Seismicity and P-wave velocity image of the Southern Tyrrhenian subduction zone

Geophys. J. Int (1995) 121, 818-826 Seismicity and P-wave velocity image of the Southern Tyrrhenian subduction zone G. Selvaggi and C. Chiarabba Isrirm Nazionale di Geofkica, Via di Vigna Muratu 60.5. 00143 Rome, Italy Accepted 1994 December S. Received 1994 December 5; in original form 1994 May 30 SUMMARY This paper focuses on the deep Earth structure and earthquake distribution within the Southern Tyrrhenian subduction zone. We discuss seismological observations which provide insight into the mechanics of subduction processes for an unusual compressional margin. The motivation for this work derives from the recognition that little is known about one of the most intriguing geodynamic features of the Italian Peninsula, and the fact that a large amount of information can be extracted from data collected by the Istituto Nazionale di Geofisica National Seismic Network. We show that the hypocentres of intermediate and deep earthquakes that have occurred since 1988 image a NW-dipping plane, continuous from the Ionian Sea to the Tyrrhenian, down to 450 km depth. We also present a 3-D model of the deep structure beneath southern Italy, derived using deep earthquake seismic tomography. Tomographic images reveal a high-velocity body dipping from the Apulian and Iblean forelands towards the Tyrrhenian Basin in the crust and upper mantle (at least down to 320 km depth). This high-velocity body may be related to the cold, subducted Ionian lithosphere. Earthquake hypocentres are confined within the high-velocity body, delineating the subduction beneath the Calabrian Arc. Key words: deep structure, seismicity, Southern Tyrrhenian, subduction. INTRODUCTION The aim of this paper is to provide evidence for the active processes of the Southern Apennines-Tyrrhenian system from seismological studies. We analyse the recent deep Tyrrhenian seismicity to improve knowledge of the Benioff zone in this region, first suggested by Caputo, Panza & Postpischl (1970), Ritsema (1972), Gasparini et nl. (1982), and more recently extensively studied by Anderson & Jackson (1987) and Giardini & Veloni (1991). We used data collected by regional Italian stations (most of them belonging to the ING National Seismic Network). In our opinion, the use of these data (available only for the past six years) strongly improves the location of deep events. Moreover, arrival-time data of deep focus events allow us to image the P-wave velocity structure of the crust and upper mantle beneath the Southern Tyrrhenian area. Seismic tomography has emerged as a powerful tool with which to extract information on crustal and mantle heterogeneities in different geodynamic environments (see, for example, Iyer Lk Hirahara 1993). Images of subduction zones and continental collision areas have been computed during the last few years by using this technique (Roecker 1982; Abers & Roecker 1991; Zhao, Hasegawa & Horiuchi 1992; Hirahara & Hasemi 1993; Roecker 1993). Recently, upper mantle images of the Italian Peninsula computed using teleseisms (Amato, Cimini & Alessandrini 1990; Amato, Alessandrini & Cimini 1993a) and including regional events (Spakman 1990; Spakman, van der Lee & van der Hilst 1993) have revealed the presence of a deep high-velocity body beneath the Southern Tyrrhenian Sea, interpreted as a subducted plate. The geometry of this body still remains ill-defined, due to inadequate model resolution. In particular, Spakman et al. (1993) observed lateral velocity perturbation of only about 1 per cent, imaging velocity anomalies much smoother than usually recovered beneath subduction zones (Hirahara & Hasemi 1993). Amato et al. (1993b) found a high-velocity NW-dipping body, with velocity contrasts of about 5 per cent, but their spatial definition of the slab was limited by the teleseismic waveforms used. In these studies the dimensions of cells in the 3-D model are about 80 X 80 km (horizontally). Thus, our purpose in this study is to impove the spatial resolution of the slab image, better defining its extent and geometry. To achieve this, we use only selected events, a 3-D parameter separation inversion technique with earthquake 81 8

