The effects of faulting and basin evolution on groundwater
flow at the Central Nevada Test Area
Lee M. Liberty1 and Rex Hodges2
1
Center for Geophysical Investigation of the Shallow Subsurface (CGISS), Boise State
University, 1910 University Drive, Boise, ID 83725-1536, lliberty@boisestate.edu
2
S.M. Stoller Corp., 2597 B ¾ Rd, Grand Junction, Co 81503, Rex.Hodges@gjo.doe.gov
Key words: fluid flow, faulting, Basin and Range
Abstract
Seismic reflection profiling at the Central Nevada Test Area (CNTA) in south-central
Nevada reveal structural and hydrogeological features that influence groundwater flow in
alluvium and underlying volcanic rock aquifers. CNTA is the site of the 1968 FAULTLESS
underground nuclear test. Low measured hydraulic conductivities within monitoring wells
suggest contaminant migration of appreciable distances away from the detonation zone is
unlikely. However, the permeability distribution and the role of faulting on fluid flow are poorly
constrained at this site, and throughout the Great Basin. At the CNTA site, we identify a complex
pattern of local flow directions that likely lead to south-directed flow at blast depths. Water table
offsets near mapped scarps suggest faults are barriers to groundwater flow within the alluvium,
and possibly conduits to vertical flow within the vadose zone. A network of high permeability
channels inferred beneath the CNTA alluvial fan may provide conduits for greater south-directed
groundwater flow within the alluvium. Fracture-controlled flow within volcanic tuffs at and
below blast depths is unknown, but remains an additional source of higher permeability flow.
Assuming groundwater flow parallels the axis of Hot Creek Valley, contaminated groundwater
may eventually discharge in southern Hot Creek Valley and not Railroad Valley as currently
modeled. Seismic interpretations, along with a network of monitoring wells can identify and
characterize contaminant migration and constrain the role of faulting and basin evolution on fluid
flow throughout the Great Basin.
Introduction
In 1968, the Atomic Energy Commission, now the U.S. Department of Energy, detonated
a 200 to 1,000 kiloton nuclear device at 975 m depth at the Central Nevada Test Area (CNTA) in
Hot Creek Valley, Nevada (Figure 1). The explosion created an extimated 200m diameter cavity
while a normal fault bounded graben formed and ruptured the surface as the cavity collapsed
(McKeown and Dickey, 1969; Thordarson, 1987). This nuclear test, termed FAULTLESS, was
designed to study the environmental and structural effects that might be expected should
subsequent higher yield underground nuclear tests be conducted in this vicinity. The event
produced an unexpected, large subsidence area with up to 4.5 m of vertical displacement along a
greater than two square km area (Figure 2). Much of the displacement occurred along existing
faults, many with Holocene (pre-blast) displacement (McKeown and Dickey, 1969; Sawyer and
Anderson, 1998). The purpose of our study was to determine the configuration of faults and
basin geometry, and the influence of geologic and tectonic site conditions on fluid flow both
within the valley fill alluvium and underlying volcanic rock aquifer.
Here, we present the results from a seismic reflection survey designed to help position
new downgradient alluvial wells to more accurately assess the risks of contaminant migration.
The 12.5 km survey consisted of five vibroseis seismic profiles that pass near six deep wells and
all extend near the central UC-1 blast well (Figure 2). We first describe the CNTA geologic
setting, relevant details of the FAULTLESS blast, and the seismic survey. We then present
seismic interpretations where we identify water table, alluvium, volcanic rock strata, and preand post-blast faults. Finally, we discuss the influence of structures and stratigraphy on local and
regional groundwater flow directions and rates, and the implications of faulting and basin
evolution on fluid flow throughout the Great Basin.
Geologic and Tectonic Setting
The CNTA site is located along the western margin the 200 km long Hot Creek Valley,
part of the Basin and Range physiographic province of central Nevada (Figure 1). The north-
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south oriented valley is located between the Pancake Range and Smokey Valley to the east and
the Hot Creek Range and Little Fish Creek Valley to the west. Mountain ranges adjacent to the
CNTA contain large volumes of extension-related Tertiary volcanic rocks overlying Paleozoic
marine strata (Ekren et al., 1973). The basins of the region contain upwards of a few km of
Neogene and younger sediments upon older strata exposed in the adjacent ranges (e.g., Effimoff
and Pinezich, 1981; Anderson et al., 1983; Liberty et al., 1994). The region is actively extending
approximately 3 mm/yr (Thatcher et al., 1999) and maximum vertical slip rates on late
Quaternary normal faults within Hot Creek Valley are estimated at 0.2-1.0 mm/yr (Sawyer and
Anderson, 1998). The CNTA surface topography slopes to the east approximately 2-3 degrees
along a broad alluvial fan that extends along the western margin of Hot Creek Valley (Figure 2).
The ephemeral Moores Station Wash, located at the base of the CNTA alluvial fan, drains Hot
Creek Valley to the south. The elevation difference between the adjacent Hot Creek Range and
the CNTA site is more than a 1,000 m.
