To cite package 'hht' in publications use:

Bowman D and Lees J (2013). “The Hilbert-Huang Transform: A High Resolution Spectral Method for Nonlinear and Nonstationary Time Series.” Seismological Research Letters, 84(6), pp. 1074-1080. http://dx.doi.org/10.1785/0220130025.

Corresponding BibTeX entry:

  @Article{,
    title = {The {Hilbert-Huang} Transform: A High Resolution Spectral
      Method for Nonlinear and Nonstationary Time Series},
    author = {Daniel C. Bowman and Jonathan M. Lees},
    journal = {Seismological Research Letters},
    year = {2013},
    number = {6},
    pages = {1074-1080},
    volume = {84},
    doi = {10.1785/0220130025},
  }