Southern Tyrrhenian subduction zone 819 relocation at each iteration step, and a robust 3-D tracer (see Zhao el a/. 1992). GEOLOGIC OUTLINE Different models have been proposed to explain the very complex geodynamic system of the Southern Apennines- Tyrrhenian area (Fig. 1 ). Two main simultaneous processes act in the region (Patacca & Scandone 1989). An extension in the Tyrrhenian Sea started about 14Ma, producing a crustal stretching of about 350km, and development of oceanic crust in the internal region (Scandone 1980; Malinverno & Ryan 1986). Contemporaneously, compressional tectonics due to the convergence of the African and European plates produced a crustal shortening and thickening of the Apenninic chain (Patacca, Sartori & Scandone 1990). Thus, while the hinterland is actually controlled by extension related to the Tyrrhenian spreading, the external units undergo compression (Malinverno & Ryan 1986). From Pliocene. this extensional tectonics developed along the whole Tyrrhenian margin of the Apennines. producing several grabens, and Quaternary volcanoes (Lavecchia 1988: Beccaluva et al. 1989). In particular, our study area (Fig. 1) seems to be controlled by the back arc migration of the Tyrrhenian Sea. due to the sinking of the oceanic Ionian lithosphere beneath the Calabrian arc (Malinverno & Ryan 1986; Makris, Nicolich & Weigel 1986; Patacca & Scandone 1989: Royden, Patacca & Scandone 1987; Doglioni 1991). Calc-alkaline Quaternary volcanism related to the subduction process formed the Aeolian Arc (Barberi et a/. 1973). The definition of active processes within the slab is of primary importance for our purpose. In fact, one of the main problems regards the sinking of the subducted lithosphere, and the presence of a slab detachment. We will demonstrate that while an aseismic Apulian subducted plate is evident in the deep structure beneath Apulia and Campania, the subduction of the cold Ionian lithosphere is seismically active beneath the Calabrian Arc. SEISMOLOGICAL OBSERVATIONS The Southern Tyrrhenian subduction zone is the region of the Italian peninsula of highest seismic energy release, where many mh 3 5 earthquakes are recorded each year. During this century, the largest earthquake (mh = 7.1) occurred on 1938 April 13, at 290km depth. Tens of intermediate and deep earthquakes, as deep as 500km, 42' - \ /f 41"- t 40-39"- Jyrrhenian Basin..-I --I.- Calabriab metamorphib 38" - 37" - 0 km I L~ Foredb- --+ L, - Thrust front itania nnn iunian -n, - Plate \Forel$nd 10", 11", 12" 13" 1 4" 15" 16" 17" 18" Figure 1. Sketch of the tectonic units of the Tyrrhenian-Southern Apennines system.

820 G. Selvaggi and C. Chiarabba occur yearly. Prior to 1988, due to an inadequate number of seismic stations that recorded deep events, the hypocentre locations were not reliable. A well-distributed seismic network close to the hypocentral area is necessary for a good depth-control in the calculation of hypocentral coordinates. The quality of ING data and the number of seismic stations increased after 1988, allowing us to analyse a more complete data set for the Southern Tyrrhenian seismicity. In this study, we selected 185 intermediate and deep earthquakes recorded since 1988 for which at least more than 16 P-wave arrivals at 'close stations' (i.e. array width comparable with respect to the hypocentral depths of the seismicity) are available. We use 63 stations, mainly vertical component, belonging to the Centralized National Network of the ING, and to other local networks. Two three-component stations, one in south Calabria and the other in eastern Sicily, provide good-quality S-wave arrivals. Figs 2 and 3 show the overall network geometry. Since our attention is concentrated on the intermediate and deep events, we removed all the shallow crustal seismicity from our analysis. In order to reliably locate deep earthquakes, we optimized a l-d model, derived from the Herrin tables (1968), perturbing the velocity values to minimize the P-wave residuals. We used these minimized residuals for computing the 3-D model (see next section). Table 1 summarizes the difference between the starting and the final model. We observe slight velocity variations, mainly in the deepest part of the model (-2 per cent), and a variance improvement of only 1 per cent. This l-d model is also used as a starting point for the final 3-D inversions. The errors associated with the hypocentral coordinates are generally within a few kilometres (-5.0 km), the rms of the residuals less than 0.9 s and the azimuthal gap less than 180". We compared our solutions with the locations reported in the ISC bulletins, finding a general significant improvement in the hypocentral errors. The map view of Fig. 2 shows that the intermediate-depth earthquakes occur mainly in the Ionian Sea and in the Calabrian Arc, while in these two zones there are no deep events. Only one earthquake occurred inland at 