Transcript Powerpoint file - The Nevada Seismological Laboratory

Refraction Microtremor
for Shallow Shear Velocity
in Urban Basins
John Louie, Nevada Seismological Lab
(at GNS & VUW through July 2006– [email protected])
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
J. Louie 18/8/2005
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
J. Louie 18/8/2005
Refraction Microtremor for Shallow Shear Velocity
ReMi measures Rayleigh dispersion with linear
refraction arrays (paper by Louie, April 2001 BSSA).
100-m depth resolution
J. Louie 18/8/2005
Initial funding from SCEC, UNR, VUW,
Optim LLC
Refraction Microtremor for Shallow Shear Velocity
Low-frequencies, 1-20 Hz, so bad geophone plants
still work.
J. Louie 18/8/2005
Initial funding from SCEC, UNR, VUW,
Optim LLC
Refraction Microtremor for Shallow Shear Velocity
Fieldwork is quick and simple; best results in cities.
J. Louie 18/8/2005
Initial funding from SCEC, UNR, VUW,
Optim LLC
Refraction Microtremor for Shallow Shear Velocity
Fieldwork is quick and simple; best results in cities.
J. Louie 18/8/2005
Initial funding from SCEC, UNR, VUW,
Optim LLC
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.
J. Louie 18/8/2005
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
J. Louie 18/8/2005
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
J. Louie 18/8/2005
ReMi-Borehole Comparison
No surface method can match log details.
J. Louie 18/8/2005
ReMi-Borehole Comparison
Depth-averaged velocities are a good match.
But CCOC’s LVZ is a problem.
J. Louie 18/8/2005
ReMi-Borehole Comparison
Joyner et al. (1981) quarter-wavelength spectra similar at important frequencies.
J. Louie 18/8/2005
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
J. Louie 18/8/2005
Los Angeles Transect
J. Louie 18/8/2005
We Follow Field’s (2001) Amplification-Mapping Strategy
Two Inputs for Microzonation: V30 and Basin Depth (Z1.5?)
J. Louie 18/8/2005
Shallow Shear-Velocity Transects
July 2003 San Gabriel Valley & Los Angeles
B-C
C-D
D
Transect
mapped on
NEHRP
hazard class
map by Wills,
from SCEC
Phase 3
Report
D-E
J. Louie 18/8/2005
Supported by USGS, NEHRP ERP and
IRIS-PASSCAL
Los Angeles Transect: V30 Results
J. Louie 18/8/2005
Los Angeles Transect: Full Section
SG Mts
•
•
•
•
Whittier
Narrows
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
J. Louie 18/8/2005
Seal Beach
Boreholes in Open-File Reports
Borehole Database Comparison —Data points within 1 km of transect


Four within 1
km of transect
Also an
incomplete
posting at
ROSRINE,
Pico Rivera 2
J. Louie 18/8/2005
Source, Transect
Array Number
Gibbs et al. (2000)
This study, 186
This study, 187
This study, 188
This study, 189
Distance from
Borehole
----------------850 m
269 m
345 m
930 m
30-m Shea r
Velocity
226.9 m/s
309 m/s
301 m/s
284 m/s
251 m/s
% Diff erence
---------------36.18%
32.66%
25.17%
10.62%
NEHRP
Class
D
D
D
D
D
Gibbs et al. (2001)
This study, 140
This study, 141
This study, 142
----------------661 m
442 m
955 m
298.7 m/s
401 m/s
338 m/s
424 m/s
---------------34.25%
13.15%
41.95%
D
C
D
C
Gibbs et al. (2001)
This study, 121
This study, 122
This study, 123
----------------870 m
673 m
997 m
544.7 m/s
580 m/s
538 m/s
498 m/s
---------------6.48%
-1.23%
-8.57%
C
C
C
C
Wills and Silva
(1998)
This study, 158
-----------------
339.06 m/s
----------------
D
939 m
317 m/s
-6.51%
D
This study , 159
973 m
306 m/s
-9.75%
D
ROSRINE Borehole
-----------------
241.54 m/s
----------------
D
This study, 153
This study, 154
492 m
221 m
425 m/s
381 m/s
75.95%
57.74%
C
C
This study, 155
775 m
337 m/s
39.52%
D
Rosrine/USGS Pico Rivera 2

Good correlation with transect below 8 m depth.
0
200
Shear-wave Velocity, m/s
400
600
800
0
Depth, m
50
100
150
200
ROSRINE picoriv2 borehole
SGRiv Survey Station 155A: ReMi
SGRiv Survey Station 156A: ReMi
SGRiv Survey Station 157A: ReMi
250
J. Louie 18/8/2005
1000
1200
Los Angeles Transect: V30 Results
Nearby borehole results in red
J. Louie 18/8/2005
Measured V30 vs Wills et al. (2000)
800


Average
measurements within
ranges for classes BC, D, and D-E
N. San Gabriel Val.
Measurements
average above
predicted C-D range
60 new C-D data
points
700
Measured 30-m Shear Velocity

