On August 23, 2011, a magnitude 5.8 earthquake occurred with its epicenter approximately 118 miles from the National Aquarium, Baltimore. A short time before the dolphin pavilion started shaking from the earthquake, an Aquarium volunteer logging the activities of four dolphins noticed that they all started to swim very quickly in close formation, something she could not recall ever having seen before. She had enough time to note this behavior in her handwritten log before the building suddenly started shaking. At the same time all this was happening, the underwater sounds in the dolphin pools were being recorded using a pair of hydrophones (i.e. underwater microphones). The combination of the in-person observation and the hydrophone recordings provides valuable insight into dolphin behavior.
When an earthquake occurs, seismic waves radiate out from the focus of the earthquake at different velocities. The fastest of these, called the primary wave or P-wave, can travel at speeds of 15,000 miles per hour. However, although very fast, P-waves often are unnoticed by humans. The S-wave and surface waves, the ones that shake everything and cause the worst destruction, travel at much slower speeds.
Although no humans at the Aquarium that day reported feeling the P-wave, its trace did show up in our hydrophone recordings almost 22 seconds before the arrival of the S and surface waves. In view of the P-wave’s appearance in the recordings and the dolphins’ behavior, marine mammal researcher Mark Turner believes the dolphins felt the P-wave, and the volunteer observed their reaction to it.
This is a clip of the underwater sounds in the dolphin pools when the August 23, 2011, Virginia earthquake occurred. Two hydrophones were recording at the time. The left stereo channel is the recording from the hydrophone in the front pool where a dolphin presentation was in progress. The right channel is from the back holding pool where fast swimming in an unusual configuration was observed. In the video that accompanies the sound clip, the top two panels show the raw signal picked up by each hydrophone. The top panel is from the front pool and the bottom one is from the holding pool.
The bottom two panels are spectrograms. A spectrogram is a visual representation of sounds in which the x-axis is time and the y-axis is frequency. In a spectrogram a dolphin whistle will appear as a dark, wavy line, and a squawk can sometimes appear as a stack of parallel wavy lines.
The sound clip begins at almost exactly the time the earthquake started in VA. The various seismic waves traveled from the earthquake’s focus to Baltimore at different velocities, with the P-wave arriving first, 27 seconds into the clip. Although the very low frequency vibrations induced by the P-wave are visible in the upper panels, they are inaudible, although you might hear some water splashing. The S and surface waves (the ones that are very loud and shook everything) did not arrive until almost 22 seconds later, 49 seconds after the beginning of the clip.
You may hear some of the presentation music, a bit louder in the left channel. If you listen carefully you will also hear (and see in the spectrograms) dolphin clicks, squawks and whistles. And, of course, you will hear the loud noises made by the earthquake surface waves as they sounded underwater.
An excellent overview of the different seismic waves with animations can be found by clicking here.
All signal displays were generated using Raven Pro, Interactive Sound Analysis Software, Bioacoustic Research Program at the Cornell Lab of Ornithology.
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