Control of geometry and kinematics on the state of stress of subduction zones: an application to the Mediterranean region

Knowing the stress field at subduction zones is fundamental as here is released most of the seismic energy in the Earth. In particular, most M W > 8.0 earthquakes originate at shallow depths along the frictional interface between subducting and overriding plates. This observation emphasizes the crucial role played by the geologic-time scale dynamics of convergent margins over the short-time scale seismogenic processes. Despite an obvious relevance to seismic hazard, knowing the driving forces generating the stress field at subduction zones is a long-standing problem. In this thesis, by means of 2D and 3D numerical viscoelastic models, I simulated the stress field in convergent plate margins to evaluate which properties control subduction dynamics. Models are built to evaluate the contribution of plate kinematics, geometry of the system, rheology and gravitational forces to the definition of the present-day stress field at different subduction zones. This has been achieved with the development of several sets of generic (i.e., not simulating specific subduction zones) 2D and 3D models. The aim is to analyze the interaction between the subducted slabs and the geodynamic forces (e.g., slab pull, mantle flow, plate convergence) that stress the system, to reproduce the observed stress fields measured in different subduction zones worldwide for both the upper and lower plates at crustal depths and for intermediate and deep subducted lithosphere. The interaction between subducting slabs and the viscosity jump at upper-lower mantle transition has been also investigated. Although generic, model geometries are consistent with natural geometries observed in real subduction zones worldwide. Modelling results are compared with stress data available in the world stress map database for different convergent margins. To define the stress field affecting the subducting plates, special attention must be paid to the choice of the righteous initial parameters, since from them depend the delicate balance between the applied tectonic forces and the geometric characteristics of the whole system. For this reason and to validate or reject the observations made for the general cases, the central Mediterranean subduction system was chosen to model a natural subduction zone. Building and constraining a model requires the knowledge of the real system. Subduction zones are primarily described by their geometry, and today the slab interfaces in the Mediterranean are still uncertain. I defined them reviewing and integrating literature data from various disciplines, collecting geometries into a specifically designed database. Unlike similar databases already available in the literature, in the database that I contributed to build the subduction interfaces are fully-parametrized, i.e., characterized by geometric (strike, dip, depth), kinematics and dynamic (rake, slip rate, seismic coupling, maximum earthquake magnitude) parameters. The database so designed, and its on-line publication makes it a valuable tool for the geometric description of active subductions in the Mediterranean area and provide the basis to investigate their seismic hazard

Normal fault earthquakes or graviquakes

Earthquakes are dissipation of energy throughout elastic waves. Canonically is the elastic energy accumulated during the interseismic period. However, in crustal extensional settings, gravity is the main energy source for hangingwall fault collapsing. Gravitational potential is about 100 times larger than the observed magnitude, far more than enough to explain the earthquake. Therefore, normal faults have a different mechanism of energy accumulation and dissipation (graviquakes) with respect to other tectonic settings (strike-slip and contractional), where elastic energy allows motion even against gravity. The bigger the involved volume, the larger is their magnitude. The steeper the normal fault, the larger is the vertical displacement and the larger is the seismic energy released. Normal faults activate preferentially at about 60° but they can be shallower in low friction rocks. In low static friction rocks, the fault may partly creep dissipating gravitational energy without releasing great amount of seismic energy. The maximum volume involved by graviquakes is smaller than the other tectonic settings, being the activated fault at most about three times the hypocentre depth, explaining their higher b-value and the lower magnitude of the largest recorded events. Having different phenomenology, graviquakes show peculiar precursor

Evaluating the role of seagrass in Cenozoic CO2 variations

Marine seagrass angiosperms play an important role in carbon sequestration, removing carbon dioxide from the atmosphere and binding it as organic matter. Carbon is stored in the plants themselves, but also in the sediments both in inorganic and organic forms. The inorganic component is represented by carbonates produced by calcareous organisms living as epiphytes on seagrass leaves and rhizomes. In this paper, we find that the rate of seagrass epiphyte production (leaves and rhizomes) averages 400 g m−2 yr−1 , as result of seagrass sampling at seven localities along the Mediterranean coasts, and related laboratory analysis. Seagrasses have appeared in the Late Cretaceous becoming a place of remarkable carbonate production and C sequestration during the whole Cenozoic era. Here, we explore the potential contribution of seagrass as C sink on the atmospheric CO2 decrease by measuring changes in seagrass extent, which is directly associated with variations in the global coastal length associated with plate tectonics. We claim that global seagrass distribution significantly affected the atmospheric composition, particularly at the Eocene-Oligocene boundary, when the CO2 concentration fell to 400 ppm, i.e., the approximate value of current atmospheric CO2

Ground deformation and source geometry of the 30 October 2016 Mw 6.5 Norcia earthquake (Central Italy) investigated through seismological data, DInSAR measurements, and numerical modelling