B
600
500
583
580
B-C
400
300
C
330
C-D
255
2003 UNR Measurements
200
100
E
Measured Averages
Wills et al. (2000) Averages and
Ranges
D
D-E
Wills et al. (2000) Predicted Hazard Class
J. Louie 18/8/2005
V30 vs Geologic Unit

Large V30 variation inside each unit
 Large V30 variation between units
J. Louie 18/8/2005
V30 vs Soil Type

In general, large V30 variation within units


Units 2 and 5 may be NEHRP D
Large V30 variation between units
J. Louie 18/8/2005
V30 vs Riverbank Elevation

Fast, bouldery alluvium at higher elevations on River’s alluvial fan
River Profile vs. 30-m Shear Velocity
300
800
30-m Shear Velocity
Elevation (m)
250
700
200
600
150
500
100
400
50
300
0
200
0
10
20
30
40
50
Distance from San Gabriel Range Front (km)
J. Louie 18/8/2005
60
30-m Shear Velocity (m/s)
Levee & Path Elevation Profile
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?
J. Louie 18/8/2005
San Gabriel River Vs30 Transect Spatial Power Spectra
1.E+08
SGRiv: y = 20823x -1.5913
1.E+07
Power, m^2/s^2/km

R2 = 0.6705 D=1.70
(D=1.78 w/o noise floor)
1.E+06
Noise
Floor
1.E+05
1.E+04
1.E+03
0.01
0.1
1
Spatial Frequency, /km
10
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.
J. Louie 18/8/2005
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
J. Louie 18/8/2005
Las Vegas Transect
J. Louie 18/8/2005
Las Vegas Shaking Computation, 2-sec
QuickTime™ and a
Photo - JPEG decompressor
are needed to see this picture.
E3D synthetic-seismogram code courtesy of Shawn Larsen, LLNL
J. Louie 18/8/2005
Las Vegas Shaking Computation, 2-sec
Las
Vegas
Little
Skull
Mtn.
33 seconds after Little Skull Mtn. earthquake, as Rayleigh wave enters Las Vegas.
J. Louie 18/8/2005
Las Vegas Transect
J. Louie 18/8/2005
Las Vegas
Transect
Most of Strip,
Downtown;
south side of
Basin only
79 sites total
1145 well logs
& geologic
mapping
J. Louie 18/8/2005
Basin-depth contours in meters
Las Vegas Transect
Some correlation to faulting, soil type?
J. Louie 18/8/2005
Geologic Info to Predict Vs

Can soil maps predict Vs?
NSL, July ‘03, sponsored by LLNL
J. Louie 18/8/2005
How to Extrapolate Shallow Vs


Correlate
transect
measurements
against Soil
Map.
Courtesy W. Taylor, UNLV, and J. Wagoner, LLNL
Soil
Correlate 75
Vs values
against a
stratigraphic
model from
1145 waterwell logs.
Stratigraphy
J. Louie 18/8/2005
How to Extrapolate Shallow Vs

Predictions are good where many measurements exist.
Comparison of Las Vegas Vs30 Data and
700
UNR Transect Vs30 Measurements
Soil-Map Predictions
Stratigraphic-Model Predictions
Vs30, km/s
600
500
400
300
200
0
2
4
6
8
10
Transect Distance South of Cheyenne, km
J. Louie 18/8/2005
12
14
How to Extrapolate Shallow Vs
Off-Transect Measurements and Predictions


Predictions are not
good where there
only sparse
measurements.
Soil map
predictions are not
conservative.
Stratigraphic model
predictions are, at
least, conservative.
1000
Predicted Vs30, m/s

Soil-Map Predictions
Stratigraphic-Model Predictions
Not Conservative
Conservative
800
600
400
200
200
400
600
800
Measured Vs30, m/s
J. Louie 18/8/2005
1000
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
J. Louie 18/8/2005
Building a Las Vegas Seismic Model
J. Louie 18/8/2005
Model Rendered as Amplification Map

Geology, Basin Depth, Geotech, Geophysical data into ModelAssembler
Deep
Volcanic
Rifts
Las Vegas
Basin
Little
Skull Mtn.
J. Louie 18/8/2005
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.
J. Louie 18/8/2005
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.
J. Louie 18/8/2005
Detailed Model Makes a Difference

Max. ground motion ratio, models with and without geotechnical model
Las Vegas
Basin
Little
Skull Mtn.
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
J. Louie 18/8/2005
6% computed for 0.1 Hz
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.
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
J. Louie 18/8/2005
Supported by USGS, NEHRP ERP and
IRIS-PASSCAL
Los Angeles Transect:
Levee Effects
 V30 levee: 245 m/s
 V30 non-levee: 241
m/s
J. Louie 18/8/2005
Basin Depth Model from USGS Gravity

Includes volcanic rift basins up to 9 km deep.
Deep
Volcanic
Rifts
Las Vegas
Basin
Little
Skull Mtn.
Death Valley
J. Louie 18/8/2005