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

斜滑构造背景下的岩脉侵入机制：揭示亚速尔群岛圣乔治岛 2022 年 3 月罕见地​​震群的原因

• 批准号: NE/X006298/1
• 负责人: Ana Ferreira
• 金额: $ 1.73万
• 依托单位: University College London
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2022
• 资助国家: 英国
• 起止时间: 2022 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FX006298%2F1
• 关键词: Mechanics; dyke; intrusion; oblique; slip

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.

地质岩脉--通常垂直于或陡峭地倾斜于先前存在的岩石的层面的岩石片--通常会侵入，因为应力要么1）克服岩石强度，要么2）利用先前构造活动产生的现有裂缝。通常情况下，不可能区分这两种可能性，因为侵入发生在沿着裂谷带-即，与裂谷内的断层方向相同。更一般地说，也很少知道有多少类型的裂缝由于类似的原因而尺寸增加以形成更大的断层。现有的一些力学模型可以解释断层的位移如何与其长度成比例。然而，他们留下了一个悬而未决的问题，即没有显示出这种缩放的裂缝是如何发展的。预先存在的裂缝的作用，在创造途径的堤坝传播可能是重要的指导传播。这种潜在的“非理性”的堤坝入侵是至关重要的解释性质（和来源）强烈地震危机的火山系统，并最终管理火山危机时，知识的潜在喷发地点，否则将是一种资产。例如，如果堤坝被证明优先遵循预先存在的结构弱点，那么详细的断层图可以为火山喷发危险图和火山喷发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，现在是一个罕见的，强烈的地震危机在地球上最极端的几何形状，裂缝喂养的火山之一，圣乔治岛（亚速尔群岛）的火山脊，其中包含断层斜向裂谷带。从2022年3月19日开始，该地区的地震活动水平从2001年1月至2003年18月记录的仅5次地震上升到3月19日至今记录的27，000多次M2 -3.3事件。不幸的是，由于现有地震网络的几何限制，目前的地震位置基本上是不确定的，现有地震网络只包括岛上的地震站。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，现有网络的数据覆盖范围和质量有限，阻碍了该地区详细三维地震层析成像图像的构建，只能基于土地数据获得一维速度模型。为了解决这些问题，我们建议在圣乔治周围部署一个由五个海底地震仪（OBS）和十个陆地宽带（BB）站组成的临时地震网络。这将大大提高该区域的地震数据覆盖面，从而产生一个前所未有的数据集：（1）显示与岩脉侵入有关的地震活动如何与已知断层有关;（2）能够在这一罕见的岩脉环境中构造地壳和火山建筑物的第一个详细的三维地下图像，该岩脉环境中的断层与裂谷带倾斜。更一般地说，该项目将为São Jorge等高大狭窄的裂缝供给火山系统的结构和管道网络带来关键的新见解。它也将揭示新的光脉侵入体的力学和它们的运动学演化一般。