1973 a law called the Alquist-Priolo Earthquake Fault
Zoning Act was passed in
We conducted a
high resolution refraction investigation in a valley that contained several
parcels of rural ranch land. Most of the valley consisted of flat grassy
fields used for horse and cattle grazing. There were low hills to the east and
west. The geology consisted of young alluvium overlaying a
Quaternary-Tertiary bedrock. The most striking geomorphic feature of the area
was a northwest-southeast lineament formed by the base of the hills on the east
side of the valley. Several geologic studies suggested the lineament was caused
by an eroded escarpment. Some investigators attributed the escarpment to
ground rupture. Direct evidence of ground rupture was found in trenches
excavated two miles southeast of the site. There was good evidence that a fault
might run through the site though different geologists placed the fault in
different locations. A 1958 regional investigation inferred the fault ran
under the west side of the valley and regional investigations in 1980 and 1981
inferred the fault ran under the east side of the valley. The general consensus
was that the fault was on the east side of the valley.
Inferred Fault Traces
Plans called for subdividing the valley. Approximately 3500 feet by 1500 feet of the proposed development fell into a designated earthquake fault zone on the valley's east side. Many trenches were dug along the different inferred fault traces but only one trench found indications of ground rupture. The problem was the fault did not disrupt the young alluvium and the bedrock was too deep for trenching. The geologists needed to find shallow bedrock along the fault trace where they could dig a trench and obtain more evidence for building setbacks. Without this information, additional trenching was too costly and unproductive.
To locate the
fault, we performed a seismic refraction survey using the generalized
reciprocal method (GRM) for data collection. GRM differs from standard
refraction in that more shot points and geophones are used and a greater amount
of raw data is collected. Using this method we can map lateral changes in a
bedrock's compressional (P) wave velocity. An
abrupt change in velocity is an indication that a fault has juxtaposed to
materials with different seismic properties. GRM is a great technique for
finding faults when the bedrock on one side of the fault has a different P-wave
velocity than the bedrock on the other side.
We began this
investigation by collected four seismic refraction lines near the trench where
the fault was found. Based on the data from these lines another trench
was dug that successfully encountered the fault. The general strike of the
fault was determined from the four seismic lines and the two trenches. We
collected an additional 10 refraction lines using the initial data to predict
the fault's location. The lines varied from 200 feet to 600 feet in
length. Compressional waves were generated
using a hammer and plate for the shorter lines. Small explosive charges
were used to create P-waves for the longer lines. The data collected along some
lines showed very strong indications of faulting. There were offsets in the
arrival time diagrams, low P-wave velocity zones and offsets within the
bedrock. Data from other lines only showed small changes in the P-wave
Velocity Profiles Showing Strong Fault Indications
Velocity profiles Showing Possible Fault Indications
Collecting multiple lines across the fault was important. This method works best when there is a change in P-wave velocity on either side of the fault. Along one refraction line the materials on either side of the fault had the same P-wave velocity and the fault was not detected. Along four refraction lines, there was a small change in the P-wave velocity. We could not be sure if the small velocity changes were caused by the fault, bedding or by changes in weathering. Along the remaining nine lines there were a significant changes in the P-wave velocity across the fault. These changes were most likely caused by displacement across the fault. Using the nine significant anomalies and the four weaker ones, we could trace the fault across the eastern side of the valley.
Based on the
results of the refraction data, four additional trenches were dug that
successfully found ground rupture. In areas too deep to trench, boreholes were
drilled that confirmed the bedrock depths calculated from the refraction data.
A fault zone and building setbacks were successfully established from the
trenching and refraction data.
This case study illustrates how seismic refraction along with traditional trenching was used to locate a fault. The geophysical data gave the geologists the information needed to successfully locate the fault. Seismics is just one of many geophysical techniques offered by J R Associates. Please contact us to discuss how our services could benefit you.