350km depth. The deep seismicity is mainly concentrated in a north-westward dipping 'tongue' offshore the Calabrian Arc, 200 km laterally extended and 50 km thick (see Fig. 2). A NW-SE cross-section of the Tyrrhenian subduction zone is plotted in Fig. 2 in which a continuous 70" dipping plane is well delineated by seismicity distribution at depth. A small gap of seismicity is located between 180 and 200 km depth. The deepest earthquake we could locate well occurred at 450 km depth. We observe no apparent 'spoon-shaped' geometry of the Tyrrhenian Benioff zone nor the reduction in steepness of the slab to a dip of -45" at a depth of approximately 250 km as delineated by different authors using ISC or USGS data in the period between 1961 and 1988 from local, regional and mainly teleseismic stations (Gasparini et al. 1982; Iannaccone, Scarcella & Scarpa 1985; Anderson & Jackson 1987; Giardini & VelonA 1991). P-wave residuals The P-wave traveltime residuals provide qualitatively relevant information on the velocity pattern within the studied region. Fig. 3 shows the average traveltime residuals for all stations recording the 185 earthquakes. Since the average residuals are not azimuth-independent, they do not only reflect shallow structures, but also maintain a significant amount of information on the anomalies along the whole ray path. We observe strong negative residuals, commonly related to high-velocity paths, along the Apulian foreland with values ranging between - -1.2 s and - -0.8 s. Early arrival times are also observed in the Iblean foreland. Along the Apenninic chain, we recognize a belt of delayed arrivals (up to 0.8 s) continuous from central Italy to western Sicily. An inner arc of weak negative residuals borders the Tyrrhenian coast (-0.3 s to -0.6 s). The residual distribution and the strong difference between the three alternate belts of negative and positive residuals are indicative of large heterogeneities in the crust and upper mantle, which could be modelled using the 3-D tomographic inversion. The strong negative residual belt that characterizes the foreland may be related to fast ray paths travelling within the high-velocity slab, while the positive residuals observed at the Apenninic stations may be generated by slow ray paths travelling in a low-velocity crust and upper mantle beneath the Apenninic chain. 3-D MODELLING: INVERSION PROCEDURE In the tomographic inversion, we used the technique developed by Zhao et a/. (1992), in which the velocity model is defined by both a 3-D grid of nodes and velocity discontinuities. The gridded model space allows us to represent the earth structure reasonably well, with velocity values continuously interpolated, while the use of the velocity discontinuities within the medium ensures a good modelling of seismic ray paths at regional distance. The computation of traveltimes is supported by a fast and robust approximate 3-D ray tracing. The starting ray path is perturbed with a pseudo-bending algorithm for the continuous model, and according to Snell's law for the discontinuous part. The details of the technique, and the ray-tracer accuracy, are given in Zhao et al. (1992). The ray tracer allows us to obtain a reasonable approximation for the arrival times of regional phases through the whole region. We believe that this kind of parametrization and the approximate 3-D ray tracing are favourable when regional ray paths are used, as in our case. We used the LSQR algorithm of Paige & Saunders (1982) to invert the large and sparse matrix resulting from our data. The non-linear problem is solved by iteratively computing linear equations. In each iteration, hypocentral parameters and velocity perturbations are simultaneously determined. Details of the local earthquake technique are summarized in Zhao et al. (1992). Results We parametrized the volume underneath the Southern Tyrrhenian region, using the available a priori information, with five layers located at 25,80, 160, 240 and 320 km depth, and the velocity discontinuities of the l-d model. We chose a spacing of 30 km between horizontal nodes in the grid, optimizing the model parametrization. We inverted 2777 arrival times from 178 (out of the 185 events in Fig. 2) deep

Southern Tyrrhenian subduction zone 821 - [ Depths (km) I 0 0 v 100.) 0 150 200 250 8 300 NW Calabrian SE Arc E h Y W 5 Q E 200-300 500 (b) I I I I 0 100 200 300 400 Distance (km) Figure 2. (a) Hypocentral map of Southern Tyrrhenian seismicity, from 1988 to 1993. (b) NW-SE vertical section, showing the events located within the box on (a).

