Integrating near-surface geophysical methods and remote sensing techniques for reconstructing fault-bounded valleys (Mellieha valley, Malta)

Tectonophysics 875 (2024) 230263 Available online 2 March 2024 0040-1951/© 2024 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/bync-nd/4.0/). Integrating near-surface geophysical methods and remote sensing techniques for reconstructing fault-bounded valleys (Mellieha valley, Malta) Luciano Galone a , * , Fabio Villani b , Emanuele Colica a , c , Davide Pistillo d , Paola Baccheschi b , Francesco Panzera e , Jesús Galindo-Zaldívar f , g , Sebastiano D’Amico a a Department of Geosciences, University of Malta, MSD2080 Msida, Malta b Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy c Research & Planning Unit, Public Works Department, Ministry for Transport, Infrastructure and Public Works, FRN1700 Floriana, Malta d Dipartimento di Scienze della Terra, University of Rome “La Sapienza”, 00185 Rome, Italy e Department of Biological, Geological and Environmental Sciences, University of Catania, Italy, Catania, Italy f Departamento de Geodin´ amica, Universidad de Granada, 18071 Granada, Spain g IACT (CSICUniversidad de Granada), 18071 Granada, Spain. ARTICLE INFO Keywords: Malta Near-surface geophysics Remote sensing Normal fault Horst and graben Red beds Extensional tectonics ABSTRACT The island of Malta (central Mediterranean) is dissected by several WSW-trending fault-line valleys related to Miocene-Pliocene extensional tectonics. Some valleys host remnants of alluvial deposits that could provide information on possible Quaternary faulting, but the thickness of these deposits and their subsurface extent is poorly constrained. Our study aimed to investigate the structural configuration of the southern sector of the Mellieha valley, which is located in the north-west part of the island and is limited by the ENE-WSW general trending Mellieha and Ghadira faults, and their relation with a thin layer of infill sediments. We employed different near-surface geophysical techniques (electrical resistivity tomography, active and passive seismic methods, ground-penetrating radar), as well as remote sensing techniques (unmanned aerial vehicle digital photogrammetry and ground-based Light Detection and Ranging) to complement classic structural surveys. The valley structure has an asymmetric graben configuration, being bounded by a normal fault to the southeast (Mellieha Fault) and other to the north-west (Ghadira Fault). Our study provides insights of possible Quaternary fault displacements and revealed the presence of an additional fault splay, the previously unmapped NNWdipping Mellieha Fault 2 at the south-eastern edge of the valley. Additionally, by integrating results from geophysical surveys, we estimated the thickness of the valley’s recent deposits, being thicker towards the Mellieha Bay sector, with maximum values of 8–10 m. Our study aligns with the general model of horst and graben structuring with associated regional tilting since the late Miocene, and supports the hypothesis that some segments of NW-trending normal faults within the North Malta Graben exhibit Quaternary activity, although with minimal throw rates (<< 0.1 mm/yr). 1. Introduction Normal faulting in extensional settings entails the generation of fault-bounded basins that may host syn-tectonic sediments, useful to provide constraints on the long-term evolution of their bounding faults. In continental settings, the sedimentary record in extensional basins is usually a fraction of the real amount of deposited material, due to the interplay between fault slip, regional vertical movements, erosional processes and discontinuous sedimentation (Gawthorpe and Hurst, 1993; Gawthorpe and Leeder, 2000; Burbank and Anderson, 2011). In these peculiar settings, particularly where slow fault slip rates are coupled with low amounts of sediment discharge, it becomes challenging to infer the fault evolution. Moreover, fault traces may be concealed by recent cover and their relation with syn-rift sediments may not be clear. In these situations, near-surface geophysics is an essential tool for * Corresponding author. E-mail address: [email protected] (L. Galone). Contents lists available at ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto https://doi.org/10.1016/j.tecto.2024.230263 Received 27 June 2023; Received in revised form 19 February 2024; Accepted 27 February 2024 Tectonophysics 875 (2024) 230263 2 imaging and locating subsurface faults and mapping the depth to the bedrock in tectonic basins (e.g., Butler, 2005; Everett, 2013). Among the various techniques, electrical resistivity tomography (ERT -e.g., Loke, 2002; Giocoli et al., 2015; Improta et al., 2010) is a powerful and efficient tool capable of providing electrical resistivity images of the subsurface that in most cases can be interpreted in terms of lithology and structural discontinuities. The advantage of this technique is the speed of execution and the fast-computing time of a robust model. However, caution is necessary to the interpretation, especially since the tomographic models are usually obtained through linearized inversion techniques that produce smooth resistivity images. Shallow active-source seismic profiling also yields excellent results in structural imaging (e.g., Gold et al., 2013; Stephenson et al., 2012), although the rough surface topography at the basin borders and very sharp velocity discontinuity in the near-surface hampers shallow reflection profiling in these settings. Refraction tomography in this case is a valuable imaging tool (e.g., Villani et al., 2015). It enables obtaining detailed subsurface velocity models that can in turn be compared with average lithological properties, due to the broad correlation existing between P-wave velocity and rock type (e.g., Sowers and Boyd, 2019). Among the passive seismic techniques, the Horizontal-to-Vertical Spectral