howdy everyone and thank you for continuing on with module four in this very brief video we're going to look a little bit more into the details of elastic rebound theory all right so here's this um slide again for you about stress and strain so it's important we review this one more time so strain is a force per unit area that is exerted on rocks or other materials that can be in the form of compression which is squeezing the rocks tension which is stretching of the rocks and shear which is sliding of the rocks past one another now strain is the deformation induced by stress and we have elastic strain which we're now going to get into more with elastic rebound theory but we also have ductile and brittle deformation which we already talked about when we looked at faults in the last video so now we're going to talk more about elastic strain okay so that brings us to the elastic rebound theory which was actually developed after the great 1906 san francisco earthquake during that event the pacific plate lurched northward relative to the north american plate by as much as 32 feet in some places now you probably recognize the small photograph shown here that's showing the movement along a fault that displaced that landowner's fence line now the figure in the middle shows how that actually happened the rocks on either side of the fault are subjected to stresses and the rock will respond by deforming or bending now that deformation is stored and it gets stored as elastic strain energy or potential energy and this strain energy is stored in the rocks primarily the rocks right along plate boundaries as the rock deforms slowly until their strength is eventually exceeded causing rupture and an earthquake to occur now the earthquake rapidly releases the stored strain energy during the movement along the fault and the rock that is not broken or fractured will return to its original shape as shown in this figure again this is most common along plate boundaries but not always here is another figure showing this elastic rebound you can see in a or b the movement of the tectonic plates causes the rocks to bend and store elastic energy over tens to hundreds of years this is similar to bending a stick slowly and watching it bend then at c and e the stick breaks causing it to vibrate just like the earth does when the frictional resistant at a fault is exceeded causing sudden movement along a fault that produces an earthquake now the rocks return to their original shape just like the stick but they now exist in different locations and this happens within seconds to minutes this is the easiest way for me to remember this theory and in canvas you'll also notice a quick one minute video demonstration that shows this elastic rebound because seeing demonstrations helps me learn and remember things a lot easier than just listening to me talk and looking at words on a screen so i highly recommend taking a look at that quick one minute demonstration video all right now moving on to the seismic waves that are produced during these fault ruptures i'll see you in our next video we'll explore that further