J. Louie 18/8/2005J. Louie 18/8/2005
Refraction MicrotremorRefraction Microtremorfor Shallow Shear Velocityfor Shallow Shear Velocity
in Urban Basinsin Urban Basins
Refraction MicrotremorRefraction Microtremorfor Shallow Shear Velocityfor Shallow Shear Velocity
in Urban Basinsin Urban Basins
John Louie, Nevada Seismological Lab(at GNS & VUW through July 2006– [email protected])UNR students: J. B. Scott, T. Rasmussen, W. Thelen, M. Clark
Collaborators:S. Pullammanappallil & B. Honjas, Optim LLCW. J. Stephenson, R. A. Williams, & J. K. Odum, USGS
Support from:IRIS-PASSCAL Instrument Center at NMT
More details at www.seismo.unr.edu/hazsurv
John Louie, Nevada Seismological Lab(at GNS & VUW through July 2006– [email protected])UNR students: J. B. Scott, T. Rasmussen, W. Thelen, M. Clark
Collaborators:S. Pullammanappallil & B. Honjas, Optim LLCW. J. Stephenson, R. A. Williams, & J. K. Odum, USGS
Support from:IRIS-PASSCAL Instrument Center at NMT
More details at www.seismo.unr.edu/hazsurv
J. Louie 18/8/2005J. Louie 18/8/2005
OutlineOutlineOutlineOutline
1.1. Refraction Microtremor for Shallow VsRefraction Microtremor for Shallow Vs
2.2. ReMi-Borehole ComparisonReMi-Borehole Comparison
3.3. Los Angeles TransectLos Angeles Transect
4.4. Las Vegas TransectLas Vegas Transect
5.5. Effect of Shallow Vs on Shaking ModelsEffect of Shallow Vs on Shaking Models
1.1. Refraction Microtremor for Shallow VsRefraction Microtremor for Shallow Vs
2.2. ReMi-Borehole ComparisonReMi-Borehole Comparison
3.3. Los Angeles TransectLos Angeles Transect
4.4. Las Vegas TransectLas Vegas Transect
5.5. Effect of Shallow Vs on Shaking ModelsEffect of Shallow Vs on Shaking Models
J. Louie 18/8/2005J. Louie 18/8/2005
ReMi measures Rayleigh dispersion with linear refraction arrays (paper by Louie, April 2001 BSSA).
Refraction Microtremor for Shallow Shear VelocityRefraction Microtremor for Shallow Shear VelocityRefraction Microtremor for Shallow Shear VelocityRefraction Microtremor for Shallow Shear Velocity
100-m depth resolution
Initial funding from SCEC, UNR, VUW, Optim LLC
J. Louie 18/8/2005J. Louie 18/8/2005
Low-frequencies, 1-20 Hz, so bad geophone plants still work.
Refraction Microtremor for Shallow Shear VelocityRefraction Microtremor for Shallow Shear VelocityRefraction Microtremor for Shallow Shear VelocityRefraction Microtremor for Shallow Shear Velocity
Initial funding from SCEC, UNR, VUW, Optim LLC
J. Louie 18/8/2005J. Louie 18/8/2005
Fieldwork is quick and simple; best results in cities.Refraction Microtremor for Shallow Shear VelocityRefraction Microtremor for Shallow Shear VelocityRefraction Microtremor for Shallow Shear VelocityRefraction Microtremor for Shallow Shear Velocity
Initial funding from SCEC, UNR, VUW, Optim LLC
J. Louie 18/8/2005J. Louie 18/8/2005
Fieldwork is quick and simple; best results in cities.Refraction Microtremor for Shallow Shear VelocityRefraction Microtremor for Shallow Shear VelocityRefraction Microtremor for Shallow Shear VelocityRefraction Microtremor for Shallow Shear Velocity
Initial funding from SCEC, UNR, VUW, Optim LLC
J. Louie 18/8/2005J. Louie 18/8/2005
ReMi has classified hard and soft sites around the world by measuring V30, average shear velocity to 30 m depth.
