High resolution seismic imaging of the M w 5.7, 2002 Molise, southern Italy, earthquake area: Evidence of deep fault reactivation

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1 Click Here for Full Article High resolution seismic imaging of the M w 5.7, 2002 Molise, southern Italy, earthquake area: Evidence of deep fault reactivation Diana Latorre, 1 Alessandro Amato, 1 and Claudio Chiarabba 1 Received 7 August 2009; revised 18 December 2009; accepted 24 February 2010; published 31 July [1] We investigate the seismic structure of the M w 5.7, 2002 Molise earthquake area in order to understand the role of E W trending strike slip faults in the tectonics of the southern Apennines. We apply an innovative seismic migration technique to a high quality data set of earthquakes recorded at a dense local network. SP converted waves are migrated in depth to image the high impedance contrast of the Apulian Platform top buried under the Apennines allochthonous cover. The continuity of the migrated seismic horizon is broken by vertical steps that we systematically picked along 200 cross sections. The best location points of these structures define two main tectonic features. ThefirstoneisrelatedtoNW SEoriented normal faults and is consistent with the SW flexure of the foreland lithosphere beneath the orogenic belt. The second one indicates that shallow E W oriented trans tensional faults are concentrated directly above the deeper (10 20 km) strike slip fault, delineating the geometry of a negative flower type structure. This fault system delimits a depressed sector of the Apulian Platform, whose geometry is consistent with a pull apart basin inherited from a previous left lateral strike slip tectonic regime. The buried structure is analogous to those outcropping in the Apulian foreland and in the Adriatic offshore, to the east. This correlation brings new support to the hypothesis of a regional E W trending shear zone cutting the Adria plate and suggests that other earthquakes could occur on this or on parallel E W trending strike slip faults. Citation: Latorre, D., A. Amato, and C. Chiarabba (2010), High resolution seismic imaging of the M w 5.7, 2002 Molise, southern Italy, earthquake area: Evidence of deep fault reactivation, Tectonics, 29,, doi: /2009tc Centro Nazionale Terremoti, Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy. Copyright 2010 by the American Geophysical Union /10/2009TC TECTONICS, VOL. 29,, doi: /2009tc002595, Introduction [2] On 31 October and 1 November 2002, the Molise region experienced two M w = 5.7 earthquakes that were unexpected for this area since neither historical nor instrumental seismicity had been reported in literature before 2002 (Figures 1 and 2). Fault plane solutions of the two events indicate E W trending, right lateral strike slip faults [Di Luccio et al., 2005a]. A seismic sequence of aftershocks followed the main shock for about one month and clustered along a 15 km long, E W oriented structure [Chiarabba et al., 2005a]. [3] The 2002 Molise events occurred just east of the southern Apennines belt, where large earthquakes mainly develop along NW SE striking normal faults [Montone et al., 2004]. Nevertheless, fault geometry and kinematics of the 2002 Molise earthquakes are analogous to the E W trending strike slip structures that are both well exposed in the Apulian foreland and buried beneath the foredeep deposits to the east. These structures are considered from many authors as potential seismogenic sources of moderateto large magnitude earthquakes [e.g., Meletti et al., 2008, and references therein]. [4] Up to now, the role of the E W trending seismogenic faults in the Apennines foredeep and Apulian foreland is not clear yet. In spite of intense geophysical and geological investigations, the crustal structure of deep strike slip seismogenic sources, as those responsible for the 2002 Molise earthquake [Chiarabba et al., 2005a] and the 1990 and 1991 Potenza earthquakes [Di Luccio et al., 2005b], is scarcely known. Moreover, other structures as the causative fault of the M 6.7, 1627 earthquake are still matter of study [e.g., Patacca and Scandone, 2004; Valensise et al., 2004]. In the current debate, E W strike slip faults have been interpreted either as second order transfer faults between NW SE trending master normal faults, or as primary lithospheric ruptures of the Adria plate [e.g., Di Bucci and Mazzoli, 2003; Patacca and Scandone, 2004; Valensise et al., 2004; D Agostino et al., 2008]. [5] Some information on the seismogenic faults associated to the two M w 5.7, 2002 Molise earthquakes only comes from focal mechanism solutions and aftershock studies, whereas previous investigations in the epicentral area did not image these deep structures. Recently, Nicolai and Gambini [2007] reported a map of the Apulian Platform top below the southern Apennines. Their map shows a very smooth surface where the 2002 Molise fault system ruptured, probably due to its regional character. [6] After the 2002 Molise earthquake, a dense local seismic network was deployed, recording thousands of aftershocks (Figure 3). In order to investigate in detail the crustal structure of this area and provide new constraints to its tectonic setting, we have exploited this large amount of high quality seismic data. We have applied an innovative 1of13

