Waveform data from continuously recording seismometers, such as those we use for research and share through community consortia like EarthScope, include signals from a gamut of events and calms. Events include earthquakes as well as hurricanes. Hurricanes represent dramatic spatial variations in atmospheric pressure and violent winds over sea surfaces, which excite forceful waves and interferences. These tempestuous waves excite strong seismic Rayleigh waves in the Earth’s crust beneath the sea, which propagate over long distances and in all directions. In fact, such Rayleigh waves are continuously excited by steady wave action in coastal seas and the ones with the longer wavelengths become amplified in hurricanes. They have a dominant frequency between 100 and 300 mHz, are labeled “secondary microseisms”, and are well recorded by our sensitive seismometers.
Recently, Hurricane Milton crossed into central Florida, where broadband seismometer DWPF is located. Tracking the seismic noise at this station over three consecutive days indeed shows a nearly ten-fold amplification of these secondary microseisms during Milton’s devastating journey. This amplification factor looks a bit smaller in a three-day record of raw waveform data shown below because it also contains a lot of seismic wave energy at higher frequencies. If this phenomenon occurred at audible frequencies, most of our ears would hear the amplification and shift to lower pitches associated with the longer wavelegths of the stormy waves. This is where the Earthtunes App (https://sites.northwestern.edu/earthtunes/) comes in handy: Start from scratch, add station IU.DWPF.00.BHZ to “your stations”, select it, select 8 October 2024 at 04h00 UTC, select a duration of 24 hours, load the data, and listen to it in different ranges of octaves. Then repeat for October 9 and 10. By compressing 24 hours of seismic data into about a minute of audio, we can actually listen to the seismic waves. You can view and hear a version of this explorative interaction with real-world seismic data in the video below. Do you hear the increase in volume over the three days? Do you hear the decrease in pitch as the secondary microseisms become more dominant?