Refraction Microtremor for Shallow Shear VelocityRefraction Microtremor for Shallow Shear VelocityRefraction Microtremor for Shallow Shear VelocityRefraction Microtremor for Shallow Shear Velocity
J. Louie 18/8/2005J. Louie 18/8/2005
OutlineOutlineOutlineOutline
1.1. Refraction Microtremor for Shallow VsRefraction Microtremor for Shallow Vs
2.2. ReMi-Borehole ComparisonReMi-Borehole Comparison
3.3. Los Angeles TransectLos Angeles Transect
4.4. Las Vegas TransectLas Vegas Transect
5.5. Effect of Shallow Vs on Shaking ModelsEffect of Shallow Vs on Shaking Models
1.1. Refraction Microtremor for Shallow VsRefraction Microtremor for Shallow Vs
2.2. ReMi-Borehole ComparisonReMi-Borehole Comparison
3.3. Los Angeles TransectLos Angeles Transect
4.4. Las Vegas TransectLas Vegas Transect
5.5. Effect of Shallow Vs on Shaking ModelsEffect of Shallow Vs on Shaking Models
J. Louie 18/8/2005J. Louie 18/8/2005
Four deep suspension logs in Santa Clara Valley
Collaboration with Stephenson, Williams, Odum (USGS), and Pullammanappallil (Optim), BSSA in press
Refraction, MASW, and ReMi at each hole
ReMi-Borehole ReMi-Borehole ComparisonComparison
ReMi-Borehole ReMi-Borehole ComparisonComparison
J. Louie 18/8/2005J. Louie 18/8/2005
No surface method can match log details.ReMi-Borehole ComparisonReMi-Borehole ComparisonReMi-Borehole ComparisonReMi-Borehole Comparison
J. Louie 18/8/2005J. Louie 18/8/2005
Depth-averaged velocities are a good match.But CCOC’s LVZ is a problem.
ReMi-Borehole ComparisonReMi-Borehole ComparisonReMi-Borehole ComparisonReMi-Borehole Comparison
J. Louie 18/8/2005J. Louie 18/8/2005
Joyner et al. (1981) quarter-wavelength spectra similar at important frequencies.ReMi-Borehole ComparisonReMi-Borehole ComparisonReMi-Borehole ComparisonReMi-Borehole Comparison
J. Louie 18/8/2005J. Louie 18/8/2005
OutlineOutlineOutlineOutline
1.1. Refraction Microtremor for Shallow VsRefraction Microtremor for Shallow Vs
2.2. ReMi-Borehole ComparisonReMi-Borehole Comparison
3.3. Los Angeles TransectLos Angeles Transect
4.4. Las Vegas TransectLas Vegas Transect
5.5. Effect of Shallow Vs on Shaking ModelsEffect of Shallow Vs on Shaking Models
1.1. Refraction Microtremor for Shallow VsRefraction Microtremor for Shallow Vs
2.2. ReMi-Borehole ComparisonReMi-Borehole Comparison
3.3. Los Angeles TransectLos Angeles Transect
4.4. Las Vegas TransectLas Vegas Transect
5.5. Effect of Shallow Vs on Shaking ModelsEffect of Shallow Vs on Shaking Models
J. Louie 18/8/2005J. Louie 18/8/2005
Los Angeles TransectLos Angeles TransectLos Angeles TransectLos Angeles Transect
J. Louie 18/8/2005J. Louie 18/8/2005
We Follow Field’s (2001) Amplification-Mapping StrategyWe Follow Field’s (2001) Amplification-Mapping StrategyWe Follow Field’s (2001) Amplification-Mapping StrategyWe Follow Field’s (2001) Amplification-Mapping Strategy
Two Inputs for Microzonation: V30 and Basin Depth (Z1.5?)
