Scientists believe that Planet Earth has tectonic plates which move. This image, from the U.S. Geological Survey (USGS) depicts the location of those plates. PD
Because Earth’s tectonic plates rest on the mantle’s molten layer, those plates also move, although scientists do not exactly understand that process. Sometimes, when tectonic plates move, earthquakes occur or volcanoes erupt.
Scientists describe the deepest part of Earth - its core - as two separate sections. The outer core is a swirling liquid composed mostly of a nickel-iron alloy. Experts think the Earth’s magnetic field is controlled by this part of the core.
Earth’s solid inner core, made mostly of iron, spins independently from the rest of the planet and is about 4,000 miles below its crust. Scientists believe pressures at that level are about 45 million pounds per square inch. That is equivalent to 3 million times the air pressure at sea level.
How the earth’s core impacts the tectonic plates (which "float" on top of it) has a lot to do with natural occurrences like earthquakes. During the years leading up to the San Francisco quake, however, few people really understood how, and why, earthquakes happened. They just understood the tremendous damage that could result from such a catastrophic event.
How frequently do earthquakes actually occur?