Wednesday, February 25, 2009

...and then two more.

I was tempted, in light of specific sentences said by a particular politician yesterday, to forgo my planned Accretionary Wedge post and instead write about how every geologist, at some point in his or her life, should engage in a little volcano monitoring. I totally stand by my statement (and will be taking a class next quarter that absolutely involves volcano monitoring), but I also recall that we're collectively trying to come with 100 things, and there is certainly enough room in the budget of geological curiosity/exploration for me to write about those things in addition to snarking about things that other members of the geoblogosphere have already addressed so eloquently.

And now, the post I was planning to write before this madness went down.

The original 100 Places Meme lists the San Andreas Fault as a Must-See. I absolutely agree with this, but as I was marking the text on the list in bold, I got to thinking about how those three words mean a whole slew of very different places, spread out over hundreds of miles. The ultimate San Andreas experience is, of course, to drive the whole thing (or, well, the parts that are on land). I can't check off the full 800 miles yet, but I've been from the southern end of the Fault at the Salton Sea up through Point Reyes, mostly on roads that cling pretty close to the surface trace. I definitely recommend this drive as a means to check off the San Andreas on the original meme, but I also feel like there are several specific must-see sites along its trace (and within its system of associate faults) that deserve space on the expanded list.

The first place that came to mind, without me even having to really think about it, is Carrizo Plain National Monument, in rural San Luis Obispo County. The quintessential image of the San Andreas, the one that pops up in all sorts of textbooks and popular science books about earthquakes, the first photo that turns up when one does a google image search for the San Andreas, is from the Carrizo. It's an aerial photo of the Elkhorn Scarp, also known as the Dragon's Backbone, a 17-mile-long pressure ridge that, from above, looks like a gaping chasm into the depths of the Earth. (The original Superman movie actually does use the scarp as Supes' entry point into the fault...) It is a spectacular photo, but the view from the ground is equally breathtaking, and has the added bonus of debunking the faults-as-gaping-chasms myth.
The Carrizo Plain is one of the best places to see a wide variety of features associated with continental strike-slip faults. In addition to the outstanding pressure ridge, there is the famous Wallace Creek. This streambed crosses the San Andreas, but not without getting pulled around a bit first. Wallace Creek takes a sharp right angle turn as it meets the Fault, then parallels it for some 130 feet before heading back on its course. This shows the displacement power that the San Andreas has, and is all the more staggering when taking into account that some people think a good percentage of that displacement came from the 1857 earthquake alone. There are plenty of other offset streambeds that are less dramatic, as well as some channels that have been "beheaded" by the fault. The Carrizo also boasts more typical scarps, some spots of more trenchlike fault expression, stepovers with traceable surface expression, and a few (mostly dry) sag ponds.

If faults aren't really your thing, the Carrizo Plain is still worth a visit. The northernmost part of the Plain contains Soda Lake, a huge alkaline lake out of which many spectacular mineral samples (some of which are in the Visitor's Center, which is not open all the time) have been extracted from not very deep. The Carrizo is also thought to be an example of what the native Californian landscape might have looked like before the Europeans did anything to it - fields dominated by wildflowers and scrubby plants. For this reason, late March and early April are the best time to visit, since the wildflowers are out in full force, and the entire Plain is painted in shades of green, yellow, orange, and blue. (Don't go in the middle of the summer, though, unless you like triple digit temperatures.)

The next place I want to add to the list doesn't strictly feature the San Andreas; the town of Hollister's bisector is the Calaveras Fault, though the Calaveras branches off from the San Andreas just south of the town. The seat of San Benito County may lack scenes of breathtaking beauty that could make even a non-geologist drool, but it's also one of the best places to see the bizarre California-centric phenomenon known as aseismic creep. Creep is a phenomenon in which a fault moves slowly and steadily at around the tectonic loading rate, constantly releasing stress in a way that does not radiate seismic waves and generally prohibits enough buildup to cause a large quake (though some creeping faults may have asperities within the creeping zone that still accumulate stress and rupture seismically, even though creep continues around that zone). The main place (if not the only - I haven't heard of it elsewhere) this has been observed is in California, specifically on the Hayward, Calaveras, and Rogers Creek Faults in the Bay Area, the San Andreas between Parkfield and San Juan Bautista, and the Superstition Hills Fault near the Mexican border.
Creep can be measured with all sorts of complex instrumentation, but its easiest for the layperson to see when some sort of manmade structure gets put on the fault, and proceeds to get bent and deflected and cracked. The cities of Santa Rosa, Berkeley, Hayward, Fremont, and others all have the trademark offset curbs of a creeping fault, but I personally think that Hollister is the best city to look for offset because it looks like they've put less effort (or money) into patching up the distortion. Some sidewalks are bent several feet out of line, many streets are crossed by sets of en echelon cracks, and some houses and garages have obvious bulges in the walls and cracks in the foundation.

So Hollister is a good place to see creep as a phenomenon in and of itself, but it's also a clear exhibition of the interaction between the Earth itself and its inhabitants, of how people keep living in some places in spite of the geology. I suspect the fault was not known to be there when Hollister was first founded, which could explain some of the spectacularly bad locations of houses, but towns like this can be an example of how the hazards of the landscape should be taken into account when planning future development.
Not to mention that such intersections of humanity and seismicity make it clear that Fault Monitoring is also pretty darn important...

No comments: