Research press release


Nature Geoscience

Heightened possibility of large earthquakes at high tide



井出哲(いで さとし)たちは、過去20年間に起きた大地震(マグニチュード5.5以上)の2週間前における(満潮の時期や位相だけでなく)潮汐応力の大きさと振幅を再計算した。潮汐応力と小さな地震との間には明らかな相関は見られなかったが、2004年インドネシア・スマトラ地震、2010年チリ・マウレ地震、2011年日本・東北地震を含むいくつかの大地震は、たしかに潮汐応力が大きいときに起きていた。彼らはまた、潮汐応力の振幅が増加するにつれて、小さな地震に比べて大きな地震の割合が増加することも発見した。


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.

doi: 10.1038/ngeo2796


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