Ratio method (HVSR) has gained increased popularity for its ease of application, particularly in the case of mapping the thickness of shallow low-velocity sediments overlying a rigid basement (e.g., Molnar et al., 2022; Paolucci et al., 2015). In the geological sciences, the emergence of technologies such as structure-from-motion photogrammetry, paired with the decreasing prices of unmanned aerial vehicles and Light Detection and Ranging (LiDAR) sensors (Colica et al., 2021a, Colica et al., 2021b; Bistacchi et al., 2022), together with other remote sensing techniques (e.g. Foody et al., 2009) has enabled to increase the efficiency of data collection through the construction and use of digital outcrop models, capable of providing large amounts of structural and stratigraphic information. In this work, we present an application of combined near-surface geophysical surveying and digital outcrop models to reconstruct the geometry of a fault-bounded valley in the north-western sector of Malta, close to the village of Mellieha, between the Anchor and Mellieha bays (Figs. 1 and 2). The possibility of unraveling evidence of Quaternary fault activity in Malta is challenging because of the meager amount of preserved inland Quaternary sediments that may document any syn-rift deposition, and the poor exposures of continental deposits. Moreover, subsurface data constraining geometry and thickness of deposits filling fault-line valleys are generally lacking. This situation is further complicated by the massive urbanization of the island, as a result of which suitable sites for the study of continental sediments are reduced to a few valleys, mainly located in the north-western sector of Malta. The Mellieha fault-line valley represents a good site to test this multidisciplinary approach because it is one of the few that is fairly large in relation to the modest size of the island, and according to official geological maps, it contains thin remnants of continental sediments (Pedley, 1993). In particular, our focus is locating and imaging the shallow subsurface of the bounding faults, and estimating the thickness of recent continental deposits preserved in the valley. These data are used to infer the overall structure of the valley and the relation of the bounding faults with recent continental sedimentation. The latter in turn could provide some indication of the possible Quaternary fault activity. We additionally use digital outcrop analysis to map fracture patterns and to obtain information on fault geometry, and also use updated bathymetric datasets to infer the possible offshore extent of the surveyed faults. To our knowledge, this is the first application of an integrated geophysical and geomatic approach for structural geology studies in Malta. Since previous studies have been conducted only based on surface geological data, this work may contribute to a further understanding of the structures of fault-bounded valleys in Malta and their relations with its regional extensional tectonics. 2. Geological setting and the study area The Maltese archipelago comprises three main islands (Malta, Gozo and Comino) located in the central Mediterranean, in the foreland of the Sicilian Thrust Belt. This region represents the uplifted part of a wide extensional crustal domain known as Pantelleria Rift (Fig. 1). The latter changes in strike from NW-SE to nearly W-E moving to the east and developed since the late Miocene with a major phase of crustal stretching during the Plio-Quaternary (Dart et al., 1993; Micallef et al., 2019). The Maltese archipelago is made of Oligocene-Miocene shallow water carbonates and deep-water clays forming a sub-horizontal to gently dipping multi-layer sequence (e.g., Pedley, 1993). The principal outcropping formations, from the oldest to the youngest, are the following: Lower Coralline Limestone Fm. (late Oligocene); Globigerina Limestone Fm. (early Miocene); Blue Clay Fm. (middle-late Miocene); Upper Coralline Limestone Fm. (late Miocene). The generalized post-Miocene uplift explains the complete absence Fig. 1. Location map of the study area (Mellieha Valley). (a) Simplified structural map of the central Mediterranean (modified after Palano et al., 2012), and Malta location in the red square. (b) Topo-bathymetric map of the Maltese archipelago. The main structural features and the Mellieha Valley are shown. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) L. Galone et al. Tectonophysics 875 (2024) 230263 3 of Pliocene marine deposits as well as the scarcity of Quaternary sediments on the islands, which experienced different episodes of extensional tectonics that are well documented in the literature (e.g., Pedley et al., 1976; Pedley, 1993; Dart et al., 1993). This gave rise to two main tectonic trends that characterize the structural setting and geomorphology of the islands. The first trend is due to WSW-ENE trending normal faults that form a series of horst and graben structures, particularly well developed between Gozo and the northwestern part of Malta, and is responsible for the generation of the North Malta Graben (Fig. 1b). The second trend is related to NW-SE trending faults that are mostly developed in the southern sector of the island (e.g., Maghlaq fault in Fig. 1b, Bonson et al., 2007). Several works deal with the relations and timing of activity of these two fault systems (e.g., Dart et al., 1993; Bonson et al., 2007; Martinelli et al., 2019). The main phases of rifting began in the Late Miocene during the initial stages of the opening of the Pantelleria Rift and probably ceased during the Pliocene. Dart et al. (1993) inferred an almost uniform N-directed extensional regime affecting the archipelago since the Late Miocene. Martinelli et al. (2019) propose that the N-directed extension was preceded by a short-lived phase of WNW-directed extension in the Miocene. The vast majority of faults outcropping in the archipelago