A Bayesian 3-D linear gravity inversion for complex density distributions: application to the Puysegur subduction system

We have developed a linear 3-D gravity inversion method capable of modelling complex geological regions such as subduction margins. Our procedure inverts satellite gravity to determine the best-fitting differential densities of spatially discretized subsurface prisms in a least-squares sense. We use a Bayesian approach to incorporate both data error and prior constraints based on seismic reflection and refraction data. Based on these data, Gaussian priors are applied to the appropriate model parameters as absolute equality constraints. To stabilize the inversion and provide relative equality constraints on the parameters, we utilize a combination of first and second order Tikhonov regularization, which enforces smoothness in the horizontal direction between seismically constrained regions, while allowing for sharper contacts in the vertical. We apply this method to the nascent Puysegur Trench, south of New Zealand, where oceanic lithosphere of the Australian Plate has underthrust Puysegur Ridge and Solander Basin on the Pacific Plate since the Miocene. These models provide insight into the density contrasts, Moho depth, and crustal thickness in the region. The final model has a mean standard deviation on the model parameters of about 17 kg m⁻³, and a mean absolute error on the predicted gravity of about 3.9 mGal, demonstrating the success of this method for even complex density distributions like those present at subduction zones. The posterior density distribution versus seismic velocity is diagnostic of compositional and structural changes and shows a thin sliver of oceanic crust emplaced between the nascent thrust and the strike slip Puysegur Fault. However, the northern end of the Puysegur Ridge, at the Snares Zone, is predominantly buoyant continental crust, despite its subsidence with respect to the rest of the ridge. These features highlight the mechanical changes unfolding during subduction initiation

Coupled surface to deep Earth processes: Perspectives from TOPO-EUROPE with an emphasis on climate- and energy-related societal challenges

Understanding the interactions between surface and deep Earth processes is important for research in many diverse scientific areas including climate, environment, energy, georesources and biosphere. The TOPO-EUROPE initiative of the International Lithosphere Program serves as a pan-European platform for integrated surface and deep Earth sciences, synergizing observational studies of the Earth structure and fluxes on all spatial and temporal scales with modelling of Earth processes. This review provides a survey of scientific developments in our quantitative understanding of coupled surface-deep Earth processes achieved through TOPO-EUROPE. The most notable innovations include (1) a process-based understanding of the connection of upper mantle dynamics and absolute plate motion frames; (2) integrated models for sediment source-to-sink dynamics, demonstrating the importance of mass transfer from mountains to basins and from basin to basin; (3) demonstration of the key role of polyphase evolution of sedimentary basins, the impact of pre-rift and pre-orogenic structures, and the evolution of subsequent lithosphere and landscape dynamics; (4) improved conceptual understanding of the temporal evolution from back-arc extension to tectonic inversion and onset of subduction; (5) models to explain the integrated strength of Europe's lithosphere; (6) concepts governing the interplay between thermal upper mantle processes and stress-induced intraplate deformation; (7) constraints on the record of vertical motions from high-resolution data sets obtained from geo-thermochronology for Europe's topographic evolution; (8) recognition and quantifications of the forcing by erosional and/or glacial-interglacial surface mass transfer on the regional magmatism, with major implications for our understanding of the carbon cycle on geological timescales and the emerging field of biogeodynamics; and (9) the transfer of insights obtained on the coupling of deep Earth and surface processes to the domain of geothermal energy exploration. Concerning the future research agenda of TOPO-EUROPE, we also discuss the rich potential for further advances, multidisciplinary research and community building across many scientific frontiers, including research on the biosphere, climate and energy. These will focus on obtaining a better insight into the initiation and evolution of subduction systems, the role of mantle plumes in continental rifting and (super)continent break-up, and the deformation and tectonic reactivation of cratons; the interaction between geodynamic, surface and climate processes, such as interactions between glaciation, sea level change and deep Earth processes; the sensitivity, tipping points, and spatio-temporal evolution of the interactions between climate and tectonics as well as the role of rock melting and outgassing in affecting such interactions; the emerging field of biogeodynamics, that is the impact of coupled deep Earth – surface processes on the evolution of life on Earth; and tightening the connection between societal challenges regarding renewable georesources, climate change, natural geohazards, and novel process-understanding of the Earth system