822 G. Selvaggi and C. Chiarabba Residuals distribution 10", 11", 12" 13" 14" 15" 16" 17" 18" Figure 3. Distribution of the average P-wave residuals for all the seismic stations. Negative values (early arrivals) are evident in the Apulian foreland, the Iblean foreland, and in the peri-tyrrhenian area. Positive residuals (delayed arrivals) characterize the Apenninic chain. Table 1. Starting and final I-D models. The optimized 1-D model (right) is used for the 3-D inversions. Starting Vp (km/s) Top of layer (km) Final Vp (km/s) 6.4 0.0 6.5 8.0 35.0 8.0 8.1 100.0 8.1 8.3 180.0 8.2 8.6 280.0 8.4 91 400.0 8.8 events that occurred in the Tyrrhenian Sea between 1988 and 1994. The earthquakes have been selected considering hypocentral errors (less than 10 km), rms residuals (less than 0.9s), and at least 16 P-wave arrival times. We chose the damping parameter for the 3-D inversion looking at the trade-off between data and model variance at the first iteration (see Fig. 4). The residual rms is 0.19s with a variance improvement of 65 per cent achieved after five iterations. Fig. 5 shows the 3-D velocity model. Layer I, at 25 km depth, resembles the main crustal heterogeneities. We find high-velocity anomalies beneath the Apulian and Iblean foreland. Note that these anomalies are shifted toward the Tyrrhenian Sea with respect to the foreland location at the surface, suggesting a high-velocity crust deepening toward the Tyrrhenian. Adjacent to these features, we observe an arc of low velocities beneath the Apennines. This arc is extended from the the Gela-Catania Foredeep in Sicily, to Calabria. The sharp velocity contrast between these two (high-velocity and low-velocity) regions is inferred to be the transition between two different crust types, one belonging to the external Ionian Plate, the other to the Apenninic Chain. A similar feature has been found by crustal refraction studies (Nicolich 1989). In Layer 2, at 80 km depth, a high-velocity body is located beneath the Apennines from north-eastern Sicily, to Calabria (where we observe the highest velocities) and slightly to Campania. The low-velocity anomaly beneath the Gela-Catania Foredeep is still present. This low-velocity zone has already been observed by Amato et al. (1993a, b) from teleseismic tomography, and is interpreted to be related to a thermal anomaly in the upper mantle. Layer 3, at 160 km depth, is our best image of the deepening slab. We find that high velocities characterize a wide area beneath the Tyrrhenian Sea. A NNW-SSE trending high-velocity body is continuous from Campania to Calabria, bending towards west to the north-eastern side of Sicily. Velocity values for the P wave are -8.6 kms-'. Beneath the Calabrian Arc, lower velocities are imaged (V, - 7.9 km s-'), probably related to the presence of a low-velocity asthenospheric channel. In Layer 4, at 240km depth, we observe that the high-velocity slab is shifted towards the centre of the

Layer 1 = 25 km Layer 2 = 80 km Figure 5. Velocity tomograms of the five layers. Earthquakes occurring within the layers are plotted on the velocity images. The seismicity concentrates within the high-velocity regions.

Layer 5 = 320 km Figure 5. (Continued.) Figure 8. NW-SE vertical section of earthquake hypocentres and velocity structure between the Tyrrhenian and Ionian Seas. Hypocentres are confined within the high-velocity descending slab.

