From March 13-19, 1998, the UNR
Geol 453/653 Geophysical Applications class conducted a
geophysical study of the Dixie Valley fault,
sponsored by the National Science Foundation. The view at left is of the study
area, looking east from the 1954 fault break down Cattle Rd. in southern Dixie
Valley.
The study area is a one and a half hour drive from Fallon, the closest town,
so the class had to make camp at the piedmont fault scarp, halfway between
the Highway 121 and the 1954 main break at the Stillwater Range front.
This view shows the western part of the study area, leading from Highway 121
up to the 1954 rupture near the mouth of Willow Canyon.
In this westward view 350 m east of the 1954 break you can see shot hole flags
leading up to the scarp.
This southwest view 100 m from the 1954 break shows the site of the high-resolution
hammer source points at 2 m intervals (orange flags).
The alluvial fan emanating from Willow Canyon is relatively fine-grained,
except for debris-flow deposits of granitic boulders. The most recent flow
left these natural levees.
Our study area in southern Dixie Valley is 50 km south of the geothermal
field developed by Oxbow Power, which is at the base of the second eastward
promontory of the Stillwater Range front in this northward view. Geophysical
data analyzed by CEMAT suggest steep normal faults in
the geothermal field; but the character of the fault may change to the south.
You can download the 350 kb panoramic movie of the study area, taken from the
1954 scarp, by clicking here.
There is also an anotated panorama.
The panoramas are in Apple's
QuickTime VR format (click to download free viewer software for
Mac and Windows 95 and NT machines). A much more detailed, 10 Mb panorama
is available on the Seismology Suns
here.
The subsurface dip and geometry of the Dixie Valley fault was investigated
with gravity measurements, and with a medium-resolution seismic reflection
line extending 3 km east of the 1954 rupture at Willow Canyon along Cattle
Rd. This rock drill from Cal-Nevada Precision Blasting placed 2 kg charges at 3 m
depth, at 60 m intervals along the south side of the road. A blue geophone
cable take-out is visible in the foreground, on the north side of the road.
In the low-lying playa area the shot holes were quickly drilled through sand and
silt. Up on the fan and in the debris-flow deposits, the drill had to penetrate
occasional granite boulders, which did not significantly delay this equipment.
Cal-Nevada Precision Blasting loaded each hole with ANFO and a PETN booster.
An armored geophone placed atop the hole triggered the seismograph recorder
upon the blast, via a radio link (recording equipment was donated to UNR by the
W. M. Keck Foundation).
Blasts were recorded by 8-Hz single geophones buried a few inches, connected
to a 48-channel cable totaling 730 m in length.
A Bison Galileo-21 recorded 4-second shot records on hard disk, which were
transferred on Zip disks to a Powerbook G3 for field processing using Louie's
RG system under
MachTen UNIX.
Results are posted on the Dixie Valley project
page.
In addition to medium- and high-resolution reflection, gravity, magnetics, and
shallow conductivity surveys, we also conducted a profile of time-domain
electromagnetic soundings across the 1954 fault break. Ron Petersen
kindly provided the equipment and training to us. At left is the transmitter
set at the corner of a 50 m loop, with the 1 m receiver loop in the center.
Ana Cadena (left), in addition to her responsibilities as a 653 student, ably provided
wonderful meals to the crew of a dozen students and faculty. Here she is assisted
by Christine Mann, a Geophysics program senior.