Also, the Nankai Trough, which will be the focus of the Japanese drilling project in the future, has a very interesting history.
(FIG-7)
I think you probably know that along the Nankai Trough, historical earthquakes have been documented very well. And the most recent earthquakes were in 1944 and 1946 along the west-south-west coast of Japan from Kii Peninsula to Shikoku. Then in 1854 two events happened, almost simultaneously, and in 1707 another big earthquake happened, and then 1605. So if you look at this kind of history, in general once every 120 years or so, there has been a great earthquake along the Nankai Trough here.
One interesting thing is that they are all different. In particular for the 1605 earthquake, the evidence is that there were very large tsunamis but no shaking. So in a way this is a tsunami earthquake. Of course you can have different interpretations, but one interpretation here is even for the same tectonic boundary, the behaviour of earthquakes can be different from sequence to sequence. Sometimes a brittle strong earthquake occurs, but sometimes a slow tsunami earthquake occurs. Again this variation can be controlled by the property along the plate boundary, which can be studied by drilling projects.
(FIG-8)
Well, another very interesting problem is this. This is the slide which I borrowed from Dr. Ando of Kyoto University. This shows the epicentres of large earthquakes in the Nankai Trough since 1900. There are quite a few earthquakes on this boundary here. However, there is a very interesting pattern.
(FIG-9)
The bottom figure shows all these events since 1900 to the present time. The upper figure shows the standard frequency-magnitude relationship. The horizontal axis is the magnitude from 5.5 to 8, and the vertical axis is the cumulative number of earthquakes- how many earthquakes you have. It is well known that there is a very standard relationship - straight-line relationship like this. So, the frequency of earthquakes will increase ten-fold if the magnitude is decreased by one unit, say from 6.5 to 5.5; then the number increases from 10 to 100. So in many parts of the world this relationship goes straight, all the way to a large earthquake. However, in this particular zone, very surprisingly, there is no single earthquake above 7, until 8. Up to here, the behaviour is very normal, but for some unknown reason there is no single earthquake 7 or larger until it gets to 8. So when the event gets bigger than 7, it gets all the way to 8, and somehow it can't stop in the middle. This is a very important aspect of this kind of seismic sequence. What causes this rather unusual behaviour? The Nankai Trough is a very mature seismic boundary. Mature means that the fault boundary is very well developed-there is a distinct boundary between the subducting plate and the continental lithosphere, so in a way there is a distinct zone over a large area.
(FIG-10)
The same kind of situation in a different tectonic environment is found in California. We have the San Andreas fault which is a strike-slip boundary and is a very mature fault; if you look at the relationship - the same relationship for the Nankai area- you have the same situation. This is for the San Andreas fault - and the magnitude goes from 2, 3, 4, 5, 6, 7 and so on, and this is the number of events, and from small earthquakes up to maybe 6 or so there is a straight line, but then it disappears. There is no earthquake along the San Andreas proper which has a magnitude between 6 and 7.5 or so. Yet, historically, like the 1906 San Francisco earthquake, and the 1854 Fort Tejon earthquake, there are magnitude 8 type earthquakes. So the same thing - somehow earthquakes can happen up to 6 or so, and they can't be 7. When it happens beyond that, it gets up to 8. And this is a very important observation. We have been talking about this for years, but I don't think there is any really good explanation.
