Mechanics of dyke intrusion in oblique-slip tectonic settings: Unravelling the causes of the March 2022 rare seismic swarm in Sao Jorge Island, Azores

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.