Earthquakes occur when stored energy is suddenly released by movement along a fault, a fracture that allows blocks of rock to slide past each other after tectonic forces build stress beyond the friction holding the fault locked. In southwestern Montana, earthquakes are part of the Intermountain Seismic Belt, a zone of frequent activity extending about 800 miles from northwest Montana southward through Yellowstone National Park, the Salt Lake City area, and southwestern Utah, with a branch extending west from Yellowstone toward the Idaho-Oregon border, caused by gradual stretching of the North American tectonic plate as it interacts with the Pacific tectonic plate. On August 17, 1959, a magnitude 7.3 earthquake struck Hebgen Lake about 12 miles northeast of here, rupturing the Hebgen Lake and Red Canyon faults, causing parts of the Hebgen Lake basin to subside as much as 22 feet, tilting the lake so that a large seiche washed back and forth across it, overtopped Hebgen Dam, and swept shoreline cabins off their foundations. The earthquake also triggered the 37 million cubic yard Madison Canyon landslide, which dammed the Madison River to form Earthquake Lake, buried part of a campground, and killed 26 people. Montana is one of the more seismically active states in the United States, averaging five earthquakes a day, though most are too small to detect without a sensitive seismograph, and the Hebgen Lake earthquake was felt across 600,00 square miles from Banff, Canada, to Provo, Utah, and from western North Dakota to Seattle, Washington.