Refraction Microtremor for Shallow Shear Velocity in Urban Basins

Transcription

1 Refraction Microtremor for Shallow Shear Velocity in Urban Basins John Louie, Nevada Seismological Lab 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

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

4 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

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

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

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

8 Outline 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 ReMi-Borehole Comparison 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

10 ReMi-Borehole Comparison No surface method can match log details.

11 ReMi-Borehole Comparison Depth-averaged velocities are a good match. But CCOC s LVZ is a problem.

12 ReMi-Borehole Comparison Joyner et al. (1981) quarter-wavelength spectra similar at important frequencies.

13 Outline 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 Los Angeles Transect

15 We Follow Field We Follow Field s (2001) Amplification-Mapping Strategy s (2001) Amplification-Mapping Strategy Two Inputs for Microzonation: V 30 and Basin Depth (Z 1.5?)

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

17 Los Angeles Transect: V30 Results

18 Los Angeles Transect: Full Section SG Mts Whittier Narrows Seal Beach 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

19 Boreholes in Open-File Reports Four within 1 km of transect 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) m/s D This study, m 309 m/s 36.18% D This study, m 301 m/s 32.66% D This study, m 284 m/s 25.17% D This study, m 251 m/s 10.62% D Also an incomplete posting at ROSRINE, Pico Rivera 2 Gibbs et al. (2001) m/s D This study, m 401 m/s 34.25% C This study, m 338 m/s 13.15% D This study, m 424 m/s 41.95% C Gibbs et al. (2001) m/s C This study, m 580 m/s 6.48% C This study, m 538 m/s -1.23% C This study, m 498 m/s -8.57% C Wills and Silva (1998) m/s D This study, m 317 m/s -6.51% D This study, m 306 m/s -9.75% D ROSRINE Borehole m/s D This study, m 425 m/s 75.95% C This study, m 381 m/s 57.74% C This study, m 337 m/s 39.52% D

20 Rosrine/USGS Pico Rivera 2 Good correlation with transect below 8 m depth.

21 Los Angeles Transect: V30 Results Nearby borehole results in red

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

23 V 30 vs Geologic Unit Large V 30 variation inside each unit Large V 30 variation between units

24 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 V 30 vs vs Riverbank Elevation Fast, bouldery alluvium at higher elevations on River s s alluvial fan

26 Line in log-log spectrum means fractal spatial distribution Spatial Statistics on V 30 V30 less predictable as distance from measurement increases Noise Floor - minimum variance reached at 700- m separation Noise Floor Incorporate fractal dimension into PSHA?

27 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 measurements in LA match predictions,, and add to class C-D data.

28 Outline 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 Las Vegas Transect

30 Las Vegas Shaking Computation, 2-sec E3D synthetic-seismogram code courtesy of Shawn Larsen, LLNL

31 Las Vegas Shaking Computation, 2-sec Little Skull Mtn. Las Vegas 33 seconds after Little Skull Mtn. earthquake, as Rayleigh wave enters Las Vegas.

32 Las Vegas Transect

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

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

35 Geologic Info to Predict V s Can soil maps predict V s? NSL, July 03, sponsored by LLNL

36 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 Courtesy W. Taylor, UNLV, and J. Wagoner, LLNL Stratigraphy

37 How to Extrapolate Shallow V s Predictions are good where many measurements exist.

38 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 Outline 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 Building a Las Vegas Seismic Model

41 Model Rendered as Amplification Map Geology, Basin Depth, Geotech, Geophysical data into ModelAssembler Deep Volcanic Rifts Las Vegas Basin Little Skull Mtn.

42 Max. Ground Motion Computed 0.5 Hz E3D elastic finite-difference solution, by Shawn Larsen, LLNL Deep Volcanic Rifts Las Vegas Basin Little Skull Mtn.

43 Max. Ground Motion Computed 0.1 Hz E3D elastic finite-difference solution, by Shawn Larsen, LLNL Deep Volcanic Rifts Las Vegas Basin Little Skull Mtn.

44 Detailed Model Makes a Difference Max. ground motion ratio, models with and without geotechnical model Las Vegas Basin Little Skull Mtn.

45 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 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.

47 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 Los Angeles Transect: Levee Effects V 30 levee: 245 m/s V 30 non-levee: 241 m/s