The data are classified with the "epoch" variable,
which separates the series into pre-seismic and seismic
data; this is defined relative to the predicted P-wave
arrival time from a traveltime model.
The original dataset contained NA values, which
were imputed using zoo::na.locf, which fills
NA with the last previous observation.
pspectrum, hfsnm,
magnet TauP.R for an R-implementation of the traveltime
calculations: