Earthquake recorded by hydrophone at 16N 43W

Seismic waves can also propagate through water as a pressure wave. Consequently, earthquakes on or below the seafloor can be heard in the water. There are a few monitoring stations in the oceans, some of which are specifically built to record earthquake activity near mid-ocean ridges. This seismogram is from to one of them.

Unlike rock, water has different qualities that affect sound propagation. The major difference is in the frequency of the ambient noise. Ships, marine mammals, and earthquakes all contribute to the background noise. These are higher frequency than most earthquake sources, so there is a whining noise, modulated by what sounds like a lot of people talking in a crowded room. In the middle of this is an earthquake arrival. Its signal is much more broad-band than the noise, lending it a whooshing sound as it passes midway through the recording. The final difference between seismic waves in rock and in water is their slow travel times in the water. The average speed in water is around 1500 meters per second as compared to around 6400 meters per second in the rock at the seafloor. Consequently, seismic waves in water are called T-waves, or tertiary waves, because they arrive after the P-waves and S-waves (primary and secondary waves).

Above the seismogram is a display of the frequency content of the signal (vertical axis) through time (horizontal axis), called a spectrogram. The spectral intensity is represented by the color: Red for intense frequencies, grading to blue for faint ones. You can see the constancy of the noise frequency as streaks running horizontally across the spectrogram. In contrast, the seismic wave arrival is spread across all frequencies up to around 40 Hz, showing its broad-band nature.
Earthquake recorded by hydrophone

Data courtesy PMEL/NOAA.