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J. Louie 18/8/2005 Refraction Microtremor for Shallow Shear Velocity in Urban Basins John Louie, Nevada Seismological Lab (at GNS & VUW through July 2006–

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Presentation on theme: "J. Louie 18/8/2005 Refraction Microtremor for Shallow Shear Velocity in Urban Basins John Louie, Nevada Seismological Lab (at GNS & VUW through July 2006–"— Presentation transcript:

1 J. Louie 18/8/2005 Refraction Microtremor for Shallow Shear Velocity in Urban Basins John Louie, Nevada Seismological Lab (at GNS & VUW through July 2006– louie@seismo.unr.edu) UNR students: J. B. Scott, T. Rasmussen, W. Thelen, M. Clark Collaborators: S. Pullammanappallil & B. Honjas, Optim LLC W. 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– louie@seismo.unr.edu) UNR students: J. B. Scott, T. Rasmussen, W. Thelen, M. Clark Collaborators: S. Pullammanappallil & B. Honjas, Optim LLC W. 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

2 J. Louie 18/8/2005 OutlineOutline 1.Refraction Microtremor for Shallow Vs 2.ReMi-Borehole Comparison 3.Los Angeles Transect 4.Las Vegas Transect 5.Effect of Shallow Vs on Shaking Models 1.Refraction Microtremor for Shallow Vs 2.ReMi-Borehole Comparison 3.Los Angeles Transect 4.Las Vegas Transect 5.Effect of Shallow Vs on Shaking Models

3 J. Louie 18/8/2005 ReMi measures Rayleigh dispersion with linear refraction arrays (paper by Louie, April 2001 BSSA). Refraction Microtremor for Shallow Shear Velocity 100-m depth resolution Initial funding from SCEC, UNR, VUW, Optim LLC

4 J. Louie 18/8/2005 Low-frequencies, 1-20 Hz, so bad geophone plants still work. Refraction Microtremor for Shallow Shear Velocity Initial funding from SCEC, UNR, VUW, Optim LLC

5 J. Louie 18/8/2005 Fieldwork is quick and simple; best results in cities. Refraction Microtremor for Shallow Shear Velocity Initial funding from SCEC, UNR, VUW, Optim LLC

6 J. Louie 18/8/2005 Fieldwork is quick and simple; best results in cities. Refraction Microtremor for Shallow Shear Velocity Initial funding from SCEC, UNR, VUW, Optim LLC

7 J. Louie 18/8/2005 ReMi has classified hard and soft sites around the world by measuring V 30, average shear velocity to 30 m depth. Refraction Microtremor for Shallow Shear Velocity

8 J. Louie 18/8/2005 OutlineOutline 1.Refraction Microtremor for Shallow Vs 2.ReMi-Borehole Comparison 3.Los Angeles Transect 4.Las Vegas Transect 5.Effect of Shallow Vs on Shaking Models 1.Refraction Microtremor for Shallow Vs 2.ReMi-Borehole Comparison 3.Los Angeles Transect 4.Las Vegas Transect 5.Effect of Shallow Vs on Shaking Models

9 J. 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 Comparison

10 J. Louie 18/8/2005 No surface method can match log details. ReMi-Borehole Comparison

11 J. Louie 18/8/2005 Depth-averaged velocities are a good match. But CCOC’s LVZ is a problem. ReMi-Borehole Comparison

12 J. Louie 18/8/2005 Joyner et al. (1981) quarter-wavelength spectra similar at important frequencies. ReMi-Borehole Comparison

13 J. Louie 18/8/2005 OutlineOutline 1.Refraction Microtremor for Shallow Vs 2.ReMi-Borehole Comparison 3.Los Angeles Transect 4.Las Vegas Transect 5.Effect of Shallow Vs on Shaking Models 1.Refraction Microtremor for Shallow Vs 2.ReMi-Borehole Comparison 3.Los Angeles Transect 4.Las Vegas Transect 5.Effect of Shallow Vs on Shaking Models

14 J. Louie 18/8/2005 Los Angeles Transect

15 J. Louie 18/8/2005 We Follow Field’s (2001) Amplification-Mapping Strategy Two Inputs for Microzonation: V 30 and Basin Depth (Z 1.5 ?)

16 J. Louie 18/8/2005 July 2003 San Gabriel Valley & Los Angeles Shallow 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

17 J. Louie 18/8/2005 Los Angeles Transect: V30 Results

18 J. Louie 18/8/2005 Los Angeles Transect: Full Section Fast bouldery alluvium near ranges Low-velocity near-surface layers thicken toward sea Vs constraint to 200 m depth Z 1.0 only constrained over 1/3 of transect– deep basin SG Mts Whittier Narrows Whittier Narrows Seal Beach

19 J. Louie 18/8/2005 Boreholes in Open-File Reports  Four within 1 km of transect  Also an incomplete posting at ROSRINE, Pico Rivera 2

20 J. Louie 18/8/2005 Rosrine/USGS Pico Rivera 2  Good correlation with transect below 8 m depth.

21 J. Louie 18/8/2005 Los Angeles Transect: V30 Results Nearby borehole results in red

22 J. Louie 18/8/2005 Measured V 30 vs Wills et al. (2000)  Average measurements within ranges for classes B- C, D, and D-E  N. San Gabriel Val. Measurements average above predicted C-D range  60 new C-D data points B B-C C C-D D D-E E

23 J. Louie 18/8/2005 V 30 vs Geologic Unit  Large V 30 variation inside each unit  Large V 30 variation between units

24 J. Louie 18/8/2005 V 30 vs Soil Type  In general, large V 30 variation within units  Units 2 and 5 may be NEHRP D  Large V 30 variation between units

25 J. Louie 18/8/2005 V 30 vs Riverbank Elevation  Fast, bouldery alluvium at higher elevations on River’s alluvial fan

26 J. Louie 18/8/2005 Spatial Statistics on V 30  Line in log-log spectrum means fractal spatial distribution  V30 less predictable as distance from measurement increases  “Noise Floor”- minimum variance reached at 700-m separation  Incorporate fractal dimension into PSHA? Noise Floor

27 J. Louie 18/8/2005 Conclusions I  Long ReMi transects can geophysically characterize spatial variations in shaking hazard.  Soil and geologic units must be specifically mapped for velocity, to reliably predict measured V 30.  210 measurements in LA match predictions, and add to class C-D data.  Long ReMi transects can geophysically characterize spatial variations in shaking hazard.  Soil and geologic units must be specifically mapped for velocity, to reliably predict measured V 30.  210 measurements in LA match predictions, and add to class C-D data.

28 J. Louie 18/8/2005 OutlineOutline 1.Refraction Microtremor for Shallow Vs 2.ReMi-Borehole Comparison 3.Los Angeles Transect 4.Las Vegas Transect 5.Effect of Shallow Vs on Shaking Models 1.Refraction Microtremor for Shallow Vs 2.ReMi-Borehole Comparison 3.Los Angeles Transect 4.Las Vegas Transect 5.Effect of Shallow Vs on Shaking Models

29 J. Louie 18/8/2005 Las Vegas Transect

30 J. Louie 18/8/2005 Las Vegas Shaking Computation, 2-sec E3D synthetic-seismogram code courtesy of Shawn Larsen, LLNL

31 J. Louie 18/8/2005 Las Vegas Shaking Computation, 2-sec 33 seconds after Little Skull Mtn. earthquake, as Rayleigh wave enters Las Vegas. Las Vegas Little Skull Mtn. Little Skull Mtn.

32 J. Louie 18/8/2005 Las Vegas Transect

33 J. Louie 18/8/2005 Most of Strip, Downtown; south side of Basin only 79 sites total 1145 well logs & geologic mapping Las Vegas Transect Basin-depth contours in meters

34 J. Louie 18/8/2005 Some correlation to faulting, soil type? Las Vegas Transect

35 J. Louie 18/8/2005 Geologic Info to Predict V s NSL, July ‘03, sponsored by LLNL  Can soil maps predict V s ?

36 J. Louie 18/8/2005 How to Extrapolate Shallow V s  Correlate transect measurements against Soil Map.  Correlate 75 Vs values against a stratigraphic model from 1145 water- well logs. Soil Stratigraphy Courtesy W. Taylor, UNLV, and J. Wagoner, LLNL

37 J. Louie 18/8/2005 How to Extrapolate Shallow V s  Predictions are good where many measurements exist.

38 J. Louie 18/8/2005 How to Extrapolate Shallow V s  Predictions are not good where there only sparse measurements.  Soil map predictions are not conservative.  Stratigraphic model predictions are, at least, conservative. Not Conservative Conservative

39 J. Louie 18/8/2005 OutlineOutline 1.Refraction Microtremor for Shallow Vs 2.ReMi-Borehole Comparison 3.Los Angeles Transect 4.Las Vegas Transect 5.Effect of Shallow Vs on Shaking Models 1.Refraction Microtremor for Shallow Vs 2.ReMi-Borehole Comparison 3.Los Angeles Transect 4.Las Vegas Transect 5.Effect of Shallow Vs on Shaking Models

40 J. Louie 18/8/2005 Building a Las Vegas Seismic Model

41 J. Louie 18/8/2005 Model Rendered as Amplification Map  Geology, Basin Depth, Geotech, Geophysical data into ModelAssembler Las Vegas Basin Las Vegas Basin Little Skull Mtn. Little Skull Mtn. Deep Volcanic Rifts Deep Volcanic Rifts

42 J. Louie 18/8/2005 Max. Ground Motion Computed– 0.5 Hz  E3D elastic finite-difference solution, by Shawn Larsen, LLNL Las Vegas Basin Las Vegas Basin Little Skull Mtn. Little Skull Mtn. Deep Volcanic Rifts Deep Volcanic Rifts

43 J. Louie 18/8/2005 Max. Ground Motion Computed– 0.1 Hz  E3D elastic finite-difference solution, by Shawn Larsen, LLNL Las Vegas Basin Las Vegas Basin Little Skull Mtn. Little Skull Mtn. Deep Volcanic Rifts Deep Volcanic Rifts

44 J. Louie 18/8/2005 Detailed Model Makes a Difference  Max. ground motion ratio, models with and without geotechnical model Little Skull Mtn. Las Vegas Basin Las Vegas Basin

45 J. Louie 18/8/2005 Detailed Model Makes a Difference  But not in any way that can be predicted from the model alone– basin geometry, source, and propagation path all matter! 73% predicted for 2-4 Hz 6% computed for 0.1 Hz

46 J. Louie 18/8/2005 Conclusions II  In tectonic areas, the regional distribution of basins affects shaking.  We have built a ModelAssembler for Nevada to create 3-d computation grids from geological and geotechnical data.  Surprisingly, geotechnical details affect even 10-sec computations in ways difficult to forecast.  In tectonic areas, the regional distribution of basins affects shaking.  We have built a ModelAssembler for Nevada to create 3-d computation grids from geological and geotechnical data.  Surprisingly, geotechnical details affect even 10-sec computations in ways difficult to forecast.

47 J. Louie 18/8/2005 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

48 J. Louie 18/8/2005 Los Angeles Transect: Levee Effects  V 30 levee: 245 m/s  V 30 non-levee: 241 m/s

49 J. Louie 18/8/2005 Basin Depth Model from USGS Gravity  Includes volcanic rift basins up to 9 km deep. Las Vegas Basin Las Vegas Basin Little Skull Mtn. Little Skull Mtn. Deep Volcanic Rifts Deep Volcanic Rifts Death Valley

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