Near-field and directionality issues for ground motion selection Jack W. Baker 35 minutes
Transcription
Near-field and directionality issues for ground motion selection Jack W. Baker 35 minutes
J. Baker 1 Near-field and directionality issues for ground motion selection Jack W. Baker 35 minutes J. Baker 2 Language from ASCE 7-10 16.1.3.2 At sites within 3 miles (5 km) of the active fault that controls the hazard, each pair of components shall be rotated to the fault-normal and faultparallel directions of the causative fault and shall be scaled so that the average of the fault-normal components is not less than the MCER response spectrum for the period range from 0.2T to 1.5T. J. Baker 3 Language from the BSSC proposal 16.2.3.3: When the MCER ground motion level is controlled by events for which near-fault effects are expected, the site shall be identified as a nearfault site and a suitable number of the ground motions shall include near-fault and directivity effects including direction of fault rupture and velocity pulses as appropriate. 16.2.5.1: For sites identified as near-fault in Section 16.2.3.3, each pair of horizontal ground motion components shall be rotated to the faultnormal and fault-parallel directions of the causative fault and applied to the building in such orientation. At all other sites, each pair of horizontal ground motion components shall be applied to the building at arbitrary orientation angles. J. Baker 4 When are you at a “near-fault” site? • Two phenomena to consider when selecting time series: – Directivity pulses – Polarization of ground motions J. Baker • REDi: • For sites located 20 km or closer to a fault capable of producing a M6.5, the site- specific seismic hazard calculations should account for near-fault directivity effects. 5 J. Baker 6 (slide with velocity pulses, NGA W-2 predictions?) J. Baker 7 Observations from the 1979 Imperial Valley earthquake The algorithm identifies ground motions with clear pulses, and the identified motions are generally from locations where directivity is expected J. Baker 8 Example classifications from past earthquakes 1979 Imperial Valley 1992 Landers The algorithm identifies ground motions with clear pulses, and the identified motions are generally from locations where directivity is expected. The algorithm has also been extended to 3D. J. Baker 9 How many pulses should you pick? Shahi, S. K., and Baker, J. W. (2011). “An Empirically Calibrated Framework for Including the Effects of Near-Fault Directivity in Probabilistic Seismic Hazard Analysis.” Bulletin of the Seismological Society of America, 101(2), 742–755. P( pulse | R, s ) = Predictive model 1 1 + e0.642+ 0.167 R −0.075 s Model predictions J. Baker 10 How many pulses should you pick? • Hayden, C., Bray, J., and Abrahamson, N. (2014). “Selection of Near-Fault Pulse Motions.” Journal of Geotechnical and Geoenvironmental Engineering, 140(7), 04014030. • For example, if R=10 km for an Mw = 7 governing event and ɛ=1:0 for the 5% damped spectral acceleration at a period of 1s, the proportion of ground motions that should be pulse motions is 0.40. J. Baker 11 Orientation of ground motions J. Baker Orientation of SaRotD100 (using α as angle to strike parallel) T* = 1.5 s Site α Strike parallel orientation Fault rupture • Dependence of α on various parameters was studied • A parametric model to predict the distribution of α is proposed T’ = 3 s Oscillator responses to 1979 Imperial Valley-06, El Centro Differential Array recording 12 J. Baker 13 Dependence of α on M and Rrup Rrup bins J. Baker Distribution of α for varying T, with Rrup between 0 and 5 km Apparent division at 0.5 or 1 second 14 J. Baker Why maintain fault-normal and fault-parallel orientations? 15 J. Baker 16 Conclusions J. Baker 17 J. Baker 18 Extra J. Baker 19 Measuring single-orientation (pseudo) response spectra x(t ) J. Baker 20 Measuring direction-dependent response spectra z (t ) = z (t ) x(t )cosθ + y (t )sin θ J. Baker 21 Measuring direction-dependent response spectra Sa(θ,1s) / SaRotD100(1s) Displacement / Maximum displacement SaRotD100 orientation (T ) SaGeoMean = SaRotD100 = 2 SaRotD 50 Sa X (T ) 2 + SaY (T ) 2 SaRotD0 orientation SaGMRotI 50 (T ) = ... SaRotD50 orientation Gilroy Array #6, 1984 Morgan Hill J. Baker 22 Example 1-second oscillator responses to multi-component motions Sa(θ,1s) / SaRotD100(1s) Displacement / Maximum displacement SaRotD100 = 2 SaRotD 50 SaRotD100 ≅1 SaRotD 50 HWA031, 1999 Chi-Chi-04 Gilroy Array #6, 1984 Morgan Hill J. Baker 23 Ground motion model for SaRotD100 at a specified period SaRotD100 = SaRotD100 SaRotD 50 SaRotD 50 ln SaRotD100 ln( SaRotD100 / SaRotD 50 ) + ln( SaRotD 50 ) = “Max direction factor” ln( SaRotD100 / SaRotD 50 ) = a + η '+ ε ' Simple prediction (constant?) Independent of primary GMPE Primary GMPE ln( SaRotD 50 ) f ( M , R,VS 30 ,...) + η + ε = Complex prediction