Climate Change

Earthquakes: the oceanographers you didn’t know about

Source: Wu, Wenbo, Zhongwen Zhan, Shirui Peng, Sidao Ni, and Jörn Callies. “Seismic Ocean Thermometry.” Science 369, no. 6510 (September 18, 2020): 1510–15.

A warming ocean

The impacts of global warming have reached from the tiniest phytoplankton to the largest hurricane. While these impacts were prompted by rising temperatures, the ocean has steadily chipped away at heat trapped in the atmosphere. Surface waters of the ocean absorb heat from the air and eventually mix into the deep ocean, trapping 90% of the excess heat from global warming deep below the surface. While we know that the effects of climate change would likely be much worse if the ocean didn’t act as a heat sink, understanding exactly where and how much the ocean is warming is critical if we are going to prepare our societies for future change.

Hearing temperature changes

On land we can chart changes in the atmospheric temperature through fixed weather stations. But the ocean is much larger, covering 70% of Earth’s surface and extending to an average depth of 5000 meters – the equivalent of 10 Empire State buildings. Consistently measuring temperatures throughout the ocean is an impossible task for humans – but not for sound waves. Sound waves released by the shifting of marine tectonic plates can travel thousands of kilometers in a few seconds. The speed of these waves traveling across the ocean is impacted by the depth of the ocean floor, the water salinity, and most importantly, the water temperature. Researchers from the California Institute of Technology used the relationship between warmer temperatures and faster sound waves to examine temperature change over a decade in the Indian Ocean.

Tectonic plate boundaries with plate motion. Image from Wikipedia.

Good data comes in pairs

The researchers chose the Indian Ocean because it is home to one of the most seismically active regions on the planet: the Australian tectonic plate. While tectonic plates are always slowly moving, earthquakes only occur when regions of two plates abruptly move against each other. To track the changing speed of a sound wave over time, researchers needed to make sure that the sound waves were originating from the same region of the tectonic plates. Researchers identified pairs of waves from the 4272 earthquakes measured at coastal seismic stations between December 2004 and June 2016. The similar shape of the wave in each pair showed researchers that the waves originated from the same source. Then, they could compare the speed of each wave to find the temperature change. For one specific pair, they found the first wave on October 21, 2006 and the second on March 3, 2008. They found that the second wave traveled 0.27 seconds faster than the first, which is equivalent to a 0.051 ˚C temperature change.

Make way for earthquakes

Fitting all the data to a statistical model, the researchers found a temperature change of 0.044  ± 0.002 ˚C per decade. This is a significantly higher result than trends measured from traditional ocean profiling floats which have recorded changes of 0.026 ± 0.001 ˚C per decade. These slow-moving floats – which sample every 10 days and don’t go deeper than 2000 meters – are very limited when compared to the fast-moving and broad reach of the seismic waves from a submarine earthquake. There is a huge potential for using this method to understand temperature changes across the global ocean, as 10,000 shallow submarine earthquakes happen every year. Classifying the magnitude and distribution of ocean warming is critical to develop plans for adapting to climate change. It seems that researchers need to make way for the newest oceanographer in town – earthquakes.

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