More About the Hazard for Tokyo & Vicinity

My post on August 12 did not tell the whole story about the hazard for Tokyo. It suggests that shaking like what was felt on August 9 occurs every two years. But the real concern is the hazard from earthquakes that are much less frequent, but cause much larger levels of shaking.

One of the big concerns for Tokyo is what will happen when the Tokai seismic gap breaks. A seismic gap is, to put it simply, a part of an active fault that has not had a large earthquake for a long time.

I found the figure below on the web site prepared by Prof. Terry Tullis for an introductory class in Earth sciences at Brown University. This shows the parts of the subduction zone in southern Japan that have ruptured in earthquakes over the past 300 years. The entire zone ruptured in 1707. It ruptured completely again in two earthquakes on two days in 1854. Most of it ruptured again in earthquakes in 1944 and 1946. However, the 1944 earthquake left what looks on this map to be a short segment at its northern end unbroken.


What looks like a short segment on this map, labeled the Tokai Gap, is actually 100 to 150 km long. This is easily a big enough fault to cause an earthquake with magnitude close to 8.

Another web site that describes the Tokai Gap is:
http://cais.gsi.go.jp/Virtual_GSI/Seismology/Tokai_Slow/1t.html