Earthquakes are caused by plate movement

Geologists know that the surface of the Earth is made up of a dozen or so large fragments called "plates." Most of these plates are more than a thousand miles across and are more than 40 miles thick. The Earth has had moving plates for at least several billion years, and these will continue to shift in the future. The plates move steadily, but slowly, past each other at rates currently up to 4 inches per year. Most earthquakes occur at the boundaries where plates are sliding past each other, but some are internal to plates. If it were possible to stop the movement of the plates, there would be no earthquakes, but because the plates will continue moving as they have for billions of years, there undoubtedly will be future earthquakes. Southern and southeastern Alaska lie at the boundary between two plates, the North American plate on the north and east and the Pacific plate on the south and west.

These immense plates move at a steady rate, but at their edges the sliding motion is neither smooth nor constant. The motion of the plates strains or deforms the rocks at their boundaries until the rocks can no longer withstand the strain. Then, a sudden slip along a fault releases energy that causes earthquake shaking at the Earth's surface. When the plates are not slipping by each other, but are "locked" together, no earthquakes occur. Eventually, enough strain will build up, the locked section will break, the two plates will slide past each other, and we experience an earthquake.

The whole process is much like pulling a concrete block with a bungee cord. At first you pull on the bungee cord and it stretches out and the block does not move. Eventually, the pull on the cord is enough to get the block moving, it slides forward with a jerk, then stops. If you keep pulling, the cycle repeats itself, just like the earthquake cycle.

Sudden slip during earthquakes occurs on different parts, or segments, of faults at different times. The places on major faults with the highest earthquake hazard are where there has not been recent large earthquakes; these are called "seismic gaps." The Shumagin seismic gap and the Yakutaga seismic gap are two places in Alaska that are considered to have a very high probability of a major earthquake in the next few decades. Eventually, there will be a very large earthquake in each of these two areas. Scientists are studying these areas in order to better understand what happens before a large earthquake.

There are three ways that plates can move relative to each other

2. The plates slide by each other. This is the geologic setting offshore of southeastern Alaska, where the North American plate and the Pacific plate slide past each other on the Fairweather­Queen Charlotte fault. The same type of movement on the San Andreas fault in California is what causes many damaging earthquakes there.

3. The plates move away from each other. This situation occurs mostly in the deep oceans, and does not typically generate large earthquakes.