Recent large Japan quakes are aftershocks of the 2011 Tohoku Earthquake

temblor.net -- May 08
Three major earthquakes have struck offshore Sendai and Fukushima, Japan, in the past three months.

The first two, both magnitude-7.1, occurred on February 13 and March 20. The latest, a magnitude-6.9, occurred on May 1. The three quakes struck within 60 miles (100 kilometers) of each other and were strong enough to rattle major population centers of northern Honshu, Japan’s main island. These were just a few of the thousands of quakes that have struck the region in the last decade — since the Great Tohoku Earthquake.

The extent to which the 2011 magnitude-9.0 Tohoku earthquake changed the seismic landscape throughout northern Honshu and the Japan Trench is truly astonishing. Both coastal and offshore areas show a much higher earthquake rate during the 10 years following the Tohoku event than the 10 years before. This can be seen by comparing the area around each epicenter in the right panel to those same areas in the left panel below. Even today, the rate of earthquakes is well above the pre-2011 level.

When one plots the cumulative number of earthquakes with time (below), an immediate jump and ‘Omori’ decay is seen that is typical of aftershocks. Omori decay is named after its discoverer in 1894, Fusakichi Omori. Immediately after the Tohoku earthquake, which struck 62 miles (100 kilometers) to the east of the recent magnitude-7 shocks, the seismicity rate of magnitude-3.0 and larger quakes jumped by more than a factor of 100. In February 2021, when the first in this latest series occurred, it was still five times higher than before the Tohoku quake. During the 90 years before the Tohoku quake, the rate of magnitude-6.8 and larger shocks within the box above was 0.58 per year; in the decade since the Tohoku shock, it has been 2.04 per year (3.5 times higher), and during the short span since February 13, it has been 13.5 per year (another 6.5 times higher).

- temblor.net