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

Recent Developments and Trends in Volcano Gravimetry

The aim of this chapter is to take a look at some developments and new trends in volcano gravimetry. First, we will review the objectives of the research work within this subfield of geophysics, discuss the data and methods it uses, and outline the outputs it strives for. Then, we will turn our attention to three areas where innovative approaches possibly can forward this field of study. The first has to do with the coupling between vertical deformations of the topographic surface (elevation changes) and the observed gravity changes or, in other words, with the removal of the deformation-induced gravimetric signal from the observed gravity changes to obtain the net gravity changes caused by volcanic signals. The second and third areas regard the inversion of the observed gravity changes and deal with two recently or newly developed inversion approaches that both are characterized by the ability to produce a suite of diverse solutions that can be analyzed and discriminated based on additional independent constraints stemming from other earth science disciplines or from the cognition of the interpreter. With this in mind, the final goal is a better understanding of the mechanisms and processes of volcanic unrest or reawakening of a volcano and forecasting the threat of consequent activity and impacts

Local Site Effects in Archaeoseismology: Examples from the Mycenaean Citadels of Tiryns and Midea (Argive Basin, Peloponnese, Greece)

The archaeological community has gained knowledge on how to document and diagnose damage by earthquake shaking to ancient man-made structures and how to estimate the intensity of past earthquakes, but has paid little attention to local site effects and its implications for the dynamic response of those structures. Qualitative studies of damage by earthquakes to ancient constructions surpass the amount of research on local site effects in the archaeoseismological literature. Yet, archaeoseismic observations are often based on a limited part of the mesoseismal area, on loosely constrained dated events, and sometimes on ambiguous evidence of earthquake damage. This mix of factors may lead to imprecise estimates of the size of past earthquakes and/or unrealistic earthquake environmental impacts if local site effects are ignored or over/undervalued. Hence, it is important not to rely solely on intensities based on archaeologically documented coseismic damage without a quantitative estimate of local site effects. The present multidisciplinary study focuses on the Mycenaean citadels of Tiryns and Midea located in the Argive Basin (Peloponnese, Greece). The study is a key contribution to archaeoseismology because it provides a quantitative and deterministic method for estimating ancient local site effects and seismic hazard at an archaeological site. The proposed method permits the calculation of site-specific ground-motions, which are transformable into intensity values. The method requires input from archaeological, geoarchaeological, geophysical, geological, geotechnical, and historical studies. The over-or-underestimation of local site effects is minimized by removing accrued soils younger than the ancient walking horizon of interest. The method is applicable to archaeological sites worldwide with clear or unclear evidence of ancient earthquake damage, is scalable to any area size, and can help to decide on the location of new excavations targeting earthquake damage. The estimation of local site effects is carried out by computing synthetic seismograms for a reference rock-site located at each citadel, which are then used to accelerate regolith models for calculating surface amplifications factors and related ground-motions. Earthquake source parameters of the hypothetical earthquakes are constrained from a seismotectonic model of the area. This study shows how to estimate ancient local site effects to test the Mycenaean earthquake hypothesis, which is based solely on archaeological and geomorphological field observations. The hypothesis suggests repeated earthquake damage to the Cyclopean fortification walls and enclosed buildings of Tiryns, Midea, and Mycenae during the end of the Late Bronze Age (LBA). The hypothesis has lacked evidence of written records of ancient earthquakes and of a town-wide devastation pattern; has left unexplained the strength and location of the potential causative earthquake(s); and has ignored the impact of local site effects. The results of the present study reveal new findings: the Tiryns and Midea citadels settled on weathered hard limestone while the outer constructions settled on cohesive-or-granular soils with variable shear strength and seismic site class categories corresponding to a lower and higher seismic hazard, respectively. Data from two field campaigns during the project coupled with available upfront information from the geophysical, geological, and geotechnical literature and developed subsurface models show that the LBA ground conditions outside the fortification walls had a higher hazard than inside the walls, but archaeological findings do not reflect this. Active seismic sources at a distance greater than 40 km play a minor role. Local seismic sources in the Argolis are however critical, but are not confirmed seismically active. These findings weaken the plausibility of the Mycenaean earthquake hypothesis for Tiryns and Midea

