Scientific American - USA (2020-12)

December 2020, ScientificAmerican.com 21

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SEISMOLOGY

Earthquake Sounds

Natural vibrations could let scientists measure ocean warming

Sound may offer a creative way to take

the ocean’s temperature. Climate change

is steadily warming the seas, which have

absorbed about 90 percent of the excess

heat trapped by greenhouse gases. This

warming contributes to sea-level rise,

imperils marine species and influences

weather patterns.

But tracking the warming is tricky.

Ship-based observations capture only

snapshots in time over a tiny portion of the

water. Satellite observations cannot pene-

trate very deep below the surface. The

most detailed picture of ocean heat comes

from Argo, a flotilla of autonomous probes

that have peppered the seas for more than

a decade and can drop down to around

6,500 feet. But there are only about 4,000

such floats, and they cannot sample deeper

parts of the oceans.

In Science, researchers at the California

Institute of Technology and the Chinese

Academy of Sciences compared the travel

speeds of sounds produced by undersea

earthquakes to detect ocean warming over

wider areas. Because sound travels faster

in warmer water, differences in speed can

reveal changing temperatures. “They’re

opening up a whole new area of study,”

says Princeton University geophysicist

Frederik Simons, who was not involved in

the research.

Oceanographers proposed measuring

ocean heat with sound in 1979, but sea-

based acoustic emitters were expensive

and raised concerns about disturbing

marine animals. Inspired by those early

efforts, Caltech researcher Wenbo Wu

thought to monitor low-frequency acoustic

waves emitted by earthquakes below the

seafloor. “I know these earthquakes are

very powerful sources,” Wu says. “So why

not try to use the earthquakes?”

He and his team tested the idea near

Indonesia’s island of Nias, where the Indo-

Australian Plate is bumping under the Sun-

da Plate. The researchers gathered acous-

tic data from 4,272 earthquakes of magni-

tude 3 or greater from 2004 to 2016, and

they compared acoustic wave speeds from

quakes that originated in the same spot

over the years. By modeling the differenc-

es, often just fractions of a second, they

found that the ocean near Nias was warm-

ing by about 0.08 degree Fahrenheit per

decade—more than the 0.047 degree F

suggested by Argo’s data. Less than one

degree F does not sound large, but it takes

considerable heat to warm the entire east-

ern Indian Ocean.

The new acoustic method is promising,

says University of Hawaii oceanographer

Bruce Howe, who was not involved in the

work. Researchers may even be able to get

a longer ocean-temperature history from

seismological data taken decades ago,

although older seismometers did not record

the sound waves’ timing as precisely as cur-

rent GPS-based ones do.

Simons and his col-

leagues are exploring an

alternative technique, de -

ploying dozens of under-

water microphones called

hydrophones to catch

more earthquake sounds.

He notes that pinpointing

the floats’ precise locations

will be challenging, how-

ever. Overcoming such

challenges would fill in

important gaps, Wu says:

“We really need different

methods of [gathering] the

data as much as possible.”

LINDSAY IMAGERY — Stephanie Pappas

Getty Images

SEISMOLOGY

Earthquake Sounds

Natural vibrations could let scientists measure ocean warming

Sound may offer a creative way to take

the ocean’s temperature. Climate change

is steadily warming the seas, which have

absorbed about 90percent of the excess

heat trapped by greenhouse gases. This

warming contributes to sea-level rise,

imperils marine species and influences

weather patterns.

But tracking the warming is tricky.

Ship-based observations capture only

snapshots in time over a tiny portion of the

water. Satellite observations cannot pene-

trate very deep below the surface. The

most detailed picture of ocean heatcomes

from Argo, a flotilla of autonomous probes

that have peppered the seas for more than

a decade and can drop down to around

6,500 feet. But there are only about 4,000

such floats, and they cannot sample deeper

parts of the oceans.

In Science, researchers at the California

Institute of Technology and the Chinese

Academy of Sciences compared the travel

speeds of sounds produced by undersea

earthquakes to detect ocean warming over

wider areas. Because sound travels faster

in warmer water, differences in speed can

reveal changing temperatures. “They’re

opening up a whole new area of study,”

says Princeton University geophysicist

Frederik Simons, who was not involved in

the research.

Oceanographers proposed measuring

ocean heat with sound in 1979, but sea-

based acoustic emitters were expensive

and raised concerns about disturbing

marine animals. Inspired by those early

efforts, Caltech researcher Wenbo Wu

thought to monitor low-frequency acoustic

waves emitted by earthquakes below the

seafloor. “I know these earthquakes are

very powerful sources,” Wu says. “So why

not try to use the earthquakes?”

He and his team tested the idea near

Indonesia’s island of Nias, where the Indo-

Australian Plate is bumping under the Sun-

da Plate. The researchers gathered acous-

tic data from 4,272 earthquakes of magni-

tude 3 or greater from 2004 to 2016, and

they compared acoustic wave speeds from

quakes that originated in the same spot

over the years. By modeling the differenc-

es, often just fractions of a second, they

found that the ocean near Nias was warm-

ing by about 0.08 degree Fahrenheit per

decade—more than the 0.047 degree F

suggested by Argo’s data. Less than one

degree F does not sound large, but it takes

considerable heat to warm the entire east-

ern Indian Ocean.

The new acoustic method is promising,

says University of Hawaii oceanographer

Bruce Howe, who was not involved in the

work. Researchers may even be able to get

a longer ocean-temperature history from

seismological data taken decades ago,

although older seismometers did not record

the sound waves’ timing as precisely as cur-

rent GPS-based ones do.

Simons and his col-

leagues are exploring an

alternative technique, de -

ploying dozens of under-

water microphones called

hydrophones to catch

more earthquake sounds.

He notes that pinpointing

the floats’ precise locations

will be challenging, how-

ever. Overcoming such

challenges would fill in

important gaps, Wu says:

“We really need different

methods of [gathering] the

data as much as possible.”

—Stephanie Pappas

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