Southern Tyrrhenian subduction zone 823 0.590 0.350-0 10 0.075 0 Squared model length (km/s)**2 Fiqure 4. Trade-off between residual data variance and squared model length computed using different values for the damping parameter (100.50,...). A damping factor of 20 has been used for the 3-D inversion. 50 Tyrrhenian Basin and in the Campania offshore, and is characterized by smaller horizontal dimensions. This body is still surrounded by a ring of low-velocity anomalies, probably indicating the deeper part of the asthenosphere. In Layer 5, at 320 km depth, the high-velocity zone moved further towards the north-west. Velocity values are -9.0 kms-'. Low-velocity anomalies south of the main high-velocity zone probably constrain the location and dimension of the former feature. Below 320 km depth, the velocity images became ill-defined due to a poor sampling of ray paths. Thus. any velocity contrast deeper than 320 km depth cannot be recovered by our data. Resolution A verification of the reliability of the depicted images is crucial in seismic tomography. This need led us to analyse the model resolution, assessing the adequacy of ray coverage. The total number of hits for each node within the modelled medium is generally more than 100 (mostly between 400 and 900). However, although we use a large number of observations (2777) with respect to the number of inverted nodes (279). the ray-path geometry (almost close to the vertical) suggests that our computation suffers from vertical smearing of anomalies. In order to define the spatial resolution limits better, we executed synthetic tests. We assigned positive and negative velocity perturbations of 2 per cent to each node in a checkerboard fashion (Humphreys & Clayton 1988: Zhao, Hasegawa & Horiuchi 1992; Chiarabba. Malagnini & Amato 1994) We used the actual configuration of earthquakes and stations. Traveltimes for this model were computed, and inverted subsequently (adding random noise to the data) using a homogeneous starting model. Although Leveque, Rivers & Wittlinger (1993) demonstrated that such a test may fail in 'not unrealistic circumstances', we believe that the checkerboard test is still a powerful tool to image the ray sampling within the model. Figure 6 shows the comparison between the original checkerboard model and the inversion result for two of the tive layers (the others three layers present similar features). Although we locally observe a slight horizontal coupling between adjacent nodes in the periphery of the inverted area, and vertical smearing of anomalies is evident in part of the model. we are pleased with the resolution of the gross structure. The main central volume appears to be adequately sampled by the ray paths, suggesting a reasonable reliability of the depicted images. EARTHQUAKES AND EARTH STRUCTURE Slab event hypocentres have been considerably improved using the heterogeneous 3-D velocity model. We find that the rms residuals strongly decrease (with respect to the rms achieved using the laterally homogeneous velocity model) for earthquakes in the depth range 300-450 km (positive values in Fig. 7), while exhibiting no large differences with

824 G. Seluaggi and C. Chiarabba RMS differences Figure 7. Differences between RMS of the residuals obtained using the homogeneous 1-D model, and using the heterogeneous 3-D model. Positive values (dark colours) imply an improvement of earthquake location using the 3-D model. A clear benefit is gained for events deeper than 200 km. Figure 6. Checkerboard test: comparison between the synthetic model (in the centre) and the recovered images at 25 km (top) and 160km (bottom). In the modelled area, we resolve the gross features, although we note a smearing of anomalies, due to the ray-path geometry. respect to the 1-D locations for the shallower events (see Fig. 7). Hypocentral errors using the 3-D model are generally less than 2 km in the horizontal directions and 5 kin in depth. Intermediate and deep earthquakes are concentrated in a narrow zone, 200 km laterally extended and 50 km thick, beneath the Calabrian Arc, approximately down to 450 km depth. We observe that the events between 40 and 100 km depth are located beneath the Ionian Sea and the Calabrian Arc, delineating a subhorizontal seismic zone. The deep seismicity is confined in a -70" NW-dipping zone beneath the Southern Tyrrhenian Sea, mainly between 100 and 320km, while below this depth only few and sparse earthquakes occur. The Apulian and Iblean forelands seem to be aseismic, and the main deep seismic release occurs only beneath the Tyrrhenian Sea. The 3-D inversion results show a high-velocity dipping body, approximately continuous in the upper mantle. At 25 km depth, the Apulian and Iblean forelands are characterized