Monday, August 18, 2008

Pull-apart basins of the Inland Empire

Last Wednesday, despite the ludicrous August-in-SoCal temperatures, those of us who are hanging around the UCR geology building this summer had the opportunity to go on a field trip to look at pull-apart basins on the San Andreas and San Jacinto Faults. The trip was led by one of the senior faculty members here, who worked at USGS for a long time (and whose name is on quite a few USGS maps of the area) before coming to teach here; I'd heard he leads some awesome field trips, so how could I pass this up, weather aside? Long story short, there was no disappointment whatsoever, and I learned almost more than I thought possible for a four-hour air-conditioned car ride.

This trip managed to tell three different stories. The first was structural; the pull-apart basins we visited on this trip come from extensional stepovers within single faults, in this case right-lateral strike slip faults that step to the right of each other. As the faults pull, the land between them subsides, creating a basin that's ripe for flooding. Sediment accumulates in these basins, and the rapid subsidence also causes the sides mountains surrounding the basin to slough off in landslides. Most of the mountains we saw showed rolled landslide toes, rather than clean faceted ridges. The San Jacinto/Hemet basin is still actively sinking, and at this time of year, that sediment shows itself as lots and lots of dust. I've been past this area before, and at the time, I laughed at the thought that a completely dry area was marked as a river overflow zone, but this field trip told me why! The subsidence is uneven enough that the San Jacinto River, when it has water in it, runs right up against the base of the mountains. That barrier makes pooling up all the easier, and it doesn't take much water to start spilling over toward neighborhoods and farms. We followed this up by visiting the inactive Mill Creek basin on the San Andreas, near Yucaipa and Crafton Hills. Here again is a body of water (with actual water in it this time!) right up against the mountains. Natural exposures and roadcuts alike show thick layers of sandstone, some with ample evidence of soft-sediment deformation (likely coseismic), between the basement rock on either side of the basin. The extension that formed this basin is also responsible for a series of normal faults in Crafton Hills; the San Jacinto basin will likely develop these as well, given enough time.

The second story here is about mapping. The area around both of these extensional basins has been mapped many times, by several different agencies, and there has been a tendency to ignore features specifically related to extension and assume that the faults are more literally responsible for every geomorphic feature in the area. Subsidence scarps have, in the past, been marked as fault scarps. The sharp boundaries between basement and accumulated sediment - ie, the edges of the basin - are also often taken as faults. The latter isn't just a pull-apart basin problem: the Perris Block, on which Riverside sits, has not deformed much internally in recent geologic time, but it had plenty of internal topography, and the lower points have accumulated sediment as the entire block moves up and down. The boundaries between this sediment and any mountains sticking out of it have been marked as faults, but tracing them reveals that they're essentially round! (Older maps actually put a fault at the end of my street, thanks to this misinterpretation. Boy am I glad this is wrong!) Assumptions that faults sit in front of the mountains also don't quite work for pull-apart basins, since landslides roll off the mountains and cover the trace. There have been several incidents of people trenching the "fault" at the base of the mountain, coming up with nothing, and being happy with that answer. Sometimes, a landslide will re-expose a fault scarp, as with above Soboba Springs, and that scarp seems to slice the mountain almost through its middle. Smaller landslides also obscure several much larger ones that bound the basin; the small ones get mapped and the large ones don't, a "forest for the trees" scenario, to use our guide's term. Fortunately, now that the structure and function of pull-apart basins is better understood, maps can be corrected and updated to better represent the region and its underlying processes.

The final story is one of people building things in really stupid places. The active San Jacinto basin is full of houses, many quite new. They're slightly higher up than the San Jacinto river, but slightly isn't going to help, when the whole area is so much lower than everything around it. And it's not that this is a slow process, either! There are stories of houses that were built 50 years ago, carefully planned so that the high water mark was several feet below where the house stood. Recent floods have dumped four feet of water into those houses. Add in the landslides off the mountains and the being wedged between two strands of one of the most active faults in California, and you've got a big problem. We passed a bunch of houses on fault scarps on the course of this trip, though many of these houses looked older and fairly isolated, likely the personal decision of a landowner rather than a clueless developer. Crafton Hills, however, is a different story. Fault scarps serrate this landscape, and those scarps are liberally dotted with brand new development of large and presumably spiffy houses. There is no way these developments could fit in with Alquist-Priolo regulations, but there they are. Were those normal faults not evaluated as hazardous compared to nearby strike-slip faults? Did nobody bother to trench to begin with? Did the developers blatantly ignore the rules and warnings? I have no idea, but I know I wouldn't want to live there.


