  The high-velocity features (HVFs) in optical spectra of type Ia supernovae
(SNe Ia) are examined with a large sample including very early-time spectra
(e.g., t < -7 days). Multiple Gaussian fits are applied to examine the HVFs and
their evolutions, using constraints on expansion velocities for the same
species (i.e., SiII 5972 and SiII 6355). We find that strong HVFs tend to
appear in SNe Ia with smaller decline rates (e.g., dm15(B)<1.4 mag), clarifying
that the finding by Childress et al. (2014) for the Ca-HVFs in
near-maximum-light spectra applies both to the Si-HVFs and Ca-HVFs in the
earlier phase. The Si-HVFs seem to be more common in fast-expanding SNe Ia,
which is different from the earlier result that the Ca-HVFs are associated with
SNe Ia having slower SiII 6355 velocities at maximum light (i.e., Vsi). This
difference can be due to that the HVFs in fast-expanding SNe Ia usually
disappear more rapidly and are easily blended with the photospheric components
when approaching the maximum light. Moreover, SNe Ia with both stronger HVFs at
early phases and larger Vsi are found to have noticeably redder B-V colors and
occur preferentially in the inner regions of their host galaxies, while those
with stronger HVFs but smaller Vsi show opposite tendencies, suggesting that
these two subclasses have different explosion environments and their HVFs may
have different origins. We further examine the relationships between the
absorption features of SiII 6355 and CaII IR lines, and find that their
photospheric components are well correlated in velocity and strength but the
corresponding HVFs show larger scatter. These results cannot be explained with
ionization and/or thermal processes alone, and different mechanisms are
required for the creation of HVF-forming region in SNe Ia.
