Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Jamie Farrell, assistant research professor with the University of Utah Seismograph Stations and chief seismologist of the Yellowstone Volcano Observatory.
It has been well documented that the interaction of ocean waves and the seabed causes seismic shaking that is recorded by seismometers around the world. This seismic energy is referred to as the earth's "microseism." Some people also refer to it as the earth's hum.
There are two ways that the ocean's waves cause seismic shaking:
- The direct interaction of ocean waves and the solid ground near the shoreline (single frequency microseism)
- The interaction of two sets of wave trains travelling in different directions in the open ocean (double frequency microseism)
The double frequency microseism can be due to two storm systems, and their associated wave trains, interacting in the open ocean, or caused by waves interacting with reflected waves near the coastline. Due to the size of the world's oceans and the costs of deploying ocean-bottom seismometers, there are very few observations of the source of the oceanic microseism.
Recently, it was documented that many lakes have their own microseism energy. Data was collected near lakes in Canada and China, the Great Lakes, and Yellowstone Lake. It was shown that the dominant period of the lake microseism is around one second (in other words, it takes about one second for two consecutive peaks of the seismic wave to pass). This is in direct contrast to the dominant periods of the oceanic microseism, which is about three to 20 seconds.
The University of Utah, in conjunction with the Woods Hole Oceanographic Institute and Yellowstone National Park, deployed 40 land seismometers around the main basin of Yellowstone Lake, four weather stations and two wave gauges in the summer of 2018. This deployment was in conjunction with the ongoing HD-YLAKE project that deployed 10 lake-bottom seismometers in the northern portion of the lake. The project was funded by the National Science Foundation.
There are two main goals of the project:
- To identify the way microseism energy is created in Yellowstone Lake and find the location of the microseism generation
- To document how the microseism energy propagates through the relatively unconsolidated lake sediments and in the more consolidated volcanic rocks surrounding the lake
Although not a main goal of the project, we can also use the microseism energy to image the subsurface of Yellowstone Lake, which contains large lake-bottom geyser basins.
The seismic data show that the microseism energy produced by Yellowstone Lake is most likely the double-frequency type created by wave-wave interaction. It is still unclear where exactly the microseism is being produced in the lake or if the location varies with changing wind direction. When comparing a full day of seismic data from Stevenson Island to the wind speed as measured at the nearest weather station, there is a clear correlation between high wind speed and the generation of microseism energy.
Future analysis of these seismic data will provide an opportunity to advance the understanding of the generation of microseismic energy as well as illuminate the robust hydrothermal system on the floor of Yellowstone Lake. In addition, since microseismic energy is only generated during warmer months when the lake is not frozen, we can use the permanent seismic network to record the date of when microseismic energy is generated (lake thaw) and the date where the microseismic energy ceases to exist (lake freeze) through time. A long-term record of these data can provide additional data related to the effects of climate change in the Yellowstone region.