2 Figure 1. Geological sketch map of the southern Apennines and the Apulian Foreland, showing the traces of the main E W trending structural lineaments. From east to the west: (1) the Gondola fault zone, (2) the Mattinata fault system, (3) the Chieuti high, and (4) the Apricena fault [after Di Bucci et al., 2006]. The trace of the Tremiti fault system is from Doglioni et al. [1994]. Hypocenter location and CMT focal mechanism solution of the two M w 5.7, 2002 Molise earthquakes are from Chiarabba et al. [2005a]. seismic converted wave migration, which matches data processing currently used in earthquake seismology and basic principles of classical depth migration techniques peculiar of seismic exploration [Latorre et al., 2008; Blacic et al., 2009]. A similar technique has been successfully applied to microearthquake data recorded at the SAFOD Pilot Hole site [Chavarria et al., 2003]. We use this migration technique to image major discontinuities in the Molise region and provide a possible interpretation of these structures in the framework of the tectonic evolution of the southern Apennines front belt. 2. Tectonic Setting [7] The southern Apennines (Figure 1) is an east verging fold and thrust belt formed since the Neogene time in consequence of the Africa Europe plate convergence and the westward subduction of the Adriatic continental lithosphere [e.g., Malinverno and Ryan, 1986; Doglioni, 1991]. The foreland domain of the orogenic belt is represented by the outer Apulian Platform, a 6 7 km thick Meso Cenozoic sequence of shallow water carbonates overlying Permo Triassic deposits, which constitutes the sedimentary cover of a Precambrian crystalline basement [see Patacca et al., 2008, and references therein]. The Apulian Platform crops out in the Gargano peninsula and Apulian region, and it plunges toward southwest beneath the southern Apennines thrust belt where is overlain by Plio Pleistocene terrigenous sediments of the foredeep domain and a thick allochthonous cover of Miocene Pliocene deposits of the Apennines nappes. [8] In the hinterland domain of the southern Apennines, the present day tectonic regime is mainly characterized by NE SW oriented extension accommodated by normal faults parallel to the NW SE striking chain axis, as indicated by borehole breakouts and focal mechanism solutions [Amato and Montone, 1997; Montone et al., 2004; Chiarabba et al., 2005b]. Moving toward the foreland domain, geophysical ad geological data indicate a transition from normal to strike slip faulting stress regime without significant changes of the maximum horizontal extension direction [Montone et al., 2004]. In the external Apennines and Apulian foreland, active deformation is mainly accommodated along rightlateral strike slip faults, roughly oriented east west, as for recent moderate magnitude events (e.g., the M w 5.7, 1990 Potenza earthquake [Di Luccio et al., 2005b] and the M w 5.7, 2002 Molise earthquake [Chiarabba et al., 2005a]), and probably for large historical earthquakes (e.g., the M6.7, 1627 Capitanata earthquake [Gruppo di Lavoro CPTI, 2004] and the M6.7, 1930 Irpinia earthquake [Pino et al., 2008, and references therein]). [9] According to some authors, E W oriented faults along the external orogenic belt and in the Apulian foreland represent pre existing zones of weakness inherited from Mesozoic tectonics [e.g., Doglioni et al., 1994; Chilovi et al., 2000; Di Bucci et al., 2006]. However, the role of 2of13

3 the contrary, Patacca and Scandone [2004] interpret these E W oriented faults as second order crustal structures that accommodate sinistral offsets between extensional NW SE trending master faults, in the framework of the present day extensional tectonic regime. Di Bucci and Mazzoli [2003] suggest that the activity of E W striking faults in external Apennines and foreland domain is not related to the extensional stress regime acting along the chain. Rather, it is the expression of intraplate deformation affecting the Adria lithosphere and is controlled by NW SE compression due to the convergence between Africa and European plates. Finally, according to D Agostino et al. [2008], active deformation along E W trending, right lateral, strike slip faults is explained with the relative motion between the Adria and the Apulian microplates. [10] The debate on the complex tectonic setting at the outer margin of the southern Apennines and the adjacent foreland has increased after the M w 5.7, 2002 Molise earthquake. Due to its geometry and kinematics, the fault system responsible for the 2002 Molise earthquakes has been interpreted as the western segment of a larger E W trending shear zone [Di Bucci et al., 2006]. This shear zone should extend for at least 180 km from the Adriatic offshore to the chain axis and includes, from east to the west, (1) the Gondola fault zone, (2) the Mattinata fault system, (3) the possible causative fault for the 1627 Capitanata earthquake, and (4) the Molise fault system (Figure 1). The eastern portions of this shear zone, i.e., the Gondola fault zone offshore and the exposed Mattinata fault system, are fairly well known, thanks to the large number of studies and data from geological surveys, deep exploration wells, commer- Figure 2. Seismicity recorded in southern Italy from 1981 to 2009: (a) RCMT focal mechanisms [Pondrelli et al., 2006], (b) hypocenter location of events with magnitude greater than 2 (black dots) and magnitude greater than 4 (red stars), and (c) SW NE vertical cross section showing the distribution at depth of the seismicity. Data are from the CSI (1.1) catalog from 1981 to 2002 [Castello et al., 2006] and Italian Seismic Bulletin from 2002 to 2009 (INGV, available at these structures in the geodynamical framework of southern Italy is still unclear, giving rise to conflicting interpretations. According to Doglioni et al. [1994], E W trending, rightlateral structures, as the so called Tremiti transfer zone in the Adriatic offshore and the Gondola Mattinata fault system (Figure 1), are lithospheric scale ruptures of the Adria plate that accommodate the differential rollback of northern and southern portions of the subducting Adriatic slab. On Figure 3. Map view of the study area. The square box delimits the area imaged by our SP converted wave migration. Inverted triangles represent the temporary seismic network deployed during the 2002 Molise aftershock sequence [Chiarabba et al., 2005a]. Black dots are the hypocenter locations of the 2002 Molise aftershocks from local earthquake tomography (P. De Gori, personal communication, 2007). Beach balls indicate the focal mechanisms of the two M w 5.7 main shocks [from Chiarabba et al., 2005a]. Open diamonds are the locations of deep exploration wells drilled in this region (R3, Rotello 3; MN1, Monacilioni 1; CCM1, Civitacampomarano 1). 3of13

