问答题 The earth is divided into three main layers—a hard outer crust, a soft middle layer and a center core. The outer crust is broken into massive, irregular pieces called "plates." These plates move very slowly, driven by energy forces deep within the earth. Earthquakes occur when these moving plates grind and scrape against each other. In California, the Pacific Plate and the North American Plate meet. The Pacific Plate covers most of the Pacific Ocean floor and the California coastline. The North American Plate stretches across the North American continent and parts of the Atlantic Ocean. The primary boundary between them is the San Andreas Fault. It is more than 650 miles long and extends 10 miles deep. Many smaller faults, such as the Hayward Fault, branch from the San Andreas Fault. The Pacific Plate grinds northwestward past the North American Plate at a rate of about two inches per year. Parts of the San Andreas Fault system adapt to this movement by a constant "creep" resulting in frequent, moderate, earth tremors. In other areas, movement is not constant and strain can build up for hundreds of years resulting in strong earthquakes when it is released. Unlike other natural disasters, there is no warning for earthquakes. Future earthquakes are a serious threat to Californians, which is why the Fire Department recommends preparing before an earthquake hits.