The western region of Yellowstone National Park in southern Montana is highly susceptible to natural hazards such as volcanic eruptions and earthquakes. Within the park is the Yellowstone Caldera, a supervolcano, which is a volcano that is capable of an eruption of more than 240 cubic miles of magma (National Park Service, Volcano). Evidence of the Caldera’s activity has been found through hydrothermal tracking of heat below the region, and through the presence of earthquakes. Increased heat on the crust causes the discharge of magma from the Caldera; in turn, this is directly correlated to the continuous seismic activity in West Yellowstone. Around 1,000 to 3,000 earthquakes occur in Yellowstone National Park per year, making Montana the fourth most seismically active state in the United States (Montana is Earthquake Country). The large amount of seismic activity is attributed to the number of faults in the region (Fig. 1). The Hebgen Lake, Splay, Graben, and Colluvial Wedge faults (Benton, 2013) lie within a region in West Yellowstone where a devastating earthquake will occur on August 17th, 1959. The terrain of West Yellowstone itself poses a potential hazard. The terrain is mountainous, with large boulders and trees in the surrounding area. Hebgen Lake, Madison River, and Madison Canyon are all located in the region where the earthquake will occur. This makes the region susceptible to floods and landslides, which are secondary effects to earthquakes (Lecture 3).
Earthquake swarms, or the presence of multiple earthquakes over a short period of time in one localized area, are extremely common in West Yellowstone. The largest earthquake swarm took place in 1985, when over 3,000 earthquakes occurred over the course of only three months (National Park Service, Earthquakes). Yellowstone National Park and its surrounding area has had thousands of recorded earthquakes, with eight identified as “historic”, meaning quakes with magnitudes ranging from 6.0-7.3 (Fig. 3). Four of the recorded historic earthquakes took place in 1959, with the remaining having taken place in 1947, 1964, 1975, and 1976.
On August 17th, 1959, beginning at 11:37 P.M., an earthquake will occur in West Yellowstone. The earthquake will have a magnitude of 7.3 MW, comparable to the magnitude of the 1906 San Francisco Earthquake that killed nearly 3,000 people (Healy, 1999). The epicenter of the earthquake will be just within Madison Canyon, east of Hebgen Lake (Fig. 3). This earthquake will be the result of a sudden slip on one (or more) of the faults in West Yellowstone. When a fault slips, seismic energy is released, causing an earthquake. The specific type of fault slip that will occur in West Yellowstone is known as a dip-slip, a fracture in the earth occurring on normal faults that causes blocks of the earth to shift vertically (Lecture 3). A surface rupture will occur, causing the ground along the north shore of Hebgen Lake to drop about 20 feet and form a fault scarp (Fig. 4) (Farrell, 2019). The shockwaves formed from the earthquake will form standing waves of water in Hebgen Lake, known as seiches. These seiches will continuously form for nearly 12 hours (Healy, 1999). The aftershock waves from the earthquake will range from magnitudes of 5.8 to 6.3 (Healy, 1999). The earthquake will also cause a severe overflow of Hebgen Lake. Water will spill over and form an entirely new lake, 170 feet deep, known as Quake Lake (Holden, 2018). The seismic waves from the earthquake will cause debris flow landslides, one of the fastest moving landslides. The landslides will consist of 50 million cubic yards of rock, mud, and other debris that will slide down the south side of Madison Canyon at 100 miles per hour and will slam into the side of the canyon within one minute (Fig. 2) (Farrell, 2019).
The social effects of the 1959 Hebgen Lake earthquake will be devastating. For context, there is a difference between a natural hazard and a geologic catastrophe. A natural hazard is merely a catastrophic natural event, while a geologic catastrophe is a catastrophic natural event that causes destruction to humans and the man-made environment (Lecture 1). The earthquake will be a large geologic catastrophe because of the amount of people that will be present for the event. It will take place in mid-August, which is peak camping season (Rademacher & Hellweg, 2019). In terms of infrastructure impacts, a portion of Highway 287 will fall into Hebgen Lake, and the majority of roads and highways leading in and out of the area will be closed due to large boulders and excess debris from the landslides (Fig. 4) (Farrell, 2019). Unfortunately, 28 lives will be lost as a result of the earthquake and the subsequent debris flow landslide that will follow (Holden 2018). 19 of the 28 will be buried from the debris flow landslide, while the latter 9 will drown from the run-up of water from Hebgen Lake in the form of the seiches (Rademacher & Hellweg, 2019). Over 300 people will be trapped in Madison Canyon as a result of the movement of the terrain and the debris flow landslide (Healy, 1999). The economic impacts, although incomparable to the social impacts, will be severe. Repairs in infrastructure (roads, highways, bridges, lodging) will cost 11 million U.S. dollars, which is equal to 96.48 million U.S. dollars in 2019 (Benton, 2013).
Accurately forecasting when an earthquake will occur and what its magnitude will be is not really possible. However, it is possible to predict a general timeframe for when an earthquake is expected to occur, based on past occurrences. Seismometers are extremely sensitive devices that can be used to measure any ground vibrations, displacement, velocity, and acceleration of primary and secondary waves (Lecture 3). Because primary waves arrive first, the seismometer can sense the presence of these p-waves, and a governmental agency can notify the public in the West Yellowstone area. To be transparent, because surface waves (the most destructive of seismic waves) follow minutes after p-waves, there cannot be much time left for evacuation. Nonetheless, notifying the public even minutes before the surface waves arrive can allow time for people to take precautionary measures without necessarily evacuating. To avoid the economic impacts of the earthquake, it is recommended to completely avoid building infrastructure in West Yellowstone. However, because West Yellowstone is a popular tourist destination, it is inevitable that infrastructure will be constructed in the area. In this case, it is important to construct buildings with “good building materials” that can easily flex in the event of seismic activity; wood and reinforced steel are good examples (Lecture 3). Materials that do not flex, such as brick or concrete, should not be used (Lecture 3). For people to protect themselves when the earthquake occurs, it is imperative to duck, cover, and hold (Lecture 3). It is also important to avoid being near things that can fall, such as boulders, trees, and other potential debris. To best prevent injuries and fatalities, tourists should remain home on August 17th, 1959.