4 cial seismic lines, historical and instrumental seismicity (see, for example, Di Bucci et al. [2006], Ridente et al. [2008], and Argnani et al. [2009] for a complete bibliography). Both seismic investigations offshore along the Gondola line [Ridente et al., 2008] and structural analyses of the Mattinata fault system [Chilovi et al., 2000] have highlighted the complex deformation history of these systems. These two major structures seem to have a Mesozoic origin, subsequently reactivated, first as sinistral strike slip faults (late Miocene to early Pliocene) and then as dextral strike slip faults (late Pliocene to present day), following the rapid change of the regional stress regime in this region [Chilovi et al., 2000]. 3. Data and Method [11] In order to obtain a high resolution seismic image of crustal features in the study area, we analyzed the seismograms of 1,156 well located earthquakes of the 2002 Molise aftershock sequence, which were recorded at 28 three component seismic stations (Figure 3). The analyzed seismograms contain strong secondary signals that we interpreted as transmitted SP converted waves, since they show clear polarization on the vertical component and arrival time between the P and S direct waves [Latorre et al., 2008]. Seismic studies for oil exploration in southern Apennines indentified a strong impedance contrast at the top of the Apulian Platform, which generates clear reflections in seismic profiles [e.g., Nicolai and Gambini, 2007, and references therein]. Since the secondary signals were observable at all the stations of our network and the hypocentral depths are mostly concentrated between 20 and 8 km, we reasonably suppose that the observed converted phases are SP transmitted waves that were generated at the top Apulian Platform, in agreement with the interpretation of Demanet et al. [1998]. For these reasons, we investigated the sub surface seismic structure of the Molise area by migrating earthquake records and assuming a SPtransmission propagation mode. [12] The imaging method we applied is a kinematic depth migration based on the Kirchhoff summation technique that we have appropriately designed to process converted waves from local earthquake data. The migrated image is obtained by (1) dividing the target volume into a three dimensional fine grid of image points and (2) assuming that seismic conversions may occur at each image point of the model grid. Then, the construction of the migrated image is performed point by point. For each image point, we apply the finite difference eikonal solver of Podvin and Lecomte [1991], and we compute the SP wave raypaths by following the gradient of the P and S travel time fields. Then, we re calculate accurate travel times along the raypaths [Latorre et al., 2004]. Travel time computation is accomplished using three dimensional P and S wave velocity models and accurate earthquake hypocenter locations provided by P. De Gori et al. (personal communication, 2007). These velocity models have been computed by inverting 21,594 P and 14,441 S arrival times with the Simulps code [Thurber, 1993]. More details about the background velocity models used in our kinematic migration are given by Latorre et al. [2008]. For a given image point and earthquake station pair, the computed travel time of a possible SP converted wave identifies a trace segment on the seismogram. The energy contained in this trace segment is scaled for a kinematic weight factor based on Snell s Law. After scaling, trace segment energies of all earthquakestation pairs are added together and assigned to the current image point. When the image point corresponds to the likely origin point of the SP converted wave, the sum of the trace segment energies provides high total energy value. Therefore, the final migrated image constitutes a collection of summed seismic energy values, each one associated to an image point. The focusing of the seismic energy along contiguous image points allows us to identify the distribution at depth of potential seismic horizons where SP transmitted converted waves may originate. [13] Migration parameter tuning, as well as data selection and data pre processing criteria, were previously tested on a small subset of data, allowing us to widely discuss robustness and potentialities of our method [Latorre et al., 2008]. Taking into account (1) the earthquake distribution, (2) the receiver distances, and (3) the signal frequency content, the grid nodes of our migrated model were 500 m spaced in the horizontal direction and 100 m spaced in the vertical direction. Since the dimension of the target volume was km 3, calculations were executed over 362,691 grid nodes, implying a notable computational effort. The aim of our data selection was to identify waveforms with high signal to noise level and choose only seismograms whose P and S wave first arrival time pickings were kinematically consistent with our background velocity models. This is accomplished by computing P and S arrival travel times in the background 3 D velocity models and rejecting data with residual times greater than 0.3 s. Data pre processing consisted in (1) band pass filtering seismograms between frequencies of 25 Hz and 40 Hz, (2) normalizing the traces with three component trace equalization, and (3) muting the energies associated to the arrivals of the P and S direct waves. 