Istituto di Geologia Ambientale e Geoingegneria, CNR, c/o Università Sapienza, P.le Aldo Moro 5, 00185 Rome (Italy)

We present a marine paleoseismology analysis of a dense network of very high resolution seismic profiles along the Gondola Fault Zone (GFZ), a right-lateral, E-W–striking, active fault system in the Adriatic foreland. This case-study aims to show how time and space variations in the activity of a dominantly right-lateral fault system can be assessed based on the vertical component of slip alone. The GFZ has been investigated for a length of 50 km. It includes two parallel subvertical fault sets and two main anticlines. The distribution of the late Middle Pleistocene to Holocene vertical component of displacement along-fault is bell-shaped, suggesting that in the long-term the fault zone acts as a single, kinematically coherent structure. Slip rates on individual fault segments, however, suggest that they may rupture independently. Vertical slip rates calculated for late Middle Pleistocene-Holocene intervals fall in a narrow range and are consistently small (0-0.18 mm/a).

Historical earthquakes of the Gargano Promontory, an uplifted foreland sector in southeastern Italy, have been usually regarded as generated by inland faults. Some have been associated with activity of the Mattinata Fault, a section of a regional E-W shear zone. The 10 August 1893, Mw 5.4 is one of such earthquakes, but its current onshore location is only loosely based on the damage pattern. Regions that were hit by offshore earthquakes are also known to be affected by a methodological bias such that offshore historical events appear to be located onshore. To test this condition for the 1893 earthquake we pursued an alternative hypothesis for its location. The earthquake occurred near the Gondola Fault Zone, a right-lateral active fault system representing the offshore counterpart of the Mattinata Fault and hence capable of producing sizable earthquakes along the Gargano coast. We focused on its westernmost segment, suggesting that it could be the causative fault of the 1893 earthquake, in agreement with both the damage distribution and reported environmental effects. The approach we present works side by side with the recent developments of the algorithms used to compile historical catalogues, providing a fine-scale, geologically-based method to define or confirm the dubious location of historical earthquakes. Marine Paleoseismology is a new field stemming from the increased capabilities of high-resolution marine techniques in supporting classical paleoseismological analyses for the exploration of the seismogenic potential of offshore faults. Based on Late Pleistocene and Holocene individual or cumulative earthquake records, the potential of offshore faults can now be constrained in terms of expected magnitude and recurrence intervals. We stress the importance of revisiting historical earthquakes in coastal zones using marine paleoseismological data to assess regional seismic hazard, particularly in tectonic settings where regional-size seismogenic areas straddle the onshore and the offshore.

Recent seismicity in and around the Gargano Promontory, an uplifted portion of the Southern Adriatic Foreland domain, indicates active E–W strike-slip faulting in a region that has also been struck by large historical earthquakes, particularly along the Mattinata Fault. Seismic profiles published in the past two decades show that the pattern of tectonic deformation along the E–W-trending segment of the Gondola Fault Zone, the offshore counterpart of the Mattinata Fault, is strikingly similar to that observed onshore during the Eocene–Pliocene interval. Based on the lack of instrumental seismicity in the south Adriatic offshore, however, and on standard seismic reflection data showing an undisturbed Quaternary succession above the Gondola Fault Zone, this fault zone has been interpreted as essentially inactive since the Pliocene. Nevertheless, many investigators emphasised the genetic relationships and physical continuity between the Mattinata Fault, a positively active tectonic feature, and the Gondola Fault Zone. The seismotectonic potential of the system formed by these two faults has never been investigated in detail. Recent investigations of Quaternary sedimentary successions on the Adriatic shelf, by means of very high-resolution seismic–stratigraphic data, have led to the identification of fold growth and fault propagation in Middle–Upper Pleistocene and Holocene units. The inferred pattern of gentle folding and shallow faulting indicates that sediments deposited during the past ca. 450 ka were recurrently deformed along the E–W branch of the Gondola Fault Zone. We performed a detailed reconstruction and kinematic interpretation of the most recent deformation observed along the Gondola Fault Zone and interpret it in the broader context of the seismotectonic setting of the Southern Apennines-foreland region. We hypothesise that the entire 180 km-long Molise–Gondola Shear Zone is presently active and speculate that also its offshore portion, the Gondola Fault Zone, has a seismogenic behaviour.

The active tectonics of the Southern Apennines of Italy (Calabrian Arc excluded) is mainly characterized by SW-NE extension, which accounts for large earthquakes generated by NW-SE striking normal faults. However, the 2002 Molise earthquakes occurred along an E-W striking right-lateral seismogenic structure located to the NE of the Southern Apennines axis. This and other lines of evidence suggested that the frontal part of the chain and the adjacent foreland are affected by E-W striking, right-lateral active faults systems. The 2002 Molise seismic sources, in particular, are located along the western part of a regional fault system, the Molise-Gondola shear zone (MGsz). On land, this system is mainly represented by the Mattinata Fault, an important structure of the foreland that has already been intensely investigated from a regional, structural and seismotectonic point of view. A polyphase activity (since Mesozoic times) has been recognized, and the complex fault kinematics is still matter of debate. Nevertheless, most investigators agree on a present-day activity with right-lateral sense of motion, as confirmed by the focal mechanism of the 19 June 1975 earthquake, GPS data, geomorphological and paleoseismological investigations. Indeed, the Mattinata Fault has already been interpreted as the source of historical earthquakes (e.g., 493 AD, 1875), and instrumental seismicity is normally recorded within the first 25 km of the crust of the Gargano area. These data indicate that inherited E-W striking high-angle fault systems are solicited under the present-day stress field. Off-shore the Gargano Promontory, the Mattinata Fault seems to be aligned with a regional (ca. 150 km in extent) E-W to NW-SE oriented deformation belt (known in the literature as Gondola Line), including a main fault and fold system known as Gondola Fault and Gondola Ridge, respectively. In the past, this structure has been investigated using multi-channel seismic reflection profiles and well-log data. Several investigators proposed a Mesozoic origin for the Gondola Line, followed by a complex pattern of repeated re-activation during the Cenozoic. Kinematics and timing of post-Mesozoic re-activation are still debated; however, most investigators agree that only deposits older than Miocene appear severely deformed, whereas Plio-Pleistocene units yield little or no deformation at all. This multi-history deformation pattern shown along the Gondola Line closely resembles the long-term complex evolution recorded along the Mattinata Fault, except for the lack of significant seismicity. Therefore, although one could expect the Gondola Line to be subjected to the same stress field responsible for recent re-activation of the Mattinata Fault, direct evidence is not available from historical and present-day seismicity. However, in recent years, evidence of recent tectonic deformation off-shore Gargano has arisen from very-high resolution seismic stratigraphy based on a dense grid of Chirp-Sonar profiles. These data allowed the identification of low amplitude fold systems and shallow sub-vertical faults propagating in middle-late Pleistocene and Holocene deposits, particularly along the E-W (on the continental shelf) and NW-SE (on the slope) segments of the Gondola Line. Several of these faults either affect Holocene units younger than 5.5 ka (based on bio-chronostratigraphic analyses from core samples), or even offset the seafloor. Altogether, both recent seismicity related to E-W dextral strike-slip tectonics along the westernmost part of the MGsz and along the Mattinata Fault itself, and very recent (< 5.5 ka) deformation features along the Gondola Line, suggest that the MGsz as a whole is being actively deformed, although variably along-strike. In order to verify this hypothesis, we attempt a comparison between on- and off-shore data supporting recent activity along E-W oriented foreland structures. The integration of such heterogeneous yet complementary datasets may contribute to discuss late Quaternary tectonics of the Southern Apennines foreland domain, and provide comprehensive (on-shore / to / off-shore) scenarios for investigating recent / active tectonics of the MGsz and evaluating its possible seismogenic character.

A dense network of Very High Resolution seismic profiles along the Gondola Fault Zone (GFZ), in the Adriatic foreland (Italy), reveals the geometry and Middle Pleistocene-Holocene activity of this inherited, E-W, strike-slip fault system. The GFZ is >50 km long and includes two parallel fault sets, characterized by subvertical planes displaying a vertical component of motion, associated with two main anticlines. The northern fault set is organized in three branches, whereas the southern one includes two branches. The overall geometry of the GFZ suggests dextral slip. The distribution of the vertical displacement is bell-shaped, suggesting a long-term behavior as a single structure. However, individual branches show different deformation histories, implying that they can slip independently. The vertical slip rates, calculated for late Middle Pleistocene to Holocene intervals, are consistently small within a limited range (0-0.19 mm/a).