Check out this visualization of the 833 quakes that have hit Japan—since March 11.
The Japan Quake Map on this website presents a time-lapse visualisation of the Sendai earthquake and its aftershocks, primarily to help those outside the affected area understand what the people of Japan are experiencing. It plots earthquake data from USGS on a map using the Google Maps API, with the size of the circle denoting the magnitude (the higher the magnitude, the larger the circle) and the colour showing the focal depth (see the legend below the map).
This is well worth a look.
[H/t Marie Ève, b/c]