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| 2002 |
I've been leading geology field studies trips to lots of places in the American West for 30 years and started to take digital pictures in 2001. I sometimes struggle to find new things to photograph when I visit a place for the 30th time, but in some cases it is not a problem. There are geologic changes that happen on a yearly basis, and with seventeen years of photos, the changes become obvious.
This is a continuing update from a post in 2013, and I'll probably continue updating for the foreseeable future.
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| 2004 |
Highway 25 in the California Coast Ranges connects the town of Hollister with the access road to Pinnacles National Park (formerly Pinnacles National Monument). Along the way the highway crosses the San Andreas fault in a section where the fault creeps an inch or so each year (36°35'54.27"N, 121°11'40.19"W). Most years we've stopped to have a look at the effect the movement has on the pavement. In 2002 and 2004, the damage was obvious.
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| 2008 |
By 2008 someone had patched the road, and no fault motion was evident.
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| 2009 |
Little damage was evident in 2009 either. But by 2010 cracks had begun to appear as the fault stressed the pavement.
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| 2010 |
The fact that the fault creeps in this region is a good thing. It means that stress is not building along the fault surface, but instead is being released gradually. The sections of the fault to the north and south of the creeping section are locked by friction, and are building up the ominous stress that will eventually produce quakes with magnitudes in the range of 7.5 to 8.0. The quakes are coming and we need to be as prepared as possible.
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| 2012 |
By 2012, the road had been completely repaved, and yet the shearing was already evident.
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| 2013 |
It became even more pronounced by 2013 and in 2014. Just by chance, the person working as a scale was the same individual as in 2004.
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| 2014 |
In 2015 the fractures were moderately larger. They'll need to start thinking of road repairs before long.
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| 2015 |
In 2016 Laura once again provided scale, as she did in 2014 and 2004.
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| 2016 |
Here in 2017, long-time trip volunteer Mary provides scale. The cracks in the road are just a bit larger, but they didn't look appreciably different than the previous year except for a twist (pun intended).
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2017 |
On Dec. 2, 2018, the break to my eye seems more continuous. It's now been six years since the road was completely repaved.
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| 2018 |
Last year the paint was deformed (twisted), but not split (below).
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2017
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The offset paint strip reminds me of illustrations of elastic rebound theory, the idea that stress builds up on a fault line over time. In that model, the land on either side of the fault is distorted over time until the frictional resistance is overcome and the rock snaps back to its original shape. That won't be happening with the paint. Last year in 2017 I said "
if they don't repair the road (as they often do; see above), it will probably show a clear break by next year." Here's what transpired:
First, a close-up on 2017's center stripe...
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| 2017 |
And here's how it looked on Sunday, Dec. 2, 2018:
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| 2018 |
As predicted, the break in the paint is complete...
And here is the new update for November 16, 2019: Long-time volunteer Paul provides scale (he has been assisting MJC with field trips for 25 years!). The crack continues to grow, and I wouldn't be surprised if it was patched by next year.
The paint on the center strip is split even more.
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| November 2019 |
These little changes that happen at a rate visible in human lifetimes add up to huge changes when multiplied by thousands or millions of years. The nearby eroded volcano of Pinnacles National Park has been displaced 195 miles (315 kilometers) in the last 20 million years or so by movement along the San Andreas.
3 comments:
Garry, is the fault really just a few inches across at that location or is just an effect of the asphalt which overlays the fault? I would think the fault to be several feet wide at a minimum, if not hundreds of feet wide in more complex areas.
The fault zone is definitely much wider in the region, as there are benches and terraces nearby. But the break in the road is exactly parallel with a precisely aligned shutter ridge just south of the road exposure, and one can see from the years of motion that it is repeatedly happening in the same location.
"Fault zone," that's a great term that describes how I would imagine it. Probably because you've taught me a lot with your blog. Regardless, if it is a zone, this one strip of asphalt seems prone to recurrent movement. It's a great demonstration through the years
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