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.
Landslides
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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R Associates,
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