The iPhone has not proven a big hit in Japan but that has not stopped a number of applications that were popular with the imode range of phones being ported to the iPhone. Here is a free application made for the iPhone that shows the earthquakes for the day. Since we get quite a lot in Japan they have a page for each day as opposed to some countries where you could probably have a page per week.
Here are today’s earthquakes. Of particular note you can see Tokyo-wan (東京湾） in at 21.22 with a M3.8
and here is the detailed map for that Tokyo Bay earthquake. The information says the earthquake was centered in Tokyo Bay 90 km below the surface and registered as Magnitude 3.8, or a level 2 quake on the Japan scale. Not really enough to shake the house in Shibuya.
Here is the search required from the App Store to get the appli. If you search with the word “earthquake” it doesn’t come up immediately. Instead you get SimCity!