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According to Bath's law, a magnitude 9.0 earthquake shouild be followed by an aftershock of about 8.0  For the first week following the mainshock, there was no evidence of such an event.  However, the magnitude 6.8 aftershock that occurred about ahalf hour after the mainshock has now been upgraded to a 7.9 magnitude quake.  This event occurred about 100 km (60 miles) northwest of Tokyo. 

According to at least one witness on the ground in Tokyo, there was substantial ground shaking associated with the event.  The most dramatic illustration of this aftershock is recorded in the GPS (Global Positioning System) ground displacement pattern.  The time history of the ground displacement is vividly illustrated in this video. 

The mainshock is shown by the large arrows (vectors).  After the mainshock, the displacements are reset to zero, so that the ground displacements associated with the aftershock near Tokyo can be seen.  Note that the ground shaking from this event has been estimated at about Mercalli Intensity VI, enough to cause substantial damage.

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