EXPERT REACTION: Large earthquakes linked with high tides ...

EXPERT REACTION: Large earthquakes linked with high tides
Japanese researchers say they have found a link between high tide and large earthquakes, which may indicate a greater likelihood of earthquakes following the new or full moon. The study involved reconstructing the size and amplitude of tidal stress from the two weeks prior to earthquakes that registered a magnitude of 5.5 or higher, over the last 20 years. The research indicated large earthquakes in Indonesia (2004), Chile (2010), and Japan (2011), among others, occurred at times of high tidal stress, and the frequency of large earthquakes compared to small earthquakes increased with tidal stress.
Media Release
Large earthquakes are more likely to occur at times of full or new Moon, according to a study published online this week in Nature Geoscience.
Although it seems intuitive that the fault lines on Earth that are already close to failure could be pushed into slipping by the gravitational forces of the Sun and Moon, firm evidence for tidal triggering of earthquakes has been lacking.
Satoshi Ide and colleagues reconstruct the size or amplitude of tidal stresses — rather than just the timing of high tide or tidal phase — in the two weeks prior to large earthquakes (magnitude 5.5 or greater) that have occurred over the past two decades. Although they find no clear correlation between tidal stress and small earthquakes, they do find that some of the largest earthquakes: including 2004 Sumatra, Indonesia; 2010 Maule, Chile; and 2011 Tohoku-oki, Japan, occurred during times of high tidal stress amplitude. They also find that the fraction of large earthquakes compared to small earthquakes increases as the amplitude of tidal stress increases.
Precisely how large earthquakes initiate and evolve is not fully understood, but they may grow via a cascading process whereby a tiny fracture builds up into a large-scale rupture. If so, the authors’ results imply that the likelihood of a small fracture cascading into a large earthquake are greater during the Spring tide. Thus, knowledge of the tidal stress state in seismic regions could help in assessing the probability of an earthquake.
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Expert Reaction
These comments have been collated by the Science Media Centre to provide a variety of expert perspectives on this issue. Feel free to use these quotes in your stories. Views expressed are the personal opinions of the experts named. They do not represent the views of the SMC or any other organisation unless specifically stated.
Mark Quigley, Associate Professor in Active Tectonics and Geomorphology, University of Melbourne
Tanaka and colleagues have been working on this problem for many years and have published 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 utilization in the context of coastal seismic hazard and public safety. Here is why.
The Tohoku earthquake occurred on March 11, 2011. There was a new moon on March 4th, a quarter moon on the 12th, and a full moon on the 19th. So this timing is exactly the opposite of what one would expect if the timing of this earthquake was associated with full or new moons. Ide et al acknowledge this lack of temporal correlation in their paper.
The tidal shear stresses Ide et al estimate, resolved on to the fault plane that ruptured in the Tohoku earthquake, were higher in the 30 days both before and after the occurrence of the Tohoku earthquake. So there is not a clear correlation between the peak tidal shear stress and the timing of earthquake nucleation. Perhaps more importantly, these tidal stress changes are also very small (less than +/- 0.3 kPa) compared to the stress changes that occur at the front of a propagating earthquake rupture, which may be several MPa. In other words, earthquake-induced stress changes at the front of a propagating rupture are probably 1000 to 10,000 times greater than tidal ones.
Interestingly, there was a magnitude 7.2 earthquake two days before the Tohoku earthquake, with a rupture plane that was very close to the nucleation point of the Tohoku earthquake. This event also did not correlate with a full moon or any anomalous tidal stresses. The static and dynamic stresses induced by this foreshock on the Tohoku earthquake hypocentral region would have greatly exceeded any sort of tidal effect. A seismic hazard warning on the basis of this foreshock would have had more scientific justification than one based on the approaching full moon (8 days after the Tohoku earthquake actually occurred) or tidal stress perturbations.
Ide et al also suggest that the magnitude frequency distributions of earthquakes from some settings show some sort of correlation with tidal stresses. I cannot evaluate this hypothesis fully, but I can say that this relationship is least convincing in the setting I consider is most analogous to the Canterbury region (California), where the ‘b values’ (binned by tidal shear stress) are essentially within error across the spectrum of lunar-induced tidal stresses.
It is important to recognize that I am not saying that tidal stresses are unimportant things to consider within the variety of processes that may influence earthquake behaviour. But when we consider how such a phenomenon could be practically considered within a coastal hazard perspective, what is the recommendation here? That we stay away from beaches close to subduction zones on full moons? This certainly would not have helped in the Tohoku example.
Many countries like Japan have earthquake early alarm systems, seismic building codes, well-engineered sea walls, and evacuation strategies in place; these are the measures that help to reduce seismic and tsunami risk. There were definitely some shortcomings (e.g., inadequate sea wall heights to deal with a larger than estimated tsunami wave height, coastal land development in high hazard regions, the Fukushima Daiichi nuclear disaster), but certainly we are not so naïve to think that the solution to these sorts of problems lies in the extraction of a tidal signal from seismicity. With fairness, Ide et al do not claim to do this in their study.
Last updated: 03 Nov 2016 4:46pm
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