Active faults and fluid flow: constraints from near-surface seismic reflection data
Lee M. Liberty, Center for Geophysical Investigation of the Shallow Subsurface (CGISS), Department of
Geosciences, Boise State University, Boise, Idaho, USA
Summary
Seismic reflection imaging of stratigraphic and
hydrogeologic targets in the upper few hundred meters
provides key insights to earthquake hazards, contaminant
transport, and aquifer characterization studies. Slip rates,
fault lengths, fault properties, and lithologies can be
estimated from these data. Here, I present examples of
near-surface studies that have helped identify the risks and
resources that impact local communities.
Introduction
A critical need for urban centers throughout the world is an
assessment of hazards and groundwater resources. Active
crustal faults that extend through densely populated regions
may be a considerable risk and must be evaluated. Seismic
reflection methods provide key insights to slip rates and
fault lengths that paleoseismologists and urban planners
require. Near-surface seismic reflection methods can be
adapted to work in high-traffic areas at low cost. Similarly,
seismic reflection methods are well suited to characterize
groundwater resources and contaminant migration. With a
tie to borehole and stratigraphic information, seismic
reflection data can help track aquifer sands and identify
barriers and conduits to groundwater flow.
Active faults
Seismic reflection imaging to identify and characterize
active faults is most critical in the urban centers where the
risk to life and property is greatest. In the US Pacific
Northwest, glaciation has reset surface landscape where
offsets of less than a few meters (from a single earthquake)
may represent a significant risk related to active faulting.
Imaging both shallow and deep targets provides
information to assess both recent and past fault activity to
better constrain both the modern-day earthquake hazard
and long-term regional deformation. In the Portland,
Oregon area, the Portland Hills fault shows little sign of
active faulting from surface features, but seismic imaging
shows deformation to as shallow as a few meters depth
(Liberty et al., 2003). Figure 1 shows field methods and
shot gathers from this site. Dipping and offset late
Quaternary strata provide compelling evidence to suggest
this fault is active.
The Seattle fault extends through the Puget Lowland urban
corridor of more than 1 million people. Figure 1 shows
steeply dipping strata that form a monocline that defines
the near-surface expression of the blind Seattle thrust fault.
Land-based high-resolution seismic profiles confirm this
monocline extends more than 70 km and deforms postglacial deposits across the length of the fault (Liberty and
Pratt, 2008). Seismic images, coupled with uplifted marine
terraces and LiDAR identified scarps confirm the active
fault is capable of supporting >M7 earthquakes.
Active faults and fluid flow: constraints from near-surface seismic reflection data
Faults and fluid flow
Seismic reflection profiling in south-central Nevada, part of
the US Great Basin, reveal structural and hydrogeological
features that influence groundwater flow in alluvium and
underlying volcanic rock aquifers. Although the
permeability distribution and the role of faulting on fluid
flow are poorly constrained at this site, water table offsets
greater than 10 meters are identified on seismic images
(Figure 2). These offsets, near mapped scarps, suggest
faults are barriers to groundwater flow within the alluvium,
and possibly conduits to vertical flow within the vadose
zone (Liberty and Hodges, in review). Seismic reflection
images capture both the shallow water table reflector and
deeper structures that provide key insights to groundwater
flow directions and high permeability channels.
High-resolution seismic reflection profiles to image the
structure and stratigraphy in this same area help to map
both the cold water aquifer (e.g., Barrash and Dougherty,
1997; Liberty et al., 1999) and the underlying geothermal
aquifer (e.g., Liberty, 1998). Using a low-cost trailer-
Figure 2. Seismic section from an underground blast in central
Nevada. The water table reflector is offset upwards of 40 m
along the western limits of the profile (Liberty and Hodges, in
review).
Aquifer-Scale Studies
Industry seismic data from the western US offer insights
into ground water resources by helping to identify major
structural and stratigraphic boundaries that may control or
influence groundwater flow. Figure 3 shows an industry
seismic reflection profile from the Boise, Idaho area that
shed light into the depositional style and sequence of
mudstones, silts, and sands that may extend to the surface
in places (Squires and Wood, 2001; Wood, 1994).
Unfortunately these data do not image the upper few
hundred meters due to the acquisition design and
processing interests (Figure 3).
