system, parallel to the trench beneath the mountains and perpendicular to it in the back arc basin (Fig. 9a and b), is not present in the north of our study region (Fig. 9c). To explain this observation, we relate the anisotropy pattern to the history of the trench-retreat (Faccenna et al., 2004). When the trench was still striking N–S, west of Corsica, we can suppose that the slab was retreating

eastward, normal to the Corsica–Sardinia axis (Fig. 9a). Subsequently, the process of slab-rollback and trench-retreat has proceeded but the prevalent retreat direction was E–W, normal to the southern edge of Northern Apennines (SNA in Fig. 9b and c; Lucente and Speranza, 2001). At the same time, the northernmost part of subduction (NNA in Fig. 9c) moved in an oblique direction relative to trench strike, pulled by nearly eastward rollback to the south (NNA in Fig. 9c). The most recent history of the Northern Apennines would then include subduction with a differential rollback. A maximum eastward retreat would act beneath the southern edge of Northern Apennines, perpendicular to the trench in the center of the arc; at the northernmost corner an ancillary retreat, oblique to the trench, would consequently occur. The back arc region of the corner edge was too small to produce an orogen-normal deformation. This mechanism explains our observations better than a radial homogeneous retreat along the Northern Apennines arc which would produce the pattern in Fig. 9b. We can observe also that the Northern Apennines orogen is less developed; it is about 300 km wide from the Tyrrhenian Sea to the PoPlain, compared to its southern edge, where the north-eastward retreat produced a 600 km wide system. Also at depth the sub-slab mantle deformed by rollback seems to be narrower beneath the northern most part of the Apennines, where the slab is also more vertical compared to what tomography images show beneath south-Northern Apennines (Fig. 8). The less advanced evolution of the Northern Apennines retreat may be due to several reasons among which we firstly would list its position near the end of the chain. Also it is possible that the Adria plate, neither oceanic nor transitional in lithology, resisted subduction, becoming an obstacle for the retreat process. The composition and thickness of the Adria microplate is not known to vary greatly along the Northern Apennines, however, so kinematic factors may be

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Fig. 7. Fresnel-zone analysis for some stations located along the northern transect (a and b) and in the Po-Plain sector (c and d). Different dimensions of circles correspond to Fresnel zones calculated at 50, 100, 150 and 200 km respectively. For each station–event pair also the fast-polarization azimuth is mapped at 50 km pierce point. Assuming horizontal symmetry axis, relevant circles to determine values of Z1 (depth below which difference in anisotropy are sampled) and Z2 (depth above which difference in anisotropy are sampled) are in white. Upper central inset: sketch of the determination of Z1, applied in examples a and b. Lower central inset: sketch of the determination of Z2, applied in examples c and d.

more important. The Alps lie close to the north, and may have behaved as a barrier for the retreat to proceed. The Adria domain is characterized by a N–S anisotropic fast polarization in its southern part, in agreement with data from previous studies (Margheriti et al., 1996, 2003) and by more scattered fastpolarization directions to the north, in the Po River Plain. This variability is representative of a complex structure at depth that is possibly resolved by the double-layer anisotropy pattern, found for some sites of the PoPlain. Double-layer models would suggest a lithospheric contribution, in agreement with the results obtained from Fresnel zones analysis (Fig. 7c and d) and with Plomerová et al. (2006). Another possible interpretation of our measurements could be linked with the presence of a discontinuity in subducting slab. A tear in the slab beneath the northernmost corner of the Apennines orogen, and mantle flow through it, would be a possible consequence of the shear occurring during the oblique retreat in the northernmost part of the Apennines. Such mantle flow could explain the NNE–SSW directions detected for the Po-Plain sites. However, the absence of any disruption of the NW–SE direction found for all measurements on the Tuscany side does not support a fully developed slab-window flow. If the tear exists, it is in an early stage and no mantle throughflow has occurred.

7. Conclusions A large set of new SKS splitting anisotropy measurements for the Northern Apennines adds detail to geodynamic interpretations of recent orogen history. The Tuscany and Adria domains, defined by anisotropy fast-polarization directions, appear to be closely related to the Apennines orogen that divides them, with a change in the anisotropy characteristics which seems more abrupt in the northern part of the study region. The Tuscany domain is characterized by homogeneous measurements, with NW–SE to E–W dominant directions, and it includes supra and sub-slab, as well as slab anisotropy, that can be discriminated by location and depth distribution of SKS ray-paths. The Adria domain is instead dominated by more heterogeneous measurements, with N–S to NNE–SSW directions mainly (but also others detected), which represent a complex structure at depth, with lateral and vertical variations. The characteristics of our measurements allow us to describe a history of time and space evolution of the rollback system, which includes also the different stages of mantle deformation that we detect with seismic anisotropy measurements. We suggest that slab-rollback, which created the Apennines and opened the Tyrrhenian Sea, evolved in the north boundary of Northern Apennines in a manner different

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Fig. 8. Tomographic sections across the Northern Apennines (from Piromallo and Morelli, 2003) and schematic depiction of regions sampled by SKS ray-paths ascending towards stations on the surface. 100 and 410 km depths are drawn by thin lines. Lower left: map of stations reported along the sections, with orientation of fast-polarization azimuths at these sites (in blue). Also section lines (red and green) are mapped. Lower right: section lines.

from its southern part. In particular, the trench-retreat developed primarily south of our study region, with an eastward rollback. In the northern portion of the orogen, after a first stage during which the retreat was perpendicular to the trench, it became oblique with respect to the structure. The process continued as a secondary effect of the energetic slab-rollback in the southern edge of Northern Apennines. A slowdown of slab retreat produced the characteristics evidenced here: the orogen is narrower compared to its southern portion and the deformation of the mantle due to the slab retreat is located beneath the inner part of the chain instead of beneath the crest, as occurs to the south of our field area. Acknowledgments This dataset was collected in collaboration of other members of the RETREAT Seismology Team (Alessandro Amato, Jaroslava Plomerová, Vladislav Babuška, Davide Piccinini, Nicola Piana Agostinetti, Luciano Giovani, Francesco Pio Lucente, Andrea Fiaschi, Petr Jedlicka, and Peter Ulbricht). We thank L. Vecsey (Geophysical Institute, Prague) who provided the code used in the analysis and Claudia Piromallo for the tomographic sections. We use GMT software (Wessel and Smith, 1998) to prepare the maps. V. Levin and J. Park were supported by NSF grants EAR-0208652 and EAR-0242291, respectively. References Audoine, E., Savage, M.K., Gledhill, K., 2000. Seismic anisotropy from local earthquakes in the transition region from a subduction to a strike-slip plate boundary. N. Z. J. Geophys. Res. 105 (B4), 8013–8033. Barruol, G., Granet, M., 2002. A Tertiary asthenospheric flow beneath the southern French Massif Central indicated by upper mantle seismic anisotropy and related to the west Mediterranean extension. Earth Planet. Sci. Lett. 202 (1), 31–47.

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