Delineating concealed faults and shallow subsurface geology along the Wasatch Front, Utah, USA, by integrating geophysical and trench data

Planning and development along the Wasatch front must compensate for the geological hazards associated with faulting. Studies show that future ruptures will probably occur along already existing zones of weakness. Although any surface faults can be detected by surficial geological mapping others have no visible expression and can only be located by subsurface investigations. Geophysical methods, such as seismic refraction, gravity, magnetic, and radar can be integrated with geotechnical engineering methods such as drilling and trenching to obtain a better understanding of the subsurface geology at a specific site. Seismic refraction, gravity and magnetic surveys conducted near Provo, Utah along the Wasatch Mountain front have helped delineate shallow, concealed faulting. The reduced, modelled gravity and magnetic data typically correlate well with the refraction depth sections generated using the General Reciprocal Method (GRM). Field observations coupled with the geophysical data facilitated the identification of a number of probable faults along Rock Canyon Road, allowing an estimate to be made of the magnitude and direction of throw on the fault. A normal fault associated with a prominent north-south scarp near Rock Canyon Road was chosen for a case study. This fault scarp was later trenched by the Utah Geological and Mineral Survey (UGMS) and provided a direct comparison with the geophysical data sets. As all the data sets at this site correlate well with the subsurface geology, confidence is increased in interpreting other parts of the line. The characteristics of subsurface deformation can be used to gain a better understanding of the potential for surface rupture at a site and thereby in planning and site development and in devising remedial measures to mitigate the effects of earthquakes in populated areas.