During the nearly five weeks since the Great Tohoku Earthquake in Japan, manyhundreds of aftershocks, some major quakes in their own right, have rocked theEarth's crust in and near northeastern Japan.This begs the question as to how long residents can expect to feel quakesrelated 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 couldtake a year or more for the aftershocks to fade away.

Map showing epicenters of TohokuEarthquake, 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 shockand take place within a fixed distance of the epicenter. This distance would bedetermined by the length of the rupture triggering the main earthquake.

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

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

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

By Jim Andrews, AccuWeather.com Senior Meteorologist