Saturday, November 21, 2015

Why did the Road Cross the San Andreas Fault? 13 Years of Geologic Change (an Update)

I've been leading geology field studies trips to lots of places in the American West for 27 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 27th time, but in some cases it is not a problem. There are geologic changes that happen on a yearly basis, and with thirteen years of photos, the changes become obvious. This is an update from a post in 2013, and I'll probably continue updating for the foreseeable future.
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. 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.
By 2008 someone had patched the road, and no fault motion was evident.
Little damage was evident in 2009 either. But by 2010 cracks had begun to appear as the fault stressed the pavement.
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.
By 2012, the road had been completely repaved, and  yet the shearing was already evident.
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. I've got some great people who volunteer and make these trips possible

Today we stopped at the fault crossing again. The fractures are moderately larger. They'll need to start thinking of road repairs before long.
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.
Until next year!


Dave said...

Garry, can you give a more specific idea of where this location is on Hwy. 25? Thanks!

Garry Hayes said...

36°35'54.13"N 121°11'40.24"W
It is 10.5 miles south of the village of Paicines. There is a prominent shutter ridge on the east side of the highway.

Garry Hayes said...

Here's a link to a map:,+Paicines,+CA+95043/@36.6488702,-121.3109876,23082m/data=!3m1!1e3!4m13!4m12!1m5!1m1!1s0x8092181dadd3d461:0x77994da527e2c741!2m2!1d-121.2788708!2d36.7289127!1m5!1m1!1s0x80923f60a2ba00cd:0xef5840b32d90f823!2m2!1d-121.1945169!2d36.5983784