The alluvial fan sediments below the CNTA comprise poorly sorted, low permeability
unconsolidated to consolidated silt, sand, gravel and fanglomerate deposits (Ekren et al., 1973).
The Moores Station Wash contains higher permeability well sorted gravels and sands. Channel
avulsion has likely responded to alluvial fan and tectonic evolution, possibly burying higher
permeability channel sediments beneath the alluvial fan (e.g., Peakall, 1998; Leeder and Mack,
2001). Tertiary volcanic rocks are exposed within a few km both east and west of the CNTA site
and are identified in wells below the CNTA site (Ekren et al., 1973).
The stratigraphy, defined in the central CNTA UC-1 blast well and related monitoring
wells (Figure 2) shows consolidated and unconsolidated Quaternary and Tertiary alluvium and
colluvium (QTa) to depths upwards of 1,000 m (Figure 3; Ekren et al., 1973; Dinwiddie and
Schroder, 1971). QTa, mostly derived from tuffs of the Hot Creek Range, grade into tuffaceous
sediments (Tv) that are observed to depths upwards of 1,200 m. Physical property measurements
in monitoring wells include sonic, density, neutron-derived porosity, and resistivity logs suggest
a subtle distinction between QTa and Tv units (Figure 3). Sonic logs show seismic velocities of
approximately 2,500 m/s at water table depths with a general increase of 1.5 m/s/m to more than
3,200 m/s at the depth of Tv. Density measurements at equivalent depths range from 2.2-2.4
g/cc. Both sonic and density measurements are consistent with unconsolidated to consolidated
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coarse-grained deposits, with the general velocity and density increase related to sediment
compaction (e.g., Mavco et al., 1998).
Beneath Tv, 25 Ma and older semi-welded and welded tuffs and rhyolites (Tov) are
identified in six CNTA wells (Ekren et al., 1973; Dinwiddie and Schroder, 1971). Again,
geophysical log measurements suggest strong similarities between Tv and Tov, but the thinbedded welded tuffs show a sharp increase in seismic velocity, density, and resistivity compared
to semi-welded tuffs, and provides a strong reflection coefficient (Figure 3). Paleozoic strata,
observed in the adjacent mountain ranges, likely underlie Tov at the CNTA, but boreholes do not
extend to adequate depths to estimate the total volcanic rock thickness or underlying
stratigraphy.
Faultless Explosion and related faulting
The 975 m depth FAULTLESS blast occurred on January 19, 1968 in well UC-1 (Figure
2). The shot, emplaced in the Tv strata, fractured this unit and the overlying alluvium with the
equivalent energy release of a M6-6.5 earthquake (McKeown and Dickey, 1969). The detonation
produced an estimated 200 m diameter cavity, 600 m chimney, and an asymmetric collapse
graben bounded by several fractures and surface scarps (Figure 2). The ground surface near UC1 is a nearly triangular subsidence block bounded by fresh fault scarps up to 3,600 m long. Some
of the displacement occurred at the time of detonation, with additional displacement related to
post-shot subsidence. The bulk of blast-related slip occurred primarily along 3 lineaments, all
containing a surface expression prior to the blast that suggests Holocene tectonic motion. The
maximum vertical displacement that resulted in the FAULTLESS blast was 4.5 m along a ~2 km
southeast perimeter scarp here termed Fault A. The northwest perimeter blast-related scarp, here
termed Fault B, has measured vertical displacements upwards of 2 m. Upwards of 3 m of vertical
displacements appear along a scarp adjacent to well UC-1. Horizontal displacements upwards of
1 m were also observed on these faults.
Surface ruptures followed underground nuclear explosions of similar magnitude at the
nearby Nevada Test Site (McKeown and Dickey, 1969). However, vertical displacements from
these blasts should not have exceeded a few cm (Bucknam, 1972). The remaining displacements
likely result from the release of tectonic strain that had accumulated at both CNTA and Nevada
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Test Site. In fact, the relationship between rupture lengths, fault displacements, and energy
release from the CNTA and Nevada Test Site blasts with typical of earthquakes of similar
magnitude (McKeown and Dickey, 1969; Wells and Coppersmith, 1994). However, the shallow
depth of blast-related faulting compared to deeper naturally occurring earthquakes, and the
paucity of aftershocks related to the blast suggest strain accommodation may have been
restricted to the upper few km (Bucknam, 1972). It is unclear how the present day strain affects
fault zone and formation permeability, but here we assume the processes that activated the fault
scarps are similar to naturally occurring earthquakes, and that groundwater flow across these
blast-induced faults is similar in process to naturally occurring earthquake-activated faults (e.g.,
Bruhn et al., 1994; Caine et al, 1996).
Hydrogeologic Setting
CNTA surface water flow is directed locally to the east along alluvial fan deposits and
southward down the valley axis along the Moores Station Wash (Figure 2). Typical
sedimentation related to alluvial fan formation is fine- to coarse-grained semi-continuous
deposits extending from the mountain range (e.g., Weissmann et al, 2004), consistent with well
log observations (Figure 3). Ground water flow associated with the alluvial fan deposits likely
follows coarse-grained deposits to the east with vertical connectivity between independent
meandering channels. Deep groundwater flow within Paleozoic strata has been modeled as
flowing eastward toward the topographically low Railroad Valley (Dinwiddie and Schroder,
1971; Prudic et al., 1995).
Volcanic tuffs (Tv) at blast depths have measured hydraulic conductivities of 0.00005
meters/day or less while hydraulic conductivities in overlying QTa can span orders of magnitude
(e.g., Weissmann et al, 2004), but have lower measured values (Lyles et al., 2006). Estimates of
higher permeability within Tov strata likely result from more developed joint and fracture
systems that provide greater ease of groundwater flow compared to overlying strata (Pohlmann
et al., 2000). However, poor connectivity between more permeable tuffs likely due to
discontinuities between generally thin tuff layers.
North of the CNTA, hydraulic head measurements suggest a recharge environment,
characterized by hydraulic heads that decrease with depth (Figure 1; Dinwiddie and Schroder,
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1971). Head measurements approximately 15 km south of the CNTA within Hot Creek Valley
show hydraulic heads increasing with depth and suggest ground water discharge conditions are
present. Although the valley does not contain strong vertical hydraulic gradients (Dinwiddie and
Schroder, 1971; Prudic et al., 1995), horizontal and vertical connectivity via faults within the
tuffs and overlying alluvium is possible.
Detonation depressed water levels in the immediate vicinity of the cavity are still
recovering to pre-shot levels (Thordarson, 1987). Contaminated groundwater from the blast has
not been observed in nearby monitoring wells and is believed to be presently contained within
the blast radius. However, given the estimated 600 m chimney height that resulted from the blast,
contaminated groundwater has likely entered the overlying alluvium (Thorardson, 1987). Water
levels from wells screened in the alluvium in and around the CNTA range from 78 – 229 m
depth while the estimated chimney height places contaminants possibly as shallow as 600 m
depth.
The influence of faulting on fluid flow is complex, with control factors that range from
chemical to mechanical processes (e.g., Caine et al., 1996). Sigda and Wilson (2003) suggest
faults in poorly lithified basin fill in the Rio Grande rift may act as conduits to vertical flow
within the vadose zone while large-offset faults in similar materials often are barriers to lateral
and vertical flow under saturated conditions (e.g., Sigda et al., 1999; Rawling et al., 2001). The
continued recovery of water levels 40 years after the FAULTLESS blast is further support of
little to no connectivity along fractures within the alluvium. Fractures in more competent rocks
like volcanic tuffs at and below CNTA blast levels may act as barriers or conduits to ground
water flow depending on local geologic conditions. The discontinuities between mapped faults
and tuff layers (Ekren et al., 1973) suggest that even if conduit flow is locally present,
contaminated groundwater may not migrate large distances along fault surfaces.
Seismic Methods
We acquired 12.5 km of seismic reflection data along 5 profiles using a 7,700 kg
vibroseis truck (Figure 2). We acquired all seismic data with a 5 m source and receiver spacing
along existing roads within the CNTA. We recorded uncorrelated, off-end, 10 s sweeps from 20160 Hz with a 120 channel Geometrics RX seismograph. We used 10 Hz geophones at offsets up
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to 600 m to optimally image strata in the upper one km. Processing steps included vibroseis
correlation, velocity analyses, band pass filters, mutes, static corrections, and post-stack Kirchoff
migration (e.g., Yilmaz, 2001). The seismic wavefield from the origin to travel times less than
the air wave direct arrival (<330 m/s) were removed due to the strong ground roll and air wave
energy overwhelming the seismic reflection energy. Due to the fast velocity of the reflections
below the water table, this data mute significantly improved the quality of the stacked sections.
We converted two-way travel time seismic sections to depth using average velocity values
obtained from refracted head wave, stacking velocity values, and borehole geophysical logs
(Figure 3). The sections are corrected to a 1900 m datum to tie each seismic profile to nearby
well log information. Each seismic section shows surface elevation, measured water table
elevation, unmigrated travel time, and migrated and depth converted seismic sections (Figures 4
and 5).
The water table generally provides a large impedance contrast within alluvium due to the
large velocity increase of saturated sediments compared to unsaturated sediments (e.g., Mavco et
al., 1998). Although water content below the water table is not directly measured with seismic
methods, changes in basin stratigraphy are interpreted from our seismic reflection data in part
from borehole geophysical measurements and interpretations (Figure 3), and hydraulic
conductivity and permeabilities are tied to these interpreted lithologies. Our seismic
interpretations away from borehole locations are tied to reflector continuity and amplitudes that
are dependent on seismic velocity and density values.
East-West Seismic Profiles
Two seismic profiles cross the CNTA from east to west (Figure 2). Profile 5 crosses
Fault B near the highest relief scarp of 2.0 m while Profile 4 crosses Fault B where only a few
cm of vertical relief is observed. The 4.2 km long Profile 4 extends east from the topographic
center of the valley west along the main CNTA access road to the foothills of the Hot Creek
Range (Figure 2). Profile 4 crosses Fault A at positions 4358, approximately 50 m south of UC-1
at position 4475, and Fault B at either position 4680 or position 4695. The elevation rises nearly
200 m from east to west and terminates within 1,000 m of Tertiary rock exposures (Figure 4;
Ekren et al., 1973). The 1.2 km long Profile 5 crosses Fault B at position 5144 with an increase
in surface topography of nearly 100 m (Figure 5).
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The unmigrated east-west Profiles 4 shows reflections to more than 1.0 s, equivalent to
depths greater than the adjacent borehole logs (Figure 3). Two prominent high amplitude
reflection zones separate a region of few coherent reflections (Figure 4). A prominent shallow
reflector within 0.3 s of the surface datum shows a consistent eastward dip that mimics the
surface topography, with a shallower apparent dip along most of the profile. We observe
significant down to the east steps on this reflector, especially at positions 4350, 4650, and 4720.
Outside the central well area, the reflector is more consistent in amplitude, while diffuse within a
~250 m radius of blast well UC-1. First arrival or refracted wave velocity measurements
associated with this boundary increase from 500-800 m/s to more than 2,500 m/s, consistent with
a transition from unsaturated and saturated sediments.
We identify a second reflection package on Profile 4 between 0.5-1.5 s (Figure 4).
Reflectors within this deeper zone dip asymmetrically toward the central well UC-1 area. We
observe westward dips from 5-15 degrees along the eastern portions of Profile 4, and we observe
eastward dips from 15-25 degrees along the western portions of both Profiles 4 and 5. Reflector
dips and continuity increase with increasing travel time. The transition from the seismically
transparent zone to the higher amplitude and laterally continuous reflectors is consistent in depth,
seismic velocity, and amplitude with the transition from QTa to Tv (Figure 3). The transition
from shallower dipping, less continuous reflectors to steeper dipping reflectors with a greater
lateral continuity is consistent in depth with a transition from Tv to Tov. The thickness of the Tv
zone increases toward the center of the profile near the change in reflector dip near position
4550, consistent Tv deposition after Tov deformation via faulting.
South-North Seismic Profiles 1, 2, and 3
Profiles 1, 2, and 3 all extend through the central CNTA with a roughly south-north
orientation (Figure 2). These profiles extend across a number of CNTA monitoring wells, and
lithologic and geophysical measurements from these nearby wells are used to constrain our
interpretations (Figure 5). The 2.75 km long Profile 1 trends north-northwest approximately 1.25
km southeast of central well UC-1, crosses Fault A at position 1182 and Fault B at position 1440.
Profile 1 rises in elevation to the northwest approximately 100 m and extends past wells HTH-1,
HTH-2 and MV-3 (Figure 2). The 2.5 km long Profile 2 is located along the south-north CNTA
access road, crosses Fault A near the maximum displacement of 4.5 m at position 2170, crosses
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approximately 50 m east of well UC-1 at position 2270, and merges with Profile 3 at position
2230. The 1.5 km Profile 3 splits from Profile 2 at position 2229 and extends beyond MV-1 to
the northeast. North of well UC-1, Profile 3 parallels the surface contours of the CNTA alluvial
fan.
Two prominent reflection zones of on all three south-north profiles separate a zone of
relative seismic transparency (Figure 5), similar in character to the east-west seismic profiles.
The prominent reflector located within 0.3 s of the surface dips to the south or east along the
northern portions of each south-north profile, while this reflector is near flat-lying south of Fault
A. We measure seismic velocities of 500-800 m/s above this reflector and velocities greater than
2,500 m/s below this reflector, consistent with the transition from unsaturated to saturated
sediments. Reflector steps near Faults A and B and a decrease in reflector continuity within the
central blast zone suggest lithologic and/or chemical processes related to the blast are influencing
fluid flow at the CNTA.
A second set of reflectors on Profiles 1, 2, and 3 appear between 0.8-1.4 s beneath the
seismically transparent zone (Figure 5). These reflectors dip to the southeast upwards of 15
degrees along the northwestern portions of the profiles, and dips flatten to less than 5 degrees
along the southeastern portions of the profiles. The transition from transparent zone to semicontinuous reflectors is consistent with the transition from QTa to Tv and below from Tv to Tov
(Figure 3).
Interpretation and Discussion
Geologic Structure
When combining the interpretations from all seismic profiles, we characterize Hot Creek
Valley in the vicinity of the CNTA is an asymmetric half graben with extension focused
primarily along the western portions of the basin (Figures 4, 5, and 6). This basin geometry is
similar to adjacent basins that reflect Neogene and younger extension and filling (e.g., Effimoff
and Pinezich, 1981; Anderson et al., 1983; Liberty et al., 1994). The basin-controlling normal
fault(s) trend N35E and dip eastward between 60-65 degrees, typical of normal faults throughout
the region. We calculate a 25 degree eastward Tov dip northwest of this fault and an oblique
southwestward Tov dip of 15 degrees southeast of this fault. Dips on Tv and QTa strata increase
9
with increasing depths due to growth faulting and basin filling. At Tv and Tov depths, the normal
faults are located within 0.5 km west of the FAULTLESS blast.
