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Earthquake faults and active tectonics around the Eastern Basin and Range region: (1) the Wasatch Fault

Jim McCalpin brought us -a group of 22 lucky guys- all around the eastern Basin-and-Range province, through Utah, Idaho, Montana and Wyoming. We could admire beautiful landscapes and discover stunning geological traces of historical and recent tectonic/volcanic activity.
This road trip occurred before the 7th INQUA PATA Days that was held in Crestone, Colorado.

The Wasatch Range and its ~300 km long fault system

The Tour started in Salt Lake City, Utah. The city is bounded to the east by the Wasatch Range. The so-called Wasatch Fault Zone, running at the toe of the range, is an active fault system (around 100 km long) divided in several segments.

The Wasatch Mountain Range Front from the Salt Lake City airport area
A view of the range from the southern part of the SLC segment. The fault line might be traced somewhere at the toe of the hillslpeThe evidences of earthquake activity of the Wasatch Fault has long been recognized by geologists. Perhaps the first one is the notorious US geologist Gilbert (1890).
Illustration of the fault morphology at the toe of the mountain range (Gilbert, 1890)
The fault scarps, corresponding to the most recent earthquake(s), are still visible along the Wasatch Boulevard, on the eastern edge of the city.
Black arrows underline the Wasatch Fault displacing the Pleistocene moraineIn 1999, Jim McCalpin and colleagues (NEHRP study) dug a "megatrench" across the 6m-high Little Cottonwood Canyon scarp to study the earthquake history of the fault segment. This trench was 68 meters long and 3-4 m deep in some places. The authors could evidence a series of ~10 events in 20 ky.

Houses are built ontop of the recent fault scarp (Little Cottonwood canyon), where McCalpin et al. dug and studied the megatrench in 1999The Wasatch range and the SLC basin have experienced a peculiar event around the end of Pleistocene and the dawn of Holocene. The permanent supply of Bear River water increased the water level of the Bonneville Lake which flooded then the whole basin of SLC. Below, the two "terraces" correspond to two high-stand paleo-shorelines of this paleo-lake, respectively at +160 m above current lake (so-called Provo level) and +270 m (so-called Bonneville level).
The two horizontal lines on the Wasatch hillslopes correpond to high-stand level of the Lake, before its (natural) draining
Fault map from DuRoss et al. (2016). White dots
with labels represent the location of trenches.
Yellow lines are the inferred segment boundaries

Seismic Hazard

Several trenches have been dug in the area to capture the earthquake history of the main fault segments of this "silent" fault system. In 1999, McCalpin et al. carried out in 1999 the first mega-trench. Later, Olig et al. (2011) excavated another one in 2003 across the Provo segment and DuRoss another one on the the East Bench fault. These works enriched the (paleo)earthquake catalog.
A synthesis on the Wasatch fault paleoearthquake information has been compiled in a recent paper by DuRoss et al. (2016). One of the big deal of such a segmented fault system is: "could the fault segments rupture all together to produce a big earthquake?"

However, according to our field guide J. McCalpin, the question of multi-segment ruptures remains open because of the lack of relevant data. These should be collected at the junctions between segments (for instance where the blue circles are on the map to the left).

Probably, field geology will have never a conclusive answer, because of the inherent uncertainties of dating. Recent history has shown in several cases that ruptures can occur in minutes or in hours (e.g. well known cases in the western USA are the Pairview and Dixie peaks in 1954 which occurred within minutes).
However, paleoseismological trenches at the segment junctions would tell whether or not the rupture would have propagated during events recognized on neighboring segments.

The Wasatch fault zone area is almost deprived of recorded seismicity. However, it has to be mentioned that a surface-rupturing M6.6 quake hit the Hansel Valley (1934) in the very northern part of the basin. The ruptured fault is an antithetic one of the Wasatch Range main fault.

An illustration of the surface rupture associated with the M6.6 quake in Hansel Valley swampy area
Retrofit of the Utah Capitol in Salt Lake City

The local guide was Jerod Johnson, one of the authors of the reference paper that describes the retrofit of the Capitol building.
The Capitol is a massive building (120 x 65 m² in map view) with a dome rising up to 65 m above ground surface. In the late 1990's, an engineering study revealed that the building had not optimal seismic performance and would suffer a lot (even collapse) during a "characteristic" earthquake on the Wasatch Fault system, and it was decided to retrofit the building.
Various components of the edifice were reinforced (façades, walls, roof, etc). It was also chosen to set up a series of seismic isolators at the bottom of the structure, so that the natural period of the building was shifted from 0.6s to 3s; the building is thus protected against quakes on close faults (Wasatch fault).

This big building was built during the first years of the 1900's. It is in concrete behing marble façade.Two seismic isolators of the Capitol

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