High-resolution imaging beneath the Santorini volcano

Volcanoes are surface expressions of much deeper magmatic systems, inaccessible to direct observation. Constraining the geometry and physical properties of these systems, in particular detecting high melt fraction (magma) reservoirs, is key to managing a volcanic hazard and understanding fundamental processes that lead to the formation of continents. Unfortunately, unambiguous evidence of magma reservoirs has not yet been provided due to the limited resolving power of the geophysical methods used so far. Here, a high-resolution imaging technique called full-waveform inversion was applied to study the magmatic system beneath the Santorini volcanic field, one of the most volcanically and seismically active regions of Europe. Quality-controlled inversion of 3d wide-angle, multi-azimuth ocean-bottom seismic data revealed a previously undetected high melt fraction reservoir 3 km beneath the Kolumbo volcano, a centre of microseismic and hydrothermal activity of the field. To enable the above method to handle land data, two major algorithmic improvements were added to the high-performance inversion code. First, to simulate instrument response of land seismometers, a pressure-velocity conversion has been implemented in a way that ensures reciprocity of the discretised 2nd-order acoustic wave equation. Second, the immersed-boundary method, originally developed for computational fluid dynamics, was implemented to simulate the wave-scattering off the irregular topography of the Santorini caldera. These advancements can be readily used to provide a higher-resolution image of the melt reservoir beneath the Santorini caldera already detected by means of travel-time tomography.Open Acces

Coupled surface to deep Earth processes: Perspectives from TOPO-EUROPE with an emphasis on climate- and energy-related societal challenges

Understanding the interactions between surface and deep Earth processes is important for research in many diverse scientific areas including climate, environment, energy, georesources and biosphere. The TOPO-EUROPE initiative of the International Lithosphere Program serves as a pan-European platform for integrated surface and deep Earth sciences, synergizing observational studies of the Earth structure and fluxes on all spatial and temporal scales with modelling of Earth processes. This review provides a survey of scientific developments in our quantitative understanding of coupled surface-deep Earth processes achieved through TOPO-EUROPE. The most notable innovations include (1) a process-based understanding of the connection of upper mantle dynamics and absolute plate motion frames; (2) integrated models for sediment source-to-sink dynamics, demonstrating the importance of mass transfer from mountains to basins and from basin to basin; (3) demonstration of the key role of polyphase evolution of sedimentary basins, the impact of pre-rift and pre-orogenic structures, and the evolution of subsequent lithosphere and landscape dynamics; (4) improved conceptual understanding of the temporal evolution from back-arc extension to tectonic inversion and onset of subduction; (5) models to explain the integrated strength of Europe's lithosphere; (6) concepts governing the interplay between thermal upper mantle processes and stress-induced intraplate deformation; (7) constraints on the record of vertical motions from high-resolution data sets obtained from geo-thermochronology for Europe's topographic evolution; (8) recognition and quantifications of the forcing by erosional and/or glacial-interglacial surface mass transfer on the regional magmatism, with major implications for our understanding of the carbon cycle on geological timescales and the emerging field of biogeodynamics; and (9) the transfer of insights obtained on the coupling of deep Earth and surface processes to the domain of geothermal energy exploration. Concerning the future research agenda of TOPO-EUROPE, we also discuss the rich potential for further advances, multidisciplinary research and community building across many scientific frontiers, including research on the biosphere, climate and energy. These will focus on obtaining a better insight into the initiation and evolution of subduction systems, the role of mantle plumes in continental rifting and (super)continent break-up, and the deformation and tectonic reactivation of cratons; the interaction between geodynamic, surface and climate processes, such as interactions between glaciation, sea level change and deep Earth processes; the sensitivity, tipping points, and spatio-temporal evolution of the interactions between climate and tectonics as well as the role of rock melting and outgassing in affecting such interactions; the emerging field of biogeodynamics, that is the impact of coupled deep Earth – surface processes on the evolution of life on Earth; and tightening the connection between societal challenges regarding renewable georesources, climate change, natural geohazards, and novel process-understanding of the Earth system

