Montana is one of the most seismically active States in the Union. Since 1925, the State has experienced five shocks that reached intensity VIII or greater (Modified Mercalli Scale). During the same interval hundreds of less severe tremors were felt within the State. Montana's earthquake activity is concentrated mostly in the mountainous western third of the State which lies within a seismic zone that also includes southeastern Idaho, western Wyoming, and central Utah.
Although earthquakes are common in Montana, the early history of felt shocks is incomplete. Only four felt earthquakes that occurred before 1900 are on record. The first was a shock on May 22, 1869, that reached intensity VI at Helena. In 1872 Helena was shaken again, this time by two earthquakes, one on December 10 and the other on December 11, both intensity VI. The fourth pre-1900 earthquake was an intensity VI shock that struck Dillon November 4, 1897.
The first significant 20th century Montana earthquake occurred on June 27, 1925, when a magnitude 6 3/4 shock caused violent shaking over a 1,600 square kilometer area in southwestern Montana. The earthquake was felt over a 803,000 square kilometer area extending from the North Dakota line to Washington and from the Canadian border to central Wyoming. Since the population of the region is sparse, damage was relatively light for such a large earthquake. The towns of Manhatten, Logan, Three Forks, and Lombard sustained the greatest damage. School buildings in these towns suffered most because of the unreinforced brick construction. Reinforced concrete, well designed masonry, and framed buildings for the most part escaped damage. Two light foreshocks and a great many aftershocks were associated with the main shock. The aftershocks continued for several months, the strongest being an intensity V shock on July 10, 1925. Since 1925, earthquakes of intensity V to VI have occurred in the region every few years.
A series of severe earthquakes in the Helena area during October and November 1935 caused four deaths, several injuries, and property damage exceed $4 million. The first shock of the series came on the evening of October 3 in the form of a hard vertical jolt (intensity V). On October 12, a stronger shock occurred that caused some damage in Helena and Fort Harrison (intensity VII) and had a total felt area of 181,000 square kilometers. Smaller shocks continued until October 18 when a magnitude 6 1/4 earthquake occurred. This tremor, the strongest of the series, caused damage in varying degrees to about 300 buildings and was widely felt in Wyoming, Idaho, eastern Washington, and adjacent parts of Canada, an area of about 596,000 square kilometers. The newly completed Helena High School suffered the greatest damage of any single structure. Damage to the State capitol, Federal building, and the St. Helena Cathedral was slight. Telephone, telegraph, and electrical services were stopped for about 1 hour. East of Helena ground cracks up to 45 meters long and 1 meter deep were formed. In spite of the great damage from this shock, there were only two fatalities and few injuries. Another shock on October 27 caused additional damage to weakened structures (intensity VI). On October 31, a magnitude 6 earthquake cause more damage and two fatalities. Many buildings, weakened by the previous shocks, were demolished, including the new high school. Damage was most severe in the business section. The October 31 earthquake was felt in the same States as the October 18 shock, but the total felt area was somewhat less (approximately 363,000 square kilometers). Following the October 31 earthquake, aftershocks continued to occur. A fairly strong tremor occurred on November 21 and another on November 28, both intensity VI. The first was felt over 34,000 square kilometers and the other over 233,000 square kilometers. The latter tremor caused additional damage to previously weakened buildings. Helena again suffered minor damage from an aftershock on February 13, 1936. A total of 1,347 shocks from this series were listed up to the end of 1935.
Most strong earthquakes in Montana have occurred in the western third of the State. The only significant shock outside this area was an intensity VI earthquake on June 24, 1943, in southern Sheridan County, in the northeastern corner of the State. A well-constructed granary at Froid cracked so severely that wheat spilled out. Plaster cracks and minor chimney damage were reported at Homestead, Redstone, and Reserve.
The southwestern portion of Montana was struck by a magnitude 6 1/4 earthquake on November 23, 1947. Maximum intensity reached VIII and brick, masonry, and concrete structures suffered considerable damage. Alder, Cameron, Ennis, Laurin, and Virginia City, in central Madison County, received the most damage. The total felt area was about 388,000 square kilometers of western Montana, northwestern Wyoming, Idaho, and eastern Washington, an area comparable to that of the October 31, 1935, Helena aftershock.