are considered currently inactive, whereas hints of recent faulting along some offshore segments mapped in the Gozo channel and to the west of the archipelago are reported by Micallef et al. (2019), who point out an active NE-extension affecting the study region. According to some authors, specific segments of the major WSW-ENE faults outcropping in Malta (Victoria Lines fault, Fig. 1b) may have experienced some episodes of Quaternary activity as well (e.g., Villani et al., 2018). Putz-Perrier and Sanderson (2010) demonstrated, by means of several geological profiles, that the amount of extension accomplished by the WSW-ENE faults increases towards the North Malta Graben, so that the faults bounding this graben are the best candidates for Quaternary activity. Pedley et al. (1976) and Illies (1981) suggested that recent uplift to due fault activity may have affected the coastal regions to the north of Mellieha, attested by raised beach deposits and notches (see also Pedley, 2011). The limestone backbone of these islands is dissected by a large number of fault-line valleys related to horst-and-graben parallel structures oriented mostly WSW-ENE (Alexander, 1988; Pedley, 1993). Being very close to the sea, and due to the limited dimensions of the drainage watersheds, the sediments laid by the short rivers were for the most part completely erased, particularly during the strong incision related to the sea level drop during the latest cold climatic phases (e.g., Hunt, 1997), leaving a discontinuous thin layer of fluvial and alluvial deposits. The preserved deposits are mostly confined in caves, or they constitute relics of conglomerates with a reddish matrix generally attributed to the Middle-Late Pleistocene (the so-called red beds; e.g., Hunt, 1997; Pedley, 2011). The Mellieha valley (Fig. 2) is about 1.4 km wide and 1.5 km long. It represents a nearly symmetrical graben structure bounded by two structural highs exposing an almost complete section of the Upper Coralline Fm., locally showing the underlying top of the Blue Clay Fm. The official geological maps (Pedley, 1993; Continental Shelf Department, 2022) report at the borders of the graben two main WSW-ENE trending faults with opposite dips, which we name Mellieha fault (NNW-dipping) and Red Tower fault (SSE-dipping). In the central part of the graben, a small structural high is present, labeled as Ghadira hill, also made of Upper Coralline Fm. Our study is based on measurements Fig. 2. Geological map of the Mellieha Valley study area and location of the main geophysical measurements. DR: digital reconstruction; HVSR: horizontal-tovertical spectral ratio measurement; GPR: ground penetrating radar B-scan; ERT: electrical resistivity tomography; GL: Globigerina Limestone Formation; UGL: Upper Globigerina Limestone member; UCL, Upper Coralline Limestone Formation. Members of the UCL Fm. are indicated as follows: GM: Għajn Melel member; T-M: Tal-Mas member; Mt.: Mtarfa member; T-P: Tal Pitkal member; GI: Gebel Imbark member; BC: Blue Clay Formation; R-B: red beds. Coordinates in WGS84 system, zone UTM-33 N. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) L. Galone et al. Tectonophysics 875 (2024) 230263 4 and observations conducted in the southern valley, which is limited by the Ghadira and Mellieha faults. We chose this site because of the documented presence of old alluvial sediments with a reddish matrix (red beds; see Pedley, 2011), whose thickness was unknown until now, that were laid down by an old fluvial network and subsequently deactivated by climatic changes and drastic sea level fluctuations occurred over the late Quaternary (e.g., Hunt, 1997). 3. Methodology 3.1. Proximity and remote sensing for digital reconstruction and structural data collection To gain insight into the structural setting of the study area, we conducted geological observations and structural measurements in the areas where the primary basin-bounding fault zone is visible, as well as in Mellieha Bay (Fig. 2). To achieve this, we used a hybrid approach that combined traditional structural compass measurements with the creation of two digital reconstructions - DR1 and DR2 - using different methods. DR1 was reconstructed over a rural road where fractures associated with the fault zone of the Mellieha Fault were observed, over the sector where the ERT E5 was performed (Fig. 2). Given the small size of the area, data acquisition was carried out using an iPhone 12 Pro max device equipped with a LiDAR sensor capable of reconstructing coloured and geo-referenced dense point clouds within a range of 5 m. The point cloud was analyzed by the Compass plugin (Thiele et al., 2019) installed in the Cloud Compare software v. 2.13 (Girardeau-Montaut, 2016), which allowed to digitize discontinuities interpreted as fractures and to obtain their strike. The digital dataset was validated by comparing the obtained fracture attitude values with compass measurements taken in a traditional manner. The results indicate that these two types of measurement provide azimuth values that can be considered very similar, as they usually fall within a range <5◦. For this reason, fracture mapping from digital models was useful to increase the density of structural data collection. DR2 was performed in Anchor Bay (Fig. 2), where direct observation of the vertical cliff is challenging due to its sub-vertical morphology. Digital photogrammetry was used with photographs acquired using a DJI Phantom 4 Pro UAV, and 20 ground control points were measured by using a survey-grade Global Navigation Satellite System (GNSS). A total of 1953 images, with a pixel resolution of 5472 ×3648, were acquired using automated flight planning with the app Pix4D Capture with an 80% forward overlap and 70% side overlap. Images were inspected and those that were defective (e.g., out of focus or overexposed) were removed, to prevent processing failures. Subsequently, Structure from Motion processing was performed with Agisoft Metashape software v. 2.0.0, following the steps used by Colica et al., 2021a,Colica et al., 2023). A third fault zone outcrop in the Mellieha harbor (Fig. 2) was measured by conventional structural methods. This outcrop is not mapped as a fault in official geological maps, and may represent the easternmost part of a second fault bounding the southern sector of the valley. Results were plotted on stereographic diagrams and rose diagrams for analysis and interpretation. 