May 8, 2013

Wasatch! Part 1 – Geological evidence of a fearsome fault

Posted by Austin Elliott

Two weeks ago I had the pleasure of attending a fieldtrip along the Wasatch Fault around and within Salt Lake City, Utah. Utah Geological Survey scientists Bill Lund and Chris DuRoss organized an impressively comprehensive tour as part of the 2013 annual meeting of the Seismological Society of America. We visited sites of interest for geology, seismology, engineering, and emergency management–a visit for each of the primary fields involved in understanding and dealing with earthquakes.

Salt Lake City and the abrupt, planar front of the Wasatch range screaming, "earthquake hazard" to any geomorphologist who sees it. ...As seen from our posh 14th floor hotel room.

The geological and geomorphic evidence of this fault is extraordinary. In subsequent posts I’ll summarize the seismological and engineering facets of our tour; this post is dedicated to the triangular facets. The field guide is available for free online if you’re in town and want to take a self-guided tour. You can also enjoy a remote introduction to the Wasatch Fault with this informative fly-through from the Utah Geological Survey.

As explained in that fly-through video, the Wasatch Fault is the world’s best-studied normal fault, a major seam in the crust separating the relatively rigid North American interior from the extending, collapsing crust of the Basin and Range [Nevada] to the west. Across this boundary Salt Lake Valley is dropping away to the west, sliding off of the Wasatch Mountains one earthquake at a time. The results of this separating process in the landscape are spectacular.

They’re so spectacular in fact that well before we really understood how faults produce earthquakes repeatedly over time, geomorphology pioneer Grove Karl Gilbert gazed upon the Wasatch Front escarpment and inferred that the towering Wasatch Range grew steadily by periodic cataclysmic separations of the valley and mountains along this boundary (Gilbert, 1890, p. 359-361). In Gilbert’s honor the UGS dedicated a geology park overlooking the spectacular landscape features from a suburb of Salt Lake City. The G. K. Gilbert Geologic View Park was our first stop on the tour.

Looking south at the mouth of Little Cottonwood Canyon, southeast of Salt Lake City. High-piled moraines are conspicuously offset by the Wasatch Fault, slicing straight up the middle of the scene, dropping the valley on the right relative to the mountains on the left.

The park is a lovely spot with a great view, and has a half dozen informative placards covering all aspects of the landscape from the bedrock to the glaciers to Pleistocene Lake Bonneville, and of course the Wasatch Fault that slices across all of it. Moraines–giant piles of silt and boulders carried down from the mountains by glaciers 13,000 years ago–reach out from the mouths of canyons, where the fault slices them and drops their downstream ends farther and farther with the valley floor.

Chris DuRoss explains the geologic setting to a cold, wet mob while the Wasatch Fault looms behind him (demarcated by red arrows), offsetting moraines and neighborhoods alike.

Although recognized as early as Gilbert’s 1890 monograph, the earthquake risk remained unaddressed until just the past few decades, so Salt Lake City’s suburbs encroach all the way up to the fault. When you know what you’re looking at, the juxtaposition is terrifying.

The "before" picture. The houses across the way are perched directly atop the fault scarp (same as in last image), where the Earth will split every time this segment of the Wasatch produces an earthquake

SLC and a few other mountainfront communities have only recently enacted setback ordinances requiring structures to be built outside of the zone of demonstrated surface rupture hazard–essentially finally requiring that buildings not be built where we know the ground will be wrenched apart during earthquakes in the future. These ordinances don’t apply to buildings “grandfathered in”, which is any built before their enactment (1989 in the case of SLC), and so a great many homes and businesses face direct threat. If you live in SLC it pays to check out the hazards of a home you own or may own. Many localities have maps and such you can freely access, like this:

As we progressed through our field trip stops, the threat of surface rupture hazard to homes, businesses, and municipal infrastructure (roads and utilities) in Salt Lake City became painfully evident. You can see it for yourself in the UGS video above, or from a Google Earth tour with the vertical exaggeration turned up. To a geomorphologist the fault scarp could scarcely be more obvious as it hoists streets, sidewalks, and whole neighborhoods to the coveted, pricey valley views of the East Bench.

In the distance the scarp abruptly lofts this road up to the state capitol grounds

Thankfully Utahns are particularly receptive to suggestions of preparation and self-protection. The state recently held a ShakeOut drill that gained the voluntary participation of 860,000 citizens–a third of the state!–as well as enabling exercises and simulations of statewide emergency operation. Scientists of all stripes are working to better understand the behavior of the Wasatch Fault and to simulate the effects of potential earthquakes along its length. Equipped with this information from scientists, policy-makers and engineers have made huge strides in preparing the city and state for its next earthquake. In Part 2 I’ll review some of the exemplary ways in which they’ve prepared.

Gilbert, G.K., 1890, Lake Bonneville: U.S. Geological Survey Monograph 1, 438 p. [pdf]