April 17, 2015

Tearing through California Part 1: the Central San Andreas

Posted by Austin Elliott

Intersection of the Pacific-North America tectonic plate boundary with a winding State Route through California's oak savanna

The intersection of the Pacific-North America tectonic plate boundary with a winding State Route through California’s oak savanna. The astute geomorphologist will spot the sag pond nestled at the base of the brown-colored scarp at center-left.

On display in central and northern California is the rare and troublesome phenomenon that’s the mischievous cousin to sudden, wrenching earthquakes: slow, steady fault creep. Rather than remaining pressed firmly together until they lurch past each other in violent earthquakes, the two sides of a creeping fault glide gradually along, generally silently carrying along everything above them. The good news is that this process takes up strain that would otherwise be accomplished abruptly, reducing the frequency and probably the size of potential earthquakes (wouldn’t it just be *too* lucky if it brought the hazard down to nil–alas! Creeping faults are probably still storing up enough energy to pop off some pretty serious earthquakes). The bad news, other than that large sudden earthquakes probably still happen along creeping faults, is that the normally limited hazard of primary ground rupture isn’t just a rare risk to be lumped in with devastating ground-shaking during a once-in-a-lifetime disastrous temblor; it’s a daily headache for the communities built innocently astride their faults, and an expensive, relentless antagonist to the companies and agencies that maintain the networks of infrastructure crisscrossing California.

To witness this almost-McPhee-worthy battle of man vs. nature (man can’t hope to try and stop this one–just to do damage control), I recently took a trip to Pinnacles National Park (“America’s newest National Park!“), a beautiful preserve in Central California’s Coast Ranges, with its own impressively built and maintained infrastructure of drilled, dynamited, and cemented trails around craggy peaks and exhilaratingly claustrophobic talus caves.

Boy, do they know how to entice visitors.

Boy, do they know how to entice visitors.

I highly recommend the park, just a short, beautiful drive south from the Bay Area, or a slightly longer but equally–or more–beautiful drive up from SoCal, and the perfect size for a weekend getaway. Although the geology of the park bears great significance with respect to the San Andreas Fault, my interest lay just outside its entrance gates, where the quintessential California Coast Ranges “highway” State Route 25 snakes back and forth across the creeping fault no fewer than seven times.

I came to a screeching halt as the car thumped over this tell-tale isolated crack stretching obliquely across the roadway. 36.483705°N, 121.070682°W

I came to a screeching halt as the car thumped over this tell-tale isolated crack stretching obliquely across the roadway. You can even see right-lateral offset of the fresh yellow line. 36.483705°N, 121.070682°W

The area is a gorgeous reach of California to visit, and especially if you’re an earthquake tourist. The stretch of SR 25 between Paicines and King City Road is a classic destination for giddy geomorphologists and other fault-studiers. Here, the relentless wrenching of the fault tears open brand new fissures in the highway on a yearly basis, beginning to deform the road anew the moment any asphalt repair is complete. The rate of creep measured here is almost exactly an inch per year and hasn’t changed markedly in four decades. A nice time-series of visits through the years is written up at GeoTripper. I’ll add to that my most recent observations, from March 25, 2015, documented in the super giddily acquired photos below. I’ve captioned each of them with very precise coordinates so that you can at least do the virtual tour yourselves. In fact, at each location the cracks shown in these photos are JUST detectable in the imagery on Google Maps, and definitely visible in StreetView. For the more keenly interested among you, and perhaps for the geomorphology classes, you can see that the positions align perfectly with adjacent geomorphic signatures of the fault in the fields. For a better perspective on that, download the USGS active fault kml, and/or the NorCal Earthscope lidar hillshades for display in Google Earth. I’ve also added these to Panoramio for display in Google Earth.


We got out for a closer inspection of the characteristic en échelon pattern of cracks. …and to see if we could feel the road’s pain. 36.483705°N, 121.070682°W


The en échelon, or stepped, series of cracks is a distinct indicator of shear, not simply wear, tear, and weather. The road is being split and pulled in two different directions. 36.416043°N, 121.001285°W


Small bends in the cracks produce openings that reveal the amount of shear that has already ripped up the road. This is probably ~two years worth of fault slip. My notorious and ubiquitous field Chacos for scale.


Here the fault forms a prominent scarp, where an oak tree soaks up fault zone fluids. The fence, though hidden in shadow, is definitely being offset… by about half a meter here. 36.406630°N, 120.993859°W

Trees must be one established fault zone occupant that can deal with the inexorable sliding of fault creep. Their roots probably grow and force their way through the soil at comparable rates to the San Andreas.


Adjacent to the oak tree, the fault crosses State Route 25 its southernmost instance, where the road is substantially offset both right-laterally and vertically. There’s been no earthquake here; it just creeps along steadily. 36.406489°N, 120.993761°W


From where it cuts across Hwy 25, it steps to the right a smidge and then slices off southward through a field of cattle, forming the steep embankmant on the west side of this pasture. 36.404009°N, 120.991773°W

Although subterranean water and gas lines are of concern where they cross this inexorably moving seam in the Earth, the main victim of the fault is Caltrans, whose crews have to patch up potholes and such annually anyway. Cattle ranchers can’t care too much about their fence lines… at least not for a few more decades until property lines have to be rehashed. The nuisance of the plate boundary fault here pales in comparison to the urban havoc wrought by its northerly splay, the Calaveras Fault. …but that’s for Part II.