The Flathead Lake - Swan Lake area in the northwestern portion of the State experienced a damaging earthquake on March 31, 1952. This shock was felt over an area of 91,000 square kilometers and caused minor damage along the eastern shore of Flathead Lake (intensity VII).
The largest earthquake in Montana's history was the magnitude 7.3 Hebgen Lake earthquake of August 17, 1959. At 11:37 p.m., Mountain Standard Time, the earth beneath Hebgen Lake suddenly warped and rotated, generating a seiche that continued for about 11 1/2 hours. The first few waves were over 1 meter in height, large enough to flow over Hebgen Dam, a concrete core earthfill structure that was completed in 1914. Although the dam's concrete corewall cracked in 16 places, only a minor amount of seepage occurred. The surface of the lake, which contained 324,000 acre-feet at the time of the earthquake, dropped more than 3 meters because of the violent geologic changes.
The main tremor triggered a major landslide in the Madison River Canyon, about 9 kilometers downstream from Hebgen Dam. An estimated 80 million tons of rock jarred loose by the earthquake slid down the south wall of the canyon. The slide's volume was estimated at 37 to 43 million cubic yards. Nearly 2 kilometers of the river and highway (Montana 287) were buried to depths as great as 120 meters. At least 26 people in the Rock Creek Campground were buried by the slide. Two other campers were killed by a rolling boulder at Cliff Lake, west of Madison Valley. The slide formed a natural dam in Madison Canyon which blocked the flow of the Madison River and created a new lake which within a few weeks was about 60 meters deep, and extended almost to Hebgen Dam. It has been appropriately named "Earthquake Lake."
Many summer homes in the Hebgen Lake area were badly damaged and there was considerable cracking and shifting of roadways. State Highway 287 broke away and slid into Hebgen Lake at four different places. Damage to roads and timber was estimated at over $11 million.
On the night of the earthquake about 18,000 people were vacationing in nearby Yellowstone National Park. Although buildings in the park were jarred by the tremor and huge boulders smashed down onto roads, no one was killed or badly injured. The earthquake disrupted the well-known thermal features in the park. Old Faithful's eruptions slowed slightly from an average 61-minute cycle to 65 minutes. Other geysers changed eruption times, new ones began to erupt, and many bubbling springs burst into violent activity.
A maximum intensity of X was assigned to the Hebgen Lake earthquake. Major new fault scarps were formed along existing normal faults northeast of Hebgen Lake. A maximum vertical displacement of 7 meters was observed near Red Canyon Creek. Minor to moderate damage occurred throughout southern Montana, northeastern Idaho, and northwestern Wyoming. The felt area extended as far as Seattle, Washington, to the west, Banff, Canada, to the north, Dickinson, North Dakota, to the east, and Provo, Utah, to the south, covering eight States and British Columbia, Alberta, and Saskatchenwan, Canada, a total of about 1,500,000 square kilometers.
The Forest Service of the U.S. Department of Agriculture later established the Madison River Canyon earthquake area to preserve the earthquake features and provide for public use and safety. A visitor center which includes a visible-recording seismograph is maintained by the Forest Service. Also, there is a memorial marker to those whose lives were lost during the earthquake. Although the scene of large-scale destruction and tragedy, the locality is of great scientific and general interest because it provides a dramatic example of mountain-building and earth-shaping processes.
A magnitude 4.7 earthquake in the Flathead Lake area on April 1, 1969, cause damage and reached intensity VII at Big Arm, Dayton, and Proctor. Some damage was also noted in the Lake Mary Ronan area and a water well near Polson went dry. Several wells in the Proctor area increased their flow or became muddy. The shock was felt over 26,000 square kilometers and was followed by at least 21 felt aftershocks from April 1 to April 24. The strongest aftershock, on April 5, reached intensity V at Lake Mary Ronan. Over 325 minor aftershocks were reported felt from May 1969 through December 1971, mainly in the Big Arm - Polson area.
Abridged from Earthquake Information Bulletin, Volume 6, Number 4, July - August, 1974, by Carl A. von Hake.
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