The microseism mystery

DAVID HAMBLING listens out for the tremors that make up the planet’s background noise

Renewed interest in fracking in the UK will bring more attention to bear on microseisms, the faint earth tremors which make up the planet’s background noise. This all-pervasive vibration is sometimes figuratively called the ‘Earth Hum’, but unlike the many mystery hums around the world, the Earth Hum is much too low-pitched for human ears. While some microseisms are caused by human activity, the vast majority are the result of natural phenomena – though exactly which phenomena is not always known, and some regular signals remain enigmatic.

The most famous of these regular microseism sources was first recorded in 1961 by Jack Oliver of Columbia University, using the seismographs at Palisades, New York, instruments so sensitive they could pick up activity on the other side of the world. A few years earlier, scientists at Palisades had detected a nuclear test at the Marshall Islands, over 7,000 miles (11,265km) away. Oliver saw a new source, one which was producing a regular pulse every 26 seconds. Unlike other sources, like explosions or volcanoes, it continued at the same level with only minor variations in intensity, like the slow heartbeat of some colossal subterranean or subaquatic creature slumbering in the depths.

These instruments did not allow Oliver to locate the “persistent, localized microseism source” with any accuracy. He could only say that it was coming from somewhere in the southern or equatorial Atlantic Ocean. Oliver also noted the pulse was stronger in the Northern Hemisphere’s summer months. The discovery attracted little interest and appeared to have been virtually forgotten.

Almost two decades later in 1980, Gary Holcomb, a geologist with the US Geological Survey, found the same microseism source and noted that it increased in intensity during storms. In 2005 it was the turn of Greg Bensen of the University of Colorado, Boulder, to rediscover the persistent pulse, and with more modern instruments he triangulated data to locate the source: it was coming from the Gulf of Guinea, off West Africa.

In 2011, another seismologist, Garrett Euler of Washington University in St Louis, pinned down the source more precisely to an area known as the Bight of Bonny (or Bight of Biafra) close to Sao Tome and Principe, and hazarded a guess about its cause.

The majority of microseisms are caused by ocean swell, in particular in shallow water on continental shelves where the variation in the weight of water on the supporting geology below causes it to flex fractionally. These are called primary microseisms. Another type is caused by ocean waves coming in opposite directions interacting, known as secondary microseisms, and can be used to infer sea state in distant locations. Unlike the localised source seen on the Gulf of Guinea, this type of wave noise is spread over a wide area, and is completely dependent on weather conditions.

Euler theorised that the 26-second source was the result of ocean waves reaching a particular section of coastline. However, the wave theory faced immediate opposition from a team at the Institute of Geodesy and Geophysics in Wuhan, China, led by Yingjie Xia, who argue for a volcanic source. Then things started to get more complicated.

Since the original discovery in the 1960s, other persistent localised microseism sources have been identified. A 26-second source similar to the one on the Gulf of Guinea was found off Vanuatu in the South Pacific. In 2012, another one was found with a 10-second pulse time off Kyushu in Japan. One conspicuous feature that these two have in common is that they are both the sites of active volcanoes – the Japanese one corresponds to Aso Volcano, one of the most active in the world with 167 documented eruptions. Vanuatu is a chain of volcanic islands, many of them active.

In addition, closer examination of data from the Gulf of Guinea revealed that there were multiple microseism sources at work. The Chinese team identified a 28-second source in addition to the original 26-second source, and in 2020 separated out a third source pulsing at 16-second intervals.

In their interpretation, the similarity to microseisms seen off the east coast of Africa that have been traced to the Mayotte volcanic system strongly suggests that the multiple sources are also volcanic. There are not known to be any active volcanoes in the Gulf of Guinea; the persistent microseism sources may be a sign that active volcanoes are lurking somewhere underwater. There may be still more persistent microseisms out there, if their pulses can be separated out from the Earth’s continuous background noise of volcanic activity, shifting tectonic plates and storms. New AI-based filters may turn up many more of them.

Earlier this year, a paper from Yongyan Chen of the University of Science and Technology of China proposed a wave-based solution which would account for the 26and 28- second sources. This is based on a mathematical analysis which found their magnitudes matched the Gutenberg-Richter Law governing the distribution of earthquake intensities. In addition both microseisms were modulated by the size of swell at the time – hence Oliver’s original observation about the seasonal variation in the intensity of the microseisms.

According to this latest work, the cause of the microseisms is “underground complex crack networks featuring fractal characteristics, and are dominantly driven by temporally random dynamic processes”. According to this theory, the microseisms are essentially caused by geological processes, but modulated by waves above. However, the matter is far from settled.

This theory, though, takes us back to fracking: hydraulic fracturing to release trapped hydrocarbon causes some level of micro-seismicity because it produces a spiderweb of underground cracks in the underlying rock formation. As the continued attempts to pin down persistent sources show, we do not yet have a full understanding of how geology interacts with other factors to produce microseismic activity.

Renewed interest in fracking in the UK will bring more attention to bear on microseisms