PEER has just published Report No. 2018/02: “Update of the BC Hydro Subduction Ground-Motion Model using the NGA-Subduction Dataset.” It was authored by Norman Abrahamson, Nicolas Kuehn, Zeynep Gulerce, Nicholas Gregor, Yousef Bozorgnia, Grace Parker, Jonathan Stewart, Brian Chiou, I.M. Idriss, Kenneth Campbell, Robert Youngs.
An update to the BCHydro ground-motion model for subduction earthquakes has been developed using the 2018 PEER NGA-SUB dataset. The full NGA-SUB database includes over 70,000 recordings from 1880 earthquakes. A subset of 8144 recordings from 181 earthquake is used in this study. The update modifies the BCHydro model to include regional terms for the VS30 scaling, large distance (linear R) scaling, and constant terms, which is consistent with the regionalization approach used in the NGA-West2 ground-motion models. A total of six regions were considered: Cascadia, Central America, Japan, New Zealand, South America, and Taiwan. Region-independent terms are used for the small-magnitude scaling, geometrical spreading, depth to top of rupture (ZTOR) scaling, and intraslab/interface scaling. The break in the magnitude scaling at large magnitudes for intraslab earthquakes is based on thickness of the intraslab and is subduction-zone dependent. The magnitude scaling for large magnitudes is constrained based on finite-fault simulations as given in the 2016 BCHydro model. Nonlinear site response is also constrained to be the same as the 2016 BCHydro model. The sparse ground-motion data from Cascadia show a factor of 2–3 lower ground motions than for other regions. Without a sound physical basis for this large reduction, the Cascadia model is adjusted to be consistent with the average from all regions for the center range of the data: M = 6.5, R = 100 km, VS30 = 400 m/sec. Epistemic uncertainty is included using the scaled backbone approach, with high and low models based on the differences in the average ground motions for the different regions. The lwer range of the epistemic uncertainty does not encompass the the full range of the very low short-period ground motions in Cascadia. For the Cascadia region, the ground-motion model is considered applicable to distance up to 800 km, magnitudes of 5.0 to 9.5, and periods from 0 to 10 sec. The intended use of this update is to provide an improved ground-motion model for consideration for use in the development of 2020 U.S. national uniform-hazard maps. It is not intended to be used for other regions around the world. This updated ground-motion model will be superseded by the suite of NGA-SUB ground-motion models when they are completed.