Refraction microtremor for shallow shear velocity in urban basins
Download
1 / 49

Refraction Microtremor for Shallow Shear Velocity in Urban Basins - PowerPoint PPT Presentation


  • 76 Views
  • Uploaded on

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

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'Refraction Microtremor for Shallow Shear Velocity in Urban Basins' - lieu


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
Refraction microtremor for shallow shear velocity in urban basins
Refraction Microtremorfor Shallow Shear Velocityin 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


Outline
Outline

Refraction Microtremor for Shallow Vs

ReMi-Borehole Comparison

Los Angeles Transect

Las Vegas Transect

Effect of Shallow Vs on Shaking Models


Refraction microtremor for shallow shear velocity

100-m depth resolution

Refraction Microtremor for Shallow Shear Velocity

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

Initial funding from SCEC, UNR, VUW, Optim LLC


Refraction microtremor for shallow shear velocity1
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


Refraction microtremor for shallow shear velocity2
Refraction Microtremor for Shallow Shear Velocity

Fieldwork is quick and simple; best results in cities.

Initial funding from SCEC, UNR, VUW, Optim LLC


Refraction microtremor for shallow shear velocity3
Refraction Microtremor for Shallow Shear Velocity

Fieldwork is quick and simple; best results in cities.

Initial funding from SCEC, UNR, VUW, Optim LLC


Refraction microtremor for shallow shear velocity4
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.


Outline1
Outline

Refraction Microtremor for Shallow Vs

ReMi-Borehole Comparison

Los Angeles Transect

Las Vegas Transect

Effect of Shallow Vs on Shaking Models


Remi borehole comparison
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


Remi borehole comparison1
ReMi-Borehole Comparison

No surface method can match log details.


Remi borehole comparison2
ReMi-Borehole Comparison

Depth-averaged velocities are a good match.

But CCOC’s LVZ is a problem.


Remi borehole comparison3
ReMi-Borehole Comparison

Joyner et al. (1981) quarter-wavelength spectra similar at important frequencies.


Outline2
Outline

Refraction Microtremor for Shallow Vs

ReMi-Borehole Comparison

Los Angeles Transect

Las Vegas Transect

Effect of Shallow Vs on Shaking Models



We follow field s 2001 amplification mapping strategy
We Follow Field’s (2001) Amplification-Mapping Strategy

Two Inputs for Microzonation: V30 and Basin Depth (Z1.5?)


Shallow shear velocity transects

B-C

C-D

D

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



Los angeles transect full section

Whittier

Narrows

SG Mts

Seal Beach

  • Fast bouldery alluvium near ranges

  • Low-velocity near-surface layers thicken toward sea

  • Vs constraint to 200 m depth

  • Z1.0 only constrained over 1/3 of transect– deep basin

Los Angeles Transect: Full Section


Boreholes in open file reports
Boreholes in Open-File Reports

  • Four within 1 km of transect

  • Also an incomplete posting at ROSRINE, Pico Rivera 2


Rosrine usgs pico rivera 2
Rosrine/USGS Pico Rivera 2

  • Good correlation with transect below 8 m depth.


Los angeles transect v30 results1
Los Angeles Transect: V30 Results

Nearby borehole results in red


Measured v 30 vs wills et al 2000

B

B-C

C

C-D

E

D

D-E

Measured V30 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


V 30 vs geologic unit
V30 vs Geologic Unit

  • Large V30 variation inside each unit

  • Large V30 variation between units


V 30 vs soil type
V30 vs Soil Type

  • In general, large V30 variation within units

    • Units 2 and 5 may be NEHRP D

  • Large V30 variation between units


V 30 vs riverbank elevation
V30 vs Riverbank Elevation

  • Fast, bouldery alluvium at higher elevations on River’s alluvial fan


Spatial statistics on v 30

Noise

Floor

Spatial Statistics on V30

  • 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?


Conclusions i
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 V30.

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


Outline3
Outline

Refraction Microtremor for Shallow Vs

ReMi-Borehole Comparison

Los Angeles Transect

Las Vegas Transect

Effect of Shallow Vs on Shaking Models



Las vegas shaking computation 2 sec
Las Vegas Shaking Computation, 2-sec

E3D synthetic-seismogram code courtesy of Shawn Larsen, LLNL


Las vegas shaking computation 2 sec1
Las Vegas Shaking Computation, 2-sec

Las

Vegas

Little

Skull

Mtn.

33 seconds after Little Skull Mtn. earthquake, as Rayleigh wave enters Las Vegas.



Las vegas transect2

Basin-depth contours in meters

Las Vegas Transect

Most of Strip, Downtown; south side of Basin only

79 sites total

1145 well logs & geologic mapping


Las vegas transect3
Las Vegas Transect

Some correlation to faulting, soil type?


Geologic info to predict v s

NSL, July ‘03, sponsored by LLNL

Geologic Info to Predict Vs

  • Can soil maps predict Vs?


How to extrapolate shallow v s

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

Soil

Stratigraphy

How to Extrapolate Shallow Vs

  • Correlate transect measurements against Soil Map.

  • Correlate 75 Vs values against a stratigraphic model from 1145 water-well logs.


How to extrapolate shallow v s1
How to Extrapolate Shallow Vs

  • Predictions are good where many measurements exist.


How to extrapolate shallow v s2

Not Conservative

Conservative

How to Extrapolate Shallow Vs

  • Predictions are not good where there only sparse measurements.

  • Soil map predictions are not conservative.

  • Stratigraphic model predictions are, at least, conservative.


Outline4
Outline

Refraction Microtremor for Shallow Vs

ReMi-Borehole Comparison

Los Angeles Transect

Las Vegas Transect

Effect of Shallow Vs on Shaking Models



Model rendered as amplification map

Deep

Volcanic

Rifts

Las Vegas

Basin

Little

Skull Mtn.

Model Rendered as Amplification Map

  • Geology, Basin Depth, Geotech, Geophysical data into ModelAssembler


Max ground motion computed 0 5 hz

Deep

Volcanic

Rifts

Las Vegas

Basin

Little

Skull Mtn.

Max. Ground Motion Computed– 0.5 Hz

  • E3D elastic finite-difference solution, by Shawn Larsen, LLNL


Max ground motion computed 0 1 hz

Deep

Volcanic

Rifts

Las Vegas

Basin

Little

Skull Mtn.

Max. Ground Motion Computed– 0.1 Hz

  • E3D elastic finite-difference solution, by Shawn Larsen, LLNL


Detailed model makes a difference

Las Vegas

Basin

Little

Skull Mtn.

Detailed Model Makes a Difference

  • Max. ground motion ratio, models with and without geotechnical model


Detailed model makes a difference1
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


Conclusions ii
Conclusions II

  • In tectonic areas, the regional distribution of basins affects shaking.

  • We have built a ModelAssemblerfor 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.


Los angeles transect1
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


Los angeles transect levee effects
Los Angeles Transect:Levee Effects

  • V30 levee: 245 m/s

  • V30 non-levee: 241 m/s


Basin depth model from usgs gravity

Deep

Volcanic

Rifts

Las Vegas

Basin

Little

Skull Mtn.

Death Valley

Basin Depth Model from USGS Gravity

  • Includes volcanic rift basins up to 9 km deep.