backhoe at trenchsite
Corner Canyon site trench excavation. Photo by Scott Bennett, 2014.

What's really involved in planning and completing a paleoseismology project like this one on the Wasatch fault zone? It's more time-consuming and interesting than you might think.

Selecting and Preparing the Site

The first order of business is selecting a trench site. The trench site location depends on the scientific questions to be answered and the ability to gain permission to dig a trench at the site. In this Wasatch fault zone study, the scientists needed to select site locations that were near the end of known fault segments, so a good bit of staring at maps and wandering around in the field preceded site selection. After a few potential sites are identified, scientists need to get permission from the landowner to dig a paleoseismic trench across the fault; the land may be private property, or owned by the Local, State, or Federal government. Once permission is granted, a detailed topographic survey is completed to record the natural site conditions before any trenching is started.

Now it's time to bring in the excavator and dig the trench. A typical paleoseismology study onthe Wasatch fault zone requires a single trench about 40-50 meters (130-160 feet) long, 5 meters (16 feet) side, and up to 5 meters (16 feet) deep. A trench usually takes one day to excavate, although this time can vary depending on the size of the trench or if more than one trench is excavated.

As soon as the excavator does its job, the paleoseismologists get to work cleaning the walls of the trench, much like archeologists cleaning an archaeological site. They want to make the trench walls as "clean" as possible and to prepare a fresh geologic exposure of the sediment layers and the faults that cut and offset these layers. Preserved in the trench walls is the geologic and tectonic record of paleo-earthquakes at this location along the Wasatch fault zone.

gridding the trenchsite
Paleoseismologists examining the trench walls at the Alpine site. Photo by Scott Bennett, 2014.
example of site before trench excavation
View of Flat Canyon site after trench is backfilled with dirt. Photo by Scott Bennett, 2014.

The trench is ready now for the scientists to set up a coordinate system on the vertical walls of the trench so that all the information can be precisely recorded. String and nails are used to create a grid on the trench walls. These horizontal and vertical grid lines are spaced exactly 1 meter apart, like a graph-paper overlay. Next, detailed photos are taken of the entire extent of the trench walls and are pieced together to create a photo mosaic of the trench. In the past this task was done manually and took several dozen person-hours. Recently, new software automatically creates a photo mosaic with more accuracy and requires fewer person hours. The software also creates a 3-dimensional model of the trench that can be rotated and manipulated to reproduce a virtual trench that can be studied long after the real trench has been filled in. Once the trench grid is set and the trench photo mosaics are printed, paleoseismologists can begin logging the trench.


Interactive 3-D Model of Alpine Trench

3dtrench

Download and interact with 3-D model of the Alpine trench.
(6.3MB PDF, Adobe Reader required).

Photos by Kendra Johnson and Ed Nissen (Colorado School of Mines). Model created by Nadine Reitman. (If PDF opens a blank page in your browser, right/control-click on it and download it to your desktop.)


Time-lapse Video of Flat Canyon Trench Excavation

by Scott Bennett