anss/nsmp strong- motion record processing and procedures christopher d. stephens and david m. boore...
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ANSS/NSMP STRONG-MOTION RECORD PROCESSING AND
PROCEDURESChristopher D. Stephens and David M. Boore
US Geological Survey
Menlo Park, CA
COSMOS/NSF International Workshop on Strong-Motion Record Processing
May 26-27, 2004
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• Network consists of both analog (265) and digital (675) recorders
• Installations include ground motion reference sites and structures
• Located in 32 states and Puerto Rico, but about half are in California
• About 320 have continuous or dial-up connections
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Digitizing Analog Records
• Scan in grayscale, 600 dpi (236pixels/sec)
• Enhance image contrast (no smoothing) and convert to black and white
• Semi-automatic trace following
• Interpolate to 200 sps for output
• Check for relative offsets and tilts of baselines for multi-segment traces
• Correct baselines using robust L1 fit
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Processing scheme is simple and ensures compatibility:
• Baseline correction
• End conditioning and padding
• Acausal filtering of acceleration only
– High cut cosine taper and instrument response correction in spectral domain
– Low cut Butterworth filter in time domain
• Integration to velocity and displacement in time domain
• Compute response spectra
• Review for physically reasonable result
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-350
0
3501999 Hector Mine (M=7.1), HEC, E-W (rjb=8.2 km)
-60
-30
0
301999 Hector Mine (M=7.1), HEC, E-W
-300
0
300
Acc
eler
atio
n(c
m/s
ec2)
1999 Hector Mine (M=7.1), HEC, N-S (rjb=8.2 km)
-30
0
30
Vel
ocity
(cm
/sec
)
1999 Hector Mine (M=7.1), HEC, N-S
-7
0
712/23/2000 Loma Linda (ML=3.4), Olive Dell Ranch, E-W
(rep=5.3 km)
0
1.5
12/23/2000 Loma Linda, Olive Dell Ranch, E-W
0 20 40 60-15
0
15
Time (sec)
02/21/2000 Loma Linda (ML=4.4), Olive Dell Ranch, E-W(rep=5.4 km)
traces aligned on first motion
0 20 40 60
0
6
Time (sec)
02/21/2000 Loma Linda, Olive Dell Ranch, E-W
traces aligned on first motion
a) b)
File
:C:\
me
tu_
03
\re
c_p
roc\
com
pa
re_
av.
dra
w;Da
te:2
00
3-0
9-1
0;T
ime
:14
:17
:26
Baseline shifts occur on recordings of strong and weak motion
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Many possible causes
• Mechanical:– Hysteresis (mechanical/ electrical)– “Popcorn” noise– Other
• Ground deformation– Tilt near earthquakes– Differential settlement– Other
• Analog-Digital Conversion (ADC)
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20 40 60 80
-50
0
50
Time (sec)
Vel
ocity
(cm
/sec
)
TCU129, E-W
t1
t2=30 sec
t2=50 sec
t2=70 sec
File
:C
:\m
etu_
03\r
ec_p
roc_
stro
ng_m
otio
n\12
9evf
it_co
lor_
xqua
d.dr
aw;
Dat
e:20
04-0
5-17
;Ti
me:
08:5
3:44
After Iwan et al, 1985
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20 40 60 80
-1.5
-1
-0.5
0
0.5
1
Time (sec)
Acc
eler
atio
n(c
m/s
2)
TCU129, E-W
Baseline corrections for different fit parameters
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0 20 40 60 80 100-200
0
200
400
Time (sec)
Dis
pla
cem
en
t(c
m)
TCU129, E-W
t2 = 30 sec
t2 = 50 sec
t2 = 70 sec
GPS, station AF11
File
:C:\
me
tu_
03
\re
c_p
roc_
stro
ng
_m
otio
n\1
29
ed
4p
p_
colo
r_xq
ua
d_
xfilt
.dra
w;
Da
te:2
00
4-0
5-2
4;T
ime
:17
:30
:58
Although the results look physically plausible, the residual displacements
can be sensitive to t1, t2
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Need for low-cut (high-pass) filtering
• There can be many reasons for the shifts, and as a result it is not possible to design a single correction scheme
• Residual displacements can be sensitive to parameters of baseline correction
• Filtering is often required (and in many cases is all that is required) to remove unwanted low-frequency noise
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Acausal filtering is preferred over causal
• At periods much shorter than the corner period, both the waveforms and response spectra are less sensitive to the corner
• Particularly true for inelastic response!
