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

地质岩墙--通常与先前存在的岩层垂直或陡峭倾斜的岩层--通常是因为应力1)超过岩石强度或2)利用先前构造活动造成的现有裂缝而侵入。通常情况下，不可能区分这两种可能性，因为侵入发生在裂谷地带--即与裂谷内的断层方向相同的方向。更广泛地说，由于类似的原因，人们也很少知道有多少类型的裂缝扩大了尺寸，形成了更大的断层。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变化的。然而，他们留下了一些悬而未决的问题，即没有显示出这种伸缩的裂缝是如何发展起来的。先前存在的裂缝在为堤坝传播创造路径中的作用对于引导堤坝传播可能是重要的。堤防入侵的这种潜在的“非理性”对于解释火山系统强烈地震危机的性质(和来源)至关重要，并最终对于管理火山危机至关重要，因为如果不这样做，对潜在喷发地的了解将是一种财富。例如，如果岩墙被证明优先遵循先前存在的结构弱点，那么对断层的详细测绘可以为火山喷发危险图和情景规划提供重要的约束。解决这一悬而未决的科学问题的一个令人兴奋的机会现在是在地球上几何形状最极端、裂缝最多的火山之一--S豪尔热岛(亚速尔群岛)的火山脊上发生的一场罕见的强烈地震危机中提出的，该火山包含与裂谷带成斜角的断层。从2022年3月19日开始，该地区的地震活动水平从01/01-18/03记录的仅5次地震异常地提高到3月19日至今记录的27,000多次2-3.3级地震。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，因为现有的地震台网只包括岛屿上的地震台。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建立该区域详细的三维地震层析成像图像，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，我们提议在S豪尔赫周围部署5台海底地震仪(OBS)和在S豪尔赫及其周围岛屿上部署10个陆地宽带(BB)台站的临时地震台网。这将大大增加该区域的地震数据覆盖面，从而产生前所未有的数据集：(1)显示与堤坝入侵有关的地震活动如何与已知断层有关；以及(2)能够在断层与裂谷带成斜角的环境中建立第一个详细的地壳和火山建筑物的三维地下图像，这是一个罕见的堤坝例子。更广泛地说，该项目将为S等狭长裂隙供养的火山系统的结构和管道网络带来关键的新见解。它还将对堤坝侵入的力学和它们的总体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.