3.2. Geophysics 3.2.1. Electrical resistivity tomography We acquired five ERT profiles at specific sections perpendicular to topographic scarps that hint at the presence of faults bounding the Anchor-Mellieha”s valley (Fig. 2). The aim was to characterize such geological structures and the very shallow sedimentary layers overlying the UCL Fm. The selection of the sites represents the best compromise between geological targets and logistical constraints imposed by the fragmentation of agricultural properties, as well as the presence of underground utilities and power lines that would cause unwanted anthropic noise. Three ERTs were performed crossing the Ghadira Fault, one on the sandy beach deposits (E1) and two on the central section (E2 and E4). One ERT was set across the Mellieha Fault (E5) and another was conducted crossing perpendicularly a >700 m-long escarpment parallel to the Mellieha Fault, where we suspect the presence of an unmapped fault (E3, roll-along array with an overlap of 50%). We used a multichannel digital resistivity meter ELECTRA (Moho srl) featuring a waveform D/A converter with continuous current and voltage control, including feedback. The applied current during measurements was 10 mA. ERT’s electrode location was measured using a GNSS system with sub-0.02 m accuracy and the altitudes corrected to the local geoid elevation were used for processing. Details of the ERT profiles are reported in Table 1 and locations in Fig. 2. For all the locations, we acquired and compared the Wenner α and Schlumberger electrode arrays because they are sensitive to different electrical resistivity distributions in the subsurface. In particular, the Wenner α array is suitable for recovering stratified media with vertical resistivity contrasts, whereas the Schlumberger array probes a greater depth with respect to the Wenner α array. Results obtained in the field depend on the local conditions and the presence of some anthropic sources of noise (power lines) that in some cases affected data quality. For these reasons, in this paper, we show only the best inversion result for each profile. The processing routine consisted of identifying and filtering outliers in the raw data with a de-spiking technique. Data inversion was performed using RES2INV software v. 4.08 (Loke and Barker, 1996; Loke, 2002) based on the least squares linearized inversion technique with smoothing constraints. Models are parameterized as regular cells whose size depends on the electrode spacing and array type. Due to the fact that the number of model parameters exceeding the measured data makes the problem underdetermined, in the linearized inversion process a regularization parameter is introduced that determines the relative weight of data misfit and model misfit terms; the latter is a smoothing constraint that is automatically tuned during the inversion to avoid an overly smooth model or conversely a model containing unreasonable artifacts. The acquired datasets were processed by incorporating topography information. The forward model was established using a horizontal mesh size of 4 nodes to enhance the accuracy of apparent resistivity values and the finite-element method was employed for forward calculation, with the program automatically adjusting the grid size. We selected robust inversion constraints, which are less susceptible to highly noisy data and are particularly well-suited for delineating sharp boundaries, as expected at the R-B – UCL Fm. contact. The model assessment was done by taking into account the data quality and absolute percentage error, which is a function that describes the difference between recorded and calculated resistivity data. We also took into account the general feature of the resistivity distribution. 3.2.2. Horizontal-to-vertical spectral ratio Ninety-six single-station ambient noise recordings were collected in the Mellieha valley. The extent of the measures is limited to areas where it was possible to access. We set up our measurements with the purpose of covering most of the valley, which was conditioned by its intense agricultural use. The seismic signals were analyzed to obtain the HVSR curves (Nakamura, 1989), computed as the ratio of the Fourier amplitude spectra between the quadratic mean of the horizontal components and the vertical component of the ambient vibrations. Following the approach of previous works in the Maltese islands (e.g., Vella et al., 2013; Leucci et al., 2021; Farrugia et al., 2016; Panzera et al., 2013), we used three Tromino (http://moho.world/), a compact three-component velocimeter with 24-bit digitizer, operating in the nominal frequency range of 0.1–300 Hz. Time series of ambient noise, having a length of 20 min, were recorded with a sampling rate of 128 Hz and, following the guidelines suggested by Bard and P.S (2004), they were divided into time windows of 20 s each not overlapping each other. A 5% cosine taper L. Galone et al. Tectonophysics 875 (2024) 230263 5 was applied to each window and the Fourier spectra were calculated. The spectra of each window were smoothed using a Konno–Ohmachi window (Konno and Ohmachi, 1998) fixing the parameter b to 40. Finally, the resulting HVSR, in the frequency range 0.3–60.0 Hz, was computed by estimating the logarithmic average of the spectral ratio obtained for each time window, selecting only the most stationary and excluding transients