 Okay, in the second part of this, we want to look at some of the mechanical and physical characteristics of the subduction zone, in particular aspects of it related to where earthquakes are occurring and the details of the three-dimensional structure. So if we think about our simple model of what a subduction zone looks like, we have an oceanic plate that's moving underneath the upper plate, oftentimes the continent. So this would be a perfect picture for looking at, for example, Japan before, during and after the recent earthquakes there. So between earthquakes, when the plates are locked together, there will be a section of the plate boundary interface that is fully coupled or locked. And so when the oceanic plate moves, in this case, to the left, it will drag with it the upper plate or Japan to the left. So let's call this Japan and call this the Pacific. If we are sitting on the top, if we're a person sitting on the top here of Japan, because of this coupling and because the oceanic plate's moving to the left, Japan itself will be getting squeezed. And so I would be moving somewhat to the left here. And if I'm standing out closer to the trench, I might be moving faster. And so if we graph that in this upper panel here, we would see that as we get closer to the trench, we are moving to the left faster than when we are further to the left. And so this is what we observe, and we'll see some data in a few minutes that show the behavior of the upper plate, what we call the upper plate, the part that we typically are on, in between earthquakes. But now during an earthquake, this region here that is locked and coupled between earthquakes will be uncoupled, will be broken. And this whole region here will be allowed to rebound, what we call the elastic rebound theory, and will be allowed to rebound back. So if we look at that in the cartoon, this area now is unlocked. This whole upper plate which was being squeezed is now going to rebound to the right. And if we're standing on the top of the land here, we're going to also move to the right. In fact, in Japan during the Tohoku earthquake, the coastline moved about five meters to the east. Unraveling all of that built up shortening that had occurred over hundreds of years before. Now it ends up that this rebound is not done instantaneously. It can take quite a period of time, but the plate interface will relock again fairly soon. So in the first 10 or 20 years after a big earthquake like this, we get a very complicated pattern of motion in that the area this is now locked. The area offshore here is actually going to begin moving this way again. But the part on land is going to still be moving to the right. And so we end up with this complicated pattern of relative plate motion. And this, as I said, occurs for about 10 to 20 years. Now the overall cycle from one earthquake to the next may be something more like 200 to 1,000 years for the earthquake cycle. And so that's going from earthquake to earthquake will occur about every 200 to 1,000 years in such a system. When we apply this to Cascadia, the area of the Pacific Northwest, it has been about 300 years since the last big earthquake there. And so we are thinking that we are now in the appropriate time zone. Now if we go and look at the data from Japan, we see exactly that pattern we just described. Before the earthquake, the pre-earthquake period, we see all these arrows moving to the left. Remember we have the plate boundary here. This section in here is the coupled section. And so all these things are moving to the left because the Japan, the Pacific plate off of Japan is moving to the left. So this is the pattern we expect to see between earthquakes. Then we see a similar pattern today in Oregon, Washington, and southern British Columbia. During the earthquake, what we call the co-sizemic period, this region through here was what ruptured and that allowed this whole area to rebound. And as I said before, this moved as much as five meters. We are now in what's term the post-sizemic period. This is that 10 to 20 year period after the earthquake. And we see that the stuff on land is still moving to the right. Please notice that the scales of each of these figures is slightly different. But some new data that's come out offshore here shows that this part is moving to the west. And so we have that pattern of it's relocked. This stuff's moving to the west, but the on land is moving to the right. So that's the standard earthquake cycle behavior. We see it very well manifest in Japan. And we see this part of the story in Oregon and Washington, but with a twist. And there's a new discovery that has caused a lot of rethinking about some of this. And this is a process that we call ETS, which is episodic tremor and slip. Now, what we would expect is that as moving to the east, because of the coupled section along the coast, the motion, if we're standing on land, and this is the data from a station right here in Southern Vancouver Island, this should be moving systematically to the east. And that's what it's doing. Moving to the east has a pattern that looks like that. But what we see is every so often, it stops moving to the east and actually starts moving to the west. And that's what it's doing. It actually starts moving to the west. Then it goes to the east again for a while, then rapidly moves to the west. But these periods of time in here, when it's moving to the west, are not instantaneous like an earthquake, but last say four to eight weeks. So this is what we call slow slip. It's occurring on a time scale that is not as fast as a normal earthquake. But otherwise it has many of the characteristics of the earthquake. So this was observed starting in the late 1990s and by about 2005 was pretty well confirmed. Now the question is what's causing it? And we don't fully understand that at all. But it is a feature that we see. And if we look at it in, for example, one of these events, here's one of these events in 2012. It started right in this area at the southern part of Vancouver Island. And then it moved both northward and southward, the color codes correspond to time, before finally completing itself down in southern Washington, south of Seattle. And so this occurred over a period of about six weeks. But other than that, it has all the characteristics of a regular earthquake just happening slow. This so-called episodic tremor and slip occurs throughout Cascadia. And we can see that it actually occurs in three distinct bands, a northern section, a northern section, a central and a southern. This is going to be part of our field trip. And each of these sections behaves apparently independently of each other, but they all show this episodic tremor and slip. And so that's one of the things. And exactly where this is occurring relative to the section that can have big earthquakes, we expect in this area that the region that will have the big earthquakes isn't that will host the big earthquakes is actually this part of the plate interface. And so the episodic tremor and slip seems to be to the east of, or what that means is down dip or down along the plate interface from the main locked zone. So it becomes very important for us to understand what the geometry of the plate boundary looks like because these earthquakes here will occur in the area that's approximately shallower than about 50 kilometers in depth. And the location of the episodic tremor and slip seems to be below that. So knowing the shape of the plate boundary will help us do that. Our primary tool for determining plate boundary geometry is earthquakes. Here's a picture from the Tonga area. The red are the shallow earthquakes. The blue are the deep earthquakes. And if we look at this in some detail we can see characteristics of the shape. So we'll look at two profiles, one across the northern section, one across the central. Those are shown here. The one across the central looks pretty standard. It comes in, it bends over and goes down. That's a typical shape and we'll look at that in more detail. The northern one is more complicated, has this little bend in it. And exactly why that's happening is a point of some debate. And so if we look at the details of that we can see this complicated shape that is thought to be because the trench is actually moving oceanward. So what we're wanting to do is to determine the actual shape of the Cascadia margin. That's gonna be your activity for this week by comparing it to other plate boundaries. We want to know the shape of this because remember the locked part, the part that produces the big earthquakes will be that part of it which is about less than 50 kilometers depth or so. And then we have the slow slip that's going on down-dip of that. So in the activity we do this week we will use plate boundaries and other subduction zones to get some insight. Because remember, a problem we have in Cascadia as you know from last week's activity we have very few earthquakes along the plate interface to help us.