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Japan Aftershocks: How Long Will They Go On?

By Story by Jim Andrews, AccuWeather.com Senior Meteorologist.
April 14, 2011, 2:45:08 AM EDT

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During the nearly five weeks since the Great Tohoku Earthquake in Japan, many hundreds of aftershocks, some major quakes in their own right, have rocked the Earth's crust in and near northeastern Japan.

This begs the question as to how long residents can expect to feel quakes related to the big, tragic magnitude 9.0 shock of March 11, 2011.

The answer, according to the U.S. Geological Survey (USGS), is that it could take a year or more for the aftershocks to fade away.


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Map showing epicenters of Tohoku Earthquake, many aftershocks (Public domain image: http://www2.demis.nl/quakes/).


The USGS define aftershocks as "earthquakes that follow the largest shock ("mainshock") in an earthquake sequence." These are smaller than the main shock and take place within a fixed distance of the epicenter. This distance would be determined by the length of the rupture triggering the main earthquake.

According to the USGS, the length of the massive "rupture plane", the tear in the Earth's crust during the Great Tohoku Earthquake, was 400 km (about 250 miles) in length and 150 km (about 95 miles) in width. In other words, the fault rupture tore the crust over an area exceeding 23,000 square miles, or nearly the size of West Virginia.

Importantly, the USGS say that aftershocks can continue, not for only weeks or years, but for years following the primary earthquake. Moreover, the most powerful main shocks tend to yield the longest-lasting sequences of aftershocks, marked by more numerous, quakes.

Incidentally, there are also earthquakes known as "foreshocks." "Foreshocks precede larger earthquakes in the same location," say the USGS. On March 9, 2011, a powerful magnitude 7.2 foreshock struck beneath the seafloor within 26 miles of the March 11 mainshock. Only after the mainshock hit was this quake shown to have been a foreshock, or precursor to an even greater event.

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