What can we learn? And of course we'd learnt a lot of things. [I'm given 5 more minutes. OK] So, the important thing is what the difference is in the fault zone. The difference is .. [unfortunately I lost the figure] Let's see, basically difference is this [yeah, I found it]. As I said, it's very possible that a fault zone is extremely heterogeneous, so we can't just use a single set of parameters to determine the fault behaviour. If you drill into one site you get a set of parameters for that particular site. If you go to somewhere else, you have a different set.
(FIG-23)
What we see is essentially integration of all of those - fault plane is a very large area that involves different environments - Sanriku may be different; Nankai Trough may be different, and each area also contains very different friction characteristic, water content, permeability, heterogeneity, and everything. So there are a lot of questions, like how much water do we have?; how big are the barriers in between fluid-filled zones ?; what is the characteristic length of the barriers?; what is the characteristic length of the fluid zones?, and so on. So all of these parameters are very important. So in this sense, drilling is very important. But just by single drilling or even multiple drilling, we can't understand everything. It's very important to think about the problem so that we can use those parameters to build a useful physical model. Then we can understand all the differences in the magnitude-frequency relationship, and also the difference between seismic and aseismic slip.
I said that along the Nankai Trough the behaviour seems to be different from sequence to sequence, and this could be really a manifestation of this non-linear process. Basically the geological or tectonic structure around this boundary remains essentially the same. However, because of this non-linear behaviour, if the initial condition is slightly different, sometimes it produces the standard brittle failure, but some other times it can produce slower tsunami earthquakes. So in a way the overall tectonic behaviour can be determined only in the average sense. But if you look at each sequence, it can be different, depending upon how it gets started. And this is a rather important realization - that we are dealing with some very complex system. We have to live with this kind of complexity and this kind of stochastic behaviour which is the very nature of a complex system.
(FIG-24)
One other important thing is this： in seismology recently there is a very important question： For the last 20 or 30 years there have been many high-rise buildings built in many parts of the world - Tokyo, Los Angeles, everywhere. The question is whether these high-rise buildings are really safe for large earthquakes nearby. And that's not really clear. The reason is we have not experienced this kind of large earthquakes since these structures were built, so the next big earthquake will be your first experience, which is a bit worrisome. So, this type of study can have important implications. This is a numerical experiment. Suppose if you have a magnitude 7 earthquake in Los Angeles. The velocity of fault motion is about 1 to 2 meter per second, so the ground motion would look something like this. If you feed this sort of ground motion at the bottom of a high-rise building like this, each floor will move like this. This is called lateral displacement and the maximum is about 3.5 meters near the roof, but near the ground it is about one half meter, and this is a large displacement and obviously if this kind of thing happens, many of the high-rise buildings may have some problems. This is a figure taken from the paper by Hall and others. The question of whether large structures are safe or not is a very important question, but we can't really have the answer for that, because we don't know what the ground motion would be for a magnitude of 7 and 8. That can be determined by the property in the fault zone. You may want to know what kind of ground motion you need to prepare for, if you have a magnitude 8 earthquake along the Nankai Trough. The answer depends on the structure and physics of fault dynamics which we do not know completely yet. Again drilling study can provide key information.
(FIG-25)
So, just to wrap up, this is a picture I sort of copied from the promotional pamphlet of JAMSTEC. Well obviously you want to understand what's happening here in a seismogenic zone, but it's very important to understand not only this particular zone but also aseismic zone in between, and also other places. But more importantly, well equally importantly, we need to have some thinking so that we can make a good use of these results to obtain a good model which we can apply to many other places, not only this particular place.
Thank you very much.
Chairperson： Thank you very much.
