Collaborative Research: The origin and propagation of shallow water microseisms: a multidisciplinary study at Yellowstone Lake

合作研究：浅水微震的起源和传播：黄石湖的多学科研究

• 批准号: 1760094
• 负责人: Jamie Farrell
• 金额: $ 17.66万
• 依托单位: University of Utah
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2018
• 资助国家: 美国
• 起止时间: 2018-06-01 至 2022-05-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1760094&HistoricalAwards=false
• 关键词: Collaborative; Research; origin; propagation; shallow

Microseisms are very small displacements (micrometers) of the Earth's solid surface that have been detected since that advent of seismometers, but the source of these vibrations was a mystery to scientists until about 60 years ago, when it began to become clear that they are largely generated by ocean waves. There are two main ways that ocean waves can generate microseism energy: 1) the interaction between waves traveling in different directions in the open ocean (secondary microseisms), and 2) the interaction of waves with the seafloor in shallow areas near the coast (primary microseisms). We presently have a good understanding of secondary microseisms, but the generation and propagation of primary microseisms is not well understood. Recently, it has been discovered that several large lakes around the world also produce microseism energy, including Yellowstone Lake in Yellowstone National Park. Yellowstone Lake provides a unique opportunity to study microseisms because, unlike the large oceans, it is relatively small and can be completely surrounded by seismic instruments, which gives us the opportunity to pinpoint the locations where microseisms are being generated and to study how they propagate into the surrounding regions. Our project will take advantage of the ongoing HD-YLAKE project, which deployed an array of seismometers on the floor of Yellowstone Lake, by supplementing those instruments with arrays of seismometers on the lake's perimeter and islands. In addition, we will also deploy instruments to simultaneously measure key atmospheric information (air temperature, pressure, wind speed, and wind direction) and the lake waves (amplitude, period, and direction) while the seismometers are recording. This combination of atmospheric, wave, and seismic data has never been collected in one place before, and it will provide an unprecedented opportunity to understand the processes that generate microseisms. Our results will have broad implications for the study of microseisms, and ultimately, we want to develop the ability to use these naturally occurring seismic waves to image the Earth's structure. This is an exciting possibility in Yellowstone Lake, because it hosts active hydrothermal systems that are believed to create large gas pockets in the lake floor sediments, and the microseism propagation velocity should be very sensitive to the presence of gas. As a broader impact, the investigators will work with Yellowstone Park educators to make their results accessible to the wider public through their exhibits.Technical Summary: It has recently been found that many lakes generate observable microseisms at periods near 1 s. Observations from land-based seismometers deployed near lakes show that the microseisms propagate as short-period Rayleigh waves (Rg), often with prograde particle motion because of the low velocity of lake sediments. However, it is unknown if the source process is linear, corresponding to primary ocean microseisms, or non-linear, corresponding to secondary ocean microseisms, or both. It is also unknown whether the source regions are predominantly in the open water or near the shoreline, and how the microseismic wavefield evolves as it crosses the shoreline. Yellowstone Lake is an excellent natural laboratory for understanding shallow water microseisms because (1) there are no potentially confounding swells from distant storms, as there are in the oceans, (2) the source region is geographically small and can essentially be surrounded with seismometers, and (3) the Yellowstone Lake microseisms are a regular, repeatable phenomenon associated with diurnal wind variation during the summer and fall. An understanding of the source mechanism of Yellowstone Lake microseisms will likely be applicable to other shallow water regions around the world and could lead to improved imaging of very shallow Earth structure. The proposed work involves a monthlong deployment of (1) 40 three-component, autonomous, 5-Hz geophones around the perimeter of Yellowstone Lake and on islands within the lake, (2) four weather stations collocated with selected geophones, and (3) two wave-height recorders in the northern portion of the Yellowstone Lake. The deployment is planned for the summer of 2018 so that it can overlap with the ongoing, NSF-funded, HDYLAKE experiment, which deployed an array of 10 lake-bottom seismometers around an active hydrothermal site. The PIs and a graduate student will analyze the interdisciplinary data set in an effort to determine the origin and generation mechanism of the short-period (~1 sec) microseisms that were recently observed to originate from Yellowstone Lake during periods of open water.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

微震是地球固体表面的非常小的位移（微米），自从地震仪出现以来就被检测到了，但是这些振动的来源对科学家来说一直是个谜，直到大约60年前，当它开始变得清晰时，它们主要是由海浪产生的。海洋波浪产生微震能量的主要方式有两种：1）在开阔海洋中以不同方向行进的波浪之间的相互作用（次级微震），以及2）波浪与海岸附近浅水区海底的相互作用（初级微震）。目前，我们对次级微震有了很好的了解，但对初级微震的产生和传播还没有很好的了解。 最近，人们发现世界上几个大的湖泊也会产生微震能量，包括黄石国家公园的黄石湖。黄石湖提供了一个独特的机会来研究微震，因为与大洋不同，它相对较小，可以完全被地震仪器包围，这使我们有机会确定微震产生的位置，并研究它们如何传播到周围地区。我们的项目将利用正在进行的HD-YLAKE项目，该项目在黄石湖的湖底部署了一个地震仪阵列，并在湖的周边和岛屿上补充了地震仪阵列。此外，我们还将部署仪器，在地震仪记录的同时，同时测量关键的大气信息（气温、气压、风速和风向）和湖波（振幅、周期和方向）。这种大气、波浪和地震数据的组合以前从未在一个地方收集过，它将为了解产生微震的过程提供前所未有的机会。我们的研究结果将对微震的研究产生广泛的影响，最终，我们希望开发使用这些自然发生的地震波来成像地球结构的能力。这在黄石湖是一个令人兴奋的可能性，因为它拥有活跃的热液系统，据信这些热液系统在湖底沉积物中产生了巨大的气袋，并且微震传播速度对气体的存在非常敏感。作为更广泛的影响，研究人员将与黄石公园的教育工作者合作，通过他们的展品向更广泛的公众宣传他们的研究结果。技术摘要：最近发现，许多湖泊产生的可观察到的微震周期接近1 s。 部署在湖泊附近的陆基地震仪的观测结果表明，微震传播的短周期瑞利波（Rg），往往与湍流粒子运动，因为低速度的湖泊沉积物。然而，它是未知的，如果源过程是线性的，对应于主要的海洋微震，或非线性，对应于二次海洋微震，或两者兼而有之。也不知道震源区主要是在开放水域还是靠近海岸线，以及微震波场在穿越海岸线时如何演变。黄石湖是一个很好的天然实验室，了解浅水微震，因为（1）没有潜在的混淆从遥远的风暴膨胀，因为有在海洋中，（2）震源区是地理上小，基本上可以与地震仪包围，（3）黄石湖微震是一个有规律的，可重复的现象与昼夜风变化在夏季和秋季。对黄石湖微地震源机制的理解可能适用于世界各地的其他浅水区，并可能导致改善非常浅的地球结构的成像。拟议的工作包括：（1）在黄石湖周边和湖中岛屿上部署40个三分量、自主、5 Hz地震检波器，（2）与选定的地震检波器一起部署的四个气象站，以及（3）黄石湖北方部分的两个波高记录仪。该部署计划于2018年夏季进行，以便与正在进行的NSF资助的HDYLAKE实验重叠，该实验在一个活跃的热液站点周围部署了10个湖底地震仪阵列。PI和一名研究生将分析跨学科的数据集，以确定最近观察到的在开放水域期间来自黄石湖的短周期（~1秒）微震的起源和生成机制。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估而被认为值得支持。