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[SSS04-P01] Effects of Earth tides on the mainshock of the 2011 Tohoku Earthquake
Keywords:Earthquake, Earth tides, 2011 Tohoku earthquake
1. Introduction
In the 2011 Tohoku earthquake, a clear correlation between the earth tide and the earthquake occurrence was observed for about 10 years just before the main shock occurred near the rupture initiation zone of the main shock (Tanaka, 2012). So, the author reports effects of the earth tides possibly related to occurrence of the mainshock.
2. Effects of earth tides to foreshock-mainshock type earthquake
The largest foreshock occurred at 11:45 on March 9 and the main shock at 14:46 on March 11, with an interval of 51 hours 1 minute. The moon has the largest influence on the earth tides with an orbital period of 24 hours and 50 minutes, or roughly 25 hours. Therefore, the mainshock occurred just after the moon orbited twice. The equation with approximate number is as follows.
Time of mainshock = Time of foreshock + 2 orbits x 25 hours + approx. 1 hour (1)
The 2016 Kumamoto earthquake was also of the foreshock-mainshock type, with an interval of 27 hours and 59 minutes, i.e., one orbit x 25 hours + approx. 3 hours. With 2 data, we have the following equation.
Time of mainshock = time of largest foreshock + number of orbits x 25 hours ± several hours (2)
It is assumed that for the mainshock, the moon was in the same position as in the foreshock, and the moon caused similar deformation to the topography and triggered the earthquake.
In the case of the 2011 Tohoku earthquake, the time for the moon in the middle was 16:12, so the mainshock occurred 1 hour ad 30miutes before it. In the case of the 2016 Kumamoto earthquake, the time for the moon in the middle was 0:33, so the mainshock occurred about one hour after the moon in the middle. In both cases, the moon was almost directly above the epicenter.
3 Effects of the earth tides on the date of occurrence
3-1. 2011 Tohoku Earthquake (Fig. 1)
The lunar phases of the major earthquakes that occurred in the central off-Sanriku area are explained below. The phase of the moon is shown by the difference in ecliptic longitude between the moon and the sun, are 0 degrees at new moon, 90 degrees at first quarter moon and so on. In this area, there is an apparent dependency on 20 to 80 degrees, and the major damaging earthquakes have occurred during this period. Or, large earthquakes occurred around 3rd to 8th day of the lunar calendar (Sue, 2008).
The 2011 earthquake occurred on the 7th day of the lunar calendar, Or 70 degrees from the ecliptic longitude difference between the moon and the sun, which means that the earthquake occurred at the same lunar phase as before (Sue, 2011).
The 2008 survey was conducted in a rectangular area across the trench axis in the central part of Sanriku, where the southern boarder is 39 degree north. The 2011 M9 earthquake, however, occurred at 38 degrees 6 minutes north, which is roughly 1 degree out of the boarder of the research area. Since the trench axis is running southward almost straight, I assume that the mechanical orientation is generally maintained. I have also conducted research to the southern area of this region, that is called land and offshore of Miyagi Prefecture. And I have confirmed that a cluster of occurrences at a lunar-solar ecliptic longitude difference of 70-95 degrees, and the 1793 earthquake (M8, 7th lunar day) occurred (Sue 2007) .
The case study of the central off-Sanriku area suggests that a area-by-area survey of past earthquakes is effective in estimating the date of occurrence of a new major earthquake that will occur in the same area later.
3-2. The 869 Jogan Earthquake
The 869 Jogan Earthquake is regarded as the similar earthquake of the 2011M9 earthquake. While this earthquake occurred on 26th of the lunar calendar, so the research shall be done carefully.
References
Sue, Y., 2007, Proc. of the Seismological Society of Japan, P2-056.
Sue, Y., 2008, JpGU, Preprints, S145-P004.
Sue, Y., 2011, JpGU, Preprints, MIS036-P87.
Tanaka, S., 2012, Geophys. Res. Lett., 39(7).
In the 2011 Tohoku earthquake, a clear correlation between the earth tide and the earthquake occurrence was observed for about 10 years just before the main shock occurred near the rupture initiation zone of the main shock (Tanaka, 2012). So, the author reports effects of the earth tides possibly related to occurrence of the mainshock.
2. Effects of earth tides to foreshock-mainshock type earthquake
The largest foreshock occurred at 11:45 on March 9 and the main shock at 14:46 on March 11, with an interval of 51 hours 1 minute. The moon has the largest influence on the earth tides with an orbital period of 24 hours and 50 minutes, or roughly 25 hours. Therefore, the mainshock occurred just after the moon orbited twice. The equation with approximate number is as follows.
Time of mainshock = Time of foreshock + 2 orbits x 25 hours + approx. 1 hour (1)
The 2016 Kumamoto earthquake was also of the foreshock-mainshock type, with an interval of 27 hours and 59 minutes, i.e., one orbit x 25 hours + approx. 3 hours. With 2 data, we have the following equation.
Time of mainshock = time of largest foreshock + number of orbits x 25 hours ± several hours (2)
It is assumed that for the mainshock, the moon was in the same position as in the foreshock, and the moon caused similar deformation to the topography and triggered the earthquake.
In the case of the 2011 Tohoku earthquake, the time for the moon in the middle was 16:12, so the mainshock occurred 1 hour ad 30miutes before it. In the case of the 2016 Kumamoto earthquake, the time for the moon in the middle was 0:33, so the mainshock occurred about one hour after the moon in the middle. In both cases, the moon was almost directly above the epicenter.
3 Effects of the earth tides on the date of occurrence
3-1. 2011 Tohoku Earthquake (Fig. 1)
The lunar phases of the major earthquakes that occurred in the central off-Sanriku area are explained below. The phase of the moon is shown by the difference in ecliptic longitude between the moon and the sun, are 0 degrees at new moon, 90 degrees at first quarter moon and so on. In this area, there is an apparent dependency on 20 to 80 degrees, and the major damaging earthquakes have occurred during this period. Or, large earthquakes occurred around 3rd to 8th day of the lunar calendar (Sue, 2008).
The 2011 earthquake occurred on the 7th day of the lunar calendar, Or 70 degrees from the ecliptic longitude difference between the moon and the sun, which means that the earthquake occurred at the same lunar phase as before (Sue, 2011).
The 2008 survey was conducted in a rectangular area across the trench axis in the central part of Sanriku, where the southern boarder is 39 degree north. The 2011 M9 earthquake, however, occurred at 38 degrees 6 minutes north, which is roughly 1 degree out of the boarder of the research area. Since the trench axis is running southward almost straight, I assume that the mechanical orientation is generally maintained. I have also conducted research to the southern area of this region, that is called land and offshore of Miyagi Prefecture. And I have confirmed that a cluster of occurrences at a lunar-solar ecliptic longitude difference of 70-95 degrees, and the 1793 earthquake (M8, 7th lunar day) occurred (Sue 2007) .
The case study of the central off-Sanriku area suggests that a area-by-area survey of past earthquakes is effective in estimating the date of occurrence of a new major earthquake that will occur in the same area later.
3-2. The 869 Jogan Earthquake
The 869 Jogan Earthquake is regarded as the similar earthquake of the 2011M9 earthquake. While this earthquake occurred on 26th of the lunar calendar, so the research shall be done carefully.
References
Sue, Y., 2007, Proc. of the Seismological Society of Japan, P2-056.
Sue, Y., 2008, JpGU, Preprints, S145-P004.
Sue, Y., 2011, JpGU, Preprints, MIS036-P87.
Tanaka, S., 2012, Geophys. Res. Lett., 39(7).