OWENS VALLEY, CALIFORNIA
The Nature of Earthquakes
John Muir had been living in Yosemite Valley for less than four years when, during the early morning hours of March 26, 1872, while asleep in a small cabin he had built near Sentinel Rock on the valley floor, he was awakened by strong shaking. "Though I had never before enjoyed a storm of this sort," he later wrote, "the strange, wild thrilling motion and rumbling could not be mistaken."
He ran outside into a clear, moonlit night, feeling "both glad and frightened." "A noble earthquake, a noble earthquake!" he shouted. He was sure that something important would be discovered.
And it was. Though it was someone else who would make the discovery. And it was twenty years before that important discovery was made.
The person who made the discovery was Grove Carl Gilbert who was conducting one of the early geologic surveys of the America West. He made his way into Owens Valley on the eastern side of the Sierra Nevada, the side opposite Yosemite Valley. He found a wall of huge boulders that continued for many miles. Those who were living in Owens Valley at the time of the 1872 earthquake said the wall had formed during the earthquake, when the ground shook so violently that the ground had ruptured. Gilbert inspected the wall and came to a fundamentally different conclusion.
The wall of boulders near Lone Pine, California. This is the surface rupture of the 1872 earthquake that caused the western block to move upward suddenly by as much as twenty feet.
Gilbert closely inspected the wall of boulders. He noticed that he could match features that indicated the ground had shifted as much as 15 feet horizontally. If ground shaking had formed the wall, then there should not have been so much horizontal movement. From that, Gilbert concluded that the wall of boulders was not an effect of the earthquake, but the cause.
That is, the earthquake was caused by the sliding of huge crustal blocks against each other. Imagine, he said, that you are in a railway car and the brake is set. Then if the car is being pushed or pulled, at first the car remains stationary, held in place by friction of the iron wheels against the rails. But eventually the pushing or the pulling becomes too great and the wheels slide a short distance along the rails, causing the entire railway car to shake momentarily.
Earthquakes are produced in a similar way. The Earth's crust is constantly being pushed and pulled. But friction holds crustal blocks stationary. That is, until the friction is overcome and the blocks slide, producing earthquakes.
The eastern side of the Sierra Nevada. The 1872 fault scarp is along the low hills in front of the mountains.