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 coverageTransportable 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.eduBroadband seismometersCombine 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 (ray-

based) code• Inversion accounts for 80.5% of the travel-

time residuals

Body Wave Inversions – hit quality(Coverage is good)

Vp in the 5-10 km layerP-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