Profile 4 best shows the structural style of Hot Creek (Figures 4 and 6). QTa strata
upwards of 700 m thick overlie Tv and Tov strata. Increasing reflector dips with depth and an
increase of Tv strata near the basin center suggest growth faulting that began prior to ~10-20 Ma.
Surface scarps that predate the FAULTLESS blast, and the magnitude of blast-related surface
ruptures, suggest the basin is still actively extending. Assuming a 60 degree fault, 1,500 m of
vertical slip from the surface to the 30 Ma Tov strata immediately west of the blast zone, and the
additional elevation difference from the Hot Creek Range, we estimate a slip rate of 0.1 mm/yr
on the basin-controlling fault system, well within the estimated slip rates for the area (Sawyer
and Anderson, 1998). This slip is distributed among a number of fault strands, including Fault B
(Figure 6).
Shallow alluvial aquifer
The strong amplitude near-surface reflector that appears on all seismic sections is
consistent in depth and character with the large velocity contrast at the water table at estimated
depths upwards of approximately 150 m. This reflector is semi-continuous across each seismic
profile and offset at a number of locations (Figures 4 and 5). Water table offsets near mapped
scarps suggest faults associated with the scarps are barriers to fluid flow. Scarp heights of less
than a meter result in water table elevation changes of more than 10 m, suggesting that even
faults with minor amounts of vertical displacement may strongly influence groundwater flow
within the alluvium.
Water table offsets do not match the surface expression of the scarps as seen across the
Fault A (Figure 2). Here, the fault scarp is down to the northwest, yet the water table deepens to
the southeast (Figures 4 and 5). This sense of offset is consistent with water table elevations
decreasing from northwest to southeast, similar to interpreted regional groundwater flow
directions (Dinwiddie and Schroder, 1971; Prudic et al., 1995). Given the unexpected vertical
relief of scarps after the blast and activation along pre-existing faults, we cannot rule out whether
the identified faults were barriers to ground water flow prior to the FAULTLESS blast. If we
assume water table offsets represents 40 years of restricted flow (the time between blast and
seismic survey), hydraulic conductivities of 0.0005-0.0025 m/day would be required to account
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for head measurements across the faults. These values are more than 10 times the maximum
borehole hydraulic conductivity measurements (Lyles et al., 2006). More likely, many of these
faults predated the FAULTLESS blast and represent long-term barriers to flow at the CNTA.
Indeed, the westernmost fault along Profile 4 was not identified as blast-ruptured fault
(McKeown and Dickey, 1969; Ekren et al., 1973), yet water table offsets across this fault
approach 40 m. The likely cause of the reduced or redirected flow across the faults is mechanical
or chemical changes within the fault zone (e.g., Bruhn et al., 1994; Sigda et al., 1999; Rawling et
al., 2001; Sims et al., 2005).
Water table offsets are significantly greater across Fault A compared to Fault B (Figures
4 and 5). This contrast may reflect greater fracturing within the zone surrounding Fault A that
results in an improved fault seal, increased horizontal flow immediately upgradient of Fault A,
the arcuate geometry of Fault A that better traps downgradient flow by inhibiting lateral flow
along the fault boundary, or it may reflect the vertical flow within the vadose zone from an
unlined storage pond previously situated between Profile 1 and Profile 4 (Figure 2). Although
this pond is presently empty, water likely remained in the storage pond for some time after the
blast. Faults acting as conduits for increased vertical flow were observed by Sigda and Wilson
(2003) in similar materials, suggesting surface water at the CNTA may more easily enter the
groundwater as a result of the FAULTLESS blast. Given the change in water table dip across
Fault A (Figures 4 and 5), lateral flow restrictions in the shallow aquifer are clear, but water table
elevation changes across the scarps may be enhanced by vertical flow within the vadose zone.
Further evidence for faults influencing groundwater flow appears along the northern
portion Profile 3 where the water table reflector (apparent) dips approximately 1 degree (20
m/km) to the south (Figures 5 and 6). This portion of the profile follows the surface contour and
contrasts the regional shallow groundwater flow directions to the east as observed along Profiles
1, 2, 4, and 5 (Figures 4 and 5). We interpret the fault extending from the blast center across
Profile 3 near position 3300 as a barrier to lateral downgradient (eastward) flow where
groundwater locally flows southwest toward the blast center. However, a high permeability
damage zone that parallels the faults may promote increased south-directed fluid flow near the
blast cavity (e.g., Bruhn et al., 1994; Caine et al., 1996; Rawling et al., 2001).