4. Discussion of Results 4.1. Migration Model [14] A collection of pixels, each one representing an image point, constitutes our three dimensional migrated image. Figure 4 shows six representative vertical crosssections of the migrated model. Three cross sections are N S oriented and perpendicular to the 2002 Molise fault system (S1, S2 and S3 sections). The other cross sections (S4, S5 and S6) are E W oriented and located around the 2002 Molise epicentral area (see the map in Figure 4 for their location). Pixel colors indicate the relative amount of focused SP transmitted energy, i.e., normalized with respect to the maximum energy value of the migrated model. High relative values (from 0.5 to 1, i.e., yellow to red colors in Figure 4) correspond to high SP energy focusing and, thus, potential seismic horizons for SP wave transmissions. Low relative values (from 0 to 0.5, i.e., blue to white colors) indicate that no SP focusing occurs, while gray pixels rep- 4of13

5 Figure 4 5of13

6 resent the model regions that cannot be covered by our station receiver geometry. [15] The migrated model images a strong seismic horizon, which appears continuous throughout the illuminated parts of the target volume. This horizon is most likely the seismic signature of the main impedance contrast observed along commercial seismic lines in southern Apennines [e.g., Roure et al., 1991; Nicolai and Gambini, 2007] and corresponds to the contact between the carbonate units of the buried Apulian Platform and the overlying Plio Quaternary terrigenous sediments [Latorre et al., 2008]. Data from the Rotello 3 well (Figure 3) indicate that the top of the Apulian Platform is located at a depth of km b.s.l. In our migration model, the seismic horizon depth is retrieved at 1.4 ± 0.5 km b.s.l., in fair agreement with well data. The slight difference between well data and our results could be due to the low resolution of the reference tomographic model in the shallowest layers of the crust. We cannot compare our model with other exploration wells in the area, since they either do not reach the Apulian Platform top (e.g., the Monacilioni 1 well, MN1 in Figure 3 [Mostardini and Merlini, 1986]) or lie outside the sampled part of our migrated volume (e.g., the Civitacampomarano 1 well, CCM1 in Figure 3). [16] The migrated SP energy appears generally well focused, delineating a clear seismic horizon. However, in the southern part of the migrated model, we observe that SP energy is distributed over a wider range of depths (Figure 4, sections S1, S3, and S6). In order to explain this result, we present two examples of data modeling (Figure 5). The sequence shows how raw data are processed and stacked to contribute with their total amount of energy to the construction of a single image point (or pixel) of the migrated image. In Figure 5a, we analyze an image point located on section S6 (the image point is represented with a black dot in the migrated image). Raw data indicate complex arrivals of SP converted waves, associated to the Apulian Platform top. In some cases, we identify two subsequent SP arrivals, but generally they are so close in time that they interfere between each other defining a long SP arrival (up to 1 s). By modeling these data, the SPconverted energy is spread over a wide depth range. On the contrary, in Figure 5b, we show how a single SP converted wave is modeled and the SP energy is well focused in a narrow depth range. This different behavior could be due to a different degree of complexity in the structure that focuses the converted energy. In the case of Figure 5b, a single, clear impedance contrast (such as the Apulian Platform top) is nicely imaged, whereas in the case of a larger pulse (Figure 5a) this can be attributed to a more layered interface, with more impedance contrasts Apulia Platform Top Reconstruction [17] Thanks to the spatial continuity of the retrieved seismic horizon and the dense sampling of our model, we reconstruct a detailed three dimensional map of the Apulian Platform top (Figure 6a). We extract a grid of average depth values of the seismic horizon from the migrated model: at each horizontal coordinate (x,y), we explore the grid of the migrated model in depth and compute the thickness of the seismic horizon, which is represented in our interpretation by normalized values of SP migrated energy between 0.5 to 1. Then, we estimate and store the average depth of the seismic horizon and its standard deviation. We obtain a dense grid of 1,529 average depth values (black dots in Figure 6a). Where possible, we constrain the depth of the Apulian Platform top using information from deep wells: depth values are fixed at km b.s.l. beneath the Rotello 3 well and at km b.s.l. beneath the Civitacampomarano 1 well (Figure 3). The final grid is imaged by using a b spline interpolation (Figure 6a). Its lateral extension indicates that the seismic horizon is well detected over a broad region around the 2002 Molise epicentral area (about km 2 ), while it is not clearly identified at the borders of our model, due to the acquisition geometry. In this external region, the reconstructed map is only extrapolated by the b spline function and it is not well constrained. For this reason, we will focus our discussion only on the well covered