Report to the GSL Tech Team, October 20, 2010.

Genevieve Atwood

Baileys Lake Fault Trenching

in the context of Earthquake Hazards and Great Salt Lake

SUMMARY: no big surprises at the trenches of Bailey’s Lake (just west of Salt Lake International Airport). The details of the trenches are still being worked out (how many movements along the fault; timing of events; consequences to tilt of the land surface; what magnitude; fault mechanisms… pretty “standard” questions to have at this stage of investigation).

Earthquake hazards and Great Salt Lake

1. We’re in earthquake territory… global tectonics. LINK USGS dynamic earth

The Basin and Range region of North America is spreading.

Using GPS, we can measure relative motion of places on Earth using GPS.

LINK to UNAVCO/BARGEN data

2. Great Salt Lake is located where it is because of tectonics (and accumulation of sediments).

The Bonneville Basin is a set of basins and ranges. The basins are closed (no surface-drainage outlets) because the basins are dropping along faults.

LINK to ESE schematic

3. We all have our faults… big, little, damaging, not so damaging, obvious, hidden…

Those that matter to management of Great Salt Lake are (a) in the lake (specifically the faults west of Antelope Island and the Promontory Mountains LINK to USGS Baskin image; (b) along the Wasatch fault zone, specifically the Salt Lake segment of the Wasatch fault and the Weber-Davis segment of the Wasatch fault LINK UGS; (c) others (mostly along margins of ranges of Basin and Range because of our setting in the Intermountain Seismic Belt LINK USGS.

4. Earthquake hazards of Great Salt Lake include:

Surface rupture: LINK to UGS image of effects … this is why we take all surface evidence of faulting seriously.

Ground shaking: LINK to UGS map showing “worst case” scenario

Ground failure, specifically, liquefaction: LINK to diagram, LINK to UGS Tooele and Salt Lake County

Tsunami (initial slosh) and continuing seiche: ethical question…

Most risks of the NW quadrant are not life threatening. The risks are significant and will be costly to address. However, a highly, very highly unlikely hazard is a killer phenomenon… movement of the Weber-Davis and/or Salt Lake segment of the Wasatch fault with ensuing sudden displacement eastward of Great Salt Lake. It’s an ethical question: is it okay to knowingly put scores of people in harm’s way of a deadly but unlikely hazard.

Tectonic deformation “tectonic tilt” (permanent relocation of GSL, Jordan River): LINK: Hebgen Lake earthquake of 1959 transposed across GSL.

Changed ground- and surface-water conditions (short- and long-term): lessons learned from effects of 1983 Borah Peak, ID.

5. At risk:

More at risk when the lake is at high levels.

Constructed environment… specific to each of the above.

Various: transportation (interstate highways / roads and bridges, rail, airport); industry (dike / impoundment failures); residential infrastructure (sewer, water, gas lines, buried tanks); structures (residential, recreation, utility, communications, other); environmental (secondary, such as ruptured oil/water lines)… and, as yet not constructed, impoundments to “control” the level of GSL (impressive concerns for life safety as well as risk to property).

6. Lines of evidence for hazards and risks

Historical records:

There has not been historic movement of the Wasatch fault;

Utah has many earthquakes LINK to UofU;

There has been movement along faults near, and perhaps in Great Salt Lake (1909 Deseret News reported that an earthquake believed to have had an epicenter near the north end of the lake produced a surge of water “that topped the Southern Pacific causeway and sent water over the bathhouse pier at Saltair.”

Geologic records (evidence of landforms/topography; sediment records (liquefaction, displaced bedding, deposition of sediments); trenching of faults; geophysical surveys; understanding of fault dynamics. Lake Bonneville sediments are helpful. If a fault displaces sediments of Great Salt Lake, we assume it’s active.

Bailey’s Lake… is a good example of (a) what can be studied at most sites with surface rupture, and (b) some of the challenges of trenching near Great Salt Lake. But… Baileys Lake faults (West Valley Fault Zone is not the major seismic threat for us to think about for management of the lake.

Images:

· Setting – fault zones LINK to UGS map

· Site – LINK to UGS image

· Ortho photo of site – LINK to UGS image

· LIDAR image of site – LINK to UGS image

· Fault scarp and site review - LINK to GA photo - scarp

Evidence of faulting (displaced lake-bed layers) – LINK to GA displaced lake beds

· Sediments that show change of environment of deposition ( Jordan River ? Tsunami/surge?) – LINK to GA photo - seds

· Site review – LINK to draft interpretation