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Within the blast zone, the water table reflector is discontinuous and more difficult to
track compared to areas outside the blast scarps (Figures 4 and 5). The poor quality of the water
table reflector within approximately ~500 m diameter of well UC-1 suggests the blast may have
highly disrupted the alluvium beyond the estimated 200 m diameter of the blast cavity. The
fractured zone within the alluvium likely resulted from displacement related to the blast and not
from tectonic strain accommodation that manifested along Faults A and B. Although an
estimated 200 m diameter cavity resulted from the blast, the zone of influence related to the blast
at water table elevationsapproaches 500 m.
Deep alluvial aquifer
The surface topography at the CNTA is typical of alluvial fan deposits building west to
east from the Hot Creek Range (Figure 2). Given that the topographic low associated with Hot
Creek Valley at the Moores Station Wash is offset from the structural center of Hot Creek Valley
immediately west of well UC-1 by more than 2 km, we infer that the sedimentation related to the
CNTA alluvial fan has outpaced basin subsidence (Figure 6). Ephemeral surface water flow in
Hot Creek Valley presently drains to the south along Moores Station Wash east of the CNTA
alluvial fan (Figure 6). Generally, these coarser-grained deposits have higher hydraulic
conductivities, and may act as conduits to greater groundwater flow compared to the alluvial fan
deposits (e.g., Mackey and Bridge, 1995; Heller and Paola, 1996; Leeder and Mack, 2001). As
the CNTA alluvial fan evolved, fluvial channels that axially drained Hot Creek Valley to the
south likely migrated eastward to the present-day location of the Moores Station Wash from the
structural center of Hot Creek Valley at the CNTA fault (e.g., Mackey and Bridge, 1995; Peakall,
1998). Westward avulsion of fluvial channels may have resulted from fault slip on the basin
bounding faults, but the positioning of paleo-axial channels is largely controlled by the alluvial
fan growth. There is no direct evidence from existing boreholes that high permeability,
continuous fluvial channels remain beneath the CNTA, and if present, their measured hydraulic
conductivities or connectivity. However, the likely path of channel migration with basin
evolution extends through the blast cavity (Figure 6).
We infer a complex pattern of groundwater flow in the alluvial aquifer at the CNTA
where faulting in the alluvium acts as a barrier to lateral flow and conduits for vertical
groundwater flow. Northwest of Fault B, we observe southeast-directed groundwater flow in the
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alluvium that mimics the surface and bedrock topography. Flow likely follows higher
conductivity alluvial channels to the southeast until faults redirect flow to the south.
Southeast of Fault A and northwest of the toe of the CNTA alluvial fan, we interpret
generally south-directed deep groundwater flow within coarse-grained meanders of the paleo
Moores Station Wash channel that parallels the axis of Hot Creek Valley. Given the continuous
reflectors and absence of faulting within Tv and Tov units on the southern and eastern portions
of Profiles 1, 2, and 4, we infer that the volcanic strata confine deep alluvial groundwater flow to
within QTa strata outside the blast radius. Tv and Tov strata dip to the southwest within this
region, and based on borehole logs (Ekren et al., 1973) and gravity data (Saltus and Jachens,
1995), the basin increases in depth generally to the south.
We characterize the groundwater flow directions between Faults A and B as
multidirectional. Temperature effects from the blast suggest that the area near well UC-1 may
still act as a sink with groundwater flowing toward the blast cavity (Thordarson, 1987; Pohlmann
et al., 2001). The southwest trend of Faults A and B and the underlying, possibly confining,
stratigraphy may direct groundwater flow to the southwest, as observed in the shallow water
table along the northern portions of Profile 3. Paleo fluvial channels that drain the valley to the
south may also direct groundwater toward the south. These directions differ from the regional
flow direction to the east and flows within the alluvial fan generally to the southeast.
Volcanic rock aquifer
Many deep reflectors from within Tv and Tov units are continuous for hundreds of
meters, but not across the length of the profiles (Figures 4 and 5). This reflection pattern is likely
due to the nature of volcanic tuff deposits. As sonic logs show, welded tuff layers show a clear
increase in sonic velocity. The thickness of these layers is generally less than 50 m, and since
many are tied to discontinuous reflectors, are likely discontinuous across the site and region. For
example, Profile 1 crosses within 100 m of wells HTH-1 and MV-3 (Figure 5). The single
welded tuff layer encountered in well MV-3 at 1101 m elevation lies stratigraphically above the
welded tuff layer was encountered in well HTH-1 at 650 m elevation. Given the very low
hydraulic conductivities within the Tv, and that these units are mostly discontinuous across the
CNTA, lateral fluid flow within the volcanic rocks is very slow, but has the potential to follow
the geologic dip of the volcanic units generally to the south (Figure 6).
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Our results do not constrain either lateral or vertical groundwater flow rates within
fractures in volcanic tuffs or within the blast zone. However, the geometry of mapped faults that
generally trend north-south (Figure 6; Ekren et al., 1973) suggest that if permeability increases in
fractures and deformation zones, fluid flow would likely follow this trend and not toward deep
groundwater flow to the east.