areas of our Apulian Platform reconstruction. [18] The standard deviation map allows us to quantify the depth uncertainty of the detected seismic horizon (Figure 6b). Depth position of the Apulian Platform top is estimated with uncertainty lower than 0.5 km in both the central and the northern part of the studied region. The example of Figure 5b shows that clear SP arrivals are recorded in this part of the model and the migration procedure provides a well focused seismic horizon. Conversely, higher values of standard deviation affect the southwestern border of the sampled area and correspond to a depth uncertainty lower than 1 km. As shown in Figure 5b, high standard deviation areas in the southern part of the model can be associated to the presence of complex SP arrivals that we cannot completely discriminate in our modeling. Therefore, we prefer to cautiously provide a single layer interpretation of the seismic horizon with an average depth and high standard deviation values, although two brighter layers can be identified at the top and the bottom of the high Figure 4. Vertical cross sections of the seismic migrated image oriented along the N S direction (S1, S2 and S3) and the E W direction (S4, S5 and S6). Traces of the sections are reported on the map (black lines). In the seismic image, pixel colors represent the relative amount of focused SP transmitted energy: yellow to red colors (normalized values from 0.5 to 1) correspond to high SP energy focusing and indicate potential seismic structures where SP wave conversion occurs. Breaks in the continuity of the seismic horizon are picked over 200 vertical cross sections (the traces are depicted with gray lines in the map). Blue dots represent the average depth values of the seismic horizon associated to the SP converted waves, and the black line is the trace of the Apulian Platform reconstruction shown in Figure 6a (see text). Black dots are the hypocenter locations of the 2002 aftershocks, obtained from tomographic inversion of P and S wave first arrival times [Latorre et al., 2008]. 6of13

7 Figure 5. Example of seismic records plotted at different steps of the SP converted wave migration. From left to right, raw data, data after modeling, stack of the data and location of the stacked energy in the migrated image are shown. Raw data are represented by the vertical component of some microearthquake records, sorted by time difference between S and P first arrivals. Each trace after modeling is shifted for the theoretical travel time of the SP converted wave computed at a grid point of the migrated volume (black dot in migrated image): traces are aligned around this arrival (zero time after modeling) and weighted for the Snell s weight factor. The stacked energy at zero time is located in the migrated volume at the grid point (black dot in migrated image). (a) The procedure applied for a grid point located in the southern part of the migrated image (section S6 in Figure 4). (b) The procedure applied for a grid point located in the northern part of the model. SP energy focusing depth range. The presence of two brighter layers in this part of the model could be explained as (1) tectonic thickening with stacking of two Apulian Platform slices (as proposed by Patacca et al. [2008] for the CROP 11 seismic profile, about 40 km to the north), (2) the presence of a wedge of pre Apennine competent sediments, dismantled during the opening of a basin structure (see below) and before the deposition of the upper allochthonous cover, or (3) fluid filled layering under the top of the Apulian Platform, sealed by the allochthonous deposits. [19] The data density map in Figure 6d represents the total number of seismic data that contribute with their amplitude to the construction of the migrated image, i.e., seismograms having computed SP wave transmitted travel time between P and S first arrival time and Snell s coefficient greater than 0.5 [Latorre et al., 2008]. The map shows that regions with low standard deviation values as the central part of the model do not correspond to few data, and indicate very consistent results. 7 of 13

8 Figure 6 8of13

9 4.3. Deformation of the Apulian Platform: Tectonic Interpretation [20] The analysis of Apulian Platform morphology highlights two main features. The first one is a regional scale structure, which indicates the gradual deepening of the Apulian Platform toward SW. In order to quantify strike and dip of this structure, we fit a bilinear trend to the Apulian Platform reconstruction using a least squares method. Figure 6c shows that the Apulian Platform top can be approximated by a surface, which roughly strikes N120, dips 7 SW in the foreland domain and 12 SW below the Apennines belt front. According to Mariotti and Doglioni [2000], the top of the undeformed Mesozoic layer of the Apulian Platform can be assumed parallel to the regional foreland monocline, whose dip values characterize the Adriatic slab behavior. In this sense, the regional trend retrieved in our study is representative of the SW flexure of the Adriatic foreland underneath the Apennines thrust belt. Our dip values for the foreland monocline are very close to those previously measured by Mariotti and Doglioni [2000], which propose a dip increasing in the Molise Gargano region from 7 to 11 toward southwest, on the basis of independent data as commercial reflection lines, unpublished industrial data and regional geologic balanced cross sections. The smooth trend of the Apulian Platform morphology recovered in this study (Figure 6c) is also consistent with the top of the Apulian Platform reconstructed at regional scale by Nicolai and Gambini [2007] (Figure 6f). [21] By removing the regional