associated with very close sources. The results revealed an HVSR peak at frequencies between 1 and 2 Hz for the measurements made on the outcropping UCL Fm. and an additional peak at frequencies >5 Hz for the measurements made on the alluvial sediments (red beds) in the valley. Both peaks have amplitudes >2 units and correspond to an eye-shaped pattern in the spectral components (Fig. 3). According to Bonnefoy-Claudet et al. (2006), the presence of pronounced peaks in the HVSR curve indicates impedance contrast at the selected site, given by a low average shear wave velocity (Vs) layer over a high Vs layer. In our case, the low-frequency peak is related to the Vs contrast between the relatively low Vs BC Fm. and the relatively high Vs GL Fm., as demonstrated in previous works (e.g., Panzera et al., 2012; Farrugia et al., 2016, 2021). On the other hand, we associate the highfrequency peak with the Vs contrast between the red beds and the UCL Fm., which has relatively high Vs (Panzera et al., 2012). We used closely spaced single-station measurements to obtain smooth 2-D pseudo-tomographic sections of HVSR as a function of frequency and distance (using the code by Wathelet et al., 2020) that can be interpreted in a qualitative way as structural sections because they show the spatial continuity of seismic impedance surfaces characterized by consistently high HVSR values. The use of HVSR peak frequencies for sedimentary thickness estimation is relatively common. The aforementioned peaks frequencies (f) Table 1 Summary of the ERT profiles presented in this study. ERT Location N◦electrodes Interelectrode spacing Array Length Array type Absolute Error % of removed data E1 Ghadira Fault – Beach deposits 32 2 m 62 m Wenner α 11.8% 11.5% Schlumberger 18.1% 49% E2 Ghadira Fault – Central sector 43 5 m 210 m Wenner α 16.2% 7.1% Schlumberger 15.4% 61.0% E3 Unmapped Fault – Central sector 64 5 m 315 m Wenner α 31.1% 22.6% Schlumberger 18.7% 19.6% E4 Ghadira Fault – Central sector 50 5 m 240 m Wenner α 14.9% 10.9% Schlumberger Too noisy data E5 Mellieha Fault – Southwest sector 64 2.5 m 155 m Wenner α 13.2% 8.1% Schlumberger Too noisy data Fig. 3. Example of HVSR processing result. In the upper panel, the continuous black line represents the average HVSR function, whereas dashed black lines indicate standard deviations. The peak at frequency of around 1.5 Hz is associated with the resonance of BC Fm. over GL Fm. (e.g., Farrugia et al., 2016). The peak at 5.5 Hz is associated with the resonance of R-B over UCL Fm. The bottom panel displays the power spectral density of each component, with the red line denoting the vertical component and the black lines representing the horizontal components. Eye-shaped patterns (increase in horizontal components energy and decrease in vertical one) correspond to the frequency of HVSR peaks. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) L. Galone et al. Tectonophysics 875 (2024) 230263 6 are related to the Vs of a layer with thickness (T) through the simple relationship f =Vs/(4 T), following the quarter-wavelength rule (Boore, 2003). One limitation of this approach is that a site-specific shear wave velocity profile is required for depth-conversion of these interfaces. To obtain insights into the distribution of red beds in the valley, we use the 1D assumption implicit in the f =Vs/(4 T) formula, by adopting Vs values for shallow soils obtained in other sites of the island with independent seismic measurements (e.g., Farrugia et al., 2016; Pischiutta et al., 2016; Villani et al., 2018). 3.2.3. Active-source seismic profiling We acquired a high-resolution seismic profile crossing the fault scarp that bounds to the ESE the Ghadira hill, along ERT E2. We used 48 vertical geophones (4.5 Hz natural frequency) spaced 2 m and connected with 24-bit seismographs of Geode Geosystem (by Geometrics Inc.). The geophones we used have a nearly flat response from about 2 Hz up to 150 Hz, therefore they are suitable for shallow seismic profiling since they accurately record high-frequency direct and refracted P-waves (frequency range of 40–100 Hz) as well as surface waves (frequency range of about 5–30 Hz). The elastic energy was input into the ground by using a 5-kg sledgehammer with a spacing of 4 m. The handpicked first arrival travel times were inverted using a non-linear multi-scale refraction tomography code that uses a finite-difference eikonal solver and mixed Monte Carlo-Simplex optimization scheme: the main feature Fig. 4. Workflow of active-source seismic tomography used in this study. The upper panel shows a recorded common shot gather (CSG) collected during seismic profiling in the Mellieha valley (48 seismic traces, only amplitude normalization applied): the red line is the envelope of the picked first arrival travel times. The latter, together with traveltime readings from all other CSGs, are inverted through a non-linear algorithm (Herrero et al., 2000) to produce a 2D P-wave velocity model (central panel). The bottom panel indicates the seismic ray coverage (indicated as hit count within 1 m ×1 m cells) obtained by back-projecting rays from receivers to source. