High tides and tidal 'stresses' linked to large magnitude quakes: Japanese study
A tsunami wave overflowed the seawall in Miyako City after the magnitude 9 earthquake struck Japan on March 11, 2011.
Some seismologists think large-magnitude earthquakes are more likely during times of high "tidal stress" - after a new or full moon.
A study published in Nature Geoscience involved reconstructing tidal size and movements for two weeks before earthquakes of magnitude 5.5 or more during the past 20 years.
Large, deadly earthquakes in Indonesia in 2004, which triggered the Boxing Day tsunami, the magnitude 8.8 quake in Chile in 2010, and the magnitude 9 earthquake and subsequent 10-metre tsunami in Japan in 2011, were all part of the study, which indicated the frequency of big quakes increased as tides increased in size and movement.
A massive magnitude 9 quake hit northeast Japan, triggering a 10-metre (33-ft) tsunami along parts of the country's coastline, in March 2011.
Theories of links between tidal forces and earthquakes have been put forward since the 19th century, but conclusive evidence has not been agreed.
People make their way through the ruins of the destroyed residential part of Kamaishi, northern Japan, destroyed by the tsunami in 2011.
Many seismologists remain sceptical of tidal research, which is a notoriously difficult arena in which to produce reliable evidence.
Tidal stresses and their effects on the Earth are tiny, but measurable, although many scientists remain unconvinced by the theory of "tidally triggered earthquakes".
University of Melbourne associate professor in active tectonics and geomorphology Mark Quigley - who lived in Canterbury during the 2010-11 quakes - said the Japanese team behind the research previously put forward some nice theories.
"But I remain unconvinced that the timing and characteristics of large earthquakes clearly correlate to lunar cycles or tidal stresses, nor do I think potentially tidally triggered seismicity has any real practical [use] in the context of coastal seismic hazard and public safety."
In the study by the University of Tokyo Department of Earth and Planetary Science, researchers said the probability of a tiny rock fracture expanding to a gigantic rupture increased with tidal stress levels.
It was important to remember the moon caused the Earth's tidal forces, but the moon did not cause earthquakes.
High tides were the result of the Earth spinning on its axis and the moon's gravitational force pulling the water on the surface, leading to high and low tides depending on the position.
The Japanese research team said they did not find a clear relationship between large tides and small earthquakes, but some of the larger earthquakes happened during periods of high tides.
Seismologists do not know exactly how large quakes work, but it is thought the effect is a "cascade" as a small fracture ruptures and enlarges into a fracture capable of generating a large earthquake.
It was possible that knowledge of large tides in earthquake-prone regions could help in probability analyses.
However, Quigley said the tidal stresses before and after the March 2011 Japan quake were higher in the 30 days before and after the quake and there was not a clear link between stress measurements and the timing of the quake.
He also said tidal stresses were tiny compared to the energy produced by an earthquake rupture.
Quake stresses were probably 1000 to 10,000 times greater than tidal forces.
Quigley said it was naive to think the answer to earthquake preparedness was tidal or that staying away from beaches during a full moon would help. The magnitude-5.7 Valentine's Day quake in Christchurch fell on a quarter moon, said Quigley.
"It is important to recognise that I am not saying that tidal stresses are unimportant things to consider within the variety of processes that may influence earthquake behaviour."
The best measures for reducing risk were already used, he said, and included seismic building codes, seawalls, early-warning systems, and tsunami evacuation plans.
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