  Just how common are mass earth shakeups like the Montana-Yellowstone Earthquake, anyhow? Geologists tell us they’re frequent, with a dozen or more major quakes, and thousands of minor tremors happening each year. Earthquakes are the natural outcome of the fact that the earth, while seeming substantial and changeless, is constantly, if most gradually, in the process of change. Mountains are thrust up. Glaciers carve them down. Volcanoes pour out their molten rock. Rivers and floods scour their erosive paths. Sediments slide and settle. The enormous masses which great internal earth forces have raised up to mountain height, create counter stresses. These forces build up for years, sometimes for centuries or longer. Eventually something has to give. When this happens on a grand, or spectacular scale, we call it an earthquake. Stress, caused by the quake, resulting from slump of surficial material caused the attractive curving of this fence along Highway 287 east of Hebgen Dam.(U. S. Geological Survey) Whether you’re a connoisseur, expert, or not, the spectacular 1959 Montana-Yellowstone Earthquake Damkeeper George Hungerford points out how much the tidal waves, or seiches, overtopped Hebgen Dam at the time of the quake. Hungerford is standing on earthfill washed down from dam-top level by the gigantic waves. The quake also cracked the dam’s concrete core as shown.(Montana Power Co.) It ranked right along with San Francisco’s 1906 shakedown as among the severest earthquakes on the North American continent. In seismic measurements, it rated 7.8 on the Richter Scale, as compared with San Francisco’s 8.2. It set up so called tidal waves, or seiches, on Hebgen Lake. There were at least three of these huge waves—20-ft. high—which overtopped the entire 721-ft. length of the dam by four feet. Eyewitness statements relate that the velocity of the tidal wave was so great that it caused the water literally to leap over the top of the dam. It filled the small generating plant with a 2 to 4 ft. deep layer of rocks. Although the dam stood, the quake caused several fractures in the core wall, one of which showed a 3- to 4-inch separation, and shattered the dam’s concrete spillway. The earthquake created three major faults, with displacement on the Red Canyon Fault running as much as 20 ft., which stacks up impressively alongside the 26-ft. maximum displacement resulting from San Francisco’s quake. (These two earthquakes differed, however, in that the Montana displacement was vertical, while San Francisco’s was horizontal.) BEFORE AFTER According to the Society of Military Engineers, surveys from benchmarks outside the earthquake-affected areas show that the earth in the Hebgen Dam Quake area near Hebgen Dam has settled between eighteen and nineteen feet from its level before the quake. It wasn’t uniform, though. The quake caused tilting, which showed up in the way the north side of Hebgen Lake had sunk eight feet, while the south side of the lake, docks, boats, etc., were sticking eight feet out of the water. The quake also caused many sink, or more properly, blow holes. These phenomena are also known as sandspouts. Water, compressed and forced up and out by quake action washes out layers of sand sub strata. The overhead surface areas naturally drop into the hole, leaving a puzzling hunk of slumped ground—separate from the normal scarps—as big as 15 × 50 ft. in area. The Montana-Yellowstone quake sent seismographs jiggling as far away as New Zealand. It caused fluctuation of water level in wells as much as ten feet in nearby Idaho, a tenth of a foot in Hawaii, 3,200 miles away, and .01 ft. in Puerto Rico. The huge concrete Hungry Horse Dam, near Columbia Falls, Montana, 250 miles NW of the quake area, showed measurable displacement as a result of the quake. In remote Seattle, the diminished tremors were still strong enough to break loose the floating amphitheatre in Lake Washington. Rock Cr. Camp Collapse of Dolomite Buttress Camp as buried by landslide But by far the most spectacular effect of Montana’s Earthquake was the huge landslide at the mouth of the Madison Canyon. At the site of the slide, a relatively strong and nearly vertical layer of dolomite rock supported a huge bank, or mountain, of comparatively unstable schist and kept it from sliding into the valley in the same way that a retaining wall keeps a hillside terrace from slipping downhill. The tremendous shock waves of the earthquake fractured this dolomite buttress, and some 43 million cubic yards, or 80 million tons of rock, timber, and other mountainside debris cascaded off the slope, hurtled into the canyon, and surged up the opposite side, carrying huge trees and house-sized boulders as if they were weightless, hollow toys. When this huge mass whomped down onto the river bed, it forced out the water and air trapped underneath at hurricane velocity. The huge slide spurted mud, air, and water with such force as to send two-ton cars sailing through the