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) L. Galone et al. Tectonophysics 875 (2024) 230263 7 of this inversion technique is that it does not require a starting reference model, which is obtained at the early stage of inversion through a massive search in the model space (Herrero et al., 2000). This approach was used on the island by Villani et al. (2018) for the shallow seismic imaging of the western sector of the Victoria fault. In this paper, we show a best-fit representative Vp model paired with a plot of ray coverage obtained with a back-ray tracing procedure (more details in Improta et al., 2002). The basic workflow of active-source seismic data is shown in Fig. 4. Using the common shot gathers at the center and on both ends of the linear array deployment, we also apply the f-k analysis to extract the Rayleigh wave dispersion curve. This was done to try to infer the shear wave velocity of the red beds. 3.2.4. Ground penetrating radar Ground Penetrating Radar (GPR) air-coupled Cobra Plug-In™ SUBECHO model (RadarTeam AB, Boden, Sweden) with a central frequency of 80 MHz was used to perform B-Scans in transects perpendicular to the major axis of the valley, with a sampling interval of 3.125 ns and 512 samples per trace over a time window of 600 ns, which corresponded to an approximate depth of 30 m in limestone with electromagnetic wave propagation velocities of 0.1 m/ns (Davis and Annan, 1989). While this approach showed good results on the UCL Fm. in other sectors of the island (Colica et al., 2023), our results were mostly very noisy, probably due to the presence of medium and high-voltage power lines near the acquisition points. Despite this, a high-quality B-scan was obtained in the site of ERT E3, which presented good resolution and was included in this study. The data were processed using Prism2® software (RadarTeam, v2.7), applying time zero correction, background removal, bandpass filter and migration using the hyperbola fitting method (Colica et al., 2021b, Persico et al., 2019, Persico and Muci, 2023) obtaining a propagation velocity for electromagnetic waves of about 0.11 m/ns, and topographic correction was also applied (Fig. 5). 4. Results 4.1. Structural analysis from field data and digital outcrops In the field, the fault outcrops are generally poorly preserved. We found evidence of an additional NNE-dipping fault, parallel to the main Mellieha fault, in the Mellieha Bay harbor (mean outcrop location E14.356785◦, N35.96676◦, blue star in Fig. 2). This fault displays a >1 m-thick NW-dipping damage zone with a mean trend of about N220◦- 225◦(also including small antithetic, SE-dipping faults) and vertically displaces the UCL Fm. of about 2 m (Fig. 6). This outcrop pinpoints the easternmost part of a subdued normal fault structure, suggested by the occurrence of a >700 m-long escarpment parallel to the main Mellieha fault and that we investigated by means of near-surface geophysics. On an outcrop located in the southern part of Mellieha Valley over the Mellieha Fault, where DR1 was performed (mean coordinates E14.34548◦, N35.95516◦), several high-angle fractures and small faults were found, suggesting the presence of a thick fault zone with a general east-west trend and dipping to the north (Fig. 7). The DR2 model allowed for precise measurements and images of the exposure of the Ghadira fault, which bounds the homonymous hill to the south, along the coast of Anchor Bay (Fig. 8). The main fault dips to the ESE with a relatively high angle (>70◦) and is roughly parallel to the limit of Anchor Bay, suggesting structural control in the shape of the bay. The damage zone associated with the fault is approximately 20 m wide and contains at least one minor synthetic fault. The main fault puts in contact the members of the UCL Fm: Tal-Pitkal, in the footwall block, and Gebel Imbark, in the hanging wall block. The throw measured in the photogrammetric model is around 20 m, taking into account the position of the Gebel Imbark member base on both sides of the fault zone. 4.2. Geophysical sections We describe the results of the geophysical sections along topographic Fig. 5. GPR B-scan processing workflow. (a) Raw data. (b) Background removal. (c) Bandpass filter. The white arrow indicates the right part of a hyperbola (indicating probably a karstic feature), the black arrow indicates a ringing effect, dash black line indicates the maximum signal penetration. (d) zoom of the more superficial part of the radargram. The portion with the ringing effect is deleted. Oblique interfaces are evident, interrupted at the central part of the radargram. (e) migrated B-scan. (f) Migrated b scan with the topographic correction. Elevation refers to the sea level. L. Galone et al. Tectonophysics 875 (2024) 230263 8 profiles I-I ′ , I-II’ and III-III’ (location in Fig. 2) from the north-eastern to the south-western part of the Mellieha valley (Figs. 9–11). The geophysical section along the I-I ′ profile was conducted on the beach deposits of Mellieha Bay (Fig. 9). It consists of an ERT profile (E1) (Fig. 9b) and a HVSR frequency tomographic plot (Fig. 9c), where the colour palette indicates the amplitude of HVSR as a function of distance (x-axis) and frequency (y-axis). These results were obtained from eight ambient seismic noise measurements spaced 15 m apart along the E1 (black triangles on Fig. 9b and c), whose position is shown on the topographic profile (Fig. 9a). The ERT E1 (Fig. 9b) revealed relatively low resistivity values ranging from 1 to 15 Ωm. In this section, distinct domains were identified, characterized by pronounced resistivity gradients. A thin, approximately 1 m-thick horizontal layer with relatively high resistivity values was observed in the shallowest portion, while a thicker domain of high resistivity values was observed towards the northern part (x = 0–30 m). Between x =30–120 m, a zone with relatively low resistivity values (<4 Ωm) was identified, characterized by a distinct lower boundary that dips towards the northwest. An additional domain of relatively high resistivity values deepening to the northwest appeared below this zone, between depths of −8 and −15 m. The HVSR values obtained along the co-located E1 (Fig. 9c) ranged Fig. 6. Evidence of the NE termination of the fault bounding the Mellieha bay to the south-east. (a) NW-dipping normal fault zone (N314 ◦/078◦) affecting the Upper Coralline Fm. at the Mellieha Harbor (Dawret it-Tunara, coordinates E14.356949◦, N35.966816◦). (b) Detail of the small-displacement fault damage zone. Fault attitude notation is dip direction/dip angle. Fig. 7. Surface structural data collected along the Mellieha Fault. (a) and (b) Details of digital