1. John G. Anderson Professor of Geophysics
Earthquake Engineering GE / CEE - 479/679 Topic 16. Predition of Ground Motions, and Other Topics in Strong Motion Seismology
John G. Anderson
Professor of Geophysics
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John Anderson: GE/CEE 479/679 Lecure 16

2. Ground Motion Prediction Equations
AKA: “Regressions”
Regression analysis is the mathematical process used to determine the coefficients in the equations.
AKA: “Attenuation Relations”
Attenuation more properly refers to the distance dependence of ground motion for a single earthquake, or perhaps a single magnitude.
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3. Some practical aspects of strong motion seismology
Access to data
Exceptional data
Data processing
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Data Access
Most important sources of strong motion data
COSMOS (
Japan K-Net and KiK-Net (
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5. John Anderson: GE/CEE 479/679 Lecure 16
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Exceedance rate per instrument-year
PGA: 1 g, ~2x10-4, or ~ once in 5000 inst-years
“Distribution” decreases faster than a power law
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8. John Anderson: GE/CEE 479/679 Lecure 16
Exceedance rate per instrument-year
PGV: 100 cm/s, ~2.5x10-4, or ~ once in 4000 inst-years
“Distribution” decreases faster than a power law
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9. John Anderson: GE/CEE 479/679 Lecure 16
Nature of data
Background noise
Need for baseline correction
Access to data
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10. John Anderson: GE/CEE 479/679 Lecure 16
Izmit, Turkey
M7.6, Aug 17, 1999
1 g full scale
Timing marks, 2/sec L V T
Trigger time, initial P-wave is missing
Trailer from the previous trigger.
Three components of ground motion, L parallel to long axis of the instrument, V vertical, T parallel to the short axis of the instrument.
Zero is not known exactly.
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11. John Anderson: GE/CEE 479/679 Lecure 16
Izmit accelerogram
Continued
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Digitization:
This is a first attempt to digitize this accelerogram.
Discontinuity where the record was shifted
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Integral of records from the previous page to velocity.
Note low frequency behavior on the accelerogram that almost certainly has nothing to do with the behavior of the ground motion.
Conclusion: digitization and baseline correction is not a trivial matter.
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John Anderson: GE/CEE 479/679 Lecure 16

14. John Anderson: GE/CEE 479/679 Lecure 16
Filter
A numerical process that is applied to a time series (in this case an accelerogram). The process removes the contributions of certain frequencies to the time series.
High pass filter: removes low frequencies (i.e. frequencies below the filter frequency ff ), but does not affect the high frequencies.
Low pass filter: removes high frequencies (above ff ), but does not affect the low frequencies.
Filter response is generally not “sharp”. In other words, there is a range of frequencies that are partly removed.
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John Anderson: GE/CEE 479/679 Lecure 16

15. Baseline correction for accelerograms
Baseline corrections generally filter the accelerograms, so that those frequencies where the raw signal is dominated by noise are removed from the time history.
The effect of filtering is small on the acceleration, as seen on the next three slides.
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Filter effects
High-pass filters generally have small effects on accelerations.
The effect is much greater on velocity and displacement.
Note: next three slides all have the same scale.
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23. Comments on baseline correction
Filter frequencies are often selected on the basis of noise models.
When a record is filtered, signal is removed as well as noise.
If a particular frequency is important to a structure, then the accelerogram you use to test it should not have that frequency filtered out.
Modern digital accelerographs require much less filtering than older analog accelerographs.
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24. Ground-Motion Prediction Equations
Handout: Abrahamson and Silva (1997)
Next Generation Attenuation Relations (NGA)
Decade-long project
Papers now posted on the web:
Database (“flatfile”) is also on that web site
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25. John Anderson: GE/CEE 479/679 Lecure 16
At this point, I switched to the presentation by Ken Campbell to an NGA working group, to give a sense of the issues NGA is working on.
NGA is the state-of-the-art in ground motion prediction equations.
I think papers on the topic will come out in the February 2008 issue of Earthquake Spectra.
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