1959 yellowstone earthquake from wikipedia date august 17 , 1959 magnitude 7.3-7.5 depth unknown epicenter location ~15 miles north of west yellowstone , montana countries or regions southwestern montana , idaho , wyoming casualties 28 plus , dead the 1959 yellowstone earthquake also known as the hebgen lake earthquake was a powerful earthquake that occurred on august 17 , 1959 at 11:37 pm ( mst ) in southwestern montana .