Spectroscopic and photometric properties of low and high-z supernovae Ia (SNe
Ia) have been analyzed in order to achieve a better understanding of their
diversity and to identify possible SN Ia sub-types. We use wavelet transformed
spectra in which one can easily measure spectral features. We investigate the
\ion{Si}{II} 4000 equivalent width (EW_w\lbrace\ion{Si}{II}\rbrace). The
ability and, especially, the ease in extending the method to SNe at high-z is
demonstrated. We applied the method to 110 SNe Ia and found correlations
between EW_w\lbrace\ion{Si}{II}\rbrace and parameters related to the
light-curve shape for 88 supernovae with available photometry. No evidence for
evolution of EW_w\lbrace\ion{Si}{II}\rbrace with redshift is seen. Three
sub-classes of SNe Ia were confirmed using an independent cluster analysis with
only light-curve shape, colour, and EW_w\lbrace\ion{Si}{II}\rbrace. SNe from
high-z samples seem to follow a similar grouping to nearby objects. The
EW_w\lbrace\ion{Si}{II}\rbrace value measured on a single spectrum may point
towards SN Ia sub-classification, avoiding the need for expansion velocity
gradient calculations.Comment: 12 pages, 5 figure