For basic reading and review of HHT algorithm, readers are referred to the references [1-4, 6].

For HHT algorithm analysis programs, two resources may be found useful: (1) the Matlab programs which perform most of the HHT algorithm functions available from [7], authored by Zhaohua Wu (2) the R package ‘EMD’ authored by Donghoh Kim and Hee-Seok Oh (published on 29/10/2012). We decide not to use ‘EMD’ package directly for our analysis, because (1) no significance test functionality available (2) no post processing functionality available (3) different stoppage rule in implementation of the EMD process (4) user manual is not very helpful.

-For basic reading and review of HHT algorithm, readers are referred to the references [1-4, 6].

For HHT algorithm analysis programs, two resources may be found useful: (1) the Matlab programs which perform most of the HHT algorithm functions available from [7], authored by Zhaohua Wu　(2) the R package ‘EMD’ authored by Donghoh Kim and Hee-Seok Oh (published on 29/10/2012).　We decide not to use ‘EMD’ package directly for our analysis, because (1) no significance test functionality available　(2) no post processing functionality available　(3) different stoppage rule in implementation of the EMD process　(4) user manual is not very helpful.