I'm not an expert of earthquakes, but a blogger has mentioned fault, so I decided to write this blog.  First, I live in Yokohama, but about six months a year, I live in San Dimas, California which is close to famous San Andreas Fault.  I don't know how close it is though.  According to my digital Britannica, the total movement along the fault during the last few million years appears to have been several miles. There are many faults all over Southern California, so we cannot avoid all.

About Plates, please see the attached photo and find two red arrows toward the bottom.

The left arrow is the Philippine Plate.  The right arrow is the Pacific Plate.  On March 11th, the Pacific Plate pushed the Land (Japan) Plate and caused the 9.0 earthquake.  "X" was the epicenter. 

On the photo, above left shows a cross section of the plates, and downward movement of the Pacific Plate (see red arrows), pushing the Land Plate upward.  The red dot shows the point the earthquake occurred.

Yearly, according to Asahi, in the western Japan, the Philippine Plate moves 4 to 5 cm, and in the eastern Japan, the Pacific Plate moves 8 to 9 cm toward the bottom of the Land Plate.  So, all these movements have been piling up over the years, and sometimes, this stress causes earthquakes.

After 9.0 erupted on March 11th, so far, five aftershocks of above 7.0 already occurred.  Experts have said this trend will continue in a diminishing way for six months to a year, but it is still possible to have 8.0.  Last few days, I felt three pretty big jolts.  In Yokohama, they were 3 and 4, but in the northeast, they were 7.1 and probably 6 and high 5.  It's so many, it's hard to keep track.

This morning, I was walking toward a commercial building and saw a worker filling a crack with cement next to a wet concrete corner of the foundation.  I chatted with him for a while.  The building had three or four cracked glass panels after the earthquake, but they were already fixed.

It must have cost the owner a lot of money to fix all that.  Concrete buildings withstood the tsunami, but for smaller earthquakes like magnitude five or so, concrete buildings cost a lot of money to fix cracks.  So far, I see wooden houses and apartments have survived with almost no problem for magnitude 5 or 4.