Project Title/ID Number  Empirical Characterization of Basin Effects on Site Response—2252002 
Start/End Dates  10/1/02—9/30/03 
Project Leader  Jonathan Stewart (UCLA/Faculty) 
Team Members  Yoojong Choi (UCLA/Grad Student), Robert Graves (Industrial Collaborator) 
Project goals and objectives  
This Year 6 project is an extension of the Year 5 project having the same title. As described in the original project proposal, the general intent of this work is to develop a statistically robust engineering model for characterizing basin effects on ground motion intensity measures (IMs). The approach taken is to evaluate residuals between data and predictions, and correlate these residuals to basin geometric parameters. The predictions used to evaluate the residuals will, to the extent possible, remove the effects of site, path, and shallow ground response in an average sense, so that the deviations from data can be attributed to basin response. 

Role of this project in supporting PEER’s vision  
The project will allow for more robust evaluations of ground motion intensity measures for use in Probabilistic Seismic Hazard Analysis (PSHA), which is an integral component of PerformanceBased Earthquake Engineering (PBEE). The results could be especially significant for regions located near the basin edge or on deep basin sediments. 

Methodology employed  
To realize the goals/objectives outlined above, the following tasks are being performed:
Work to date
has focused principally on Tasks (a)(c). We have recently begun Task
(d) for an IM that we anticipate having basin parameter sensitivity (significant
duration). Remaining work is ongoing. 

Brief description of past year’s accomplishments and more detail on expected Year 6 accomplishments  
The principal outcome of work to date is the amplification model described in Task (c) above as well as the basin parameter uncertainty evaluation in (b). The amplification model has several unique attributes. First, it is the only available model that provides statistically robust estimates of ground motion amplification as a continuous function of both Vs30 and reference peak acceleration (PHAr). The model is described by the following equation: One of the innovative features of the model is the formulation of b, which reflects the Vs30dependence of amplification as shown in the figure below. The line in red represents the model selected, the black dots indicate the results of regressions within specific Vs30categories. Another interesting feature of the model concerns the formulation of error term . We investigated the dependence of on Vs30, distance (r), and magnitude (m). We identified a significant dependence on r for r < 20 km, no significant dependence on m, and a weak dependence on Vs30. Based on this model, we find a bias in existing NEHRP site factors, as shown below. 

Other similar work being conducted within and outside PEER and how this project differs  
A project funded by SCEC (PI: Ned Field) evaluated the effect of basin depth on spectral acceleration using data from southern California. This project is examining additional basin geometric parameters and an additional region (northern California). Therefore, the results will be more comprehensive, and more thus more useful for performancebased engineering. An ongoing project funded by CSMIP (PI: Graves) is examining the generation of basin induced surface waves and their correlation with geologic parameters such as distance to basin edge. This work is similar to the present study, but is focusing on longperiod ground motions. Moreover, that study formulated correction factors relative to the Abrahamson and Silva attenuation relationship and not to amplificationadjusted ground motion predictions.


Plans for Year 7 if this project is expected to be continued  
I anticipate that the basin project will be completed within Year 6 for the IMs of spectral acceleration and significant duration. Possible Year 7 topics could include site amplification models for other IMs, interpretation of the Taiwan earthquake data, or work related to the development of vector hazard capabilities. Alternatively, my work could be redirected into research related to soilfoundation structure interaction (SFSI). Ideally, my research topic would be selected through open discussion between myself and the PEER leadership. 

Describe any instances where you are aware that your results have been used in industry  
My ground motion related work from previous phases of PEER research has been used in PG&E and Caltrans, and is being implemented within the nextgeneration attenuation (NGA) project. Previous work on soilstructure interaction partially funded by PEER has been implemented into the NEHRP provisions for new structures and is in the process of being implemented into design documents for existing structures (ATC55 project). 

Expected milestones  


Deliverables  
PEER report presenting research results along with summary journal and conference papers. 