The 1959 Hebgen Lake earthquake (also known as the 1959 Yellowstone earthquake) occurred in the western United States on August 17 at 11:37 pm (MST) in southwestern Montana. The earthquake measured 7.2 on the moment magnitude scale, caused a huge landslide, resulted in over 28 fatalities and left $11 million (equivalent to $ million in ) in damage. The slide blocked the flow of the Madison River, resulting in the creation of Quake Lake. Significant effects of the earthquake were also felt in nearby Idaho and Wyoming, and lesser effects as far away as Puerto Rico and Hawaii. The 1959 quake was the strongest and deadliest earthquake to hit Montana, the second being the 1935–36 Helena earthquakes that left four people dead. It also caused the worst landslides in the northwestern United States since 1927. Earthquake The earthquake occurred at 11:37 p.m. (MST) with a magnitude of 7.2 . The U.S. Weather Bureau reported that the quake lasted 30–40 seconds. During the quake, the surrounding landscape dropped as much as and shockwaves caused numerous seiches to surge across Hebgen Lake for 12 hours. Water pushed by the seiches poured over Hebgen Dam which did not collapse. Several aftershocks ranging from 5.8 to 6.3 were reported after the quake. The earthquake struck Madison Canyon, an area to the west of Yellowstone National Park. Several nearby campgrounds were occupied by vacationing campers and tourists at the time. Although magnitude estimates for the 1959 earthquake vary (the United States Geological Survey recorded the quake at both 7.3 and 7.5, now calculated by the ISC as 7.2 ) the 1959 earthquake is comparable to the 1906 San Francisco earthquake as one of the strongest earthquakes in North America, behind the 1964 magnitude 9.2 Good Friday earthquake in Alaska and the 1811–1812 New Madrid earthquakes in Missouri. The 1959 earthquake is also the most severe earthquake in the Rocky Mountain area of the United States along with the 6.9 magnitude earthquake which struck Idaho in 1983. The landslide caused by this quake was the largest since an earthquake in Wyoming in 1925 caused a landslide amounting to of rock and debris that left 28 people dead. The death toll from the quake was also the highest since the 1925 earthquake and most recent for the Northwestern United States since an earthquake in 1927 that left seven people dead. The 1959 earthquake was also the most