Applicability of foreign ground motion prediction equations for New Zealand active shallow crustal earthquakes
The number of instrumental ground motion records in New Zealand (NZ) has increased significantly in recent years due to an increase in the number and quality of seismometer throughout NZ. Figure 1 provides a comparison between NGA ground motion database and the NZ database developed as part of this study. Despite this increase in instrumental data, it can be seen clearly in Figure 1 that there is a lack of empirical records from large magnitude events observed at near-source distances. This is even more clear in Figure 2, which plots the cumulative number of records exceeding specific PGA values in the NZ ground motion database. There are only a total of 66 ground motion records which have PGA values above 0.1g (28 crustal, 11 interface, and 27 slab). Furthermore, the maximum PGA values recorded are 0.39g, 0.31g, and 0.28g for crustal, interface, and slab events, respectively. This lack of ground motion records from large magnitude nearsource records, which typically dominate seismic hazard analyses, makes it difficult to develop robust ground motion prediction equations used in seismic hazard analysis based on NZ data alone. In this study an alternative approach to empirical ground motion prediction equation development was taken. Firstly, the applicability of various foreign ground motion prediction equations (derived using plentiful data) to NZ were considered. The consideration was based on both the dependence of the inter- and intra-event residuals as a function of several predictor variables, and also the general predictor variable scaling of the various models. Secondly, the model exhibiting the best applicability to NZ was modified based on theoretical and empirically-driven considerations to better represent the NZ data.