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Surveying Seismic Microzonation in Urban Areas

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. <4 Hz Filter. Refraction Microtremor for Shallow Shear Velocity.

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Surveying Seismic Microzonation in Urban Areas

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  1. Surveying Seismic Microzonation in Urban Areas • Using Refraction Microtremor reconnaissance: • Rapid surveys take advantage of urban noise • More details at www.seismo.unr.edu/hazsurv

  2. Raw Data >14 Hz Filter <4 Hz Filter Refraction Microtremor for Shallow Shear Velocity Surface waves show different velocities at different frequencies– but only after some distance. Initial funding from SCEC, UNR, VUW, Optim LLC

  3. Refraction Microtremor for Shallow Shear Velocity 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). Initial funding from SCEC, UNR, VUW, Optim LLC

  4. Refraction Microtremor for Shallow Shear Velocity City traffic gives appropriate 4-16 Hz signal. Initial funding from SCEC, UNR, VUW, Optim LLC

  5. Refraction Microtremor for Shallow Shear Velocity A wavefield transform from (x, t)-space to (p, f)-space shows phase-velocity dispersion (McMechan and Yedlin, 1981). Initial funding from SCEC, UNR, VUW, Optim LLC

  6. Refraction Microtremor for Shallow Shear Velocity What if energy arrives transverse to the linear array? Initial funding from SCEC, UNR, VUW, Optim LLC

  7. Refraction Microtremor for Shallow Shear Velocity What if energy arrives transverse to the linear array? Initial funding from SCEC, UNR, VUW, Optim LLC

  8. Refraction 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! Initial funding from SCEC, UNR, VUW, Optim LLC

  9. Refraction Microtremor for Shallow Shear Velocity The minimum-velocity envelope represents the Rayleigh fundamental-mode dispersion curve. Kinematic data only, that we model for a shear-velocity versus depth profile. Initial funding from SCEC, UNR, VUW, Optim LLC

  10. 100-m depth resolution Refraction Microtremor for Shallow Shear Velocity We don’t invert for Vs(z), we forward-model interactively by hand. Initial funding from SCEC, UNR, VUW, Optim LLC

  11. Refraction Microtremor for Shallow Shear Velocity Fieldwork is quick and simple; best results in cities. Initial funding from SCEC, UNR, VUW, Optim LLC

  12. Refraction Microtremor for Shallow Shear Velocity Fieldwork is quick and simple; best results in cities. Initial funding from SCEC, UNR, VUW, Optim LLC

  13. Refraction Microtremor for Shallow Shear Velocity Low-frequencies, 1-20 Hz, so bad geophone plants still work. Initial funding from SCEC, UNR, VUW, Optim LLC

  14. Refraction Microtremor for Shallow Shear Velocity Low-frequencies, 1-20 Hz, so bad geophone plants still work. Initial funding from SCEC, UNR, VUW, Optim LLC

  15. Refraction Microtremor for Shallow Shear Velocity Fieldwork is quick and simple; best results in cities. Initial funding from SCEC, UNR, VUW, Optim LLC

  16. Refraction Microtremor for Shallow Shear Velocity Fieldwork is quick and simple; best results in cities. Initial funding from SCEC, UNR, VUW, Optim LLC

  17. Refraction Microtremor for Shallow Shear Velocity Fieldwork is quick and simple; best results in cities. Initial funding from SCEC, UNR, VUW, Optim LLC

  18. Refraction Microtremor for Shallow Shear Velocity ReMi has classified hard and soft sites around the world by measuring V30, average shear velocity to 30 m depth.

  19. Reno Transect Local reconnaissance was first large-scale test. Supported by UNR and IRIS-PASSCAL

  20. Ta West East Reno Transect Unexpected results: high V30 in active alluvium.

  21. Reno Transect Transect has results to 300 m depth, shown here to 500 m. V30 is only the shallow part that the IBC2000 requires measuring.

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

  23. Reno Transect Lack of V30 correlation to geologic or soil maps

  24. Reno Transect Basin info from 2000 gravity work.

  25. Reno Transect Most of basin fill is Tertiary.

  26. The First Reno Hazard Map

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

  28. B C D E Shallow Shear-Velocity Transects July 2003 San Gabriel Valley & Los Angeles Transect mapped on NEHRP hazard class map by Wills, from SCEC Phase 3 Report Supported by USGS, NEHRP ERP and IRIS-PASSCAL

  29. Los Angeles Transect • Approximately 60 km in length • Followed San Gabriel River Bike Path • 20 m takeout interval, 300 m array, recorded for 30 min • 4 teams, 3 people each, 4.5 days • 120 IRIS/PASSCAL “Texan” single-channel recorders mated to a vertical 4.5-Hz geophone Supported by USGS, NEHRP ERP and IRIS-PASSCAL

  30. Los Angeles Transect:Levee Effects • V30 levee: 245 m/s • V30 non-levee: 241 m/s

  31. Los Angeles Transect: Results

  32. V30 vs Geologic Unit • Large V30 variation inside each unit • Large V30 variation between units

  33. V30 vs Soil Type • In general, large V30 variation within units • Units 2 and 5 may be NEHRP D • Large V30 variation between units

  34. Shallow Shear-Velocity Transects Las Vegas route from Cheyenne to Tropicana 3-km-deep basin 1-km-deep basin Supported by DOE-LLNL and IRIS-PASSCAL

  35. Las Vegas Transect Measurements 49 velocity profiles to correlate with stratigraphy

  36. Transect 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.

  37. Las Vegas Transect Some correlation to faulting, soil type?

  38. NSL, July ‘03, sponsored by LLNL Geologic Info to Predict Vs • Can soil maps predict Vs?

  39. Does extrapolating Vs work? • Vs assigned to soil-map units using central-City measurements. • Later, outlying measurements (B. Luke, UNLV) not predicted by maps.

  40. 2 1 3 4 Does extrapolating Vs work? • Vs assigned to soil-map units using central-City measurements. • Later, outlying measurements (B. Luke, UNLV) not predicted by maps.

  41. Las Vegas Transect Most of Strip, Downtown; south side of Valley only 79 sites total 1000 well logs & geologic mapping

  42. How to Extrapolate Shallow Vs • Correlate 75 Vs measurements against stratigraphic model. • Stratigraphy interpolated between >500 water-well logs.

  43. Extrapolated vs. Measured • Comparisons along Cheyenne-to-Tropicana transect.

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

  45. Las Vegas Transect Correlation with clay soils, caliche in Las Vegas?

  46. Summary • ReMi allows rapid seismic shaking-hazard classification, as required by IBC2000. • Long ReMi transects can geophysically characterize spatial variations in shaking hazard. • Mapped soil and geologic units do not reliably predict the measured V30. • Our measurements suggest one can use river gradients, Quaternary thickness, or alluvial fan composition to predict shallow velocities.

  47. Outreach • Example of public outreach: • Discoveries and Breakthroughs Inside Science, news shorts funded by AIP, AGU, etc. KFSN-TV Fresno, CA KGBT-TV Harlingen, TX KJTV-TV Lubbock, TX KTRK-TV Houston, TX KNBC-TV Burbank, CA KNSD-TV San Diego, CA KQTV-TV St. Joseph, MO KNTV-TV San Francisco, CA KRQE-TV Albuquerque, NM KTSM-TV El Paso, TX KTVN-TV Reno, NV KWQC-TV Davenport, IA KXAS-TV Fort Worth, TX WAGM-TV Presque Isle, ME WTEN-TV Albany, NY WCAU-TV Philadelphia, PA WCMH-TV Columbus, OH WCYB-TV Bristol, VA WEWS-TV Cleveland, OH WFTS-TV Tampa, FL WLUK-TV Green Bay, WI WBBH-TV Fort Myers, FL WHIO-TV Dayton, OH WIS-TV Columbia, SC WJAR-TV Cranston, RI WLBT-TV Jackson, MS WMAQ-TV Chicago,IL WNBC-TV New York, NY WNCN-TV Raleigh, NC KTSM-TV Omaha, NE WPBN-TV Traverse City, MI WSYM-TV Lansing, MI WTVJ-TV Miami, FL WVIT-TV Hartford, CT WVTM-TV Birmingham, AL WXYZ-TV Southfield, MI WJXT-TV Jacksonville, FL WTXL-TV Tallahassee, FL WIVT-TV Binghamton, NY WEVV-TV Evansville, IN WCJB-TV Gainesville, FL WPXI-TV Pittsburgh, PA CHAT-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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