 Okay, in this week's discussion tutorial, we want to talk about subduction. And this is going to be very important to us when we go out to the West Coast and look at the tectonics and the impact on society, because in particular we're going to be looking at this region here in which we have subduction, that is the movement of the Juan de Fuca plate, this small little plate here, underneath Western North America, producing this sequence of volcanoes, and also the potential for extremely large earthquakes and volcanoes. So what we want to do today is set the scene a bit and provide some background for everyone on what goes on in these subduction zones. Now these are very impressive. I think we all can remember that about ten years ago, there was a very large earthquake in the off of the West Coast of Indonesia, and that really got the idea that we need to understand subduction zones better going. And then more recently in 2011, there was the devastating earthquake in Japan, the Tohoku earthquake. Here is just a picture of the tsunami from that earthquake coming ashore right near the Sendai Airport in the Tohoku region of Japan. Another picture of that tsunami coming across land. The airport is just off to the right of this figure, and you can see it as the tsunami has totally wiped out everything between the airport and the coast for a distance of about a mile. So we want to look at this. Now this has happened, this happened in Japan in 2011, but a similar type of event is expected to potentially happen along the West Coast of North America and the Pacific Northwest, and so understanding what causes these becomes very important. Here is a schematic picture, relatively accurate, of the plate boundary in North America. This region over on the right is North America. Seattle would sit right in here. This is the coast, and we have the Wanda Fuca plate coming in and going underneath North America. Now in the presentation today, I'm going to be predominantly talking about what's happening along this part of the plate boundary and those aspects related to earthquakes and other deformation. Later in the semester, we'll worry about what's going on down here where things such as the source region for the volcanoes that occur at the surface are produced. So for today, we're going to be mostly looking at the area in which the big earthquakes are generated and the impact that may have on things such as tsunamis and also how we may be able to observe the processes that go on in this region. And so we'll come back to this picture numerous times in the semester, but for our purposes, the key thing is that the Wanda Fuca plate is moving to the east and coming in underneath North America and along some fraction of that boundary, the two plates are locked together, building up strain, deforming, and then that will catastrophically be released during the big earthquakes. Now, subduction zones are complicated places. The geology of them is very complicated. This figure shows one model of how they are set up or what's going on in them, and you can see there are a huge number of different terms and names being used to describe them. We will come back to some of those later, but the key things to think about is, essentially, we have volcanoes and a so-called volcanic front. That's just the oceanwardmost part of the volcanoes. We have a trench. This is where the two plates meet for the first time. It oftentimes is a bathymetric deep. Adjacent to the trench, we have an area that they call the accretionary prism that is made up predominantly of sediments, either scraped off the oceanic plate as it comes in or eroded off of the land or the volcanoes that are above the plate. In this case, the oceanic plate is moving to the left. It goes down, and there is a plate boundary between the two areas. We may come back to this figure, but the key points is that we have a trench, we have a volcanic arc, and we have a region along here in which the two plates are in contact and can be building up the stress and strain that is released in major earthquakes. Now, we don't know a lot about this. We can observe things on the surface out here and along this whole surface, but the details of what's actually going on along the plate interface is very difficult to observe. We can't actually see it directly in most places. There are several ongoing experiments to try to say something more about that. In particular, there is a very large one that is going on in Japan where they are actually drilling in several places in this area. This is part of that project that was done in the vicinity of the 2011 earthquake. In that case, we have the Pacific plate moving down underneath Japan. In fact, in this part of Japan, it is really part of North America. The plate boundary between North America and Eurasia actually runs through the middle of Japan, and that's something when you look at the plate boundaries on the globe, you can see. But that's sort of a secondary issue. The main thing is that we have a plate boundary that ruptured during the earthquake, and with a drilling ship, they can go out and actually drill into the sediments right near the trench. We need that to try to understand what happened in the earthquake. This similar type drilling project is going on further south in Japan in an area that is called the Nankai Traw, and it's an area where they're trying to drill several places into faults associated with the plate boundary. The upper figure here is what we call a seismic section. It is basically a picture of what the structures look like in this area. The lower figure here is an interpretation of the data in the upper figure. So the background in both are the same. The difference is in the lower figure, they have colored in the players, who belongs to what, and they have identified the major faults. So we'll focus, principally, on the lower feature, and what we see is that the area, the plate boundary between what is Japan here and what is the Pacific here is very complicated. It's not just a single fault, but in fact there are numerous faults and one main detachment fault or one main boundary that's between the blue and the brown here, which represents the boundary between the Pacific and the Japanese crusts and lithosphere. Any one of these faults can have an earthquake on it, but the ones that we are most concerned on are the ones that are the biggest earthquakes and so those would be things that would either occur on the main boundary between the two plates or on some features that they have begun to call mega-splays and these are major faults that start on the main boundary but then splay off or diverge from the main boundary and maybe come up to the surface along other faults. So the bottom line here is that although we talk about this as a single plate boundary structure, it's made up of many different features, many different faults, any one of which could potentially host a big earthquake and depending on where and when that earthquake occurs, they may also produce other features such as a tsunami. So let's think a little bit about what happens because we have two plates moving past each other that are locked in a particular area.