Sunday, November 30, 2008

Hiking Lake Elsinore

Yesterday, as a means of combating the sloth and gluttony inherent in Thanksgiving weekend, some friends and I went on a nine mile hike in the Santa Ana Mountains, above Lake Elsinore. It was a trip intended just for walking and being outside, rather than a specifically geological venture, but I was still pretty psyched about getting to hike around in a fault zone with which I didn't have much up close experience. To my disappointment, the trail we selected was far enough up into the mountains that the fault trace itself wasn't actually visible, and all of the outcrops we encountered were between granite and granodiorite, with rounded weathering, and undeformed/unmetamorphosed. (I was not the only one slightly bummed - the only bugs we saw were termites, which was a disappointment to the entomologist on the hike, and the two plant biologists were not thrilled that most of the vegetation was invasive.) That said, it was still a wonderful hike. The weather was great, the scenery was gorgeous despite its lack of faults, and it just plain felt good to get out and walk without the purpose of going somewhere. I wish I could afford the time to go and do this more often in the middle of the quarter. Backlogged homework would kill me dead if I tried, though.

On the drive down from the trailhead, there were some fantastic views:
You can see three of southern California's most prominent peaks from here: from left to right, Mt. Baldy (highest in the San Gabriels), Mt. San Gorgonio (highest in the San Bernardinos), and Mt. San Jacinto (highest in the San Jacintos). You also get a great look at three major fault zones: the San Andreas runs at the foot of the San Bernardinos, from just to the right of Mt. Baldy off to the end of the page; the San Jacinto runs in front of its like-named mountain, branching off the San Andreas to the right of Mt. Baldy and continuing in the other direction toward the end of the image; the Elsinore is in the foreground, with one strand on either side of the lake. Lake Elsinore is, in fact, there to begin with because of this extensional stepover in the fault (and I miiight even look at the possible dynamics of that particular stepover in my thesis, maybe).

This panorama also illustrates a particularly heinous case of Houses On Faults. The houses directly on the banks of Lake Elsinore are smack dab on the fault trace, but the neighborhoods that sit between the base of the Santa Ana mountains and the lower string of hills directly opposite the lake are sandwiched in that extensional zone. That does not bode well for ground motion in and of itself, but it becomes an even sticker situation when you consider the soft lake sediment around there. I also wouldn't be surprised if the liquefaction hazard there is much higher than in pretty much the rest of the Inland Empire (though I'd have to look it up - I haven't done that yet). The houses we drove past also look pretty new, likely newer than the Alquist-Priolo act. I guess most of them fall within the fifty-feet-away clause, but really, what's fifty feet compared to the speed of P-waves? The Elsinore may not have one of the higher seismic hazards in California according to this spring's figures, and it may not be as well known of a fault, but it definitely does still move (last September's 4.7 was on the Elsinore), and I definitely wouldn't want to live on it.

I would, however, be more than glad to hike there again.

1 comment:

Silver Fox said...

Nice view! Houses on faults, indeed.