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Why earthquakes at mid-ocean ridges are more common at low-tide

Why earthquakes at mid-ocean ridges are more common at low-tide
09 Jul
2019
Researchers at Colombia University have answered a question that has confounded scientists for decades – why are earthquakes in the ocean linked to low-tide?

Earthquake WeekIn 1897, the British physicist, Arthur Schuster, turned his attention to a theory that had been circulating among those who study earthquakes for centuries. The theory went that earthquakes are linked to the tides and, by extension, that the forces of the sun and moon can influence them. Schuster examined the existing catalogue of earthquake occurrences and applied his now famous probability theory to the question. His analysis showed that there was no statistical correlation between earthquakes and the tides – a decisive blow for the proponents of the theory.

Despite this, researchers kept going, returning to the question every 20 or so years but always drawing a blank. Things finally changed with a series of studies, published over the last decade, which have demonstrated that the correlation is in fact real. Though the link appears weak when it comes to earth-bound earthquakes, it is stronger in the ocean and particularly strong at mid-ocean ridges, where tectonic movement forms great seafloor mountain systems. Nevertheless, while this research answered some crucial questions it also added to the confusion. Scientists in the field had always assumed that there would be more earthquakes at high-tide – in fact, the opposite proved to be the case.

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Christopher Scholz, a seismologist at Colombia University, explains why the low-tide correlation is so surprising. Mid-ocean faults are vertical faults. During movement, the upper block slides down with respect to the lower one. Scientists expected that at high tides, when there is more water sitting on top of the fault, it would push the upper block down and cause the earthquakes. 

Scholz and his colleagues have now answered this conundrum and in doing so have verified more clearly than ever before the link between tides and earthquakes. To do so they used data collected by ocean-bottom seismographers placed near the Axial Volcano along the Juan de Fuca Ridge in the Pacific Ocean, a patch of the mid-ocean ridge that’s highly sensitive to earthquakes and where ocean tides are particularly strong. The researchers were helped by the fact that the measuring instruments in this area are directly linked to the shore via cables (the volcano is situated around 300 miles off the coast of Oregon), which makes it possible to collect more data over longer periods and to analyse that data without the instruments needing to be brought to the surface.

Map00384 28004999920Axial Volcano off the coast of Oregon on the Juan de Fuca Ridge [Image: NOAA Vents Program]

Analysis of the data revealed that the missing component in the story is the magma that lies beneath mid-ocean ridges. The team realized that when the tide is low, there is less water sitting on top of the magma chamber (a soft, pressurized pocket below the surface) and so it expands. ‘This magma chamber underneath acts like a bellows,’ says Scholz. ‘When you reduce the force pushing down, which happens at low tide, the bellows open and that pushes the faults from below.’ Rather than the upper block being forced down, this process forces the lower block to slide up the fault which casues tremors. It’s a key finding in a series of research that has troubled scientists for centuries.

The other significant finding from the study relates to the sheer abundance of earthquakes in the region. In one three-month period the researchers recorded 60,000, albeit some at a very small magnitude. The detailed measurements from the instruments showed that even the tiniest stress could trigger an earthquake. While the tidal data helped to calibrate this effect, the triggering stress could be caused by anything – such as the seismic waves from another earthquake, or fracking wastewater pumped into the ground.

shutterstock 378131164Scholz warns that there is no way for the fracking industry to know what level of pressure is safe

According to Scholz this research has implications for all earthquakes. Most fundamentally of all, he says it reveals that there is no way to decisively predict what level of stress will set one off. ‘It depends on what the state of stress is locally. There’s nothing about the earthquake itself that produces a threshold stress.’

He notes that this has ramifications for the fracking industry, which he claims has always been keen to identify a ‘safe’ level of pressure that can be applied to the Earth. Unfortunately, such a level can never be established. ‘You couldn’t say that everywhere you go you can pump the pressure up x amount and you’ll be safe,’ warns Scholz. ‘It’s going to depend on each place. There’s no rule of thumb.’

Geographical July 2019CRACKING THE EARTHQUAKE CODE
The July 2019 issue of Geographical takes an in-depth look at the ways advancements in technology are improving the scientific art of earthquake detection. But will we ever be able to accurately predict these devastating incidents?
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