GIANT EARTHQUAKES ARE OCCURRING AT LUNAR PHASES SPECIFIC TO EACH
SUBDUCTION ZONE
*Y. Fujii, J. Kodama and D. Fukuda
Hokkaido University
N13W8
Sapporo, Japan 060-8628
(*Corresponding author: fujii6299@frontier.hokudai.ac.jp)
GIANT EARTHQUAKES ARE OCCURRING AT LUNAR PHASES SPECIFIC TO EACH
SUBDUCTION ZONE
ABSTRACT
As the example for Peru subduction zone (Fig. 1) shows, the most giant earthquakes (MW  8) during 19002013 occur during specific lunar phases for each subduction zone. The delay from spring tides is used
instead of lunar phase for better representation of the concentration of the occurrences. The dangerous
lunar phases for each subduction zone, represented as delays from spring tides, with the number of giant
earthquakes within the lunar phase/the total earthquake number and the probability of the random null
hypothesis are -3.1 to -1.6 days for Chile (4/7, 1.51%), -1 to -0.7 days for Alaska (3/6, 0.39%), -1.9 to 1.5
days for New Guinea (5/7, 2.5%), 1.9 to 3.3 days for Peru (6/7, 0.0041%), 3.5 to 4.6 days for Indonesia
(4/6, 0.35%), 5.3 to 6.7 days for Tonga (4/5, 0.31%), 3.6 to 7.7 days for Japan (5/6, 2.1%), and -1.8 to 2.3
and 8.8 to 10.5 days for Kuril (8/8, 0.36%).
Magnitude
10
9
8
0
5
10
Delay from spring tides (day)
Figure 1 - Delay from spring tides and magnitude of giant earthquakes during 1900-2013 in Peru
subduction zone
The random null hypothesis is statistically rejected for the all subduction zones since the probabilities are
less than 5%. This means that the statement "the giant earthquakes are occurring at lunar phases specific to
each subduction zones" is statistically meaningful. Paying sufficient attention to foreshocks and various
precursors such as GPS (Global Positioning System) anomalies during the dangerous lunar phases for each
subduction zone will be very effective to reduce damages from giant earthquakes.
The strike of a subduction zone affects the amplitude of the theoretical solid tidal stress acting to cause a
slip at a reverse fault in the zone. The amplitude for the near EW subduction zones in which giant
earthquakes are occurring around spring tides is much smaller than that for the near NS subduction zones
in which they are occurring around neap tides. A series of uniaxial compression creep test was carried out
on Inada granite. Slight stress disturbances were repeatedly applied simulating spring tides at NS
subduction zones to clarify whether the larger strain rate during spring tides has prohibited giant
earthquake occurrences at the near NS subduction zones.
As results, under 8.3 MPa stress disturbances which was impossibly large as tidal stress and just for
comparison, specimens failed under stress whose value was larger than the average. Under 0.83 MPa
disturbances, which was slightly larger than tidal stress amplitude considering migration of seawater and
beginning from stress increase, 3/5 specimens failed when the stress value was larger than the average.
This is a random process since the probability of the random null hypothesis is 8.8% and means that the
small stress amplitude did not affect the specimen failure. Under the same stress amplitude but beginning
from stress decrease, all 5 specimens failed under creep control (Fig. 2). Probability of the random null
hypothesis is 3.1% and the hypothesis is rejected. There should statistically be a reason and the low strain
rate would be. This supports the assumption that the larger strain rate during spring tides has prohibited
giant earthquake occurrences at the near NS subduction zones.
Stress (MPa)
170
169.5
169
168.5
2000 3000 4000 5000 6000
Time (s)
Figure 2 - An example of failure under creep control for 0.83 MPa stress disturbance beginning from
decrease
KEYWORDS
Giant earthquakes, Lunar phase, Subduction zone, Delay from spring tide, Statistical testing