model DR1 reconstructing an outcrop over a rural road across the Mellieha fault zone, co-located with ERT profile E5. Red arrows and dash lines indicate meso-faults. (c) Stereplot indicating small meso-faults measured in the field. (d) Rose diagram indicating the azimuthal distribution of faults measured in the field (red petals) and those mapped from the digital model (blue petals). Count ranges from 0 to 10. (e) Bar plot of fracture density per meter surveyed along a 96 m-long section of the profile. Two main clusters of fractures are found at 15–30 m and 40–55 m, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) L. Galone et al. Tectonophysics 875 (2024) 230263 9 from 0.5 to 9 units in the profile drawn based on ambient noise recordings. Interpolation of the values obtained at each station revealed two interfaces: one at frequencies below 2 Hz throughout the profile, and another at frequencies between 4 and 10 Hz approximately, extending from x =30 m to the end of the section. The observed discontinuities in both the ERT and HVSR sections around x =30 m were interpreted as block displacement due to the Ghadira fault, with the NW block acting as the footwall block. The overall low resistivity values were interpreted as due to the proximity of the sea, where the saline water may saturate the pore rocks and sediments, thus strongly increasing the conductivity. The relatively high thin resistivity layer at the surface was identified as dry sand, while the thick high resistivity domain between x =0–30 m corresponds to a very shallow UCL Fm. The relatively low resistivity zone may indicate the presence of non-lithified sedimentary infill material (indicated in Fig. 2 as red beds, R-B). A gentle dip of the base of these deposits towards the fault can be observed. The high-frequency HVSR interface was interpreted as low-velocity deposits above the UCL Fm., which is characterized by higher Vs values that produce a positive seismic impedance contrast. The abrupt interruption of this interface at x =30, coinciding with the lateral change of ERT resistivities, supports the interpretation of the presence of a fault (Fig. 9c). On the other hand, the low-frequency interface was interpreted as the relatively low-Vs BC Fm. above the higher-Vs GL Fm. We do not see a clear step in the lower impedance contrast (Fig. 9c) because the associated wavelength (about 350 m, considering a Vs of 400 m/s) is too large compared to the small fault displacement. Our HVSR interpretation aligns with previous studies in Malta, where HVSR peaks around 1 Hz were interpreted as due to the BC Fm. - GL Fm. contact (e.g., Panzera et al., 2012; Villani et al., 2018; Farrugia et al., 2016, 2021). HVSR results are discussed in more detail in Section 4.3. The geophysical sections in profile II-II’ were carried out in the central sector of the Mellieha valley (Fig. 10). We acquired one ERT (E2) and a co-located P-wave refraction tomography in the NW sector, and an ERT (E3) and a co-located radargram in the SE sector (Fig. 10a). The electrical resistivity values obtained in the ERTs (Fig. 10b) range from 20 Ωm to over 1500 Ωm. In both sections, a shallower layer with average thicknesses ranging from 2 to about 10 m was observed, with resistivity values <150–200 Ωm. Underneath this shallow layer, a zone of higher resistivity (up to 1500 Ωm) was recognized separated from the overlying layer by a high-gradient thin belt. In the case of the ERT located to the NW (E2), the geometry of this interface is sub-horizontal, and at x =0–20 m, high resistivity values (>600 Ωm) are also present at the surface. On the other hand, the interface in the ERT located to the SE (E3) is more irregular and exhibits a stepped geometry, with a general northwest dip. The P-wave tomography obtained from the seismic profile overlapping to E2 (Fig. 10c) shows values of Vp <600–800 m/s in the shallow 3–5 m, and >1400 m/s at greater depths. In particular, a discontinuous near-surface low-Vp layer (350–700 m/s), with a Fig. 8. Anchor Bay. Top panel, digital outcrop model DR2 interpreted from Anchor Bay. The throw measured on the base of the GI member is approximately 20 m. Bottom. Panoramic picture of the fault zone. The main fault is indicated by a thicker red line. The dash-yellow line indicates the approximate stratigraphic boundaries. BC: Blue Clay Formation; T-M: Tal-Mas member; T-P: Tal-Pitkal; GI: Gebel Imbark member. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) L. Galone et al. Tectonophysics 875 (2024) 230263 16 regional tilting, likely related to the late Miocene to recent opening of the Pantelleria Rift (e.g., Dart et al., 1993). Following the end of the main phase of uplift (which likely took place in the final part of the Pliocene), some localized activity affected a few fault segments, including those bounding the Mellieha graben, as pointed out by our geophysical profiles and the uneven distribution and thickness of red beds within the valley. The inferred resulting fault displacements achieved in the Quaternary are quite small, probably not exceeding a few meters. This novel result is the first in this sector of the island of Malta and is in broad accordance with some previous inferences (e.g., Pedley et al., 1976; Illies, 1981) on the recent tectonic activity in the area surrounding the North Malta graben. We point out that Micallef et al. (2019) mapped several fault scarps offshore, with dominant trends NWSE and WSW-ENE, and they infer that most of them show evidence of activity in the past 20 ka, in the framework of a current NE-extension. Some of these scarps represent the offshore prolongation of onshore faults, as in the case of the NE tip of the Mellieha fault. Therefore, our results are consistent with these findings, and support the hypothesis that NW-trending normal faults within the North Malta Graben display Quaternary activity and they may be still active, although with very