Next Generation Attenuation (NGA) - West
The “Next Generation of Ground-Motion Attenuation Models” for the western United States (NGA West) program was a multidisciplinary research program coordinated by the Lifelines Program of the Pacific Earthquake Engineering Research Center (PEER), in partnership with the U.S. Geological Survey and the Southern California Earthquake Center. The NGA West program concluded in 2008.
The objective of the project was to develop new ground-motion prediction relations through a comprehensive and highly interactive research program. Five sets of ground-motion attenuation models were developed by teams working independently but interacting with one another throughout the development process. The development of ground-motion models was supported by other project components, which included (1) developing an updated and expanded PEER database of recorded ground motions, including supporting information on the strong-motion record processing, earthquake sources, travel path, and recording station site conditions; (2) conducting supporting research projects to provide guidance on the selected functional forms of the ground-motion models; and (3) conducting a program of interactions throughout the development process to provide input and reviews from both the scientific research and engineering user communities.
Subsequently, two other NGA programs began in 2010, NGA East and NGA West 2. The NGA East Program is developing a new set of comprehensive & broadly accepted attenuation relationships for the Central and Eastern US.
The NGA West2 program is a followup to the original NGA program and will be investigating the modeling of directivity and directionality; verification of NGA-West models for recent small, moderate and large magnitude events; scaling of ground motion prediction equations (GMPE) for different levels of damping; development of GMPEs for vertical ground motion; treatment of epistemic uncertainty; and evaluation of soil amplification factors in NGA models versus NEHRP site factors.
The PEER NGA West project resulted in a major improvement in the prediction of ground motions in the western United States. The NGA West models provide much more robust and reliable estimates of ground motion than the previous generation of attenuation models developed in 1990s and early 2000s.
- After an extensive review in 2008 by the California Geological Survey (CGS), the USGS, and a reviewer “Tiger Team” convened by the USGS, the USGS announced that the NGA West models would be considered to supersede the previous generation of the attenuation models and that only the NGA West models would be used to develop the US National Seismic Hazard Maps for the western US. Thus, the models have influenced the current building codes and are being widely used by earthquake practitioners including structural engineers, geotechnical engineers, risk modelers and loss modelers.
- The EERI Board of Directors awarded the resulting research paper from the NGA West Program “Outstanding Earthquake Spectra Paper of 2008”. More information.
- Other organizations throughout the world are using the NGA models. For example, NGA programs are either being initiated or planned for areas such as Japan, Taiwan, and Europe.