air, and to grind others to suitcase thickness against the rocks. All this happened in seconds. It would take eight seconds for the mass at the top of the mountain to fall to the valley floor 1,200 ft. below. At the time it reached this point, the mass would be travelling 174 miles per hour. The time it took to zoom half-a-mile across the valley, up the opposite canyon wall, then split and flow three-quarters-of-a-mile up and down the valley (the slide lies one-and-a-half-miles-long in the valley), was less than thirty seconds! The fact that timber from the face of the mountain is spread in relatively uniform fashion over the entire surface of the slide is interpreted to mean that there was little tumbling action—that the slide moved as a single, if shattered, mass. One important scientific controversy has emerged from the earthquake. It relates to the time relationship, or sequence between the initial shock, the tidal waves, or seiches, how fast the huge quantities of water which overtopped the dam moved down the valley, and whether these slugs of water had rushed through the canyon in time to reach the site of the slide before the mountain fell. The stretch of the Madison running through the canyon is fresh, fast water, but normally it takes up to two hours There are two big, related questions. Could the big surges of water reach the point of the slide soon enough? And just how soon after the first shock did the mountain fall? Ruined building on the fault line. For the first couple of days after the quake, the theory persisted that the slide must have happened quite some time after the first shock—as late as 5:00 A. M., according to some theorists. But, as the facts, and the testimony of folks trapped near the slide—the Osts, Fredericks, Smiths, and Mrs. Bennett—became available, it was apparent that the slide must have closely followed the initial shock. Even if you discount the disrupted time sense of people under stress—when a minute can seem like an hour, and vice versa—it’s difficult to imagine that more than 20 minutes elapsed between the first shock and the slide. According to one set of calculations, big waves could have swept from the dam to the slide site in 18 minutes or so. A man stands in the gap left by the earthquake-caused simple fault scarp.(U. S. Geological Survey) Something slipped here! Before the quake caused the ground drop, creating this magnificent scarp, the ground surface shown here was continuous.(U. S. Forest Service) Surface elevation after quake.
Although the quake caused much settling of the earth packed against the downstream side of Hebgen Dam’s concrete core, the relatively slight displacement of the sod cover is interpreted to mean that all three tidal waves passed over the dam before this earth subsided and separated from the core. Thus the water would have begun its race down the valley before the heavy earth-settling shocks hit the dam area. Those who support the high-water-at-the-moment-of-the-slide theory point to the great volume of water damage way below the slide. If the slide had come first, it would have dammed off the tidal waves, and prevented such damage. They feel there just wasn’t enough water in the river bed’s normal content to cause the water damage done both upstream and downstream by the slide. And they argue, the mud and dust in the composition of the slide would have taken up most of the water normally found in the reach of the river buried under the slide. There’s further evidence in the numerous fish found high and dry on the flat along the river bank several feet higher than the streambed. Most of them were small, catfish-like chubs. There were numerous trout, and one 18-inch carp. There is no place in the river below the pool at the toe of the dam where carp would likely be found. Also, there was further confirmation in the fact that three of the especially made 11-inch squared timbers, eight and a half feet long, with notched ends and two U-bolts used as stop-logs in the Hebgen Dam spillway were found below the slide. Some shadow was cast on this as absolute confirmation by the Montana Power Co.’s explanation that stop-logs have been lost from time to time before the quake. Those who, in spite of such evidence, oppose the theory that the high water reached the slide area first just don’t feel that the water could have made it all the way down the canyon in so short a time. They feel that it would have taken at least 40 minutes for the big waves to traverse the seven miles. They have some support in L. D. Smith’s testimony that in driving down from Beaver Creek to Rock Creek right after the shocks, he saw no such waves. At any rate, this is one argument that geologists and hydrologists will be batting around for a long time. This view of the Madison Canyon slide gives the feeling of the up-canyon and down-canyon flow of the 80-million-ton mass of rock and debris.(Montana Highway Commission) |
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