Silver Fox said...

Sounds like it was an interesting field trip: finding out you can't find faults at the base of every mountain, a bunch of little mini Death Valleys, it sounds like. As for houses, surely they have some kind of hazard maps? That doesn't always stop development, but then a buyer can maybe pick and choose from bad and worse.

coconino said...

Julian - I worked fairly extensively in that area a few years back. The EIRs required by CEQA and the extensive planning reviews conducted under CA planning laws usually require some sort of geotech analysis. Any issues arising must be mitigated under CEQA requirements (ie., like landslide hazards). That doesn't necessarily mean the development would not get constructed, but it does mean there is an assumption of a certain amount of risk that is usually identified in the geotech report. Indications of faults are usually trenched and logged or downhole logging is conducted.

That aside, my anecdotal evidence is that the subsidence is primarily caused by groundwater withdrawal for domestic/ag use. When San Jacinto was first settled, apparently the San Jacinto River was perennial in the reach upstream of the Soboba Reservation. Flumes constructed of redwood would carry the water to fields and the town. If you can get your hands on the EIS for Eastern Municipal Water District's Hemet San Jacinto Recharge and Recovery Program, you'll note some interesting groundwater basin tidbits about the San Jacinto River.

Downstream of Lamb Canyon Road, in the reach from around Mystic Lake to the entrance to Railroad Canyon, the landscape gets very interesting from a hydrogeomorphological standpoint. As I understand it, that reach was a lakebed in Quaternary, gathering lots and lots of very fine grained lacustrine seds. The river in that reach ends up having a gradient of about a foot per mile, resulting in a very wide 100-yr floodplain (~1 mile) and large flood events trun the area more into a very large lake. This has resulted in hydric (frequently flooded) soils and a plant community that is less riparian than it is vernal pool-related. There are three endemic T&E plant species, spreading navarettia, SJ crownscale, and thread-leafed brodeia that inhabit the area as a result of the combination of geology and hydrology of the area.

As a side note, I believe research conducted by Becky Dorsey (U of O) has shown that the San Jacinto fault is 700,000 years old or less. I always thought it quite fascinating that this landscape with which I had so become so intimate was so young.

coconino said...

I should add that CA panning regulations are much stricter than most of the rest of the SW, but are not infallible. Different counties apply them more or less stringently, and it depends on public comments and lawsuits as to whether the less stringent counties get on board with the more stringent ones. It is also somewhat to fairly political, depending on the Boards of Supervisors and the political winds that blow thru the county in question.

Julian said...

Silver Fox: No mini Death Valleys here! Nor is every mountain a pressure ridge. (Though plenty of them are, around here.) I do have to wonder how much of the hazard risk is let on for prospective buyers. Someone in the department told me that, with regards to faults at least, sellers just have to tell people that their house is in a "zone," with no further detail!

Coconico: Only 700,000? I'd heard an age of 1 million for the San Jacinto (though I honestly can't remember where I got that figure), which is still pretty young, but dropping it down to six digits...woah. Count me 300,000 years more in awe of how quickly the San Jacinto has shaped the landscape. The San Andreas' hyperactive little brother indeed!

It doesn't surprise me in the least that the decision of where to build is so political, no matter what the hazard reports say. Though I'd think that'd leave room for far more political kvetching and flailing when something bad does happen...

That is very cool about the vernal pool ecosystem in the basin! Just shows I really need to learn more about local plants, if they can tell such stories about the geology. I'll also have to check out that EIS report.

coconino said...

One more comment: The flood control agencies/authorities in CA (where I am now, there are none to precious few) have significant restrictions on construction in the 100-yr floodplain. At this juncture, the levees in between San Jacinto and Lamb Canyon are at about the 50-year event. When I left the SoCal area, the local responsible agency was in the very initial stages of planning to construct set-back levees behind the existing ones to provide protection against a 100-yr event, and try to utilize the area between the two as a riparian habitat mitigation. It is unlikely that any project would have been approved within the 100-yr FP by the area planners since CEQA's passage.

Typically, that county requires that any arroyo/stream/channel through a development pass a 100-yr event with an appropriate Manning's roughness coefficient for the expected vegetation. If there is riparian habitat existing or proposed for the arroyo/stream/channel, the hydrologic calculations should include a Manning's corresponding to mature riparian vegetation and the channel still pass a 100-yr event with no overtopping.