We investigate the Mw 6.5 Norcia (Central Italy) earthquake by exploiting seismological data, DInSAR measurements, and a numerical modelling approach. In particular, we first retrieve the vertical component (uplift and subsidence) of the displacements affecting the hangingwall and the footwall blocks of the seismogenic faults identified, at depth, through the hypocenters distribution analysis. To do this, we combine the DInSAR measurements obtained from coseismic SAR data pairs collected by the ALOS-2 sensor from ascending and descending orbits. The achieved vertical deformation map displays three main deformation patterns: (i) a major subsidence that reaches the maximum value of about 98 cm near the epicentral zones nearby the town of Norcia; (ii) two smaller uplift lobes that affect both the hangingwall (reaching maximum values of about 14 cm) and the footwall blocks (reaching maximum values of about 10 cm). Starting from this evidence, we compute the rock volumes affected by uplift and subsidence phenomena, highlighting that those involved by the retrieved subsidence are characterized by significantly higher deformation values than those affected by uplift (about 14 times). In order to provide a possible interpretation of this volumetric asymmetry, we extend our analysis by applying a 2D numerical modelling approach based on the finite element method, implemented in a structural-mechanic framework, and exploiting the available geological and seismological data, and the ground deformation measurements retrieved from the multi-orbit ALOS-2 DInSAR analysis. In this case, we consider two different scenarios: the first one based on a single SW-dipping fault, the latter on a main SW-dipping fault and an antithetic zone. In this context, the model characterized by the occurrence of an antithetic zone presents the retrieved best fit coseismic surface deformation pattern. This result allows us to interpret the subsidence and uplift phenomena caused by the Mw 6.5 Norcia earthquake as the result of the gravitational sliding of the hangingwall along the main fault plane and the frictional force acting in the opposite direction, consistently with the double couple fault plane mechanism

Are normal fault earthquakes due to elastic rebound or gravitational collapse?

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The Bortoluzzi Mud Volcano (Ionian Sea, Italy) and its potential for tracking the seismic cycle of active faults

The Ionian Sea in southern Italy is at the center of active interaction and convergence between the Eurasian and African–Adriatic plates in the Mediterranean. This area is seismically active with instrumentally and/or historically recorded Mw > 7:0 earthquakes, and it is affected by recently discovered long strike-slip faults across the active Calabrian accretionary wedge. Many mud volcanoes occur on top of the wedge. A recently discovered one (called the Bortoluzzi Mud Volcano or BMV) was surveyed during the Seismofaults 2017 cruise (May 2017). Bathymetric backscatter surveys, seismic reflection profiles, geochemical and earthquake data, and a gravity core are used here to geologically, geochemically, and geophysically characterize this structure. The BMV is a circular feature ' 22m high and ' 1100m in diameter with steep slopes (up to a dip of 22 ). It sits atop the Calabrian accretionary wedge and a system of flowerlike oblique-slip faults that are probably seismically active as demonstrated by earthquake hypocentral and focal data. Geochemistry of water samples from the seawater column on top of the BMV shows a significant contamination of the bottom waters from saline (evaporite-type) CH4-dominated crustalderived fluids similar to the fluids collected from a mud volcano located on the Calabria mainland over the same accretionary wedge. These results attest to the occurrence of open crustal pathways for fluids through the BMV down to at least the Messinian evaporites at about 3000 m. This evidence is also substantiated by helium isotope ratios and by comparison and contrast with different geochemical data from three seawater columns located over other active faults in the Ionian Sea area. One conclusion is that the BMV may be useful for tracking the seismic cycle of active faults through geochemical monitoring. Due to the widespread diffusion of mud volcanoes in seismically active settings, this study contributes to indicating a future path for the use of mud volcanoes in the monitoring and mitigation of natural hazards.Published1-233SR TERREMOTI - Attività dei CentriJCR Journa

Tsunamigenic potential of crustal faults and subduction zones in the Mediterranean

Abstract We compiled a database and systematically evaluated tsunamigenic potential of all up-to-date known crustal fault systems and subduction zones in the entire Mediterranean region that has experienced several catastrophic tsunamis in historical times. The task is accomplished by means of numerical modeling of tsunami generation and propagation. We have systematically simulated all representative ruptures populating known crustal faults and subduction interfaces with magnitudes ranging from 6.1 up to expected Mwmax. Maximum tsunami heights calculated everywhere along the coasts allowed us to classify the sources in terms of their tsunamigenic potential and to estimate their minimum tsunamigenic magnitude. Almost every source in the Mediterranean, starting from Mw = 6.5, is capable to produce local tsunami at the advisory level (wave height >20 cm and ≤50 cm). In respect to the watch level (wave height >50 cm) larger magnitudes are needed (Mw ≥ 6.9). Faults behave more heterogeneously in the context of far field early warning. De-aggregation of the database at any selected coastal location can reveal relevant sources of tsunami hazard for this location. Our compilation blueprints methodology that, if completed with source recurrence rates and site-specific amplification factors, can be considered as a backbone for development of optimal early warning strategies by Mediterranean tsunami warning providers