scale trend from the Apulian Platform top, we point out a second order morphology, which corresponds to a local depression situated around the Molise fault system (Figure 6e). In Figure 6a, isolines between 4 and 5 km depth show a local deviation of the Apulian Platform trend from northwest southeast to east west, suggesting that intense deformation, probably related to E W oriented strike slip faulting, controls the topography of the buried Apulian Platform in the 2002 Molise epicentral area. In the foreland, cumulated offset of sinistral motion along the E W strike slip Mattinata fault system is estimated to be several kilometers from late Miocene to Pleistocene, whereas more recent dextral motion is significantly smaller [Argnani et al., 2009, and references therein]. According to some authors [Billi et al., 2007] the present day activity, at least in the Gargano area, is still leftlateral, but fault plane solutions of crustal earthquakes both in the Gargano promontory and in the foredeep show predominance of right lateral motion on E W trending planes [Del Gaudio et al., 2007]. Moreover, both GPS [Anzidei et al., 1996; D Agostino et al., 2008] and InSAR data [Atzori et al., 2007] support present day right lateral motion across the Mattinata fault. It is worth noting that in the southern Apennines, few kilometers to the west of the study area, E W trending, right lateral strike slip faults have been proposed as the cause for the segmentation of the normal fault belt, along which major Italian earthquakes occur [Chiarabba and Amato, 1997]. This would imply that the E W trending strike slip faults cut obliquely the region from the belt, through the foredeep, as far east as the Adriatic foreland. [22] Breaks in the Apulian Platform top continuity are well recognized in vertical cross sections (Figure 4). Seismic horizon correlations allow us to identify vertical displacements of the Apulian Platform top, which we interpret as dip slip normal faults. We picked systematically these vertical offsets on a suite of 100 E W and 100 S N trending vertical sections, ending up with a series of connected points that represent the position of faults at depth. The best identified location points of these structures are mapped in Figure 7, where their position is referred to the Apulian Platform depth. We can distinguish two main fault trends (Figure 7): the first one is NW SE oriented and is visible in the northeastern part of the study area, toward the buried front of the Apennine belt. NW SE striking normal faults are probably expression of the SW flexure of the foreland lithosphere beneath the southern Apennines and agree with similar structures previously interpreted in seismic reflection profiles [Mostardini and Merlini, 1986]. The second trend consists of E W oriented faults that are mainly concentrated around the 2002 epicentral area. Unlike the imaged NW SE striking normal faults, E W trending lineaments, which affect the buried Mesozoic layer, have never been reported in this region. In particular, we observe that two main E W striking segments extend along the Molise fault system in the upper crust (4 5 km depth), above the hypocenters of the two 2002 strike slip events, which are located at about 20 km depth [Chiarabba et al., 2005a]. Connecting the shallower faults with the deeper E W trending strike slip fault, we reconstruct the geometry of a negative flower type structure, where vertical displacements of the Apulian Platform occur along secondary faults that are reactivated by the deeper strike slip fault system (Figure 4, sections S2 and S3) Molise Pull Apart Basin [23] South of the 2002 epicentral area, both E W and NE SW oriented normal faults delimit a rhomboidal shape topographic depression, which shows average depth of 6 km, and whose geometry is compatible with that of a pullapart basin structure. Taking into account the spatial distribution of the sampled faults (Figure 7), the basin structure is Figure 6. (a) Map view of the Apulian Platform top inferred from the SP wave migration model shown in Figure 4 (see text). The regular grid of black dots represents the depth values of the seismic horizon extracted from the model. The Apulian Platform top surface is imaged by applying a b spline interpolation [Smith and Wessel, 1990]. Inverted triangles are the seismic stations used for the SP converted wave migration. Open diamonds are the deep wells drilled in this area. (b) Standard deviation map of the Apulian Platform top reconstruction. (c) Regional trend of the Apulian Platform top computed by fitting a bilinear trend to the surface shown in Figure 6a. (d) Representation of the data density, i.e., the number of data stacked at each grid point. (e) Map view of the residuals obtained by removing the regional trend from the 3D map of Figure 6a. (f) Map view of the buried Apulian Platform top (depth values from Nicolai and Gambini [2007]). 9of13

