Surveying Seismic Microzonation in Urban Areas

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


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


ReMi measures Rayleigh dispersion with linear refraction arrays (paper by Louie, April 2001 BSSA).



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).


City traffic gives appropriate 4-16 Hz signal.Refraction Microtremor for Shallow Shear VelocityRefraction Microtremor for Shallow Shear Velocity



A wavefield transform from (x, t)-space to (p, f)-space shows phase-velocity dispersion (McMechan and Yedlin, 1981).




What if energy arrives transverse to the linear array?Refraction Microtremor for Shallow Shear VelocityRefraction Microtremor for Shallow Shear Velocity



What if energy arrives transverse to the linear array?Refraction Microtremor for Shallow Shear VelocityRefraction Microtremor for Shallow Shear Velocity



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!




The minimum-velocity envelope represents the Rayleigh fundamental-mode dispersion curve.

Kinematic data only, that we model for a shear-velocity versus depth profile.




We don’t invert for Vs(z), we forward-model interactively by hand.


100-m depth resolution



Fieldwork is quick and simple; best results in cities.Refraction Microtremor for Shallow Shear VelocityRefraction Microtremor for Shallow Shear Velocity






Low-frequencies, 1-20 Hz, so bad geophone plants still work.




Low-frequencies, 1-20 Hz, so bad geophone plants still work.













ReMi has classified hard and soft sites around the world by measuring V30, average shear velocity to 30 m depth.



Local reconnaissance was first large-scale test.Reno TransectReno Transect

Supported by UNR and IRIS-PASSCAL


Unexpected results: high V30 in active alluvium.Reno TransectReno Transect

West East

Ta


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


Lack of V30 correlation to geologic or soil maps.Only Tertiary Andesite (Ta) shows a characteristic V30.



Lack of V30 correlation to geologic or soil maps



Basin info from 2000 gravity work.



Most of basin fill is Tertiary.



The First Reno Hazard MapThe First Reno Hazard Map


We Follow Field’s (2001) We Follow Field’s (2001) Amplification-Mapping StrategyAmplification-Mapping Strategy


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


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


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


Los Angeles Transect: ResultsLos Angeles Transect: Results


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


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


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


49 velocity profiles to correlate with stratigraphyLas Vegas Transect MeasurementsLas Vegas Transect Measurements


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.


Some correlation to faulting, soil type?Las Vegas TransectLas Vegas Transect


Geologic Info to Predict VGeologic Info to Predict Vss

NSL, July ‘03, sponsored by LLNL

Can soil maps predict VCan soil maps predict Vss??


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.


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


Most of Strip, Downtown; south side of Valley only

79 sites total

1000 well logs & geologic mapping

Las Vegas Las Vegas TransectTransect


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.


Extrapolated vs. MeasuredExtrapolated vs. Measured Comparisons along Cheyenne-to-Tropicana transect.Comparisons along Cheyenne-to-Tropicana transect.


No correlation between V30 and geologic map.Las Vegas TransectLas Vegas Transect


Correlation with clay soils, caliche in Las Vegas?Las Vegas TransectLas Vegas Transect


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.


Example of public outreach:Discoveries and Breakthroughs Inside Science, news shorts funded by AIP, AGU, etc.

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OutreachOutreach

KFSN-TV Fresno, CAKGBT-TV Harlingen, TXKJTV-TV Lubbock, TXKTRK-TV Houston, TXKNBC-TV Burbank, CAKNSD-TV San Diego, CAKQTV-TV St. Joseph, MOKNTV-TV San Francisco, CAKRQE-TV Albuquerque, NMKTSM-TV El Paso, TXKTVN-TV Reno, NVKWQC-TV Davenport, IAKXAS-TV Fort Worth, TXWAGM-TV Presque Isle, MEWTEN-TV Albany, NYWCAU-TV Philadelphia, PAWCMH-TV Columbus, OHWCYB-TV Bristol, VAWEWS-TV Cleveland, OHWFTS-TV Tampa, FLWLUK-TV Green Bay, WIWBBH-TV Fort Myers, FL

WHIO-TV Dayton, OHWIS-TV Columbia, SCWJAR-TV Cranston, RIWLBT-TV Jackson, MSWMAQ-TV Chicago,ILWNBC-TV New York, NYWNCN-TV Raleigh, NCKTSM-TV Omaha, NEWPBN-TV Traverse City, MIWSYM-TV Lansing, MIWTVJ-TV Miami, FLWVIT-TV Hartford, CTWVTM-TV Birmingham, ALWXYZ-TV Southfield, MIWJXT-TV Jacksonville, FLWTXL-TV Tallahassee, FLWIVT-TV Binghamton, NYWEVV-TV Evansville, INWCJB-TV Gainesville, FLWPXI-TV Pittsburgh, PACHAT-TV Medicine Hat, ALB K. Heineman, NewsProNet, Producers

Technology Transfer: Commercial ReMi is now in use by geotechnical consultants and agencies worldwide.

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Surveying Seismic Microzonation in Urban Areas

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