Al Stahler: Did you feel it? |

Al Stahler: Did you feel it?

We live on a rubber band … though it’s comprised, not of rubber, but of rock. Rock, like rubber, will stretch; and, again like rubber, if stretched too far, rock will break; and, after breaking, rock will snap back to where it was before it was stretched.

The surface of the Earth is a ginormous jig-saw puzzle. The puzzle-pieces are continents: North America, South America, Africa. And in addition to the continents, there’s another, super-important puzzle-piece: The floor of the Pacific Ocean.

The pieces in this jig-saw puzzle are rearranged constantly. The process by which the pieces are moved about is still mysterious. There seem to be currents – rivers – of flowing rock, beneath the surface, pushing – and perhaps pulling – on the undersides of the puzzle-pieces, moving them this way and that.

The San Andreas Fault — where North America butts against the floor of the Pacific — runs straight, from top left to lower right, in this Landsat image of Pt. Reyes.
Photo by Jesse Allen, NASA Earth Observatory

Hundreds of millions of years ago, Africa and South America nestled side-by-side, one into the other – their former jig-saw puzzle fit is obvious. Now, the two continents are moving apart.

Like South America, India, too, is moving away from Africa … being pushed into a headlong collision with Asia. The collision has made India a bulldozer: Pushing up bazillions of tons of rock, the collision builds the world’s highest mountains, the Himalayas.

Some of the puzzle-pieces are neither moving apart, nor smashing together, but are grinding against each other sideways. The coast of California and the floor of the Pacific grind thusly against each other. Standing here in the Sierra, looking out to sea, we see the floor of the Pacific moving northward. If we were floating in the ocean, looking back to land, we’d see North America moving toward the south.

But even as the floor of the Pacific moves north, and North America moves south, the puzzle-pieces are jammed together so hard that, where they meet … they’re stuck. As the Pacific moves north, and North America moves south, their boundaries are left behind. To accommodate the motion, rock near the boundary must stretch.

There’s a limit to how much rock can stretch.

One day, back in 1906, rock beneath San Francisco reached its limit. Able to stretch no more, it broke.

And then the two sides snapped back, generating the Great San Francisco Earthquake. A (rebuilt) fence at Pt. Reyes runs smoothly across the landscape … then suddenly stops … there’s a jag … and the fence starts up again, twenty feet away. Streams that crossed the fault were similarly carried sideways.

The ’06 earthquake allowed only some of the stretched rock to snap back to where it wanted to be.

Sit on the sidewalk outside the UC Berkeley football stadium, and check out the curb – it’s broken. As stress builds in the rock of the Hayward fault zone – inland of the San Andreas Fault – the slowly-stretching rock carries the sidewalk with it. Many earthquake scientists believe the Bay Area’s next great quake will happen, not in the west Bay, on the San Andreas Fault … but in the east Bay, on the Hayward Fault.

Further to the east is more rock with stress to relieve. Head down to Death Valley – east of the Sierra – and you can sit atop a small volcano … or, rather, you can sit atop part of a small volcano … the rest of the volcano is visible when you look a few hundred feet to the south.

This fault zone east of the Sierra is the Walker Lane. The Walker Lane stretches from Death Valley, through western Nevada, to north of Truckee.

9:35 Thursday night, a fault in the Walker Lane broke … and the rock snapped back, generating an earthquake strong enough to feel here in the foothills.

Looking at patterns in the Walker Lane, paleoseismologists (studying evidence of earthquakes in the distant past) suspect we are seeing, in the Walker Lane, something very similar to what the San Andreas Fault system looked like, some tens of millions of years ago. They hypothesize (make an educated guess) that the Walker Lane – much younger than the San Andreas Fault zone – will come to dominate the motion – and the stress-relief – between the floor of the Pacific and North America. The motions along the Walker Lane – over some millions of years – will enlarge the Sea of California, now separating Mexico from Baja. All of California will become a peninsula – a larger version of Baja … and western Nevada will become North America’s west coast.


An object 90 degrees from the horizon would be directly overhead. Friday the International Space Station will be nearly so – 89 degrees. Problem is, seeing this requires getting up before dawn. The ISS rises over the northwestern horizon at 4:49 a.m., climbing overhead a few minutes later.

A much easier chance to see the ISS – at 82 degrees, almost overhead – comes next Monday night, when it will rise over the southwestern horizon at 8:46 p.m., climbing very high into the sky within a few minutes.

There’s a total lunar eclipse – a super-moon eclipse – coming up, end-of-month – more on that next week.

Al Stahler enjoys sharing science and nature with friends and neighbors in The Union and on KVMR-FM. He teaches classes for both kids and grown-ups, and can be reached at

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