Since Revision II, when the maps were submitted to the Building Seismic Safety Council, the following additional changes are recommended for future updates of the hazard maps.
- For the CEUS background seismicity, the software computes a mean distance for a finite, random-striking fault with given moment magnitude (M) if M≥6.0. The Revision II maps used the nearest mbLg to M6, but then continued using mbLg as the magnitude for conversion of magnitude to surface-rupture length (SRL) for M>6, using the Wells and Coppersmith (1994) relation. It is more correct to always use M because that is what the original Wells and Coppersmith regression assumed. Revision III converts all mbLg to M before computing surface-rupture length. For larger values of mbLg, near the Mmax for the region (7.5 ± dM for extended margin), mbLg is lower than M, and converting to M yields a larger mean SRL, a nearer average distance, and a slightly greater hazard. For the stable craton, with lower Mmax (7.0 ± dM), the effect of using mbLg instead of M in these SRL calculations is minimal. This change results in 0.5% to 2% changes in the ground motions.
- For the New Madrid faults, the Somerville finite fault GMPE was given the distance to rupture surface (Rcd) rather than the distance projected to the earth surface (Rjb). Using Rjb in the Revision II curves increases the hazard at sites over the fault, because the top of fault is assumed to be at depth 10 km. Effects of this error diminish rapidly with distance from the fault. This change results in 2% to 4% changes over the Reelfoot fault in the New Madrid seismic zone.
- One of the instruction files for summing WUS fault-hazard contributions had a few typographic errors for the 5-Hz spectral acceleration (SA). These typos increased the hazard estimate over some WUS faults in Wyoming,New Mexico, Arizona, and Idaho. These typos are corrected in Revision III. Only 5-Hz or 0.2-s SA is affected by these typos, and only in those states. The correction reduces the hazard by a few percent near the more active faults in those states. One such example is the Teton fault in western Wyoming.
- The computation of nonlinear site response in the Chiou-Youngs relation (2008) for background seismicity had a term for inter-event uncertainty, or tau, that is magnitude but not distance dependent. The map data had introduced a distance dependence that in effect reduced tau for larger distances, thereby reducing probabilistic hazard contributions from more distant background sources. Because contributions from these distant background sources tend to be of very limited significance at most places, correcting this error has little effect on the mean hazard estimate. In a modest way, this error affected 1- and 5- Hz SA and PGA calculations in the maps of Revision II.
Figures
Figures 1 – 3 show ratios of the peak horizontal ground acceleration (PGA), 0.2-s, and 1-s spectral accelerations. The ratios represent ground motions that were developed using the changes described above (Revision III below) divided by the December 2008 hazard model (Revision II below).
Figure 1. Ratio of peak horizontal ground acceleration at a 2% probability of exceedance in 50 years. The ratios represent ground motions that were developed using the changes of Revision III divided by the December 2008 hazard model (Revision II).
Figure 2. Ratio of 5-Hz (0.2-s) SA ground motions at a 2% probability of exceedance in 50 years. The ratios represent ground motions that were developed using the changes of Revision III divided by the December 2008 hazard model (Revision II).
Figure 3. Ratio of 1-Hz (1-s) SA ground motions at a 2% probability of exceedance in 50 years. The ratios represent ground motions that were developed using the changes of Revision III divided by the December 2008 hazard model (Revision II).
References
- Atkinson, G. M. and D. M. Boore (2008) Erratum to Empirical Ground-Motion Relations for Subduction Zone Earthquakes and Their Application to Cascadia and Other Regions. Bull. Seism. Soc. Amer. 98 (5) 2567 – 2569.
- Boore, D. M. and G. M. Atkinson (2008) Ground-Motion Prediction Equations for the Average Horizontal Component of PGA, PGV, and 5%-Damped PSA at Spectral Periods between 0.01 s and 10.0 s. Earthquake Spectra 24 (1) 99 – 138.
- Chiou, B. and R. R. Youngs (2008) Chiou–Youngs PEER-NGA empirical ground motion model for the average horizontal component of peak acceleration and pseudo-spectral acceleration for spectral periods of 0.01 to 10 seconds. Earthquake Spectra 24 (1) 173 - 206.
- Wells, D. L. and K. J. Coppersmith (1994) New Empirical Relationships among Magnitude, Rupture Length, Rupture Width, Rupture Area, and Surface Displacement. Bull. Seism. Soc. Amer. 84 (4) 974 – 1002.