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This change to reduce the long period site amplification was as a result of the CB14 site amplification overamplifying over-amplifying long periods since it double counts for velocity features which are already explicitly modelled in the LF simulations (due to the high resolution 3DVM).
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This change to increase the HF path duration was a result of the durations duration being identified as too short when compared to observed ground motions.
Background and computational implementation of the two changes are provided on the pages for the versions corresponding to the individual changes:
1.Path duration model: gmsim_v18.5.1
2.Long period site amplification:gmsim_v18.5.2---More details needed here.