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 March 23, 2008 1 John Anderson: GE/CEE 479/679 Lecure 16

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. March 23, 2008 2 John Anderson: GE/CEE 479/679 Lecure 16

Some practical aspects of strong motion seismology Access to data Exceptional data Data processing March 23, 2008 3 John Anderson: GE/CEE 479/679 Lecure 16

John Anderson: GE/CEE 479/679 Lecure 16 Data Access Most important sources of strong motion data COSMOS (http://www.cosmos-eq.org/) Japan K-Net and KiK-Net (http://www.k-net.bosai.go.jp/; http://www.kik.bosai.go.jp/) March 23, 2008 4 John Anderson: GE/CEE 479/679 Lecure 16

John Anderson: GE/CEE 479/679 Lecure 16 March 23, 2008 5 John Anderson: GE/CEE 479/679 Lecure 16

John Anderson: GE/CEE 479/679 Lecure 16 March 23, 2008 6 John Anderson: GE/CEE 479/679 Lecure 16

John Anderson: GE/CEE 479/679 Lecure 16 Exceedance rate per instrument-year PGA: 1 g, ~2x10-4, or ~ once in 5000 inst-years “Distribution” decreases faster than a power law March 23, 2008 7 John Anderson: GE/CEE 479/679 Lecure 16

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 March 23, 2008 8 John Anderson: GE/CEE 479/679 Lecure 16

John Anderson: GE/CEE 479/679 Lecure 16 Nature of data Background noise Need for baseline correction Access to data March 23, 2008 9 John Anderson: GE/CEE 479/679 Lecure 16

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. March 23, 2008 10 John Anderson: GE/CEE 479/679 Lecure 16

John Anderson: GE/CEE 479/679 Lecure 16 Izmit accelerogram Continued March 23, 2008 11 John Anderson: GE/CEE 479/679 Lecure 16

John Anderson: GE/CEE 479/679 Lecure 16 Digitization: This is a first attempt to digitize this accelerogram. Discontinuity where the record was shifted March 23, 2008 12 John Anderson: GE/CEE 479/679 Lecure 16

John Anderson: GE/CEE 479/679 Lecure 16 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. March 23, 2008 13 John Anderson: GE/CEE 479/679 Lecure 16

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. March 23, 2008 14 John Anderson: GE/CEE 479/679 Lecure 16

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. March 23, 2008 15 John Anderson: GE/CEE 479/679 Lecure 16

John Anderson: GE/CEE 479/679 Lecure 16 March 23, 2008 16 John Anderson: GE/CEE 479/679 Lecure 16

John Anderson: GE/CEE 479/679 Lecure 16 March 23, 2008 17 John Anderson: GE/CEE 479/679 Lecure 16

John Anderson: GE/CEE 479/679 Lecure 16 March 23, 2008 18 John Anderson: GE/CEE 479/679 Lecure 16

John Anderson: GE/CEE 479/679 Lecure 16 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. March 23, 2008 19 John Anderson: GE/CEE 479/679 Lecure 16

John Anderson: GE/CEE 479/679 Lecure 16 March 23, 2008 20 John Anderson: GE/CEE 479/679 Lecure 16

John Anderson: GE/CEE 479/679 Lecure 16 March 23, 2008 21 John Anderson: GE/CEE 479/679 Lecure 16

John Anderson: GE/CEE 479/679 Lecure 16 March 23, 2008 22 John Anderson: GE/CEE 479/679 Lecure 16

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. March 23, 2008 23 John Anderson: GE/CEE 479/679 Lecure 16

Ground-Motion Prediction Equations Handout: Abrahamson and Silva (1997) Next Generation Attenuation Relations (NGA) Decade-long project Papers now posted on the web: http://peer.berkeley.edu/products/rep_nga_models.html Database (“flatfile”) is also on that web site March 23, 2008 24 John Anderson: GE/CEE 479/679 Lecure 16

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. March 23, 2008 25 John Anderson: GE/CEE 479/679 Lecure 16