surveying seismic microzonation in urban areas
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Surveying Seismic Microzonation in Urban Areas. Using Re fraction Mi crotremor reconnaissance: Rapid surveys take advantage of urban noise More details at www.seismo.unr.edu/hazsurv. Raw Data. >14 Hz Filter.TRANSCRIPT
J. Louie UNR Geol492/692 Spring J. Louie UNR Geol492/692 Spring 20052005
Using Refraction Microtremor reconnaissance: Rapid surveys take advantage of urban noise More details at www.seismo.unr.edu/hazsurv
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Surveying Seismic Microzonation in Surveying Seismic Microzonation in Urban AreasUrban Areas
J. Louie UNR Geol492/692 Spring J. Louie UNR Geol492/692 Spring 20052005
Surface waves show different velocities at different frequencies– but only after some distance.
Refraction Microtremor for Shallow Shear VelocityRefraction Microtremor for Shallow Shear Velocity
Initial funding from SCEC, UNR, VUW, Optim LLC
Raw DataRaw Data >14 Hz Filter>14 Hz Filter <4 Hz Filter<4 Hz Filter
J. Louie UNR Geol492/692 Spring J. Louie UNR Geol492/692 Spring 20052005
ReMi measures Rayleigh dispersion with linear refraction arrays (paper by Louie, April 2001 BSSA).
Refraction Microtremor for Shallow Shear VelocityRefraction Microtremor for Shallow Shear Velocity
Initial funding from SCEC, UNR, VUW, Optim LLC
The array has to have at least 12 channels; 150 m length; and recorded for 4 sec (for 30-m depth resolution and 1-Hz frequency resolution).
J. Louie UNR Geol492/692 Spring J. Louie UNR Geol492/692 Spring 20052005
City traffic gives appropriate 4-16 Hz signal.Refraction Microtremor for Shallow Shear VelocityRefraction Microtremor for Shallow Shear Velocity
Initial funding from SCEC, UNR, VUW, Optim LLC
J. Louie UNR Geol492/692 Spring J. Louie UNR Geol492/692 Spring 20052005
A wavefield transform from (x, t)-space to (p, f)-space shows phase-velocity dispersion (McMechan and Yedlin, 1981).
Refraction Microtremor for Shallow Shear VelocityRefraction Microtremor for Shallow Shear Velocity
Initial funding from SCEC, UNR, VUW, Optim LLC
J. Louie UNR Geol492/692 Spring J. Louie UNR Geol492/692 Spring 20052005
What if energy arrives transverse to the linear array?Refraction Microtremor for Shallow Shear VelocityRefraction Microtremor for Shallow Shear Velocity
Initial funding from SCEC, UNR, VUW, Optim LLC
J. Louie UNR Geol492/692 Spring J. Louie UNR Geol492/692 Spring 20052005
What if energy arrives transverse to the linear array?Refraction Microtremor for Shallow Shear VelocityRefraction Microtremor for Shallow Shear Velocity
Initial funding from SCEC, UNR, VUW, Optim LLC
J. Louie UNR Geol492/692 Spring J. Louie UNR Geol492/692 Spring 20052005
What is our low-frequency detection limit?Can we use standard 4-16 Hz geophones that are easy to deploy?
Cluster-test microtremor records show we get data down to 25-50% of the geophone frequency!
Refraction Microtremor for Shallow Shear VelocityRefraction Microtremor for Shallow Shear Velocity
Initial funding from SCEC, UNR, VUW, Optim LLC
J. Louie UNR Geol492/692 Spring J. Louie UNR Geol492/692 Spring 20052005
The minimum-velocity envelope represents the Rayleigh fundamental-mode dispersion curve.
Kinematic data only, that we model for a shear-velocity versus depth profile.
Refraction Microtremor for Shallow Shear VelocityRefraction Microtremor for Shallow Shear Velocity
Initial funding from SCEC, UNR, VUW, Optim LLC
J. Louie UNR Geol492/692 Spring J. Louie UNR Geol492/692 Spring 20052005
We don’t invert for Vs(z), we forward-model interactively by hand.
Refraction Microtremor for Shallow Shear VelocityRefraction Microtremor for Shallow Shear Velocity
100-m depth resolution
Initial funding from SCEC, UNR, VUW, Optim LLC
J. Louie UNR Geol492/692 Spring J. Louie UNR Geol492/692 Spring 20052005
Fieldwork is quick and simple; best results in cities.Refraction Microtremor for Shallow Shear VelocityRefraction Microtremor for Shallow Shear Velocity
Initial funding from SCEC, UNR, VUW, Optim LLC
J. Louie UNR Geol492/692 Spring J. Louie UNR Geol492/692 Spring 20052005
Fieldwork is quick and simple; best results in cities.Refraction Microtremor for Shallow Shear VelocityRefraction Microtremor for Shallow Shear Velocity
Initial funding from SCEC, UNR, VUW, Optim LLC
J. Louie UNR Geol492/692 Spring J. Louie UNR Geol492/692 Spring 20052005
Low-frequencies, 1-20 Hz, so bad geophone plants still work.
Refraction Microtremor for Shallow Shear VelocityRefraction Microtremor for Shallow Shear Velocity
Initial funding from SCEC, UNR, VUW, Optim LLC
J. Louie UNR Geol492/692 Spring J. Louie UNR Geol492/692 Spring 20052005
Low-frequencies, 1-20 Hz, so bad geophone plants still work.
Refraction Microtremor for Shallow Shear VelocityRefraction Microtremor for Shallow Shear Velocity
Initial funding from SCEC, UNR, VUW, Optim LLC
J. Louie UNR Geol492/692 Spring J. Louie UNR Geol492/692 Spring 20052005
Fieldwork is quick and simple; best results in cities.Refraction Microtremor for Shallow Shear VelocityRefraction Microtremor for Shallow Shear Velocity
Initial funding from SCEC, UNR, VUW, Optim LLC
J. Louie UNR Geol492/692 Spring J. Louie UNR Geol492/692 Spring 20052005
Fieldwork is quick and simple; best results in cities.Refraction Microtremor for Shallow Shear VelocityRefraction Microtremor for Shallow Shear Velocity
Initial funding from SCEC, UNR, VUW, Optim LLC
J. Louie UNR Geol492/692 Spring J. Louie UNR Geol492/692 Spring 20052005
Fieldwork is quick and simple; best results in cities.Refraction Microtremor for Shallow Shear VelocityRefraction Microtremor for Shallow Shear Velocity
Initial funding from SCEC, UNR, VUW, Optim LLC
J. Louie UNR Geol492/692 Spring J. Louie UNR Geol492/692 Spring 20052005
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 Velocity
J. Louie UNR Geol492/692 Spring J. Louie UNR Geol492/692 Spring 20052005
Local reconnaissance was first large-scale test.Reno TransectReno Transect
Supported by UNR and IRIS-PASSCAL
J. Louie UNR Geol492/692 Spring J. Louie UNR Geol492/692 Spring 20052005
Unexpected results: high V30 in active alluvium.Reno TransectReno Transect
West East
Ta
J. Louie UNR Geol492/692 Spring J. Louie UNR Geol492/692 Spring 20052005
Transect has results to 300 m depth, shown here to 500 m. V30 is only the shallow part that the IBC2000 requires measuring.
Reno TransectReno Transect
J. Louie UNR Geol492/692 Spring J. Louie UNR Geol492/692 Spring 20052005
Lack of V30 correlation to geologic or soil maps.Only Tertiary Andesite (Ta) shows a characteristic V30.
Reno TransectReno Transect
J. Louie UNR Geol492/692 Spring J. Louie UNR Geol492/692 Spring 20052005
Lack of V30 correlation to geologic or soil maps
Reno TransectReno Transect
J. Louie UNR Geol492/692 Spring J. Louie UNR Geol492/692 Spring 20052005
Basin info from 2000 gravity work.
Reno TransectReno Transect
J. Louie UNR Geol492/692 Spring J. Louie UNR Geol492/692 Spring 20052005
Most of basin fill is Tertiary.
Reno TransectReno Transect
J. Louie UNR Geol492/692 Spring J. Louie UNR Geol492/692 Spring 20052005
The First Reno Hazard MapThe First Reno Hazard Map
J. Louie UNR Geol492/692 Spring J. Louie UNR Geol492/692 Spring 20052005
We Follow Field’s (2001) We Follow Field’s (2001) Amplification-Mapping StrategyAmplification-Mapping Strategy
J. Louie UNR Geol492/692 Spring J. Louie UNR Geol492/692 Spring 20052005
July 2003 San Gabriel Valley & Los AngelesShallow Shear-Velocity TransectsShallow Shear-Velocity Transects
B
C
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 UNR Geol492/692 Spring J. Louie UNR Geol492/692 Spring 20052005
Los 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 UNR Geol492/692 Spring J. Louie UNR Geol492/692 Spring 20052005
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 UNR Geol492/692 Spring J. Louie UNR Geol492/692 Spring 20052005
Los Angeles Transect: ResultsLos Angeles Transect: Results
J. Louie UNR Geol492/692 Spring J. Louie UNR Geol492/692 Spring 20052005
VV3030 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 UNR Geol492/692 Spring J. Louie UNR Geol492/692 Spring 20052005
VV3030 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 UNR Geol492/692 Spring J. Louie UNR Geol492/692 Spring 20052005
Las Vegas route from Cheyenne to Tropicana
1-km-deep basin
3-km-deep basin
Supported by DOE-LLNL and IRIS-PASSCAL
Shallow Shear-Velocity TransectsShallow Shear-Velocity Transects
J. Louie UNR Geol492/692 Spring J. Louie UNR Geol492/692 Spring 20052005
49 velocity profiles to correlate with stratigraphyLas Vegas Transect MeasurementsLas Vegas Transect Measurements
J. Louie UNR Geol492/692 Spring J. Louie UNR Geol492/692 Spring 20052005
Transect DataTransect Data
High-velocity picks at >0.4 sec period are unreliable.
Picks at 0.1-0.3 sec illustrate velocity differences to 100 m depth.
J. Louie UNR Geol492/692 Spring J. Louie UNR Geol492/692 Spring 20052005
Some correlation to faulting, soil type?Las Vegas TransectLas Vegas Transect
J. Louie UNR Geol492/692 Spring J. Louie UNR Geol492/692 Spring 20052005
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 UNR Geol492/692 Spring J. Louie UNR Geol492/692 Spring 20052005
Does extrapolating VDoes extrapolating Vs s work? work? VVss assigned to soil-map units using central-City measurements. assigned to soil-map units using central-City measurements.
Later, outlying measurements (B. Luke, UNLV) not predicted by maps.Later, outlying measurements (B. Luke, UNLV) not predicted by maps.
J. Louie UNR Geol492/692 Spring J. Louie UNR Geol492/692 Spring 20052005
Does extrapolating VDoes extrapolating Vs s work? work? VVss assigned to soil-map units using central-City measurements. assigned to soil-map units using central-City measurements.
Later, outlying measurements (B. Luke, UNLV) not predicted by maps.Later, outlying measurements (B. Luke, UNLV) not predicted by maps.
1 23
4
J. Louie UNR Geol492/692 Spring J. Louie UNR Geol492/692 Spring 20052005
Most of Strip, Downtown; south side of Valley only
79 sites total
1000 well logs & geologic mapping
Las Vegas Las Vegas TransectTransect
J. Louie UNR Geol492/692 Spring J. Louie UNR Geol492/692 Spring 20052005
How to Extrapolate Shallow VHow to Extrapolate Shallow Vss Correlate 75 Vs measurements against stratigraphic model.Correlate 75 Vs measurements against stratigraphic model. Stratigraphy interpolated between >500 water-well logs.Stratigraphy interpolated between >500 water-well logs.
J. Louie UNR Geol492/692 Spring J. Louie UNR Geol492/692 Spring 20052005
Extrapolated vs. MeasuredExtrapolated vs. Measured Comparisons along Cheyenne-to-Tropicana transect.Comparisons along Cheyenne-to-Tropicana transect.
J. Louie UNR Geol492/692 Spring J. Louie UNR Geol492/692 Spring 20052005
No correlation between V30 and geologic map.Las Vegas TransectLas Vegas Transect
J. Louie UNR Geol492/692 Spring J. Louie UNR Geol492/692 Spring 20052005
Correlation with clay soils, caliche in Las Vegas?Las Vegas TransectLas Vegas Transect
J. Louie UNR Geol492/692 Spring J. Louie UNR Geol492/692 Spring 20052005
SummarySummary
ReMi allows rapid seismic shaking-hazard ReMi allows rapid seismic shaking-hazard
classification, as required by IBC2000.classification, as required by IBC2000.
Long ReMi transects can geophysically Long ReMi transects can geophysically
characterize spatial variations in shaking hazard.characterize spatial variations in shaking hazard.
Mapped soil and geologic units do not reliably Mapped soil and geologic units do not reliably
predict the measured Vpredict the measured V3030..
Our measurements suggest one can use river Our measurements suggest one can use river
gradients, Quaternary thickness, or alluvial fan gradients, Quaternary thickness, or alluvial fan
composition to predict shallow velocities.composition to predict shallow velocities.
J. Louie UNR Geol492/692 Spring J. Louie UNR Geol492/692 Spring 20052005
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OutreachOutreach
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