On March 27th 1964 Prince William Sound, Ak, experienced the largest recorded earthquake in recorded United States history, the second largest ever recorded in the world. The cause of the earthquake was compression due to the Pacific plate subducting under the North American plate. The compression on the North American plate was relieved by a quick release of stress the Latouche Island moved 18 meters (about 60 ft.) to the southeast.
Because of this earthquake, the USGS set up a system to monitor earthquakes in the United States to help us understand and better prepare for earthquake hazards.
For more information on the Great Alaska Earthquake, visit the Alaska Earthquake Information Center.
Check out the video from the USGS on the Great Alaska Earthquake!