The westward drift of the lithosphere. A tidal ratchet?

Is the westerly rotation of the lithosphere an ephemeral accidental recent phenomenon or is it a stable process of Earth’s geodynamics? The reason why the tidal drag has been questioned as the mechanism determining the lithospheric shift relative to the underlying mantle is the apparent too high viscosity of the asthenosphere. However, plate boundaries asymmetries are a robust indication of the ‘westerly’ decoupling of the entire Earth’s outer lithospheric shell and new studies support lower viscosities in the low-velocity layer (LVZ) atop the asthenosphere. Since the solid Earth tide oscillation is longer in one side relative to the other due to the contemporaneous Moon’s revolution, we demonstrate that a non-linear rheological behavior is expected in the lithosphere mantle interplay. This may provide a sort of ratchet favoring lowering of the LVZ viscosity under shear, allowing decoupling in the LVZ and triggering the westerly motion of the lithosphere relative to the mantle

Longer aftershocks duration in extensional tectonic settings

Aftershocks number decay through time, depending on several parameters peculiar to each seismogenic regions, including mainshock magnitude, crustal rheology, and stress changes along the fault. However, the exact role of these parameters in controlling the duration of the aftershock sequence is still unknown. Here, using two methodologies, we show that the tectonic setting primarily controls the duration of aftershocks. On average and for a given mainshock magnitude (1) aftershock sequences are longer and (2) the number of earthquakes is greater in extensional tectonic settings than in contractional ones. We interpret this difference as related to the different type of energy dissipated during earthquakes. In detail, (1) a joint effect of gravitational forces and pure elastic stress release governs extensional earthquakes, whereas (2) pure elastic stress release controls contractional earthquakes. Accordingly, normal faults operate in favour of gravity, preserving inertia for a longer period and seismicity lasts until gravitational equilibrium is reached. Vice versa, thrusts act against gravity, exhaust their inertia faster and the elastic energy dissipation is buffered by the gravitational force. Hence, for seismic sequences of comparable magnitude and rheological parameters, aftershocks last longer in extensional settings because gravity favours the collapse of the hangingwall volumes

Earth's rotation variability triggers explosive eruptions in subduction zones

The uneven Earth’s spinning has been reported to affect geological processes, i.e. tectonism, seismicity and volcanism, on a planetary scale. Here, we show that changes of the length of day (LOD) influence eruptive activity at subduction margins. Statistical analysis indicates that eruptions with volcanic explosivity index (VEI) ≥3 alternate along oppositely directed subduction zones as a function of whether the LOD increases or decreases. In particular, eruptions in volcanic arcs along contractional subduction zones, which are mostly E- or NE-directed, occur when LOD increases, whereas they are more frequent when LOD decreases along the opposite W- or SW-directed subduction zones that are rather characterized by upper plate extension and back-arc spreading. We find that the LOD variability determines a modulation of the horizontal shear stresses acting on the crust up to 0.4 MPa. An increase of the horizontal maximum stress in compressive regimes during LOD increment may favour the rupture of the magma feeder system wall rocks. Similarly, a decrease of the minimum horizontal stress in extensional settings during LOD lowering generates a larger differential stress, which may enhance failure of the magma-confining rocks. This asymmetric behaviour of magmatism sheds new light on the role of astronomical forces in the dynamics of the solid Earth

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

Horizontal mantle flow controls subduction dynamics

It is generally accepted that subduction is driven by downgoing-plate negative buoyancy. Yet plate age –the main control on buoyancy– exhibits little correlation with most of the present-day subduction velocities and slab dips. “West”-directed subduction zones are on average steeper (~65°) than “East”-directed (~27°). Also, a “westerly”-directed net rotation of the lithosphere relative to the mantle has been detected in the hotspot reference frame. Thus, the existence of an “easterly”-directed horizontal mantle wind could explain this subduction asymmetry, favouring steepening or lifting of slab dip angles. Here we test this hypothesis using high-resolution two-dimensional numerical thermomechanical models of oceanic plate subduction interacting with a mantle flow. Results show that when subduction polarity is opposite to that of the mantle flow, the descending slab dips subvertically and the hinge retreats, thus leading to the development of a back-arc basin. In contrast, concordance between mantle flow and subduction polarity results in shallow dipping subduction, hinge advance and pronounced topography of the overriding plate, regardless of their age-dependent negative buoyancy. Our results are consistent with seismicity data and tomographic images of subduction zones. Thus, our models may explain why subduction asymmetry is a common feature of convergent margins on Earth

Hydrogeochemical changes before and during the 2016 Amatrice-Norcia seismic sequence (central Italy)

Seismic precursors are an as yet unattained frontier in earthquake studies. With the aim of making a step towards this frontier, we present a hydrogeochemical dataset associated with the 2016 Amatrice- Norcia seismic sequence (central Apennines, Italy), developed from August 24th, with an Mw 6.0 event, and culminating on October 30th, with an Mw 6.5 mainshock. The seismic sequence occurred during a seasonal depletion of hydrostructures, and the four strongest earthquakes (Mw ≥ 5.5) generated an abrupt uplift of the water level, recorded up to 100 km away from the mainshock area. Monitoring a set of selected springs in the central Apennines, a few hydrogeochemical anomalies were observed months before the onset of the seismic swarm, including a variation of pH values and an increase of As, V, and Fe concentrations. Cr concentrations increased immediately after the onset of the seismic sequence. On November 2016, these elements recovered to their usual low concentrations. We interpret these geochemical anomalies as reliable seismic precursors for a dilational tectonic setting

