Ground motion selection

response analyses. Because of the complexity of ground motion time series, in comparison to their simple characterization, it is often found that there exists significant uncertainty in seismic response as a result of different incident ground motions. Therefore, if inappropiate/inconsistent ground motion records are selected for use in seismic response analysis then there is the potential for large biases in response estimates.This work has focused on the development of a ground motion selection method with a fundamental basis, rather than heuristic approaches that are widely used. The developed ground motion selection method is based on the Generalized Conditional Intensity Measure (GCIM) approach (Bradley 2010), which provides the multi-variate distribution of ground motion intensity measures conditional on a specific value of a single intensity measure (e.g. SA(T=2.0s)). A ground motion selection algorithm (Bradley 2012) has been developed to automatically select ground motions based on the GCIM distributions. The selection algorithm can be used for both as-recorded and simulated ground motions, and requires only the specification of a 'weight vector', representing a heirarchy of importance of various intensity measures. A principal feature of the selection algorithm is that ground motions are selected based on their explicit intensity measure values (e.g. PGA, SA, PGV, Duration, etc), and not based on implicit causal parameters (e.g. magnitude, source-to-site distance etc) which are utilized in other contemporary ground motion selection methods. In order to realize the full potential of the GCIM-based ground motion selection algorithm, numerous intensity measure correlation relationships have been developed (see relevant publications).

The ground motion selection algorithm can also be used to assess any bias in seismic response which may result from inappropiate ground motion selection.

Because of the theoretical consistency of the GCIM-based ground motion selection algorithm, it has been demonstrated that the seismic demand hazard can be computed essentially independent on the conditioning intensity measure (Bradley 2012).

Implementation of the GCIM-based ground motion selection in OpenSHA and wed-based software

In order to facilitate the incorporation of the GCIM-based ground motion selection algorithm and realize its benefits, it has been incorporated in the open-source, USGS-funded, Open Seismic Hazard Analysis (OpenSHA) software (See Software and Data, as well as codes for the selection of ground motions) (Scarr and Bradley, 2014).

Ground motion selection for scenario earthquakes

The GCIM-based ground motion selection methodology can also be considered for scenario-based ruptures (Tarbali and Bradley (2015), Tarbali and Bradley (2014)). Ground motions representative of major earthquake ruptures on the Alpine, Hope, and Porters Pass faults for Christchurch can be downloaded here; and for major ruptures of the Wellington, Wairarapa, and Ohariu faults for Wellington can be downloaded here. Full details of this study are available in a UC report.