Last evening's aftershock map. Tsukuba is about 90 km SSW from the epicenter. Since that time (13 hours), there have been 47 aftershocks of magnitude 3 or greater. Follow all the action at http://www.jma.go.jp/en/quake/quake_singendo_index.html
Words are inadequate to describe the ground shaking... not rumbling or vibrating, but shaking, trees swaying like a field of wheat. And the buildings do not fall. The electricity does not flicker. The water pipes are not sundered. Earth day, mirth day... Mother Nature is a tough gal.