On March 11, 2011, at 2:46 pm, a megathrust earthquake measuring 9.0 Mw struck 81 miles off the east coast of Japan. Among the hardest hit areas of Japan was Sendai, the capital city of Miyagi Prefecture and the largest city in the Tohoku Region, which was also struck by a 33-foot tsunami. At the time of writing, the number of dead and missing is estimated at over 20,000, and is still rising.
A megathrust earthquake of this size permanently alters the location of countries, the Earth’s rotation, and even the tilt of the Earth. After the 1995 Kobe earthquake, Japan installed the best geological monitoring system in the world, so many of the changes to the Earth resulting from the Sendai earthquake are known with great precision.
The rift
Based on preliminary data inversion, the undersea rift caused by the earthquake is believed to be 180 miles long and 90 miles long, although the Japanese Meteorological Society suspects that the fault zone may have been ruptured all the way from Iwate to Ibarki, a distance of over 300 miles. At the same time, the seafloor also rose by at least 3 metres.
On the land, 250 miles of coastline dropped vertically by 2 feet. Some parts of Ishinomaki dropped by as much as 4 feet. This drop enabled the tsunami to come as much as 6 miles inland in some places. The last time a tsunami came so far inland at this location was during the Sanriku earthquake in 869 AD.
Because of the combined coastal drop and seafloor rise, residential areas in port cities such as Onagawa, Kesennuma, and Ishinomaki now flood regularly at high tide. The flooding will be even worse after the summer rains.
Plate movements
The entire country of Japan moved east. Much of northeast Japan, which lies on the Okhotsk Plate, moved 8 feet east in a split second. The areas closest to the epicenter moved east by as much as 13 feet. Parts of Japan are now wider than they were before.
At the same time, the Pacific Ocean became smaller. The Pacific plate moved westwards, although these displacements are not known with the same precision as those in Japan. Estimates of Pacific plate movement range from 66 feet to as much as 130 feet at points nearest the rift. This is among the largest fault movement ever known to have been caused by an earthquake.
The seismic waves from this earthquake were felt around the world. Even in Antarctica, the seismic waves are believed to have caused the Whillans Ice Stream to have slipped by 1.6 feet in just a few minutes. The glacier had previously been thought to move only during tides.
Earth’s rotation and axis
The Sendai earthquake shifted the Earth’s figure axis. NASA estimates the shift at 6.5 inches, while Italy’s National Institute of Geophysics and Volcanology places it at 9.8 inches. Earth’s day has also been shortened by between 1.6 and 1.8 microseconds. These shifts were caused by the redistribution of mass on the Earth’s surface.
The figure axis is not the same as the north-south axis of rotation, which can only be influenced by exterior gravitational forces such as the sun and moon. Instead, it is the axis about which the Earth’s mass is balanced. Changes in the figure axis affect the wobble of Earth’s north-south axis, but do not influence the tilt of the north-south axis.
Final word
The Sendai earthquake is estimated to be the 5th largest earthquake measured anywhere in the world since modern recordkeeping began in the 1900s. However, it was not the big one which is due to strike Tokyo, which will be caused by a different segmented fault line bordering the Amurian Plate. A Tokai Region earthquake usually occurs every 100-150 years. The last Tokai earthquake occurred in 1854.
