[MUSIC] Now if you think about the fact that earthquakes are generated by breaking, how do we apply that concept to what is going on inside the Earth? What breaks inside the Earth? Well, except for the soil and debris that's sitting near the surface of the earth. When you get down beneath the surface, it's solid rock at least down to great depths within the planet and solid rock is hard. So if you imagine a block of rock that's really like a stick and break that rock just like the breaking of the stick, the breaking of the rock is going to cause vibrations that transmit out. We can picture this by doing a simple experiment in a laboratory. Imagine that you have tow pistons that are being able to be squeezed together. Now, imagine that I've put cylinder rock in between. As I start pushing, basically, this rock will start to break internally. A little bit later after these pistons have moved together somewhat, that rock is going to suddenly fracture. It can happen so fast and you might not even see. You can recorded in slow motion and see the fracture grow, but what happens is a fracture develops and propagates to the edge of the sample. When the fracture reaches the surface of the cylinder, the rock above the fracture is no longer connected to the rock below. The instant that this happens, the rock one side of the fracture slips relative to the rock on the other. Now, this sliding surface is called a fault or what geologists mean when they use the word fault is that it is a fracture on which sliding has occurred. So, we've just seen that vibrations are going to be generated by cracking intact rock just like when we take an intact stick and snap it. When we take an intact rock and crack it, it generates vibrations, but we can also generate vibrations when a preexisting fracture or a preexisting fault undergoes a renewed episode of sliding. So, we can say that the fault can remain stable enough until stresses become great enough to cause it to start sliding again. And therefore, we can say that the slip on a preexisting fault can cause an earth quake as well. Now, why does that happen? Well, let's take an example of this book sitting on a table. I can't blow the book across the table. If I [SOUND] blow on it, nothing happens, because the book is heavy. And what that means is that as the weight of the book is pushing down on the surface below, there is friction between the surface of the on the book and the surface of the table. Now, that friction prevents sliding from happening unless I push a certain amount. So I can start pushing and pushing, and pushing, and pushing. Nothing happens until finally, I push enough and then suddenly sliding occurs. Now, what we're seeing here is the phenomena of friction. Friction prevented the book from sliding until I pushed hard enough. Friction will prevent a fault from sliding until the push or the stress becomes great enough. Now, what's happening at a microscopic scale? Turns out that in nature, surfaces are never perfectly smooth. There's always some sort of roughness. And so if we enlarge the surface between the book and the table, what we would actually see that there are these little protrusions where the book is pushing down into the table top. And if we enlarge one of these, we can imagine that one of those little protrusions from the base of the book is pushing into the top of the table. Now this protrusion, technically it's called an asperity, but it's basically a bump. But you can think of it as being like the anchor of a ship, that's keeping the ship from moving along. Well, likewise, in this situation, this little protrusion or what we call an asperity is strong enough that it anchors the book to the table. And so therefore, the book doesn't move unless I push hard enough. And if I push hard enough, then suddenly, a fracture will develop. That asperity will snap off and the book will start to slide. Well, all this breakage generates energy. It generates vibrations and those vibrations go past through the Earth, and if they reach the surface will be felt as an earthquake. So you get the idea that when sliding occurs on a preexisting surface, it generates energy, because you're breaking off all these little piece. And each one of those pieces it breaks, it's like cracking rock and that forms a vibration. The vast majority of the earthquakes on the planet are a consequence of sliding on faults. So, imagine somewhere underground that there's a fracture and the pushes or pulls on it. Meaning, the stresses acting on it become large enough to cause the rock to fail or the asperities to break or whatever that causes sliding to occur and the fault moves. And when that happens, vibrate start to pass outwards from the fault and that's the process of forming an earthquake. [MUSIC]
