What are we doing?

example of trench excavation
A typical, benched paleoseismic trench exposure across the Wasatch fault scarp. The fault scarp at this Flat Canyon site is about 13 m high and formed as a result of several surface-faulting earthquakes. Photo by Scott Bennett, 2014.

A team of scientists from the USGS Geological Hazards Science Center, led by Mendenhall Postdoctoral Fellow Scott Bennett and Research Geologists Ryan Gold, Richard Briggs, Christopher DuRoss, and Stephen Personius are collaborating with scientists at the Utah Geological Survey to gather data from new paleoseismic trenches along the Wasatch fault zone. These new datasets will help researchers to understand if past surface-rupturing earthquakes have spanned fault segment boundaries. They are also analyzing new high-resolution airborne LiDAR topographic data to characterize previously unmapped fault traces and to measure how vertical displacements (vertical offset of the ground surface from faulting) vary, both in space (from north to south) and time (the last 20,000 years).

Down in the Trenches - Paleoseismology

The USGS and collaborators are excavating trenches near the Salt Lake City-Provo and Provo-Nephi fault segment boundaries (yellow arrows on map below). These trenches near fault segment boundaries can provide critical information about the length and displacement of past earthquakes. These data will also allow the researchers to figure out if past earthquakes repeatedly stop at a fault segment boundary or sometimes break through.

Preliminary observations include evidence for four to seven Holocene earthquakes at each trenching site. Ongoing geochronologic analysis will help determine the timing of these earthquakes. The timing of these earthquakes will be compared to earthquakes observed in trenches on adjacent fault segments to help identify through-going ruptures across segment boundaries.

Up in the Air - Lidar Analysis

Lidar, which stands for Light Detection And Ranging, is a remote sensing method that uses laser pulses, usually from a specially-equipped airplane, to create detailed topographic maps of Earth's surface. Lidar is able to "see through" the vegetation to record ground elevations and is a much faster and more precise way to gather topographical data than collecting data with a ground survey.

high altitude image of area
Oblique photo looking north across Utah Valley. Segment boundaries (yellow arrows) separate fault segments (e.g. Salt Lake City, Provo, Nephi). Previous trenches (white stars); new trenches from this study (yellow stars). Photo taken at approximately 10,000 m altitude.

The USGS, in collaboration with the Utah Geological Survey, Federal Emergency Management Agency (FEMA), Utah Division of Emergency Management, Salt Lake County Surveyor's office and partner cities, has acquired 3,683 sq. km (appox. 1,400 sq. miles) of new high-resolution (8 points per sq. meter) airborne lidar topographic data along the entire WFZ. These data are available to the public. Detailed bare-earth digital elevation models (DEMs) and hillshade models of these LiDAR data allow the scientists to identify previously unmapped fault traces, providing a more complete story of Holocene earthquake ruptures on the WFZ.

Analysis of the lidar data helps quantify fault displacements and slip rates, allows scientists to estimate the extent of fault ruptures associated with most recent earthquakes, and documents spatial and temporal slip patterns near fault segment boundaries.

Preliminary observations identify numerous previously undocumented fault scarps, including recent scarps north of Ogden, UT that may correspond with a late Holocene rupture that spanned the Brigham City-Weber fault segment boundary, as previously suggested from nearby paleoseismic trench studies.