As ocean waves rise and fall, they impact the seafloor below and generate seismic waves. These seismic waves are so powerful and widespread that they appear as a steady pulse on seismometers, and these waves have increased in strength in recent decades.
In a new study published in the journal Nature Communications, researcher Richard Aster, professor and chair of the Department of Geophysics at Colorado State University, and his colleagues tracked this increase around the world over the past four decades.
As Dr. Richard says in an article on The Conversation website, this global data, along with other ocean, satellite and regional seismic studies, has shown a decades-long increase in wave energy that coincides with an increase in storm surges attributed to rising global temperatures.
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Highly sensitive global seismic instruments and networks constantly record a huge variety of natural or human-caused seismic phenomena, including the most widespread seismic background signal globally, which is caused by the constant noise resulting from storm-driven ocean waves. Ocean waves generate microseismic signals in two different ways.
The first, and more active of the two, are known as secondary microseisms, which pulsate for between about 8 and 14 seconds. When sets of waves travel across the oceans in different directions, they interfere with each other, causing pressure variations on the seafloor. This type does not always exist.
The second way ocean waves generate global seismic signals is called primary microseismicity, which is produced by traveling ocean waves directly pushing and pulling on the seafloor. Since water movements within waves quickly descend to depth, this occurs in areas where the water depth is less than about 300 metres. These signals appear in seismic data as a constant hum with a period ranging between 14 and 20 seconds.
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In their study, the researchers estimated and analyzed the intensity of historical primary microseisms dating back to the late 1980s at 52 seismic stations around the world, and found that 41 (79%) of these stations showed very large and gradual increases in energy over the decades.
The results indicate that global average ocean wave energy since the late twentieth century has increased at an average rate of 0.27% per year. However, since 2000, the average global rate increase has increased by 0.35% per year.
This increase was most noticeable in areas of the very stormy Southern Ocean near the Antarctic Peninsula. In the North Atlantic Ocean, wave energy has increased faster in recent decades compared to its historical levels.
As Richard says in the article; The oceans have absorbed about 90% of the excess heat associated with rising greenhouse gas emissions caused by human activities in recent decades. This excess energy can translate into more damaging waves and more powerful storms.
He stresses that their findings provide another warning to coastal communities, as increased ocean wave heights could pound coastlines, damaging infrastructure and eroding land.
The effects of increased wave energy are exacerbated by the continuing rise in sea levels fueled by climate change and land subsidence. Researchers stress the importance of mitigating the effects of climate change, building resilience in coastal infrastructure, and environmental protection strategies.