10 Figure 7. Location of the main tectonic lineaments retrieved in this study. The color map is the Apulian Platform top surface of Figure 6a. Red diamonds represent the location points of breaks in the seismic horizon continuity, which are associated to vertical offsets of the Apulian Platform top. The position of these points is referred to the depth of the Apulian Platform topography. Black lines represent schematically the main structures that are related to these vertical offsets. CMT focal mechanism solutions correspond to the main strike slip events occurred (1) during the 2002 seismic sequence (the M w = 5.7, 10/31/2002 event, the M w = 5.7, 11/01/2002 event and the M w = 4.2, 11/12/02 event; hypocenter location and CMT focal mechanisms are from Chiarabba et al. [2005a]) and (2) two strike slip events occurred in 2003 and located some kilometers to the south (the M L = 4.3, 06/01/2003 event and M L = 4.3, 12/30/ 2003 event; hypocenter locations are from INGV bulletin and RCMT focal mechanisms are from Pondrelli et al. [2006]). about 10 km long and 4 km wide. This part of our Apulian Platform reconstruction is fairly well constrained, as confirmed by standard deviation map (Figure 6b) and data density map (Figure 6d). However, we cannot exclude that the topographic depression of the buried Apulian Platform further extends toward southwest, where we also observe similar average depth values (Figures 6a and 6e), but higher standard deviation (Figure 6b). [24] The geometry of the buried basin and its location above the seismogenic source of the 2002 Molise earthquake is suggestive of a releasing fault step or bend in the western portion of the fault system and indicative of a left lateral strike slip kinematics (Figure 7). Since focal mechanism solutions of the two 2002 Molise main shocks indicates right lateral strike slip movement, we suggest that this topographic depression of the Apulian Platform top has been created during a previous tectonic regime, which implied a left lateral motion on the E W striking fault system. The buried pull apart basin could be interpreted as an inherited structural feature, somehow preserved as a relict during the subsequent deformational regimes that have involved this region up to the present day. Our interpretation is supported by the analogy between this buried structure and the well exposed Pantano S. Egidio basin, which is a pull apart structure related to a left step of the Mattinata Fault System in the Gargano region, about 80 km east of the study area along the same E W lineament (Figure 1). According to Chilovi et al. [2000], the Pantano S. Egidio pull apart basin was formed from late Miocene to early Pliocene when the Mattinata fault system was reactivated as sinistral strike slip fault, in consequence of the regional stress field related to the orogenic shortening of the southern Apennines. Successively, the structure of this pull apart basin was preserved when the Mattinata fault system reactivated in a dextral strike slip sense, following a rapid change of the tectonic regime (late Pliocene to present day, according to Chilovi et al. [2000], and middle Pleistocene to present day according to Di Bucci et al. [2006]). [25] Chilovi et al. [2000] suggest that the Pantano S. Egidio basin is part of a large E W trending wrench zone, whose complex structural segments are linked to each other with left steps (e.g., the two major segments of the Mattinata fault system) or right steps (e.g., the Mattinata fault system and the Gondola fault zone offshore). These segments have been reactivated through successive deformation stages, giving rise to a variety of compressional (e.g., the Chieuti high; Figure 1) and extensional structural features (e.g., the Pantano S. Egidio basin or the Serracapriola basin). The near right lateral, strike slip Molise fault system at the outer front of the Apennines and the associated buried pull apart basin seem to have experienced very similar tectonic evolution to the E W trending structures of the Apulian foreland. Based on the shape of the basin evidenced by our study, we provide a rough estimate of the cumulative offset, ranging from 5 to 9 km. This value is consistent with the estimate of 2 20 km proposed by Billi et al. [2007] and yields an average displacement rate of about mm/yr (from late Miocene to Pleistocene) Old, Recent, and Future Earthquakes in the Region [26] The 2002 Molise aftershock sequence was mostly confined between 8 and 20 km depth [Chiarabba et al., 2005a]. Our results show that aftershocks reaches the Apulian Platform top only in the eastern part of the fault system (section S3 in Figure 4). We do not find any evidence of seismicity above the Apulian Platform top, where data from Monacilioni 1 well indicate the presence of an allochthonous cover of Miocene Pliocene terrigenous deposits [Mostardini and Merlini, 1986]. Taking into account the average thickness ( 6 km) of the Apulian Platform (drilled in some wells [see Patacca et al., 2008]), we infer that seismicity mainly nucleated in the Paleozoic crystalline basement. A similar result was found for the 1990 and 1991 Potenza earthquakes, located to the south 10 of 13

11 Figure 8. Sketch map of the study area and the contiguous foreland region where the main features inferred from this work and data from other geophysical and geological studies are summarized. The gray area limited by normal faults (red lines) highlights the location of the buried pull apart basin associated to the strike slip Molise fault system. Grey contours represent the Apulian Platform top reconstructed from our seismic image (Figure 7). Boxes A C indicate the seismogenic sources of historical events (source is DISS Working Group [2009]): A, the M6.1, 1875 event; B, the M6.7 Gargano earthquake; and C, the Frosolone seismogenic source for the M7.0, 1456 event. The black triangle is the epicenter of the 1627 Capitanata event from the CFTI catalog [Guidoboni et al., 2007]. Beach balls show focal mechanism of events with magnitude M L > 4 (CMT focal mechanism solutions are from Pondrelli et al. [2006]; earthquake hypocenter location of both the M w 5.7, 10/31/2002 and the M w 5.7, 11/01/2002 events are from Chiarabba et al. [2005a]; new earthquake locations and magnitude estimations of the M L 4.3, 06/01/2003 and the M L 4.3, 12/30/2003 events are courtesy of B. Castello, INGV). and showing a similar rupture mechanism [Di Luccio et al., 2005a]. [27] The seismic activity after 2002 is not very high, and is concentrated mostly at the edges of the 2002 fault system and south of it (Figure 8). The largest shocks recorded by the National Seismic Network (both with M L = 4.3) occurred in 2003 and were located south of the 2002 fault, with similar strike slip focal mechanisms and hypocentral depth (Figure 7). Unfortunately, their location is not as precise as the 2002 aftershocks, since in 2003 the improvement of the Italian monitoring system had just started [Amato and Mele, 2008]. We suggest that the 2003 events occurred along the 11 of 13

