Saturday, November 19, 2016

Why did the Road Cross the San Andreas Fault? 14 Years of Geologic Change (an Update)

2002
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.
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. 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.
2008
By 2008 someone had patched the road, and no fault motion was evident.
2009
Little damage was evident in 2009 either. But by 2010 cracks had begun to appear as the fault stressed the pavement.
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.
2012
By 2012, the road had been completely repaved, and  yet the shearing was already evident.
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.
2014

In 2015 the fractures were moderately larger. They'll need to start thinking of road repairs before long.
2015
And here we are in 2016. Laura once again provides scale, as she did in 2014, and 2004. It's a wonderful thing to have people that recognize the value of geological education and work year to year to volunteer their services in the name of learning (as well as following a career as a teacher). She is a National Association of Geoscience Teachers Outstanding Earth Science Teacher awardee.
2016
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!