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.
But by far the most spectacular effect of Montana’s Earthquake was the huge landslide at the mouth of the Madison Canyon.