Conclusions
High quality seismic images at the CNTA in Hot Creek Valley, Nevada show water table
reflector and underlying stratigraphy. These data provide the framework for basin-scale and local
groundwater flow directions. We identify Hot Creek Valley as an asymmetric fault graben with
normal faults identified beneath the CNTA and near the cavity of an underground nuclear blast.
Our interpretations suggest that locally complex groundwater flow directions in the shallow and
deeper alluvial aquifer leads to generally south-directed regional groundwater flow. Both preand post-blast fault scarps act as barriers to lateral groundwater flow, and these same faults may
enhance vertical flow within the vadose zone and along the fault surfaces. Buried paleochannels,
inferred from the offset of structural and topographic lows may provide conduits for higher
permeability channels. The style of faulting and basin evolution is similar throughout the Great
Basin where ground water resources are scarce. Although seismic methods do not directly
measure hydraulic conductivities, key structural and hydrostratigraphic controls can offer
considerable insight into ground water flow within basins throughout the area.
Acknowledgements
Field assistance was provided by Boise State University graduate students Kristin Pape,
Troy Brosten, and Vijay Gottumukkula and John and Elizabeth Healey from the Desert Research
Institute (DRI). Land access was granted by the Bureau of Land Management. Funding was
provided by S.M. Stoller Corp. through the U.S. Department of Energy Office of Legacy
Management contract number DE-AC01-02GJ79491. Data processing was made possible
through a Landmark Graphics software grant.
14
References
Anderson, R.E., Zoback, M.L., and Thompson, G.A., 1983, Implications of selected subsurface
data on the structural form and evolution of some basins in the northern Basin and Range
province, Nevada and Utah, Geological Society of America Bulletin, 94, 9, 1055-1072.
Bruhn, R.L. Parry, W.T., Yonkee, W.A., and Thompson, T., 1994, Fracturing and hydrothermal
alteration in normal fault zones, PAGEOPH, 142, 609-644.
Bucknam, R.C., 1972, Vertical deformation produced from some underground nuclear
explosions, Bulletin of the Seismological Society of America, 62, 4, 961-971.
Caine, J.S., Evans, J.P., and Forster, C.B., 1996, Fault zone architecture and permeability
structure, Geology, 24, 1, 1025-1028.
Dinwiddie, G.A., Schroder, L.J., 1971. Summary of Hydraulic Testing in and Chemical Analysis
of Water Samples from Deep Exploratory Holes in Little Fish Lake, Monitor, Hot Creek,
and Little Smoky Valleys, Nevada. U.S. Geological Survey Open-File Report USGS474-90.
Effimoff, I. and Pinezich, A.R., 1981, Tertiary structural development of selected valleys based
on seismic data: Basin and Range province, northeastern Nevada, Royal Society of
London, Philosophical Transactions, 300, p. 435-442.
Ekren, E.B., Hinrichs, E.N., Quinliven, W.D., and Hoover, D.L., 1973, Geologic map of the
Moores Station Quadrangle, Nye County: U.S. Geological Survey Miscellaneous
Investigation Series Map I-756, scale 1:48,000
Leeder, M.R. and Mack, G.H., 2001, Lateral erosion (‘toe-cutting’) of alluvial fans by axial
rivers: implications for basin analysis and architecture, Journal of the Geological Society,
London, 158, 885-893.
Liberty, L.M., Heller, P.L., and Smithson, S.B., 1994, Seismic reflection evidence for two-phase
development of Tertiary basins from east-central Nevada, Geological Society of America
Bulletin, v. 106, p. 1621-1633.
Lyles, B., Oberlander, P., Gillespie, D., Donithan, D., and Chapman, J., 2006, Hydrologic data
and evaluation for wells near the Faultless underground nuclear test, Central Nevada Test
15
Area, submitted to the Nevada Site Office, U.S. Department of Energy, Publication
No. 45219, 64 p.
Mavko, G., Mukerji, T., Dvorkin, J., 1998, The rock physics handbook : tools for seismic
analysis in porous media, Cambridge ; New York : Cambridge University Press, 329 p.
McKeown, F.A. and Dickey, D.D., 1969, Fault displacements and motion related to nuclear
explosions, Bulletin of the Seismological Society of America, 59, 6, 2253-2269.
Peakall, J., 1998, Axial river evolution in response to half-graben faulting: Carson River,
Nevada, U.S.A., Journal of Sedimentary Research, 68, 5, 788–799.
Pohlmann, K., Hassan, A., and Chapman, J., 2000, Description of hydrogeologic heterogeneity
and evaluation of radionuclide transport at an underground nuclear test, Journal of
Contaminant Hydrology, 44, 353-386.
Prudic, D.E., Harrill, J.R., Burbey, T.J., 1995, Conceptual Evaluation of Regional Ground-Water
Flow in the Carbonate Rock Province of the Great Basin, Nevada, Utah, and Adjacent
States. U.S. Geological Survey Professional Paper 1409-D. 93-170.
