NGA 2 GMPE’s Under Predict Long-Period Near-Source Motions from Large Earthquakes

Thomas H. Heaton, & Becky Roh

Published August 15, 2016, SCEC Contribution #6952, 2016 SCEC Annual Meeting Poster #276

The recently constructed San Bernardino Law and Justice Center is an 11-story story steel building located 6 km from the San Andreas Fault. In order to mitigate strong shaking expected during the lifetime of this expensive structure ($400M), the designers chose to use base isolation to limit the inertial forces from future earthquakes. The isolation system uses a triple pendulum design with an approximate period of 5 ½ s and a maximum isolator displacement of 1.1 m. The designers have advertised that the building is designed for the maximum ground motion expected with a repeat of 2,500 years. Somewhat embarrassingly, the Lucerne record from the 1992 M 7.3 Landers earthquake had motions too large to be accommodated by the Law and Justice Center isolators. The isolator design parameters appear to have been derived from the USGS National Probabilistic Seismic Hazard Maps. In turn, The USGS PSHA maps rely heavily on the NGA 2 GMPE’s to predict shaking as a function of magnitude, distance, and site parameters.

We attempt to clarify the long-period near-source ground motions that are predicted by the NGA 2 GMPE’s by investigating the predictions for 10-s response spectral displacements for very near-source locations in large crustal earthquakes. We assume that 10-s response spectral values are approximately equal to the peak ground displacement experienced at a site. In this case, the 10-s spectral displacements at 0-km distance to the rupture should be comparable to 2/3 the average slip that occurred in the earthquake (see Aagaard, Hall, and Heaton, 2004). There are five separate GMPE’s (developed by different research groups) and the average 10-s, 0-km, spectral displacement is 0.88 m, assuming M=8.0. This spectral displacement should be typical of the very near-source region of a shallow fault with average slip of about 1.3 m, which is far smaller than is expected from a M 8 earthquake. We discuss why the NGA 2 long-period GMPE’s are far smaller than is expected from this simple check.

Citation
Heaton, T. H., & Roh, B. (2016, 08). NGA 2 GMPE’s Under Predict Long-Period Near-Source Motions from Large Earthquakes . Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Ground Motion Prediction (GMP)