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.