Layers: San Francisco

LAYERSSanFrancisco.64x84inOne of the most beautiful and important cities in the United States, San Francisco is rent by numerous fault lines.  To simplify you can picture the faults bunched together like a deck of cards stood on edge.  Each fault is a card, lined up on its edge, as they cut through the city, making the deck itself a single, greater fault.  The “deck” of faults runs offshore in Marin County, approaching the city just beyond the Golden Gate Bridge then slices sharply through the mainland and south into the city.  To the north, the famous San Andreas Fault continues to sea and, to the south, it dives down beyond Los Angeles.

Let the deck shift and the western, Pacific side, from the upper mantle to the buildings atop the surface, rides northward on the great, slow pinwheel of the Pacific Plate. Meanwhile, the North American side shifts southward, and, where the two edges meet, the cards, of solid crust and upper mantle, grind and shear.  Over millions of years, in geologic time, this is a smooth, continuous motion, but humans experience it as sudden jolts from the powerful earthquakes with quiet times between.  On both sides of the fault, from the mobile asthenosphere, through the solid upper mantle and into the solid crust, this has played out since the Pacific Plate butted against the North American Plate.  Their movement has always been powered by the churning convection within the hot, semi-molten asthenosphere many layers below.  Friction between the solid rock locks the fault together, until the crushing pressure crumbles rock to dust and the plates are released.  Along the fault margins, rocks are ground, crushed and jerked as one plate turns counterclockwise and the other shifts south.  Humans, by our presence and our built layers atop of these moving crustal plates, have created a hazard.

At least twice in the last century, the city swayed up and down, to and fro, and the human artifacts creaked, groaned and then crashed down, burying those within.  Wood splintered as bricks crumbled and walls fell.  The constant fires of industrializing civilization, from pilot lights and light bulb filaments to electric arcs from exposed wires, met the natural gas released from broken pipes and newly created kindling of splintered buildings.  The explosions and infernos couldn’t be extinguished, not when broken pipes stopped feeding water to the hydrants.  San Francisco burned.

Today, believing themselves better prepared, people have either run to the support of a doorjamb or outside to the streets beyond the inside chaos.  But in the Marina district, the surface tension that had held the mud together was shaken.  Houses built on loose fill ground, which liquefied, descended into jelly-like quicksand.  Cars were jostled about, many collided.  The elevated section of the Nimitz highway crushed onto those driving beneath, even as the Bay Bridge dropped away from those atop it.

The sobering scenes of smoke billowing from the city in 1905 were captured in static black and white photos.  In 1985, the images were in living, moving color on television.  Twice the human built environment was destroyed, along with humans who built and lived there.  Twice the human built environment was rebuilt, stronger and more resilient, but it cannot ever completely overcome the shaking.  The rocks will grind as the plates pass again.

Text prepared by Dr. James Brey

Additional Images

 

Tags: , , , , ,

Layers: Places in Peril

Follow

Get every new post on this blog delivered to your Inbox.

Join other followers: