Fault-Rupture Hazard Zones

Alquist-Priolo Fault Study Case History

Finding the location of faults on properties with deep sections of young sediments is a common problem for site investigations. At these sites a fault can be inferred through a property even though there are no visible indications of rupture on the surface or in trenches. Should the trace of the fault be implied by drawing a line between the last known points? What is needed is a picture of the faulted bedrock below the young sediments.

We provide several techniques for locating faults when direct observations fail.  The two most common techniques are seismic refraction and magnetics.  High resolution seismic refraction looks for offsets in bedrock, low velocity zones and other telltale indications of faulting. Magnetic techniques look for offset or truncated bedrock. Magnetics is particularly useful in serpentinites and ultramafic iron bearing rocks. Other techniques include seismic reflection and resistively profiling.

Slide ProfileLandslides

High resolution seismic refraction is also useful for mapping landslides.  The diagram above shows the results of a refraction profile collected over a landslide. Some investigators thought the slide to be near surface soil slumping.  Other investigators though it was a deeper rotational slide.  The profile revealed two slide planes, shallow slumping and a deeper rotational slide.


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