Seismometers of the Transportable Array (TA) not only directly measure ground motion due to sources within the Earth's solid interior, they also indirectly provide valuable information about other phenomena that affect ground motion, such as atmospheric infrasound and gravity waves. Recently all 400 sites of the TA were retrofitted with barometers and infrasonic microphones. The Earthscope Science Plan includes as a scientific target the "cryosphere, hydrosphere, and atmosphere". Specifically, "studies of the infrasonic field allow for novel deep sounding of atmospheric structure and facilitate quantitative characterization of volcanic, oceanic, and other infrasonic sources." It has been recently shown that the density of TA sites permits one to accurately locate infrasonic sources, regardless if the infrasonic signals are registered by seismometers or microphones. This project is expanding upon previous Earthscope-funded work on the detection and location of more than 900 infrasonic pressure disturbances, or "skyquakes", that were registered by the seismic USArray during 2007-08 in the western U.S. Specifically, methods are being improved and applied to the seismo-acoustic USArray in the central U.S. Previously undetected skyquakes are being identified as are sources of gravity waves that propagate across the TA. The goals of this project are to identify the occurrence, waveform structure, and propagation statistics of these sources from 01/2009 through 03/2012. Correlations are being made between these events and atmospheric phenomena such as tornadoes and gust fronts. This work is resulting in two new databases of infrasonic and gravity wave event locations, arrival characteristics, and waveforms. The project is also illuminating infrasonic arrival branches in fine detail from several high-quality events.
Although seismology, which has achieved profound advances in the study of seismic sources and the Earth's interior structure, is well known to the general public, its close cousin, infrasound, is still relatively unknown. Infrasound is low-frequency energy in the atmosphere that cannot be heard and can only be detected using instruments. As the name suggests it is simply sound, although due to its low-frequency it is sound that allows us to "tune in" to an entirely different, previously unheard, world that surrounds us. Infrasound propagates with relatively little loss of energy. If a source is large the sound waves it spawns can travel around the world. Recently the large meteor that entered the Earth's atmosphere above Chelyabinsk, Russia, excited infrasound waves that were detected after circling the globe twice. Infrasound is not just generated by meteors but also by a very broad suite of natural atmospheric sources including volcanoes, severe storms, ocean waves, aurora and man-made sources such as explosions and rocket launches. Infrasound is used to detect volcanic eruptions, that might not be detected seismically or using satellites, to provide early warnings to Volcano Advisory Centers regarding possible ash releases that could threaten aircraft. Thanks to support from the National Science Foundation we have conducted basic research to learn more about how this energy is generated by natural phenomena as part of a larger effort to use this energy to detect faint sources and to use the energy to better understand the workings of these phenomena. Our particular focus has been ash-rich volcanic eruptions, tornadic storm systems and large meteors. Further, by studying how infrasound propagates through the atmosphere we have used recordings of this energy to probe mid-atmospheric structure that is difficult to study using ground- or space-based instruments. Atmospheric gravity waves are not well known to the general public however they are important way for the atmosphere to transport heat energy. They are very relevant to us as they are spawned by severe storms and can lead to severe turbulence. With this contract we have learned how infrasound waves are affected by gravity waves and thus how infrasound recordings can be used to probe this structure. The Earth is a system that includes a solid interior, atmosphere, oceans and cryosphere as well as all living things in the biosphere. These different parts of the earth don't exist independently of the others. All parts of the earth interact with all other parts. In other words, the Earth is an interconnected system. Infrasound provides a link between the solid Earth and the atmosphere - for example, seismic waves shake mountains which then vibrate and radiate acoustically into the atmosphere. Infrasound provides a link between the oceans and the atmosphere. Ocean waves continuously press against the atmosphere and radiate low-frequency sound waves called microbaroms which can be "heard" instrumentally at distances of thousands of kilometers from the source. Infrasound sheds light on how we in the biosphere impact our environment simply by generating sound waves that can be recorded 100's of kilometers from the source. Importantly, infrasound helps us understand how our environment interacts with us - often very negatively. In one of the most "violent" interactions between the solid Earth and atmosphere, and the biosphere, volcanoes transform from seismic sources radiating energy predominantly into the solid Earth to acoustic sources radiating mainly into the atmosphere. Infrasound and seismic data, taken together, can provide us with a much more complete understanding of the workings of these important events and help us understand the immediate threat these events pose to us. With this project we have played a small role in fleshing out our understanding of this enigmatic energy called infrasound and what it can tell us about the workings of our planet.