low throw rates (<< 0.1 mm/a). Further investigations and additional data, including an accurate estimation of the age of the continental deposits preserved in the Valley, may clarify some aspects of the recent geological evolution of the area. 6. Conclusions In this multidisciplinary study, we used various geophysical and remote sensing methods to obtain a detailed image of the geological structure of the southern sector of the Mellieha valley, located within the North Malta Graben, in the northern part of the island of Malta. We employed electrical resistivity tomography, active and passive seismic techniques and ground penetrating radar, as well as remote sensing techniques such as UAV photogrammetry, ground-based LIDAR and bathymetry data. The results enabled us to identify the structural settings of the valley, including the presence of a previously unmapped fault, which we have named the Mellieha fault 2. The valley structure has an asymmetric graben configuration, where a splay of two synthetic faults at the southeastern boundary (the Mellieha Fault and the Mellieha Fault 2) and a fault at the north-western boundary (the Ghadira Fault). Analysis of the bathymetric data allowed us to track the offshore extent of the Ghadira fault, Mellieha fault and Mellieha fault 2, evidenced by high slope gradients in the seafloor topography in the Mellieha Bay. We estimated vertical displacements of the Ghadira Fault of around 20 m in Anchor Bay and about 8 m in Mellieha Bay. This difference in vertical displacements could be explained by the differential uplift of the NE sector of the block representing Ghadira Hill, as well as by displacement transfer to the adjacent faults to the north (including the Red Tower fault). Additionally, we estimated the thickness of the recent deposits (so-called red beds) in the valley, showing a trend of increasing thickness towards the Mellieha Bay sector, partly following the regional topographic slope. Our findings suggest a certain wedging with increasing thicknesses towards the Ghadira Fault and Mellieha Fault 2, indicating possible activity of those segments of the fault during the Quaternary. The Quaternary activity of these faults is consistent with recent studies that outline some recent fault scarps offshore in the North Malta Graben, close to our study area. Overall, the different geophysical and remote sensing methods we used allowed us to obtain a detailed image of the geological structure of the area. These findings are significant for better understanding the structure and recent tectonic dynamics in the Mellieha Valley area, providing insights into the recent evolution of the North Malta Graben, which is an important regional morphostructure developed in response to the opening of the Pantelleria Rift. Moreover, this work highlights the benefits of using a multidisciplinary approach in structural studies that combines field data collection, shallow geophysical prospecting and remote sensing. CRediT authorship contribution statement Luciano Galone: Visualization, Validation, Software, Methodology, Investigation, Formal analysis, Data curation, Conceptualization, Writing – review & editing, Writing – original draft. Fabio Villani: Validation, Supervision, Software, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization, Writing – review & editing, Writing – original draft. Emanuele Colica: Software, Methodology, Investigation, Writing – review & editing. Davide Pistillo: Investigation. Paola Baccheschi: Investigation, Writing – review & editing. Francesco Panzera: Software, Investigation, Writing – review & editing. Jesús Galindo-Zaldívar: Investigation, Writing – review & editing. Sebastiano D’Amico: Validation, Supervision, Project administration, Investigation, Funding acquisition, Formal analysis, Conceptualization, Writing – review & editing, Writing – original draft. Declaration of Competing Interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: F. Villani reports financial support was provided by National Institute of Geophysics and Volcanology. S. D’Amico reports financial support was provided by Malta Council for Science and Technology and the INTERREG V A–Italy-Malta Capitalization Programme. Data availability Data will be made available on request. Acknowledgments S. Amoroso, A. Antonioli, L. Cantore, A. Cuoghi, D. Farrugia, P. Galea, G. Manzi, D. Di Naccio, P. Iregbeyen, A. Mercuri, L. Piccolini, M. Pischiutta, and A. Rovelli provided precious support during some early field surveys performed in the study area. We would also like to thank Ritienne Gauci (University of Malta) for her useful comments about the geology and structural data interpretation of the area. Topobathymetric information from ERDF 156 data, (2013), Developing National Environmental Monitoring Infrastructure and Capacity, Planning Authority was used in this work. Two anonymous reviewers provided insightful comments that helped us improve the quality of the manuscript. Part of the field activities were conducted within the framework of the INGV Project “Ricerca Libera” BR2019.23 (“Unveiling silent faults in low strain-rate regions through the integration of high-resolution geophysical and seismological analyses,” Principal Investigator: Fabio Villani), the project “Multi-disciplinary monitoring system for resilient management of coastal areas (REMACO),” financed by the INTERREG V A–Italy-Malta Capitalization Programme (local P.I., S. D’Amico), and the IPAS+(Internationalisation Partnership Awards Scheme), supported by the Malta Council for Science and Technology (https://mcst.gov. mt/). References Alexander, D., 1988. A review of the physical geography of malta and its significance for tectonic geomorphology. Quat. Sci. Rev. 7, 41–53. https://doi.org/10.1016/02773792004. 188)90092-3. Bard, P.Y., P.S, August 2004. The SESAME project: an overview and main results. 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