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Boore and Akkar, 2003
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Choosing filter corners
• Subjective
• Often guided by shape of Fourier spectrum, but this can lead to excessive removal of long periods
• Noise can be estimated from: - digitized fixed trace for analog records - pre-event of sufficient duration for digital
• In many cases, digital instruments allow choice of filter corners longer than periods of engineering interest, but peak displacement may be sensitive to the filter corner
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Mw 6.9 Loma Prieta at Anderson Dam DS, 333o comp
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Conditioning for bandpass filter• Values extending inward from each end and up to
first zero-crossing are set to 0.
• Pads are added symmetrically to both leading and trailing edges to accommodate filter transients
• Pad length at each end is determined from the empirical relation:
tpad = 1.5 * nroll / fc
• Pads are removed after final processing
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20 30 40 50 60 70-300
-200
-100
0
100
200
300
Acc
eler
atio
n(c
m/s
2)
acausal, flc = 0.05 Hz, NO PADS
1989 Loma Prieta eq; Anderson Dam DS, 333o
20 30 40 50 60 70-300
-200
-100
0
100
200
300acausal, flc = 0.05 Hz, WITH PADS
20 30 40 50 60 70
-20
-10
0
10
20
Vel
ocity
(cm
/s)
20 30 40 50 60 70
-20
-10
0
10
20
20 30 40 50 60 70-20
-15
-10
-5
0
5
10
Time (s)
Dis
plac
emen
t(c
m)
20 30 40 50 60 70-10
-5
0
5
10
Time (s)
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Selecting filter order is a compromise between:
• Effectively removing unwanted long-period noise
• Avoiding the introduction of excessive ringing by using too high an order
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0 20 40 60 80-300
-200
-100
0
100
200
300
Acc
ele
ratio
n(c
m/s
2)
acausal, flc = 0.05 Hz, nroll = 1
1989 Loma Prieta eq; Anderson Dam DS, 333o
100 120 140 160-300
-200
-100
0
100
200
300acausal, flc = 0.05 Hz, nroll = 4
0 20 40 60 80
-20
-10
0
10
20
Ve
loci
ty(c
m/s
)
100 120 140 160
-20
-10
0
10
20
0 20 40 60 80-10
-5
0
5
10
Time (s)
Dis
pla
cem
en
t(cm
)
100 120 140 160-10
-5
0
5
10
Time (s)
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20 30 40 50 60 70-300
-200
-100
0
100
200
300integrating processed pad-stripped data
1989 Loma Prieta eq; Anderson Dam DS, 333o
20 30 40 50 60 70-300
-200
-100
0
100
200
300
Acc
eler
atio
n(c
m/s
2)
acausal, flc = 0.05 Hz, WITH PADS
20 30 40 50 60 70
-20
-10
0
10
20
20 30 40 50 60 70
-20
-10
0
10
20
Vel
ocity
(cm
/s)
20 30 40 50 60 70-20
-15
-10
-5
0
5
10
Time (s)
20 30 40 50 60 70-10
-5
0
5
10
Time (s)
Dis
plac
emen
t(c
m)
Integrating pad-stripped filtered data causes distortion
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Uniform filtering (same filter corners and rolloff) is usually applied to all channels
from a particular site or structure
• Disadvantage is that long-period content is controlled by channel with weakest signal
• But motions involving more than one channel (e.g., inter-story drift, torsion) can be computed directly from processed records
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Mw 7.2 Hector Mine = 160 km, inter-station = 1.6 km
One method used to validate choice of filter parameters is to compare long-period waveforms at nearby stations