Yin-Nan Huang

Maximum spectral demands in the near-fault region

The Next Generation Attenuation (NGA) relationships for shallow crustal earthquakes in the western United States predict a rotated geometric mean of horizontal spectral demand, termed GMRotI50, and not maximum spectral demand. Differences between strike-normal, strike-parallel, geometric-mean, and maximum spectral demands in the near-fault region are investigated using 147 pairs of records selected from the NGA strong motion database. The selected records are for earthquakes with moment magnitude greater than 6.5 and for closest site-to-fault distance less than 15 km. Ratios of maximum spectral demand to NGA-predicted GMRotI50 for each pair of ground motions are presented. The ratio shows a clear dependence on period and the Somerville directivity parameters. Maximum demands can substantially exceed NGA-predicted GMRotI50 demands in the near-fault region, which has significant implications for seismic design, seismic performance assessment, and the next-generation seismic design maps. Strike-normal spectral demands are a significantly unconservative surrogate for maximum spectral demands for closest distance greater than 3 to 5 km. Scale factors that transform NGA-predicted GMRotI50 to a maximum spectral demand in the near-fault region are proposed.

NEHRP Site Amplification Factors and the NGA Relationships

The Next Generation Attenuation (NGA) relationships were developed using a large database of strong motion recordings of shallow crustal earthquakes. The functional form of the relationships provides an opportunity to assess and update if necessary the site class coefficients provided in the NEHRP Provisions and ASCE-7. Site amplification factors estimated using the Boore and Atkinson, Campbell and Bozorgnia, and Chiou and Youngs NGA relationships are computed and averaged for a combination of magnitude, distance, faulting type and period and for NEHRP/ASCE-7 Site Classes B, C, D and E and boundaries A/B, B/C, C/D and D/E. The average NGA site amplification factors show a clear dependency on period for VS30 smaller than 270 m/s and are substantially greater than the current NEHRP site class coefficients in some cases. The use of the NEHRP/ASCE-7 site class coefficients and spectral demands for the reference site condition computed using the NGA relationships may produce erroneous results. A family of site class coefficients is proposed to address these issues in a format suitable for inclusion in the NEHRP Provisions and ASCE-7.

Orientation of Maximum Spectral Demand in the Near-Fault Region

An orientation-dependent parameter, R¯ Sa (θ), is used to investigate the orientation of maximum elastic spectral demand for near-fault ground motions. The upper bound on R¯ Sa (θ), equal to 1, occurs when the spectral demands for an orientation θ are equal to the maximum demands at all periods considered. One hundred forty-seven pairs of near-fault ground motions were used to identify the orientation associated with the maximum value of R¯ Sa for each pair of ground motions. The maximum value of R¯ Sa is greater than 0.9 for over 80% of the records in the dataset, meaning that there is generally an axis where the spectral demands are equal or very close to the maximum spectral demands for a wide range of period. The strike-normal direction should not be used as a surrogate for the direction of maximum shaking for a) closest site-to-source distance less than 5 km and periods less than 1 second, and b) closest site-to-source distance greater than 5 km .

On the Calculation of Peak Ground Velocity for Seismic Performance Assessment

Ratios of pseudo-spectral velocity (averaged over the period range of 0.5–2 seconds and at a period of 1 second) to peak ground velocity (PGV) were computed for hundreds of ground motion records selected from the PEER NGA Strong Motion Database. Two-stage regression analyses were performed to develop models for the ratios. The inter-event terms of the ratios trend to moderate decrease as moment magnitude increases; the intra-event residuals do not depend on distance. The results of this study provide the technical basis for a procedure presented in the American Technology Councils ATC-58 project, Guidelines for Seismic Performance Assessment of Buildings, for estimating PGV by scaling values of pseudo-spectral velocity.