Figure 1. (top) Shot gathers showing reflector at ~10-20 m
depth using a sledge hammer source. Acquisition geometry
with geophones placed in concrete at 0.5 m spacing (from
Liberty et al., 2003). (bottom) Seismic reflection image
across the Seattle fault showing >40 degree dips and active
deformation (from Liberty and Pratt, 2008).
mounted hammer seismic source, high quality seismic
images are obtained to map groundwater aquifer targets.
The seismic reflection profile shown in Figure 3 was
acquired along a railroad line through a densely populated
urban area. Here, reflections to ~ 20 m depth are obtained
to tie to a set of water wells that identify sand aquifers. The
depositional dip observed in these data show that adjacent
water wells draw from different sand aquifers. The seismic
Active faults and fluid flow: constraints from near-surface seismic reflection data
data also show recharge efforts may not translate to
increased aquifer storage outside a small area surrounding
the injection well.
that seismic velocities strongly correlate with neutronderived porosity values at this site (Figure 4).
Figure 4. Results from a crosswell seismic reflection survey
showing level run velocity logs, a comparison of slowness
(1/velocity) vs. neutron-derived porosity values, and an
unprocessed and processed (wavefield separation) shot gather
from 21 m depth with 12.7 m borehole spacing. Note the
strong correlation between seismic boundaries and porosity.
Conclusions
Figure 3a. (top) Industry seismic reflection profile from Boise,
Idaho provides critical basin-scale information but lacks
stratigraphic details to identify sand aquifers at groundwater
depths (from Squires and Wood, 2001); (bottom) Highresolution seismic reflection profile showing stratigraphy in the
upper few hundred meters dipping to the west. A shallow
channel, unconformity, and faults provide key constraints to
the groundwater aquifer.
Crosswell seismic profiling confirms grain size (porosity)
variations in the Boise aquifer produce reflections observed
in Figure 3. The crosswell shot (Figure 4) shows three
distinct reflections in the upper 5 m of the section, from the
land surface, water table (at 2.4 m depth), and from
approximately 4.5-5 m depth. Also, reflections appear later
in the section at approximately 13 m, 17.5 m, and 21 m
depth (reflection from 21 m correlates with a basal clay and
does not appear on this shot gather due to source/receiver
geometry). Each reflection is distinct and closely ties with
hydrostratigraphic boundaries that appear on geophysical
logs. When we invert the level run data set (source and
receivers at the same depths between two wells), we see
I show that near-surface seismic reflection methods are
well suited for mapping hydrostratigraphy and structures on
both a basin scale and local scale to help communities
assess both resources and risks. These low-cost methods
can be utilized along busy roads through urban centers.
References
Barrash, W., and Dougherty, M. E., 1997, Modeling axially
symmetric and nonsymmetric flow to a well with
MODFLOW, and application to Goddard2 well test, Boise,
Idaho: Ground Water, 35, 4, 602-611.
Clement, W.P., Knoll, M.D., Liberty, L.M., Donaldson,
P.R., 1999, Michaels, P., Barrash, W., and Pelton, J.R.,
Geophysical surveys across the Boise Hydrogeophysical
Research Site to determine geophysical parameters of a
shallow, alluvial aquifer, Proceedings of SAGEEP,
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Liberty, L.M., 1998, Seismic reflection imaging of a
geothermal aquifer in an urban setting: Geophysics, 63, 4,
1285-1294.
Active faults and fluid flow: constraints from near-surface seismic reflection data
Liberty, L. M., Clement, W. P., and Knoll, M. D., 1999, A
comparison of surface seismic sources and receivers on
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Proceedings of SAGEEP, Environmental and Engineering
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Liberty, L. M., Clement, W. P., and Knoll, M. D., 2000,
Crosswell seismic reflection imaging of a shallow cobbleand-sand aquifer: an example from the Boise
Hydrogeophysical Research Site, Proceedings of SAGEEP,
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Liberty, L.M. Hemphill-Haley, M.A., and Madin, I.P.,
2003, The Portland Hills Fault: uncovering a hidden fault in
Portland, Oregon using high-resolution geophysical
methods, Tectonophysics, 368 (2003) 89–103.
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seismic reflection data, Bulletin of the Seismological
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sand aquifers: report to the Idaho Department of Water
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Wood, S. H., 1994, Seismic expression and geological
significance of a lacustrine delta in Neogene deposits of the
western Snake River plain, Idaho, AAPG Bulletin, 78, 1,
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