The Earth's surface oscillates on timescales of a few hours, both horizontally and vertically, by up to several centimetres because it deforms under the weight of the oceans which is regularly redistributed by the ocean tides. These 'ocean tide loading' deformations are too small, slow, and spatially smooth to be apparent to us humans, but they can be detected by precise satellite positioning techniques such as GPS. This allows us to investigate the Earth's rheology (deformational behaviour in response to forces) at time scales intermediate between the frequencies of seismic vibrations following earthquakes (seconds to minutes) and the Chandler wobble (an almost-regular rotational movement at near-yearly timescales). Because the ocean tides are similar over spatial scales of a few tens to hundreds of kilometres, ocean tide loading tells us most about the Earth's behaviour within the few hundred kilometres nearest the surface (corresponding to its crust and uppermost mantle). An important question is whether the Earth behaves perfectly elastically (like a rubber ball, as it does over very short seismic timescales), or if it behaves anelastically (i.e. not perfectly elastically; more like a wet sponge ball or an under-inflated football, that exhibits a time delay after the removal of the force before it returns to its original form). The way in which the Earth's behaviour changes from elastic to anelastic (or even more fluid-like over geological timescales) is not just scientifically interesting in itself, but it affects how we can infer other aspects of its behaviour from geodetic measurements of Earth's shape. The ocean tides are the only regular, well-known, phenomena that affect the Earth at these depths, and allow us to model its behaviour so we can later understand other less-regular and therefore less-tractable phenomena. Thus, the regular ocean tide forcing of the Earth's deformation, dominantly at semi-diurnal (roughly 12-hour) and diurnal (roughly 24-hour) periods, provides a way to understand Earth's behaviour in ways we could not before the advent of GPS and which are now important to the way we use geodesy to study earthquake recurrence, sea level rise, and other geohazards. Precise GPS geodesy allows us to measure ocean tide loading deformations with hitherto unsurpassed accuracy and spatial coverage (as we recently demonstrated for the dominant 'M2' tidal constituent in western Europe). However, GPS is problematic at certain tidal and near-annual frequencies corresponding to the GPS satellites' orbital and geometry repeat periods. New developments in multi-GNSS (Global Navigation Satellite Systems: GPS, GLONASS, Beidou, and Galileo) positioning offer a way around this obstacle. We will use multi-GNSS data to observe the tidal harmonic motions of the Earth's surface and infer the degree of anelastic deformation of the solid Earth over the full range of semi-diurnal and diurnal tidal timescales. Our observations will allow us to investigate the behaviour of the soft 'asthenosphere' layer of the Earth, in the uppermost mantle, at this poorly-studied timescale, which will have implications for (e.g.) the understanding of slow slip events and short-term postseismic relaxation in subduction zones (where the largest earthquakes occur). In addition to these more "blue-sky" aspects, improved forward models (resulting from our work) of the Earth's near-instantaneous response to surface mass loads will have immediate practical consequences for users measuring key climate change variables, e.g. GRACE satellite measurements of water and ice mass transfer, and GNSS measurements of tide gauge vertical land motion to correct sea level change observations.

Geological dykes - sheets of rock that are often oriented vertically or steeply inclined to the bedding of preexisting rocks - typically intrude because stresses either 1) overcome rock strength or 2) exploit existing fractures created by preceding tectonic activity. Normally, it is impossible to tell these two possibilities apart because intrusion occurs along the rift zone - i.e., in the same direction as the faults within the rift. More generally, it is also poorly known how many types of fractures increase in size to form larger faults for similar reasons. Some existing mechanical models can explain how the displacements of faults scales with their length. However, they leave open questions of how fractures not showing such scaling develop. The role of pre-existing fractures in creating pathways for dyke propagation could be important for guiding the propagation. This potential "irrationality" of dyke intrusion is crucial for interpreting the nature (and source) of intense earthquake crises in volcanic systems, and ultimately for managing volcanic crises when knowledge of potential eruption sites would otherwise be an asset. For instance, if dykes are shown to preferentially follow pre-existing structural weaknesses, then detailed mapping of faults could provide important constraints for volcano eruption hazard maps and scenario-planning. An exciting opportunity to tackle this outstanding scientific problem is now presented by a rare, intense earthquake crisis in one of the most geometrically extreme, fissure-fed volcanoes on Earth, the volcanic ridge of São Jorge Island (Azores), which contains faults oblique to the rift zone. Starting on 19 March 2022, the region's seismicity levels raised extraordinarily from only 5 earthquakes recorded in 01/01-18/03, to over 27,000 M 2-3.3 events recorded from March 19th until now. Unfortunately, current earthquake locations are substantially uncertain because of geometric limitations of the existing seismic network, which includes only seismic stations in the islands. These uncertainties prevent us from relating the earthquakes to known faults and volcanic centres. Further, the limited data coverage and quality of existing networks have hindered the construction of detailed 3-D seismic tomography images of the region, with only 1-D velocity models being available based on land data. In order to address these issues, we propose to deploy a temporary seismic network of five ocean bottom seismometers (OBSs) around São Jorge and ten land broadband (BB) stations on São Jorge and surrounding islands. This will substantially enhance the region's seismic data coverage, leading to an unprecedented dataset: (1) showing how seismicity associated with a dyke intrusion relates to known faults; and (2) enabling the construction of the first detailed 3-D subsurface images of the crust and of the volcanic edifice in this rare example of a dyke in an environment with faults oblique to the rift zone. More generally, this project will bring key new insights into the structure and plumbing network of tall and narrow fissure-fed volcanic systems such as São Jorge. It will also shed new light on the mechanics of dyke intrusions and their kinematic evolution in general.