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

微震是地球固体表面的微小位移(微米)，自地震仪问世以来一直被探测到，但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，人们开始清楚地知道，它们主要是由海浪产生的。海浪产生微震能量的方式主要有两种：1)在开阔海洋中沿不同方向传播的波之间的相互作用(次生微震)；2)海浪与海岸附近浅海海底的相互作用(原生微震)。目前我们对次生微震有了很好的了解，但对原生微震的产生和传播还不是很清楚。最近，人们发现，世界各地的几个大型湖泊也产生了微震能量，其中包括黄石国家公园的黄石湖。黄石湖为研究微震提供了一个独特的机会，因为与大洋不同，它相对较小，可以完全被地震仪器包围，这使我们有机会精确定位微震产生的位置，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，该项目在黄石湖底部署了一系列地震仪，在湖的周边和岛屿上补充了这些仪器的阵列。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)和湖浪(幅度、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，它将提供一个前所未有的机会来理解产生微震的过程。我们的结果将对微震的研究产生广泛的影响，最终，我们希望发展利用这些自然产生的地震波来成像地球结构的能力。这在黄石湖是一种令人兴奋的可能性，因为它拥有活跃的热液系统，据信这些热液系统会在湖底沉积物中产生巨大的气穴，而微震传播速度应该对气体的存在非常敏感。作为更广泛的影响，调查人员将与黄石公园的教育工作者合作，通过他们的展览向更广泛的公众公布他们的成果。技术综述：最近发现，许多湖泊在1 S附近会产生可观察到的微震。部署在湖泊附近的陆基地震仪的观测表明，微震以短周期瑞利波(Rg)的形式传播，由于湖泊沉积物的低速，微震通常伴随着前进性的粒子运动。然而，目前尚不清楚震源过程是线性的，对应于原生海洋微震，还是非线性，对应于次生海洋微震，或者两者兼而有之。也不知道震源区主要是在开阔水域还是在海岸线附近，以及微震波场在穿过海岸线时是如何演变的。黄石湖是了解浅水微震的极好的天然实验室，因为(1)不会像海洋中那样存在来自遥远风暴的潜在混杂的巨浪，(2)震源区地理上很小，基本上可以被地震仪包围，(3)黄石湖微震是一种规则的、可重复的现象，与夏季和秋季的风日变化有关。对黄石湖微地震震源机制的了解很可能适用于世界其他浅水地区，并可能导致对极浅层地球结构的改进成像。拟议的工作包括：(1)在黄石湖周围和湖内岛屿上部署(1)40个三分量自主5赫兹地震检波器，(2)与选定的地震检波器配置的4个气象站，以及(3)在黄石湖北部的两个波高记录器。部署计划于2018年夏天进行，以便与正在进行的由NSF资助的HDYLAKE实验重叠，该实验在一个活跃的热液地点周围部署了10个湖底地震仪阵列。PIS和一名研究生将分析跨学科的数据集，努力确定最近观察到的短期(~1秒)微震的起源和产生机制，这些微震起源于开放水域期间的黄石湖。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

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.