In seismology and acoustics, we often reduce the information in our measured waveforms by focusing on the properties of specific phases (packets of wave energy that travel in certain ways along specific paths). When I worked on my PhD, I focused entirely on the study of teleseismic P, PKP, and PKIKP waves (longitudinal waves that refract in the lower mantle or travel through Earth’s core). As I’ve had the opportunity to do more in my career (i.e., gotten older), I’ve been fortunate to do research on many more types of both seismic and acoustic phase. Without thinking about it this way, I’ve increased my ‘phase breadth’.
We had some visitors in my lab this week and all the students and postdocs gave briefings. I got to thinking about the phase breadth of the work we’re doing. I’m quite proud of the fact that we’re working on a really wide-range of phases that sample different parts of Earth! While not an explicit goal unto itself, phase breadth does keep things interesting and provides a useful perspective. What follows is a very brief synopsis of the phases we’re studying, organized by depth (from the waves that travel deepest into the Earth to the highest into the atmosphere). I won’t go into the details of the research, because that’s not my story to tell (look for forthcoming student and postdoc papers and presentations)!
The deepest traveling seismic waves we’re looking at are teleseismic P and S waves that are refracted in the lower mantle. Such phases are great for forensic applications of source properties, because they’re relatively simple and uncontaminated by path effects. However, while useful for studying large events, smaller events are often not observed at the longer distances associated with these lower mantle refractions (i.e., distances greater than 30 degrees from the source). One way we can drive teleseismic detection limits lower (especially important for studying seismicity in the ocean basins) is with seismic arrays and a student in our group is exploring this with machine learning using teleseismic P and S waves from a large seismic array in Warramunga, Australia.
Next deepest are the seismic waves that travel in the crust and upper mantle. Seismologists call these ‘regional’ seismic waves, and they’re often messier than teleseismic waves but can contain additional information at higher frequencies. We were pretty fortunate this past December while operating a Distributed Acoustic Sensing Interrogator Unit for a few days in North Dallas to detect signals from regional earthquakes in west Texas and California. Seismologists often joke that putting out sensors can stop earthquakes from happening. I guess we got lucky this time, because we were only out a few days and got a couple good sized regional events. Anyway, we have a postdoc working on understanding what we do with this new type of dense data in urban environments (very different from Warramunga, Australia).
At distances of 10’s of kilometers from the source, seismic waves propagate in the crust. In certain active seismic areas (e.g., volcanoes, or areas of induced seismicity), seismologists have put out dense networks of stations to study small earthquakes and explore what we can learn from them. We have a student developing a new method for detecting and locating these small events, with a variety of both fundamental and practical applications.
Moving into the atmosphere, at very local distances we can study acoustic waves that propagate in the atmospheric boundary layer (the part of the atmosphere whose dynamics are directly affected by the surface). One of our students is working on studying acoustic waves in urban boundary layers. Part of this work has involved the dense deployment of home-made infrasound sensors in the area around SMU in Dallas. Exotic signals abound, and the study of the spatiotemporal variation in these signals provides opportunity to study wave propagation.
Moving yet higher in the atmosphere, we have a few projects looking at low-frequency sound waves that refract in the stratosphere due to the presence of the ozone layer absorbing solar radiation, and a wind jet at this altitude. One of our students is exploring the study of these phases to understand short-time-scale variations in the atmosphere in the stratosphere. More about this effort can be found here: https://allthespheres.com/2021/01/measuring-explosions-in-oklahoma/
It’s not always helpful to think about phases in isolation. When an event happens, it can excite a wide-variety of seismoacoustic waves that can themselves excite new waves as they propagate. Studying these waves holistically can lead to new insights on sources and wave propagation. To this end, one of our students is focusing on developing a way to fuse seismic and acoustic data for improved event characterization. And, this past week, a review paper led by a former SMU student (now scientist at Sandia) on seismoacoustics came out in SRL: https://bit.ly/3ZDJck9 The review paper describes and categorizes the variety of different coupled seismoacoustic phases. It is a bit disconcerting that my 2010 review paper is now considered historical, but really great to see the advances in seismoacoustics in the past 11 years!