Coastal altimetry for the computation of a Mean Dynamic Topography in the Mediterranean sea

Satellite Sea Level Anomaly (SLA) observations are crucial in an operational oceanographic system due to their high coverage on sea surface currents and elevation and their strong constraint on water column integrated steric contributions. The use of Sea Surface Height (SSH) measurements by altimeter satellites in the Mediterranean Forecasting System (MFS) requires an accurate Mean Dynamic Topography (MDT) field with a high horizontal resolution which must be added to SLA observations. Here a new MDT computed through a direct method is proposed to solve the main limitations to the current MDT, evaluated from a model-dependent first guess. The direct method consists in the difference between an altimetric Mean Sea Surface Height (MSSH) and a geoid model. Moreover, a novel altimetric dataset reprocessed near the coast is adopted in order to improve the representation of coastal dynamics. Altimetric data from a single satellite, Jason-2, are used to generate a SSH dataset. This is used along with the EGM2008 geoid model to compute along track MDT observations. Optimal Interpolation algorithms are used to regrid along track MDT on MFS model grid. Derived geostrophic velocities are then computed. The validation of the altimetric dataset against the operational dataset showed improved performances in terms of time series completeness and standard mean error. From the analysis of the MDT and the retrieved geostrophic velocities we can conclude that the direct method allowed us to reconstruct basin scale and large scale MDT features but not meso/small scale and coastal dynamics. Main limitations in our results are due to the low accuracy of geoid model and the Jason-2 tracks spacing

Synergies between astroparticle, particle and nuclear physics

One overarching objective of science is to further our understanding of the universe, from its early stages to its current state and future evolution. This depends on gaining insight on the universe's most macroscopic components, for example galaxies and stars, as well as describing its smallest components, namely elementary particles and nuclei and their interactions. It is clear that this endeavour requires combined expertise from the fields of astroparticle physics, particle physics and nuclear physics. Pursuing common scientific drivers also require mastering challenges related to instrumentation (e.g. beams and detectors), data acquisition, selection and analysis, and making data and results available to the broader science communities. Joint work and recognition of these "foundational" topics will help all communities grow towards their individual and common scientific goals. The talk corresponding to this contribution has been presented during the special ECFA session of EPS-HEP 2019 focused on the update of the European Strategy of Particle Physics.Comment: Late submission to the Proceedings of the EPS-HEP 2019 Conference, Special ECFA session (https://indico.cern.ch/event/577856/sessions/291392

Fault dip vs shear stress gradient

In the brittle regime, faults tend to be oriented along an angle of about 30 relative to the principal stress direction. This empirical Andersonian observation is usually explained by the orientation of the stress tensor and the slope of the yield envelope defined by the Mohr-Coulomb criterion, often called critical-stress theory, assuming frictional properties of the crustal rocks (friction coefficient = 0.6-0.8). However, why the slope has a given value? We suggest that the slope dip is constrained by the occurrence of the largest shear stress gradient along that inclination. High homogeneous shear stress, i.e., without gradients, may generate aseismic creep as for example in flat decollements, both along thrusts and low angle normal faults, whereas along ramps larger shear stress gradients determine greater energy accumulation and stick-slip behaviour with larger sudden seismic energy release. Further variability of the angle is due to variations of the internal friction and of the Poisson ratio, being related to different lithologies, anisotropies and pre-existing fractures and faults. Misaligned faults are justified to occur due to the local weaknesses in the crustal volume; however, having lower stress gradients along dip than the optimally-oriented ones, they have higher probability of being associated with lower seismogenic potential or even aseismic behavior

Polarized Plate Tectonics.

The mechanisms driving plate motion and the Earth\u2019s geodynamics are still not entirely clari\u2423ed. Lithospheric volumes recycled at subduction zones or emerging at rift zones testify mantle convection. The cooling of the planet and the related density gradients are invoked to explain mantle convection either driven from the hot interior or from the cooler outer boundary layer. In this paper we summarize a number of evidence sup- porting generalized asymmetries along the plate boundaries that point to a polariza- tion of plate tectonics. W-directed slabs provide two to three times larger volumes to the mantle with respect to the opposite E- or NE-directed subduction zones. W-directed slabs are deeper and steeper, usually characterized by down-dip compres- sion. Moreover, they show a shallow decollement and low elevated accretionary prism, a steep regional monocline with a deep trench or foredeep, a backarc basin with high heat \u2423ow and positive gravity anomaly. Conversely directed subduction zones show antithetic signatures and no similar backarc basin. Rift zones also show an asymmetry, e.g., faster Vs in the western lithosphere and a slightly deeper bathymetry with respect to the eastern \u2423ank. These evidences can be linked to the westward drift of the lithosphere relative to the underlying mantle and may explain the differences among subduction and rift zones as a function of their geographic polarity with respect to the \u201ctectonic equator.\u201d Therefore also mantle convection and plate motion should be polar- ized. All this supports a general tuning of the Earth\u2019s geodynamics and mantle convec- tion by astronomical forces