12 southern border of the pull apart basin, and suggest that future M > 5.5 shocks might nucleate along the western portion of this fault. [28] Figure 8 summarizes the main seismogenic features of the region around the Molise area struck by the 2002 shocks and studied in this paper. To the east, we see the hypothesized sources of two remarkable historical earthquakes: the M Gargano event and the M 6.7, 1627 Capitanata earthquake, closer to the Molise region. Both source locations and sizes are determined from damage patterns (CFTI catalog [Guidoboni et al., 2007]) and geological evidence [Basili et al., 2008]. It must be considered that for the 1627 event an alternative fault location (the Apricena fault, Figure 1) was proposed by Patacca and Scandone [2004], not far from the one reported in Figure 8, but with a different mechanism. To the west, part of the 1456 fault is shown, according to Fracassi and Valensise [2007]. The Molise area lies in between, and the 2002 sequence seems to have activated only a part of this E W trending lineament, i.e., 15 km of the almost 30 km left unbroken by historical events. This implies the possibility that future M > 5.5 earthquakes will occur along the fault system (Figures 1 and 8). The most likely candidate area for future shocks is to the east of the 2002 epicentral region. The expected magnitude is between 5.5 and 6, approximately, considering the size of the 2002 epicentral region. The low displacement rate, however, could suggest long repeat times. 5. Conclusions [29] We have proposed a three dimensional reconstruction of the Apulian Platform buried under the foredeep in the Molise region, where two M w 5.7 earthquakes occurred in The high quality data analyzed in this study, consisting in several thousand seismic waveforms recorded from aftershocks at a dense local network, allowed us to recover the structure of the Apulian Platform top with unprecedented detail for passive seismic studies. This represents one of the first studies where a large amount of microearthquake data is processed using exploration imaging techniques to define the geometry of crustal discontinuities. [30] This work highlights that intense deformation modeled the topography of the buried Apulian Platform in the 2002 Molise epicentral area. We discriminate the contribution of localized deformation, which is related to the activity of the E W trending strike slip Molise fault, from that of regional scale deformation, which is associated to the SW flexure of the Apulian lithosphere. The regional trend measured in this study indicates that the Apulian Platform dips from NE to SW at increasing angles of 7 to 12, in agreement with previous interpretation of independent data [Mariotti and Doglioni, 2000]. By removing the regional field from our reconstruction we obtain a residual image that points out a local depression of the Apulian Platform located along the southwestern segment of the Molise fault system. We have detected several E W oriented breaks of the Apulian platform top continuity, demonstrating the presence of a releasing bend or step of the Molise fault system and a negative flower type structure, which extends in the upper crust down to about 20 km depth. Strike slip earthquakes in the region nucleate in the Paleozoic crystalline basement and rupture mostly up to the base of the Apulian Platform ( 10 km depth). [31] The newly discovered Molise pull apart basin suggests that the E W trending strike slips faults previously identified in the Gargano region along the Mattinata and the Gondola fault systems, continue further west and constitute a first order regional shear zone that is traceable from the Adriatic offshore to the Apennines. This regional structure was active with a left lateral motion during the NE SW compression from late Miocene to lower Pliocene (according to Chilovi et al. [2000]) or middle Pleistocene [e.g., Cinque et al., 1993; Di Bucci et al., 2006]. After the end of the orogenic compressional phase, some parts of this structure have been reactivated with a right lateral sense of motion, as shown by the current seismicity. [32] Our results demonstrate that the 2002 Molise earthquakes did not rupture a newly formed structure, but were confined within the bend or step of the regional E W fault system. This also suggests that pre existing structures control segmentation and maximum magnitude of earthquakes in such a low strain rate shear zone. Looking at earthquakes in the past centuries (1627 AD to the east and possibly 1456 AD to the west), and at seismic activity after 2002, we argue that both sides of the 2002 Molise faults might rupture in future M > 5.5 events. [33] Acknowledgments. We thank the Associate Editor, Davide Scrocca, and Nano Seeber for careful reviews that improved the manuscript. The paper benefitted from discussions with Michele Carafa, Francesca Cinti, Paola Montone, Fabio Villani, and Francesco Salvini. D.L. is supported by Project MIUR Airplane (contract RBPR05B2ZJ, UR 3). Figures were produced using the free software GMT [Wessel and Smith, 1991]. References Amato, A., and P. 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