J. Louie 18/8/2005J. Louie 18/8/2005
July 2003 San Gabriel Valley & Los AngelesShallow Shear-Velocity TransectsShallow Shear-Velocity TransectsShallow Shear-Velocity TransectsShallow Shear-Velocity Transects
B-C
C-D
D
D-E
Transect mapped on NEHRP hazard class map by Wills, from SCEC Phase 3 Report
Supported by USGS, NEHRP ERP and IRIS-PASSCAL
J. Louie 18/8/2005J. Louie 18/8/2005
Los Angeles Transect: V30 ResultsLos Angeles Transect: V30 ResultsLos Angeles Transect: V30 ResultsLos Angeles Transect: V30 Results
J. Louie 18/8/2005J. Louie 18/8/2005
Los Angeles Transect: Full SectionLos Angeles Transect: Full SectionLos Angeles Transect: Full SectionLos Angeles Transect: Full Section
• Fast bouldery alluvium near ranges
• Low-velocity near-surface layers
thicken toward sea
• Vs constraint to 200 m depth
• Z1.0 only constrained over 1/3 of
transect– deep basin
SG MtsSG MtsWhittierNarrowsWhittierNarrows Seal BeachSeal Beach
J. Louie 18/8/2005J. Louie 18/8/2005
Boreholes in Open-File ReportsBoreholes in Open-File ReportsBoreholes in Open-File ReportsBoreholes in Open-File Reports
Four within 1 Four within 1 km of transectkm of transect
Also an Also an incomplete incomplete posting at posting at ROSRINE, ROSRINE, Pico Rivera 2Pico Rivera 2
Borehole Database Comparison—Data points within 1 km of transect
Source, Transect Array Number
Distance from Borehole
30-m Shear Velocity
% Difference NEHRP Class
Gibbs et al. (2000) ----------------- 226.9 m/s ---------------- D
This study, 186 850 m 309 m/s 36.18% D
This study, 187 269 m 301 m/s 32.66% D
This study, 188 345 m 284 m/s 25.17% D
This study, 189 930 m 251 m/s 10.62% D
Gibbs et al. (2001) ----------------- 298.7 m/s ---------------- D
This study, 140 661 m 401 m/s 34.25% C
This study, 141 442 m 338 m/s 13.15% D
This study, 142 955 m 424 m/s 41.95% C
Gibbs et al. (2001) ----------------- 544.7 m/s ---------------- C
This study, 121 870 m 580 m/s 6.48% C
This study, 122 673 m 538 m/s -1.23% C
This study, 123 997 m 498 m/s -8.57% C
Wills and Silva (1998)
----------------- 339.06 m/s ---------------- D
This study, 158 939 m 317 m/s -6.51% D
This study, 159 973 m 306 m/s -9.75% D
ROSRINE Borehole ----------------- 241.54 m/s ---------------- D
This study, 153 492 m 425 m/s 75.95% C
This study, 154 221 m 381 m/s 57.74% C
This study, 155 775 m 337 m/s 39.52% D
J. Louie 18/8/2005J. Louie 18/8/2005
Rosrine/USGS Pico Rivera 2Rosrine/USGS Pico Rivera 2Rosrine/USGS Pico Rivera 2Rosrine/USGS Pico Rivera 2 Good correlation with transect below 8 m depth.Good correlation with transect below 8 m depth.
0
50
100
150
200
250
0 200 400 600 800 1000 1200Shear-wave Velocity, m/s
Depth, m
ROSRINE picoriv2 borehole
SGRiv Survey Station 155A: ReMi
SGRiv Survey Station 156A: ReMi
SGRiv Survey Station 157A: ReMi
J. Louie 18/8/2005J. Louie 18/8/2005
Los Angeles Transect: V30 ResultsLos Angeles Transect: V30 ResultsLos Angeles Transect: V30 ResultsLos Angeles Transect: V30 Results
Nearby borehole results in red
J. Louie 18/8/2005J. Louie 18/8/2005
Measured VMeasured V3030 vs Wills et al. (2000) vs Wills et al. (2000)Measured VMeasured V3030 vs Wills et al. (2000) vs Wills et al. (2000)
Average Average measurements within measurements within ranges for classes B-ranges for classes B-C, D, and D-EC, D, and D-E
N. San Gabriel Val. N. San Gabriel Val. Measurements Measurements average above average above predicted C-D rangepredicted C-D range
60 new C-D data 60 new C-D data pointspoints
583 580
330
255
100
200
300
400
500
600
700
800
Wills et al. (2000) Predicted Hazard Class
Measured 30-m Shear Velocity
2003 UNR Measurements
Measured Averages
Wills et al. (2000) Averages andRanges
B
B-C
CC-D
D D-EE
J. Louie 18/8/2005J. Louie 18/8/2005
VV3030 vs Geologic Unit vs Geologic UnitVV3030 vs Geologic Unit vs Geologic Unit
Large VLarge V3030 variation inside each unit variation inside each unit
Large VLarge V30 30 variation between units variation between units
J. Louie 18/8/2005J. Louie 18/8/2005
VV3030 vs Soil Type vs Soil TypeVV3030 vs Soil Type vs Soil Type
In general, large VIn general, large V3030 variation within units variation within units Units 2 and 5 may be NEHRP DUnits 2 and 5 may be NEHRP D
Large VLarge V3030 variation between units variation between units
J. Louie 18/8/2005J. Louie 18/8/2005
VV3030 vs Riverbank Elevation vs Riverbank ElevationVV3030 vs Riverbank Elevation vs Riverbank Elevation Fast, bouldery alluvium at higher elevations on River’s alluvial fanFast, bouldery alluvium at higher elevations on River’s alluvial fan
River Profile vs. 30-m Shear Velocity
0
50
100
150
200
250
300
0 10 20 30 40 50 60
Distance from San Gabriel Range Front (km)
Elevation (m)
200
300
400
500
600
700
800
30-m Shear Velocity (m/s)
Levee & Path Elevation Profile
30-m Shear Velocity
J. Louie 18/8/2005J. Louie 18/8/2005
Spatial Statistics on VSpatial Statistics on V3030Spatial Statistics on VSpatial Statistics on V3030 Line in log-log Line in log-log
spectrum means spectrum means fractal spatial fractal spatial distributiondistribution
V30 less V30 less predictable as predictable as distance from distance from measurement measurement increasesincreases
““Noise Floor”- Noise Floor”- minimum minimum variance reached variance reached at 700-m at 700-m separationseparation
Incorporate Incorporate fractal dimension fractal dimension into PSHA?into PSHA?
San Gabriel River Vs30 Transect Spatial Power Spectra
SGRiv: y = 20823x -1.5913
R2 = 0.6705 D=1.70(D=1.78 w/o noise floor)
1.E+03
1.E+04
1.E+05
1.E+06
1.E+07
1.E+08
0.01 0.1 1 10
Spatial Frequency, /km
Power, m^2/s^2/km
NoiseFloor
J. Louie 18/8/2005J. Louie 18/8/2005
Conclusions IConclusions IConclusions IConclusions I
Long ReMi transects can geophysically Long ReMi transects can geophysically
characterize characterize spatial variationsspatial variations in shaking in shaking
hazard.hazard.
Soil and geologic units must be Soil and geologic units must be
specifically specifically mapped for velocitymapped for velocity, to , to
reliably predict measured Vreliably predict measured V3030..
210 measurements in LA 210 measurements in LA match match
predictionspredictions, and add to class C-D data., and add to class C-D data.
Long ReMi transects can geophysically Long ReMi transects can geophysically
characterize characterize spatial variationsspatial variations in shaking in shaking
hazard.hazard.
Soil and geologic units must be Soil and geologic units must be
specifically specifically mapped for velocitymapped for velocity, to , to
reliably predict measured Vreliably predict measured V3030..
210 measurements in LA 210 measurements in LA match match
predictionspredictions, and add to class C-D data., and add to class C-D data.
J. Louie 18/8/2005J. Louie 18/8/2005
OutlineOutlineOutlineOutline
1.1. Refraction Microtremor for Shallow VsRefraction Microtremor for Shallow Vs
2.2. ReMi-Borehole ComparisonReMi-Borehole Comparison
3.3. Los Angeles TransectLos Angeles Transect
4.4. Las Vegas TransectLas Vegas Transect
5.5. Effect of Shallow Vs on Shaking ModelsEffect of Shallow Vs on Shaking Models
1.1. Refraction Microtremor for Shallow VsRefraction Microtremor for Shallow Vs
2.2. ReMi-Borehole ComparisonReMi-Borehole Comparison
3.3. Los Angeles TransectLos Angeles Transect
4.4. Las Vegas TransectLas Vegas Transect
5.5. Effect of Shallow Vs on Shaking ModelsEffect of Shallow Vs on Shaking Models
J. Louie 18/8/2005J. Louie 18/8/2005
Las Vegas TransectLas Vegas TransectLas Vegas TransectLas Vegas Transect
J. Louie 18/8/2005J. Louie 18/8/2005
Las Vegas Shaking Computation, 2-secLas Vegas Shaking Computation, 2-secLas Vegas Shaking Computation, 2-secLas Vegas Shaking Computation, 2-sec
E3D synthetic-seismogram code courtesy of Shawn Larsen, LLNL
J. Louie 18/8/2005J. Louie 18/8/2005
Las Vegas Shaking Computation, 2-secLas Vegas Shaking Computation, 2-secLas Vegas Shaking Computation, 2-secLas Vegas Shaking Computation, 2-sec
33 seconds after Little Skull Mtn. earthquake, as Rayleigh wave enters Las Vegas.
LasVegas
LittleSkullMtn.
LittleSkullMtn.
J. Louie 18/8/2005J. Louie 18/8/2005
Las Vegas TransectLas Vegas TransectLas Vegas TransectLas Vegas Transect
J. Louie 18/8/2005J. Louie 18/8/2005
Most of Strip, Downtown; south side of Basin only
79 sites total
1145 well logs & geologic mapping
Las Vegas Las Vegas TransectTransect
Basin-depth contours in meters
J. Louie 18/8/2005J. Louie 18/8/2005
Some correlation to faulting, soil type?Las Vegas TransectLas Vegas Transect
J. Louie 18/8/2005J. Louie 18/8/2005
Geologic Info to Predict VGeologic Info to Predict Vss
NSL, July ‘03, sponsored by LLNL
Can soil maps predict VCan soil maps predict Vss??
J. Louie 18/8/2005J. Louie 18/8/2005
How to Extrapolate Shallow VHow to Extrapolate Shallow Vss
Correlate Correlate transect transect measurements measurements against Soil against Soil Map.Map.
Correlate 75 Correlate 75 Vs values Vs values against a against a stratigraphic stratigraphic model from model from 1145 water-1145 water-well logs.well logs.
SoilSoil
StratigraphyStratigraphy
Courtesy W. Taylor, UNLV, and J. Wagoner, LLNL
J. Louie 18/8/2005J. Louie 18/8/2005
How to Extrapolate Shallow VHow to Extrapolate Shallow Vss Predictions are good where many measurements exist.Predictions are good where many measurements exist.
Comparison of Las Vegas Vs30 Data and
200
300
400
500
600
700
0 2 4 6 8 10 12 14
Transect Distance South of Cheyenne, km
Vs30, km/s
UNR Transect Vs30 Measurements
Soil-Map Predictions
Stratigraphic-Model Predictions
J. Louie 18/8/2005J. Louie 18/8/2005
How to Extrapolate Shallow VHow to Extrapolate Shallow Vss
Predictions are not Predictions are not good where there good where there only sparse only sparse measurements.measurements.
Soil map Soil map predictions are not predictions are not conservative.conservative.
Stratigraphic model Stratigraphic model predictions are, at predictions are, at least, conservative.least, conservative.
Off-Transect Measurements and Predictions
200
400
600
800
1000
200 400 600 800 1000
Measured Vs30, m/s
Predicted Vs30, m/s
Soil-Map PredictionsStratigraphic-Model PredictionsNot ConservativeConservative
Not
Con
serv
ativ
e
Not
Con
serv
ativ
e
Conservative
Conservative
J. Louie 18/8/2005J. Louie 18/8/2005
OutlineOutlineOutlineOutline
1.1. Refraction Microtremor for Shallow VsRefraction Microtremor for Shallow Vs
2.2. ReMi-Borehole ComparisonReMi-Borehole Comparison
3.3. Los Angeles TransectLos Angeles Transect
4.4. Las Vegas TransectLas Vegas Transect
5.5. Effect of Shallow Vs on Shaking ModelsEffect of Shallow Vs on Shaking Models
1.1. Refraction Microtremor for Shallow VsRefraction Microtremor for Shallow Vs
2.2. ReMi-Borehole ComparisonReMi-Borehole Comparison
3.3. Los Angeles TransectLos Angeles Transect
4.4. Las Vegas TransectLas Vegas Transect
5.5. Effect of Shallow Vs on Shaking ModelsEffect of Shallow Vs on Shaking Models
J. Louie 18/8/2005J. Louie 18/8/2005
Building a Las Vegas Seismic ModelBuilding a Las Vegas Seismic ModelBuilding a Las Vegas Seismic ModelBuilding a Las Vegas Seismic Model
J. Louie 18/8/2005J. Louie 18/8/2005
Model Rendered as Amplification MapModel Rendered as Amplification Map Geology, Basin Depth, Geotech, Geophysical data into Geology, Basin Depth, Geotech, Geophysical data into ModelAssemblerModelAssembler
Las VegasBasinLas VegasBasin
LittleSkull Mtn.LittleSkull Mtn.
DeepVolcanic
Rifts
DeepVolcanic
Rifts
J. Louie 18/8/2005J. Louie 18/8/2005
Max. Ground Motion Computed– 0.5 HzMax. Ground Motion Computed– 0.5 Hz E3D elastic finite-difference solution, by Shawn Larsen, LLNLE3D elastic finite-difference solution, by Shawn Larsen, LLNL
Las VegasBasinLas VegasBasin
LittleSkull Mtn.
LittleSkull Mtn.
DeepVolcanic
Rifts
DeepVolcanic
Rifts
J. Louie 18/8/2005J. Louie 18/8/2005
Max. Ground Motion Computed– 0.1 HzMax. Ground Motion Computed– 0.1 Hz E3D elastic finite-difference solution, by Shawn Larsen, LLNLE3D elastic finite-difference solution, by Shawn Larsen, LLNL
Las VegasBasinLas VegasBasin
LittleSkull Mtn.LittleSkull Mtn.
DeepVolcanic
Rifts
DeepVolcanic
Rifts
J. Louie 18/8/2005J. Louie 18/8/2005
Detailed Model Makes a DifferenceDetailed Model Makes a Difference Max. ground motion ratio, models with and without geotechnical modelMax. ground motion ratio, models with and without geotechnical model
LittleSkull Mtn.
Las VegasBasinLas VegasBasin
J. Louie 18/8/2005J. Louie 18/8/2005
Detailed Model Makes a DifferenceDetailed Model Makes a Difference But not in any way that can be predicted from the model alone– basin But not in any way that can be predicted from the model alone– basin
geometry, source, and propagation path all matter!geometry, source, and propagation path all matter!
73% predicted for 2-4 Hz73% predicted for 2-4 Hz 6% computed for 0.1 Hz6% computed for 0.1 Hz
J. Louie 18/8/2005J. Louie 18/8/2005
Conclusions IIConclusions IIConclusions IIConclusions II
In tectonic areas, the regional In tectonic areas, the regional
distribution of distribution of basinsbasins affects shaking. affects shaking.
We have built a We have built a ModelAssemblerModelAssembler for for
Nevada to create 3-d computation grids Nevada to create 3-d computation grids
from geological and geotechnical data.from geological and geotechnical data.
Surprisingly, geotechnical Surprisingly, geotechnical detailsdetails affect affect
even 10-sec computations in ways even 10-sec computations in ways
difficult to forecast.difficult to forecast.
In tectonic areas, the regional In tectonic areas, the regional
distribution of distribution of basinsbasins affects shaking. affects shaking.
We have built a We have built a ModelAssemblerModelAssembler for for
Nevada to create 3-d computation grids Nevada to create 3-d computation grids
from geological and geotechnical data.from geological and geotechnical data.
Surprisingly, geotechnical Surprisingly, geotechnical detailsdetails affect affect
even 10-sec computations in ways even 10-sec computations in ways
difficult to forecast.difficult to forecast.
J. Louie 18/8/2005J. Louie 18/8/2005
Los Angeles TransectLos Angeles TransectLos Angeles TransectLos Angeles Transect
Approximately 60 km in lengthApproximately 60 km in length Followed San Gabriel River Bike PathFollowed San Gabriel River Bike Path 20 m takeout interval, 300 m array, recorded for 30 min20 m takeout interval, 300 m array, recorded for 30 min
4 teams, 3 people each, 4.5 days4 teams, 3 people each, 4.5 days 120 IRIS/PASSCAL “Texan” single-channel recorders 120 IRIS/PASSCAL “Texan” single-channel recorders
mated to a vertical 4.5-Hz geophonemated to a vertical 4.5-Hz geophoneSupported by USGS, NEHRP ERP and IRIS-PASSCAL
J. Louie 18/8/2005J. Louie 18/8/2005
Los Angeles Transect:Los Angeles Transect:Levee EffectsLevee Effects
Los Angeles Transect:Los Angeles Transect:Levee EffectsLevee Effects
VV30 30 levee: 245 m/s levee: 245 m/s
VV30 30 non-levee: 241 non-levee: 241 m/sm/s
J. Louie 18/8/2005J. Louie 18/8/2005
Basin Depth Model from USGS GravityBasin Depth Model from USGS Gravity Includes volcanic rift basins up to 9 km deep.Includes volcanic rift basins up to 9 km deep.
Las VegasBasinLas VegasBasin
LittleSkull Mtn.
LittleSkull Mtn.
DeepVolcanic
Rifts
DeepVolcanic
Rifts
Death ValleyDeath Valley