Japan earthquake an aftershock from 2011

Only minor injuries were reported in Japan following the 6.9m earthquake that shook the area near Fukushima and brought back traumatic memories for local residents. The earthquake was an aftershock of one of the world’s strongest earthquakes to ever shake the earth, the 9.0m earthquake that hit Japan in the same region 5 years ago.

“The first earthquake was such a large earthquake that there was an expansive rupture zone,” USGS Geophysicist Julie Dutton said. “So this earthquake still resides in that rupture zone and that’s why they consider it to be an aftershock.”

The Southern California Earthquake Center says that seismologist defines an earthquake as an aftershock using two guidlines. The first being that the earthquake must occur within an aftershock zone. The second being that it must happen within the zone before seismic activity returns to its pre-mainshock level.

“The larger earthquakes, you can see aftershocks occur weeks, months, and even years later because that stress regime is still releasing pressure and readjusting to the original mainshock,” Dutton explained.

It’s for this reason that the Japan Metorological Agency warned that another large quake could be expected within the next few days.

Many remember the 2011 Japan earthquake because of the massive death toll, as well as how it moved the Japanese Coastline 8 feet, shortened the earth day by one millionth of a second, and shifted the Earth’s axis by 6.5 inches.