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

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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.