Implications of seismic data for the structural evolution and numerical modelling of the Eastern Mediterranean Basin

The Eastern Mediterranean region includes several diverse tectonic domains. Their complexity and interaction have led to competing theories for the region’s evolution. This study aims to resolve some of these issues by using interpretations of regional seismic reflection data located in the offshore waters of Cyprus, Syria, Lebanon and Israel. The extent of the available data is unprecedented for a single study in the area, and the academic nature of the study means the remit is not constrained by political boundaries. The interpretations facilitated a new regionally consistent understanding of the tectonic evolution of the area that included several key conclusions. On the basis of literature review and regional seismic data, the western boundary of the ‘Sinai Plate’ that underlies the Eastern Mediterranean was concluded to run further west than is commonly drawn. However, the ‘Sinai Plate’ may not represent a true tectonic plate as it is not fully rifted at the Gulf of Suez. This study had access to a seismic data set from offshore Syria that only a single published paper had previously investigated. This allowed numerous new observations of a relic subduction zone beneath the Cyprus Arc to be made. The structural restoration constrained by these observations required a restructured plate evolution that included the arrival of the subduction zone in the latest Cretaceous, and provided the keystone to a new explanation for a set of normal faults mapped in the Levantine Basin. These layer-bound normal faults exist spatially and temporally where one might expect compressional features. The interpretation of the regional seismic data more than doubled the area documented as being affected by this deformation, and highlights a set of anticlines that are perpendicular and contemporaneous to the faults. Numerical analysis was conducted on detailed 3D interpretations of the faults using purpose-written software. In combination with observations from seismic data, this analysis provided evidence that contradicted previously published explanations, and suggested the shear system associated with the formation of the neighbouring Levant Shear Zone could have generated the deformation. Both Wilcox-strain-ellipse and extension associated with tectonic indentation of Eurasia contributed to the deformation. Seismic lines over the enigmatic Eratosthenes Seamount with a significantly higher fidelity than those in previously published work showed features that included prograding foresets and an erosive escarpment. These previously undocumented features formed the basis for an updated evolution of carbonate growth on the feature. A depression surrounding the bathymetric high is defined by an external escarpment. Numerical modelling of halokinetic deformation supports inflation of an evaporite body as the explanation for the formation of this depression. Well-imaged internal salt reflectors also indicate an episode of halokinesis immediately after evaporite deposition, contrary to some published interpretations. The new regional insights and detailed interpretations of localised features observed in the newly available 3D datasets are summarised in a series of maps detailing the evolution of the area

Seismic tomographic full-waveform inversion for the Vrancea sinking lithosphere structure using the adjoint method.

The Vrancea region, at the south-eastern bend of the Carpathian Mountains in Romania, represents one of the most puzzling seismically active zones of Europe. Beside some shallow seismicity spread across the whole Romanian territory, Vrancea is the place of an intense seismicity with the presence of a cluster of intermediate-depth foci placed in a narrow nearly vertical volume. Although large-scale mantle seismic tomographic studies have revealed the presence of a narrow, almost vertical, high-velocity body in the upper mantle, the nature and the geodynamic of this deep intra-continental seismicity is still questioned. High-resolution seismic tomography could help to reveal more details in the subcrustal structure of Vrancea. Recent developments in computational seismology as well as the availability of parallel computing now allow to potentially retrieve more information out of seismic waveforms and to reach such high-resolution models. This study was aimed to evaluate the application of a full waveform inversion tomography at regional scale for the Vrancea lithosphere using data from the 1999 six months temporary local network CALIXTO. Starting from a detailed 3D Vp, Vs and density model, built on classical travel-time tomography together with gravity data, I evaluated the improvements obtained with the full waveform inversion approach. The latter proved to be highly problem dependent and highly computational expensive. The model retrieved after the first two iterations does not show large variations with respect to the initial model but remains in agreement with previous tomographic models. It presents a well-defined downgoing slab shape high velocity anomaly, composed of a N-S horizontal anomaly in the depths between 40 and 70km linked to a nearly vertical NE-SW anomaly from 70 to 180km