example of site before trench excavation
Collecting data at the Flat Canyon site. Photo by Scott Bennett, 2014.

Down in the Trenches & Up in the Air - Collecting Data

The trench log is a map of the trench wall drawn on a small-scale paper print out of the trench photo mosaic. Paleoseismologists record details of the trench walls onto the trench logs. These details include the location and type of geologic contacts between layers of sediment (a.k.a. geologic units), the location and orientation of faults that cut these sediments, and any other evidence for ground shaking and failure during or after a large paleo-earthquake.

Careful descriptions of color, grain size, moisture, and density are recorded for each geologic unit. Samples of important geologic units are collected and sent to a laboratory to be age-dated. The geologic age of a sample is determined by various age-dating techniques, such as carbon-14 dating and luminescence dating.

example of site before trench excavation
Trench review of the Flat Canyon site. Photo by Scott Bennett, 2014.

The deformation of sediment, faults, and other features in the trench walls tell the sequential story of past earthquakes at that location, and age-dating informs paleoseismologists when the past earthquakes happened. Collecting all the data to reconstruct the story takes a team of scientists about two weeks in the trench, carefully recording everything onto the trench log. When the job is done they celebrate with a "trench party", where interested scientists and local citizens are invited to tour the trench and discuss the geologic interpretations before it is filled in.

As part of this Wasatch fault zone project, an airborne LiDAR topographic survey was conducted of the entire Wasatch fault zone. The LiDAR survey along the length of the fault took approximately 8 weeks of flying time between the Fall of 2013 and Spring of 2014.

LiDAR data provides a vey high-resolution picture of Earth's surface so subtle features (like fault scarps) that may be missed during ground mapping can be identified from a bird's eye view. The laser scanner that does all the work is slightly larger than a large coffee can and is mounted to the underside of a small plane that flies back and forth over an area until the entire fault zone has been surveyed.

The laser pulses are able to penetrate vegetation and bounce off Earth's surface, so even if an area is covered with a dense forest, LiDAR is able to measure the surface elevation beneath the tree canopy.

image of LiDAR flight View Video of LiDAR Survey Along the Wasatch Front
North Creek trench video View Video of North Creek Trench on Wasatch Fault

open trench viewed from one end person hammering a marker into side of trench person standing in trench scientists and students on different levels of trench sitting and standing

Upper-left: Paleoseismologists collecting data from the Corner Canyon trench site.
Upper-right: USGS scientist, Ryan Gold, collecting a luminscence sample at the Corner Canyon trench site.
Lower-left: Total Stations (on yellow tripods) are used to survey the trench grid and important geologic features.
Lower-right: Paleoseismologists examine the fault zone at the Corner Canyon trench site.