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

地质堤防——通常垂直或陡然倾斜于先前存在的岩石层理的岩片——侵入通常是因为应力1)克服岩石强度或2)利用先前构造活动产生的现有裂缝。通常情况下，不可能将这两种可能性区分开来，因为入侵是沿着裂谷带发生的，即与裂谷内的断层方向相同。更普遍的是，人们也不知道有多少类型的裂缝由于类似的原因而增大尺寸以形成更大的断层。一些现有的力学模型可以解释断层的位移是如何随其长度而变化的。然而，他们留下了一个悬而未决的问题，即没有显示出这种尺度的裂缝是如何形成的。预先存在的裂缝在为岩脉扩展创造通道方面的作用可能对指导岩脉扩展具有重要意义。这种堤坝入侵的潜在“不合理性”对于解释火山系统中强烈地震危机的性质（和来源）至关重要，最终对于管理火山危机至关重要，因为潜在喷发地点的知识本来是一种资产。例如，如果堤坝优先遵循先前存在的结构弱点，那么断层的详细测绘可以为火山爆发危险地图和场景规划提供重要的约束。一个令人兴奋的机会来解决这个突出的科学问题，现在出现了一个罕见的、强烈的地震危机，地震发生在地球上几何形状最极端的、由裂缝滋养的火山之一，即<s:1>奥若热岛（亚速尔群岛）的火山脊，它包含了向裂谷带倾斜的断层。从2022年3月19日开始，该地区的地震活动水平显著上升，从1月1日至18日记录的只有5次地震，到3月19日至今记录的超过27,000次m2 -3.3事件。不幸的是，由于现有地震台网的几何限制，目前的地震位置基本上是不确定的，这些台网只包括岛屿上的地震台站。这些不确定性使我们无法将地震与已知的断层和火山中心联系起来。此外，现有网络有限的数据覆盖范围和质量阻碍了该地区详细的三维地震层析成像图像的构建，只有基于陆地数据的一维速度模型可用。为了解决这些问题，我们建议在<s:1>奥豪尔赫岛周围部署一个由5个海底地震仪（OBSs）组成的临时地震台网，并在<e:1>奥豪尔赫岛及其周围岛屿上部署10个陆地宽带（BB）站。这将大大提高该地区的地震数据覆盖范围，从而产生一个前所未有的数据集：(1)显示与堤坝入侵相关的地震活动如何与已知断层相关；(2)在断层向裂谷带倾斜的环境中，这是一个罕见的岩脉例子，能够首次构建地壳和火山大厦的详细三维地下图像。更广泛地说，该项目将为<s:1> o Jorge火山等又高又窄的裂缝型火山系统的结构和管道网络带来关键的新见解。它还将为堤防侵入的力学及其总体的运动学演化提供新的线索。

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.