Next Generation Attenuation (NGA) Models, WUS Shallow Crustal Earthquakes - 1L11

Project goals and objectives

The overall goal of the project is to develop Next Generation Attenuation (NGA) relationships for shallow crustal earthquakes in the western United States. Five attenuation relationship developer teams (Norman Abrahamson and Walter Silva; David Boore and Gail Atkinson; Brian Chiou and Robert Youngs; Kenneth Campbell and Yousef Bozorgnia; and I.M. Idriss) are developing separate sets of NGA relationships through a process that includes interaction among the developer teams and with other researchers, development of an upgraded strong motion data base, and utilization of related research results on strong ground motion.

Role of this project in supporting PEER's mission (vision)

A critical element of Performance-Based Seismic Design is the estimation of the seismic ground motion to which a structure may be subjected, including the uncertainty in the ground motion. The NGA provides improved relationships for estimating ground motion as a function of the characteristics of the seismic source, source-to-site travel path, and the local soil or rock conditions.

Methodology employed

The NGA project is jointly sponsored and conducted by PEER Lifelines Program (PEER-LL), U.S. Geological Survey (USGS) and Southern California Earthquake Center (SCEC), with each organization actively participating in different components of the project. The methodology framework for the NGA project includes five attenuation relationships developer teams developing their own sets of NGA relationships in a process that is interactive and systematic and provides improved resources for development and review of attenuation relationships. The project includes: (1) major expansion and upgrading of the PEER strong-motion data base to provide the strong motion data and supporting information needed for the development of attenuation relationships; (2) application of research results from PEER-LL, USGS, SCEC, California Geological Survey (CGS), and other organizations to provide physical constraints on factors influencing ground motions; (3) development of improved statistical methods for analyzing ground motion data; and (4) a series of workshops, working group meetings, and meetings of the developers to present and review the data base, supporting research results, and attenuation relationships. Research tasks and corresponding working groups supporting NGA relationship development are summarized in Table 1.

Brief Description of previous year's achievements, with emphasis on accomplishments during last year (Year 8)

The project accomplishments through Year 8 are summarized as follows. (1) The PEER data base of ground motion recordings and supporting information has been substantially improved through the addition of a large number of recordings, evaluation of the characteristics of recordings (e.g. better understanding of the usable period range for response spectra of the recordings), and research and quality assurance that has resulted in additional and more accurate information about the earthquake source, travel path, and recording station parameters. The addition of records to the PEER data base is shown in Figure 1. (2) Selected research and synthesis of current and previous research has developed findings from the following projects: (a) rock motion simulation; (b) basin response simulations; (c) site response simulations; (d) attenuation at moderate distances; (e) effects of rupture directivity on ground motions; (e) and synthesis of empirical studies on site response. (3) Preliminary NGA relationships were developed; and (4) An external review of NGA relationships by the USGS was initiated.

During Year 8, preliminary sets of NGA relationships were developed and discussed at Workshop 7 in December 2004 and Workshop 8 in April 2005. The Five NGA Developer teams held a series of interaction meetings to exchange information on NGA model developments by the teams as well as continue to review and update the data base. The external review process with USGS was initiated by providing preliminary reports of NGA relationships to the USGS and receiving and reviewing initial comments from the USGS during August and September 2005.

Other similar work being conducted within and outside PEER and how this project differs

The NGA project is unique in providing a focused effort to utilize the latest scientific knowledge and research results, a comprehensive ground motion data base, and advanced statistical analysis methods to develop a next generation of ground motion attenuation relationships for western U.S. shallow crustal earthquakes.

Describe any instances where you are aware that your results have been used in industry

Because the attenuation relationships are still under development, they are not yet in use in industry. It is expected that preliminary versions of the relationships will be used by industry for comparative analysis by Spring 2006 and the final relationships will be in widespread use in Fall 2006.

Expected milestones & deliverables

Expected milestones and deliverables in Year 9 include: review meetings with USGS in October 2005 (two meetings), December 2005, and March 2006; completion of final reports for NGA relationships for the average horizontal components of ground motion in May and June 2006; and completion of the USGS review process including attendance at a USGS workshop in September 2006. Work on additional components of NGA models for effects of near-source rupture directivity on ground motions and extensions of the models for the average horizontalcomponents of ground motion to include fault-strike-normal and fault-strike-parallel components and reporting of these additional model components is expected to occur by September 2006.