Rawling, G. C., Goodwin, L. B., and Wilson, J. L., 2001, Internal architecture, permeability
structure, and hydrologic significance of contrasting fault-zone types: Geology, v. 29, p.
43-46.
Saltus, R.W. and Jachens, R.C., 1995, Gravity and basin-depth maps of the Basin and Range
Province, Western United States, U.S. Geological Survey Geophysical Investigations
Map GP-1012.
Sawyer, T.L., and Anderson, R.E., compilers, 1998, Fault number 1355, Kawich-Hot Creek
Ranges fault zone, in Quaternary fault and fold database of the United States: U.S.
Geological Survey website, http://earthquakes.usgs.gov/regional/qfaults
Sigda, J.M., Goodwin, L.B., Mozley, P.S., and Wilson, J.L., 1999, Permeability alteration in
small-displacement faults in poorly lithified sediments: Rio Grande rift, central New
Mexico, in Haneberg, W.C., et al., eds., Faults and subsurface fluid flow in the shallow
crust: American Geophysical Union Monograph 113, p. 51–68.
16
Sigda, J. M., and J. L. Wilson, 2003, Are faults preferential flow paths through semiarid and arid
vadose zones?, Water Resources Research, 39, 8, doi:10.1029/2002WR001406.
Sims, D.W., A.P. Morris, D.A. Ferrill, and R. Sorkhabi, 2005, Extensional fault system evolution
and reservoir connectivity. In Sorkhabi, R. and Tsuji, Y. (Eds.), Faults, Fluid Flow, and
Petroleum Traps. American Association of Petroleum Geologists Memoir 85, American
Association of Petroleum Geologists, Tulsa, OK, USA, p. 79-93.
Thatcher, W., G. R. Foulger, B. R. Julian, J. Svarc, E. Quilty, and G. W., Bawden, 1999, Presentday deformation across the Basin and Range Province, Western United States, Science,
283, 1714–1718.
Thordarson, W., 1987, Hydrogeology of the Faultless site, Nye County, Nevada, U.S. Geological
Survey Water-Resources Investigations Report 86-4342.
Weissmann, G.S., Zhang, Y., Fogg, G.E., Mount, J.F., 2004, Influence of incised valley fill
deposits on hydrogeology of a stream-dominated alluvial fan, SEPM Special Publication
No. 80, 15-28.
Yilmaz, O. (2001). Seismic data analysis: processing, inversion, and interpretation of seismic
data, Society of Exploration Geophysicists, Tulsa, Oklahoma, 2027 p.
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Figure Captions
Figure 1. The Central Nevada Test Area is located in Hot Creek Valley, central Nevada (box). A
southeast-directed regional groundwater flow direction is derived from regional surface
and borehole measurements (from Prudic et al., 1995). Inset map shows site location in
central Nevada.
Figure 2. Aerial photographic and surface topographic map of the CNTA site with seismic line
(orange lines), every 100th shot (red circle), scarp (black lines), and borehole locations
(blue circles). Bold faults represent rupture during the FAULTLESS blast (McKeown and
Dickey, 1969; Ekren et al., 1973). The topography slopes toward the southeast along the
CNTA alluvial fan, while surface water flows south along the Moores Station wash.
Figure 3. Borehole geophysical logs including sonic, density, neutron, and lithologic logs for
MV-1 and MV-2. Note the general increase in P-wave velocity and density with depth
within the alluvium due to compaction. Also note the sharp increase in physical
properties where densely welded tuffs are identified. Red, green, and blue curves for the
same log represent separate runs as the well was drilled.
Figure 4. (a) Surface and water table elevation profiles for Profile 4, scarp and well locations,
and interpreted faults that extend from the surface. Squares represent water table
elevations along cross lines (Figure 5); (b) unmigrated travel time section; (c) interpreted
migrated, and depth converted seismic section. We identify a strong reflection from the
water table and from Tertiary volcanic rock strata. The thin dashed lines represent blastrelated faulting, while the bold dashed line represents a pre-blast fault that ties the surface
scarp to the change in Tv and Tov dip. Rectangle at UC-1 represents the inferred blast
chimney with circle denotes the blast cavity. QTa= Quaternary and Tertiary alluvium;
Tv=Tertiary tuffaceous sediments; Tov=Tertiary older volcanic rocks.
Figure 5. Surface and water table elevation profiles, unmigrated travel time section and
interpreted migrated, and depth converted seismic section for a) Profile 1, b) Profile 2, c)
Profile 3, and d) Profile 4. Scarp and well locations are labeled. Square boxes represent
water table elevations along cross lines. QTa= Quaternary and Tertiary alluvium;
Tv=Tertiary tuffaceous sediments; Tov=Tertiary older volcanic rocks.
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Figure 6. (a) Block diagram of the CNTA site, geometry of faults and stratigraphy, and surface
aerial photo. Arrows represent surface and groundwater flow directions.
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Figure 1
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Figure 2
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Figure 3
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Figure 4
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Figure 5
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Figure 6
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