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
• 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
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
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
-350
0
3501999 Hector Mine (M=7.1), HEC, E-W (rjb=8.2 km)
-60
-30
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301999 Hector Mine (M=7.1), HEC, E-W
-300
0
300
Acc
eler
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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
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Baseline shifts occur on recordings of strong and weak motion
Many possible causes
• Mechanical:– Hysteresis (mechanical/ electrical)– “Popcorn” noise– Other
• Ground deformation– Tilt near earthquakes– Differential settlement– Other
• Analog-Digital Conversion (ADC)
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
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After Iwan et al, 1985
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
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
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Although the results look physically plausible, the residual displacements
can be sensitive to t1, t2
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
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!
Boore and Akkar, 2003
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
Mw 6.9 Loma Prieta at Anderson Dam DS, 333o comp
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
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)
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
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)
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
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
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