Sound Waves from Fin Whales Help Scientists Probe Earth’s Crust

Scientists investigating a seismic fault off the U.S. West Coast have an unlikely new ally in their quest to create images of the Earth’s crust deep beneath the ocean: Fin whales.

Fin whales (Balaenoptera physalus) are 20- to 25-meter-long (60 to 85 foot) behemoths whose songs can be heard up to 1,000 kilometers (600 miles) away, booming through the seas as loudly as a ship’s engine does. New research, published Thursday in Science, finds these calls’ sound waves can help create images of the seafloor subsurface down to 2.5 kilometers (1.6 miles).

Seafloor imaging, which is important for studying things like earthquake mechanics and carbon storage capacity, typically uses large air guns that send blasts of sound downward. The sound waves penetrate the crust and bounce back to instruments on the seafloor, carrying information about the structures they travel through. But such surveys are expensive, and the guns’ concussive noise may disturb marine mammals that use sound to communicate.

Whale vocalizations often appear in such instruments’ records. Seismologists usually see these signals as a nuisance. Scientists however, noticed that some signals appeared only on the seafloor instruments’ seismometers, which measure vibrations; their audio microphones did not pick up any corresponding sounds. This meant the signals were not coming directly from the whales, but were echoes bouncing back from within the crust.

To use such signals for imaging, scientists had to pinpoint the whales’ locations. This was possible because each whale call made two sets of waves: one that traveled directly to the seismic station on the ocean floor, and another that bounced between the ocean floor and surface before hitting the same station. By comparing the arrival times of these two waves, scientists could calculate the whales’ approximate locations.

Fin whales sing in loud, one-second pulses. This pattern is good for seismic imaging because pulses are easier to analyze than continuous noise. These whales are found almost everywhere except the ice-covered portions of the Arctic, he notes—so their calls “could potentially be used in a lot of regions.”

Citations:

Kuna, Václav M., and John L. Nábělek. “Seismic Crustal Imaging Using Fin Whale Songs.” Science, vol. 371, no. 6530, 2021, pp. 731–735., https://doi.org/10.1126/science.abf3962.