Earthquakes usually occur along the edge of Earth's crustal plates, especially where they collide (India-southern Asia), subduct (the western coast of South America) or scrape along one another (Southern California). They also occur in rift zones, where continents are being pulled apart (the East African Rift) and around volcanic hotspots (Hawaii).
In some cases, earthquakes occur in mid-Continental areas where one might not expect an unstable bedrock. Such quakes are usually related to the presence of an old suture line (where land masses had fused) or an old, aborted rift. The New Madrid earthquakes of 1811-1812, which altered the course of the Mississippi River, developed along an old, deeply buried rift.
Some areas of the world, located at the conversion of numerous crustal plates, are especially prone to earthquakes. Two of these tectonic bullseyes are Indonesia and Japan; both of these countries straddle four plates (Australian-Eurasian-Philippine-Pacific for Indonesia, Eurasian-North American-Philippine-Pacific for Japan). As expected, these regions also harbor numerous volcanoes.