Body and Surface Wave Seismic
Tomography for Regional
Geothermal Assessment of the
Western Great Basin
Glenn Biasi1, Leiph Preston2, and Ileana Tibuleac1
Nevada Seismological Laboratory, University of Nevada
Reno, Reno, NV 89557
2Sandia National Laboratory, Albuquerque, NM
Overview
• Objective
– Develop crustal seismic velocity coverages for Nevada for
correlation with regional geothermal indicators
• Opportunity
– Earthscope Transportable Array is the first seismic network
to provide coverage of all of Nevada and the Great Basin.
– Combine with the Nevada permanent seismic network
• Approach
– Tomographic imaging using body waves (P and S) arrival
times and surface wave coverage
• This talk: Progress report showing results using the
combined TA and UNR seismic networks
Station coverage
Transportable Array (triangles),
400 stations on ~70 km grid,
two year station occupation
Funded by the National Science
Foundation through the
Earthscope program
(www.earthscope.org)
Station coverage and details at
http://anf.ucsd.edu
Broadband seismometers
Combine with permanent
Nevada network (green)
This work: 275 total stations
Why Combine Body and Surface Waves?
• Coverage
– Body waves (P, S) provide the highest resolution
– Transportable Array station spacing (~70 km)
means body waves travel deep in the crust.
– Surface waves sample the shallow crust but with
lower resolution
• Collateral estimates of rock physical properties
– Raleigh waves are sensitive to the shear-wave
velocity of the shallow crust.
Body Wave Tomography
• -- Model: 10x10 km blocks, 5 km thick layers
• -- 169,000 total P- and S- arrival time
measurements from ~6,900 earthquakes
• -- Invert with a 3-D Vidale-Hole eikonal (raybased) code
• Inversion accounts for 80.5% of the traveltime residuals
Body Wave Inversions – hit quality
(Coverage is good)
Vp in the 5-10 km
layer
P-wave velocities from
5.5 to 6.35 km
Body Wave Results
15-20 km depth
-- Major faults separate
high velocity blocks
-- Velocities may be
affected by fracturing or by
structural down-dropping
-- High-velocity bodies
often correspond with
Paleozoic and older
terrains.
Body Wave results
-- Vs correlates with Vp in
most cases. The Carson Sink
(Fal) is a prominent low
structurally linked to
extension and strike slip
faulting of Walker Lane
faults.
-- Volcanic centers (small
triangles) tend to edges of
higher velocity bodies
Surface Wave Tomography
• Dispersion curves (wave speed vs. frequency)
are measured from larger earthquakes and
blasts
• Dispersion curves relate to average velocity on
the earthquake-station path
• Many fewer paths are available
• Invert with 50x50 km blocks; higher resolution
is planned
Surface Wave Velocity
Estimation
Fundamental Raleigh Wave group
velocity estimate, 5.5 second period
-- Surface waves sensitive to structural
lows and shallow (<5 km) crustal
velocities.
-- Resolution is lower in the south and
east – not as many earthquakes
Note: Color scale is reversed: red is
slow.
Group velocities at 5.5 and 10 second periods.
Example physical property application
• Vp below average – composition, temperature,
lithology, fracturing, structure.
– Attenuation? Correlates with fracturing, anisotropy
• Vp/Vs up: decreases Vs more than Vp?
Fracturing, saturation, composition.
– Below 2-3 km, saturation can be assumed.
• Regional models give large scale structural
context for hand off to other methods or more
detailed inversions.
Conclusions
• Transportable Array data provides
unprecedented coverage.
• Regional scale correlations indicate underlying
major structures are being recovered:
– Velocity reductions consistent with deep-seated
crustal shear
– Paleozoic and older structural terrains
– Detailed geologic interpretations are in process