by a high-velocity anomaly, which probably reflects the shallow part of the dipping body (Fig. 5). We relate this high-velocity body to a cold slab, representing the main feature of the Southern Tyrrhenian subduction zone. The dimension of the slab yielded by our best image (Layer 3 in Fig. 5) is -60 km thick and -250 km laterally extended. The slab is characterized by velocity values ranging from 8.6 to 9.0 km s-l, in agreement with results obtained for other compressional margins (Zhou & Clayton 1990; Hirahara & Hasemi 1993; Roecker 1993). The velocity contrasts are much larger than previously determined by Spakman et al. (1993), and similar to those obtained by Amato et al. (1 993b). The joint analysis of earthquake hypocentres and velocity model (see Fig. 5) results in a better insight both into the seismogenic phenomena, and the active geodynamic processes. We find that the events occur within the high-velocity slab, deepening towards the north-west (Fig. 8). Our observations show that seismicity occurs not only at the plate boundary, but within a large portion of the descending slab (Fig. 8). Intermediate-depth events occur within a high-velocity zone between 40 and 80 km depth

Southern Tyrrhenian subduction zone 825 beneath the Ionian side of the Calabrian Arc. These earthquakes may represent the subducting Ionian Plate, indicating that the subduction process is active there. Furthermore, the earthquake distribution does not indicate the presence of any evident slab detachment, previously suggested by Spakman (1990; Spakman et al. 1993), or interruption of the subduction process along the descending slab at least from 40 to 450 km depth. The only feeble hint of detachment could be a rarity of earthquakes between 180 and 200 km depth (see Fig. 2). Anyway, we note that this rarity could also be due to the small time window that our data span (only six years), rather than a spatial seismicity gap. Thus, if the detachment exists, it could not be recovered by seismic tomography (it is also not visible in our image), but would not be larger than -20km thick. This small dimension implies that any tomographic study with a spatial resolution of more than 20 km can fail in detecting this feature. Finally, the absence of intermediate and deep seismicity within the high-velocity slab deepening from the Apulian foreland toward the Tyrrhenian may indicate that the subduction process is no longer active beneath this part of the Southern Apenninic chain, as a result of the continent-continent collision. We infer that the subduction of the Ionian oceanic lithosphere is active only beneath the Calabrian Arc. Figure 9 summarizes the geometry of the Southern Tyrrhenian subduction zone, as derived from our study. The high-velocity Ionian oceanic lithosphere, subhorizontal until the Calabrian Arc, abruptly deepens towards the north-west, offshore the Calabrian Arc. Intermediate-depth events occur within the subhorizontal part of the Ionian lithosphere, while the deep earthquakes are confined within the 70" NW-dipping high-velocity slab, down to 450 km depth (dashed area in Fig. 9). h E Y v 5 n 8 0 100 200 300 400 500 NW Tyrrhenian Sea ------ Calabrian Arc 0 100 200 300 400 Distance (km) SE Figure 9. Schematic model of the Tyrrhenian subduction zone. The shaded area represents the cluster of seismic activity within the slab. CONCLUSIONS Large residual variations are systematically observed at the 63 stations used for locating the intermediate and deep earthquakes. Residual distribution shows three alternate belts of negative and positive values from the external towards the internal areas. Velocity tomograms yield a reasonably resolved image of the subduction zone, defining the geometry of the high-velocity slab where the earthquakes are concentrated. The slab is evident down to 320 km depth beneath the Calabrian Arc (at greater depth our resolution is poor). The observed velocity values are between 8.6 and 9.0kms-', in agreement with values usually found in subduction zones. Lateral heterogeneities are -4 per cent, consistent with those of Amato et al. (1993b). Hypocentral distribution of deep seismicity depicts a NW -70" dipping plane beneath the Southern Tyrrhenian Sea, down to 450 km depth. Intermediate-depth events occur beneath the Ionian side of the Calabrian Arc, probably within the subducted Ionian Plate. In this area, the Ionian Plate is subhorizontal, bending offshore the Calabrian Arc. The occurrence of seismicity within the slab, and the intermediate-depth earthquakes beneath south-eastern Calabria may imply an active subduction process. We also recognize an aseismic subducted Adriatic lithosphere beneath the Southern Apenninic chain. 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