"Are
any earthquakes caused by somethingother than tectonic forces?"
The sudden slip that is an earthquake results from a gradual build-up of stress inside the Earth. Mechanical stress -- not the kind of stress (emotional or mental stress) we often hear about and encounter in the hassles of day-to-day life -- is what seismologists are talking about when they use the word "stress". If you've ever bent and snapped a twig between your fingers, or lost part of a corn chip in a dip or salsa, you've seen how a material responds to an excess of mechanical stress. Basically, the rocks that make up the outer layers of the Earth are no different than the examples of wood or vegetable matter given above. They too, if subjected to sufficient force, can be brought to a "breaking point". It is even easier for them to snap if certain places are already weakened, much the way a sheet of paper will tear more readily along a sharp crease. When the stress in a particular location is great enough to overcome the forces holding together the rocks below us, something, effectively, "breaks" or "gives way", and an earthquake begins.
The forces needed to cause this stress and move such large masses of rock are, as you might imagine, immense. It is quite natural to wonder, then, "What causes these forces?"
The answer can be found in the theory of plate tectonics. This theory -- a radical concept when first suggested which has since gained acceptance as an overwhelming amount of supporting data was gathered by researchers in several fields -- states that the Earth's crust is sectioned into great slabs, known as plates. These plates drift very slowly and steadily with relation to each other as they "float" on the more fluid material (the mantle) beneath them. At their edges, they may be colliding, separating, or moving laterally past each other. The nature of these plate-plate boundaries can have a tremendous effect on the geology and types of volcanic and seismic activity found along the edges (margins) of each plate.
"Are
any earthquakes caused by something
other than tectonic
forces?"
To people in the United States, one of the most familiar plate boundaries is the rather infamous San Andreas Fault. This fault represents over a thousand kilometers of the boundary between the Pacific Plate and the North American Plate, and is often portrayed as the single biggest natural threat to the livelihood of those in southern California. It is not the only fault, however, and in some ways and some places, it is less dangerous than many other faults. What are all these faults, and why are they there? To answer that, let's now return to the second of the two follow-up questions: "What exactly is a fault?"
