June 23, 2013

John Muir’s 1872 earthquake and Sierran rockfalls

Posted by Austin Elliott

A blustery late May day in Yosemite Valley

When you gaze upon the breathtaking vistas in Yosemite National Park the mind reels trying to take in and process the breathtaking natural beauty in front of you. At the same time, it strains and cringes as hordes of tourists unloading by the bus-full elbow to carve out their own little section of the crowd in which to pose for a stranger-free shot of the stunning natural scenery. It’s the rough balance we strike in the National Parks, reserved as national shrines that make our country’s astonishing natural beauty accessible to the masses, while paradoxically demeaning and overrunning them with RVs, sneakers, plastic water bottles, and innumerable warehouses worth of REI paraphernalia.

Just as the founders intended it.

Touristic impedimenta notwithstanding, the place is transcendant no matter how crowded or materially encumbered your visit is. There’s a reason classic naturalist advocates like Ansel Adams and John Muir focused so much of their output in this setting. In every view of the Park the landscape overwhelms you with evidence of processes much bigger, longer, and rarer than you often consider, and events that people collectively rarely witness. The Yosemite landscape is the epitome–the idealized type locale–of the Sierra Nevada, an incredible impediment of a mountain range lofted upward by far-flung tectonic forces and ground back down into sublime canyons by millennia of glacial scouring. Similarly grand tectonic and erosive forces are ongoing, evident in the minor–and major–catastrophic events that recur surprisingly often in the valley.

Rocks tumble from Middle Cathedral

The USGS last week released an inventory of historic rockfalls in Yosemite. The database goes back to 1857 and includes approximate sizes, dates, and eyewitness accounts of each event (PDF). They’re frighteningly yet unsurprisingly common. The Park Service has a page dedicated to them. Refreshment of the sheer cliff faces is to be expected on such suddenly exposed granite monuments. Naturally, the record becomes more and more complete with ever-increasing visitor traffic, so small ones throughout the park are now often witnessed or detected. Ground-based laser scanning and numerical modeling are used (PDF) to measure their volumes and assess the rates and risk of rockfall. All rock falls are of course dangerous, but most are small. Less frequently, large ones cascade down the cliffs and threaten dwellings on the valley floor, like a newsworthy one in 2008.

The National Park Service also put out a video discussing this phenomenon and its threat, including recent footage of a “small” one taking place:

Skip to 2:50 for the dusty thundering gravitational drama.

Yet perhaps the most famous of the Yosemite rockfalls–at least the most famous eye-witness account of a Yosemite rockfall–comes from the 1872 Owens Valley earthquake (alternatively called the Lone Pine or–a classic Eastern Sierra double-entendre favorite–Inyo earthquake). John Muir was fortuitously camped in Yosemite Valley when the earthquake trembled the vast Sierra, dislodging jointed cliff-faces and rocky parapets everywhere. His account of the earthquake captures the wonder that I suspect I would feel in that situation, if I could suppress the utter terror of cascading boulders and toppling sequoias:

…though I had never before enjoyed a storm of this sort, the strange, wild thrilling motion and rumbling could not be mistaken, and I ran out of my cabin, near the Sentinel Rock, both glad and frightened, shouting, “A noble earthquake!” feeling sure I was going to learn something. The shocks were so violent and varied, and succeeded one another so closely, one had to balance in walking as if on the deck of a ship among the waves, and it seemed impossible the high cliffs should escape being shattered.

In fact the cliffs were shattered, and came tumbling down in sparking, crashing cascades. Muir, hiding behind a tall pine, takes in the surreal sound of this violent but lonesome natural tumult:

…No sound was heard for the first minute or two save a low muffled underground rumbling and a slight rustling of the agitated trees, as if, in wrestling with the mountains, Nature were holding her breath. Then, suddenly, out of the strange silence and strange motion there came a tremendous roar. The Eagle Rock, a short distance up the valley, had given way, and I saw it falling…. The sound was inconceivably deep and broad and earnest, as if the whole earth, like a living creature, had at last found a voice and were calling to her sister planets. It seemed to me that if all the thunder I ever heard were condensed into one roar it would not equal this rock roar at the birth of a mountain talus.

Muir, in an adrenaline-filled naturalist frenzy, raced out to climb one of these fresh piles (this is not advisable). I encourage you to read in full the short chapter of his recounting these events. It includes his take on the settling boulder pile, the smell of rock dust and whole swaths of obliterated pine forest, plus some amusing accounts of the other residents of the valley, understandable more frightened than him. You can read the 6 short pages online through the Library of Congress:  John Muir, Our National Parks pp. 261-267.

In the next few months Muir rode through scores of aftershocks, and turned his keen eye to the dramatic landscape change wrought by a single fairly distant earthquake:

Nature, usually so deliberate in her operations, then created, as we have seen, a new set of features, simply by giving the mountains a shake – changing not only the high peaks and cliffs, but the streams. As soon as these rock avalanches fell every stream began to sing new songs; for in many places thousands of boulders were hurled into their channels, roughening and half damming them, compelling the waters to surge and roar in rapids where before they were gliding smoothly.

His report of the mainshock itself accurately describes the onset and duration of an earthquake with its two modes of elastic waves shaking in different ways at different times, a testament to his keen observational habits: “The blunt thunder-tones in the depths of the mountains were usually followed by sudden jarring, horizontal thrusts from the northward, often succeeded by twisting, upjolting movements.”

The 1872 earthquake was a monstrous one, officially a M7.4, but debated higher or lower as many pre-seismometer temblors are. It devastated the desert town of Lone Pine, and tore a gash up the center of Owens Valley along one of the major faults that now accommodates some of the gradual northward motion of western California as it’s dragged along by the Pacific Plate. When you drive through Owens Valley you’ll see the Owens Valley Fault slicing straight through the eastern flank of Crater Mountain.

The 1872 rupture slipped along the dramatic fault scarp that bisects Owens Valley, cutting across the flank of Crater Mountain as seen in this Google Earth view. SoCal/ECSZ LiDAR hillshade from Earthscope is available from OpenTopography: http://www.opentopography.org/index.php

The landscape here is stunning… and evidence of its ongoing formation and transformation through cataclysmic events surrounds you: volcanoes, debris-flows piled into alluvial fans, fault scarps of every stripe from the horizontal tearing of the valley floor to the basin-bounding rangefront faults beneath which the mountains slide upwards in gradual exhumation, one cliff-toppling seism at a time.

I <3 the Eastern Sierra.

an Apocalyptic cloudburst descends on the glaciated eastern front of the Sierra Nevada

Searchable text of Muir’s Our National Parks, including p. 261-267 on the 1872 earthquake is available here:
http://archive.org/stream/ournationalparks00mu/ournationalparks00mu_djvu.txt