During December 21, a magnitude 7.4 shook the seafloor 960 km (600 miles) from the coast of Japan. This earthquake brings the total number in that category (mag 7) to 20 during the year 2010.

During the middle of the night the earthquake shook awake residents of Tokyo while tsunami alerts warned of possible 1 meter high waves, fortunately small.

Not  surprisingly, many strong aftershocks have been jolting the region and are expected to continue for some time. After approximately 70 powerful aftershocks, a very strong magnitude 6.3 surprisingly jolted the region and alarmed locals who scrambled to higher ground in fear of tsunami.


The location of the 7.4 earthquake is in very close proximity to the pacific plate, which itself is moving approximately 75 mm per year to the west in that area.

An interesting observation of the numerous aftershocks is the fact that most all of them are located at a depth of 10 km. This is the same observation of the recent earthquake swarm that occurred off the coast of Yemen in the Gulf of Aden.

The following illustration shows the depths of the various earthquakes of each event (Japan and Gulf of Aden). The first earthquake begins on the left side of the graph and subsequent earthquakes continue to the right.


Why are so many earthquake aftershocks all located at depths of 10 km?

Logical thinking indicates that this is where the most stress is located and being released after the initial “big one” which was located at 14.9 km depth.

The cross-sectional area of the location of aftershocks affecting the Japan – Bonin Islands region is approximately 3,500 square km (2,200 square miles). It is as though a huge horizontal sheet of this size is stressed at 10 km below the seafloor.

Or, perhaps there is another explanation, to be determined.

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