In this article, they explain that the earth 'jiggles' in a random walk on a very small scale. They use my favourite thing, that 'Everything is fractal!". I've written quite a bit on this. But really, there is no such thing as a solid block of granite, even though those nuclear waste people imagine there is. All the rock is full of fractures, and they are full of fluids. As the fluids flow, there are little shifts. In a big fault zone, such as Hamilton, there is a concentration of fractures, and an intense fluid flow. That's when we have earthquakes. But 'stable' zones have little silent 'teensy quakes' all the time.