High-Resolution Seismic Imaging Investigations along the Wasatch Front, Utah

Seismic Profile near Salt Lake, Base map courtesy of F. Ashland, Utah Geological Survey
Seismic Profile near Salt Lake, Base map courtesy of F. Ashland, Utah Geological Survey

In collaboration with researchers at the Utah Geological Survey, University of Utah, Utah State University, Brigham Young University, San Diego State University, and private industry, the Geologic Hazards Team Intermountain West MegaProject has conducted several high-resolution seismic imaging investigations along the Wasatch front to better characterized earthquake hazards and ground motion in this region. These studies, to date, have been focused in the Salt Lake and Utah Valleys. The information gathered from the various seismic surveys will be able to fill in gaps about the subsurface geologic structure from 100-1000 feet, as well as illuminate the dip of the  Wasatch  fault and depth to bedrock.

Article in Fault Line Forum Newsletter - vol 22, 2006 (1.36MB PDF)

USGS Minivib III seismic source truck
The USGS Minivib III seismic source truck has a pad between the front and back wheels that is hydraulically pushed against the road and gently vibrates the ground. View is to the northeast between Spanish Fork and Mapleton, Utah with the Wasatch Mountains in the background.

Two different types of shaker trucks have been used in the studies.  The small ones called “minivibes” gently shake the ground and provide information about the ground structure down to about 100 feet (? m) below the surface.  The larger-scale shaker trucks call “Liquidators” generate long-period (low-frequency) seismic waves that penetrate deeper into the crustal structure down to 1000 feet (? m).  The shaker trucks can shake the ground both in the vertical direction to generate P-waves, or in the horizontal direction to generate S-waves.From the echoes that are recorded as the seismic waves bounce back to the surface from the various geologic structures beneath, we can create a subsurface image and estimate how fast seismic waves travel beneath a site. Both the image and the wave speeds are important to more accurately determine how the soils and rock will behave during an earthquake.

P-wave Seismic Imaging In Western Salt Lake Valley

P-wave Seismic Image Beneath Western Salt Lake Valley
P-wave Seismic Image Beneath Western Salt Lake Valley

As part of an initial pilot experiment to determine if the minivibe would work for imaging in the Salt Lake Valley, we conducted a P-wave imaging experiment along a busy residential arterial street in western Salt Lake Valley. Our target beneath the two-km-long profile was to image two important subsurface layers beneath the valley, known as the R1 and R2 reflectors, which are believed to affect earthquake ground motion.

Minivibe generating P-waves in Western Salt Lake Valley
Minivibe generating P-waves in Western Salt Lake Valley

The image reveals a series of reflections (echoes) from soil and rock layers to as deep as about 800 meters depth beneath the heavily-trafficked street. The R1 and R2 reflectors, along with a third deeper reflector, are interpreted on the image. This interpretation will be incorporated in three-dimensional models of the basin for earthquake simulations.

S-wave Seismic Imaging in the Salt Lake Valley

Minivibe generating S-waves in the Salt Lake Valley
Minivibe generating S-waves in the Salt Lake Valley

In addition to the P-wave study, we have also obtained S-wave velocities and images at seven sites along the Wasatch front. Shallow S-wave velocities have a large effect on the ground motion experienced at a site during an earthquake. The goals of this experiment are: 1) Obtain S-wave velocities and images to as deep as possible with the minivibe in the urbanized sedimentary basins along the Wasatch front; 2) Incorporate these deeper S-velocities and geologic structure into the next-generation earthquake hazard calculations along the Wasatch front.

S-wave Seismic Image South of the Great Salt Lake
S-wave Seismic Image South of the Great Salt Lake

The S-wave seismic image and velocity structure from the southern end of the Great Salt Lake, west of the Salt Lake airport, reveals flat lying soil and rock layers. Using different types of "echoes" (reflections and refractions), the velocity structure is determined to over 200 m depth. Each of the colored bands represents a different soil or rock layer, each getting progressively older from top to bottom.

Seismic Imaging in the Utah Valley

USGS Minivib III seismic source truck along roadway at mouth of Rock Canyon in Provo, Utah
The USGS Minivib III seismic source truck along roadway at mouth of Rock Canyon in Provo, Utah.

In the Spanish Fork-Mapleton areas of the Utah Vallley, we collected a 3-mile long, high-resolution P-wave seismic-reflection survey using a Minivibe.  The profile extends west from the mountain front to the middle of Utah Valley along Highway 147 and E1600S in Mapleton. These images will allow us to determine the dip of the Wasatch fault and the depths to bedrock and other layers in the 50- to 1500-m depth range, thus constraining both the faulting history and subsurface models for earthquake simulations.

USGS Minivib III seismic source truck along roadway between Spanish Fork and Mapleton, Utah
USGS scientists collecting high-resolution seismic data along the highway between Spanish Fork and Mapleton, Utah. View is looking east with the Wasatch Mountains in the background.