On page 124 of "SEED Reference Manual, Standard for the Exchange of
Earthquake Data, SEED Format Version 2.4, August, 2012" with
respect to [1001] Data Extension Blockette (8 bytes):

> field 4: BYTE: µsec has the data start time down to the
>   microsecond. The SEED format handles down to 100µsecs. This
>   field is an offset from that value. The recommended value is
>   from -50 to +49µsecs. At the users option, this value may be
>   from 0 to +99µsecs.