*terp- is glossed as 'erstarren' in LIV and *(s)dʰerbʰ- is glossed as 'fest/starr werden'.
*terp- is found in Lat. torpeō 'to be numb, paralysed. Also found in Lith. tir͂pti 'to coagulate, grow stiff'; OCS. trƅpĕti 'to suffer'; In germanic this root is also found OHG derb/p, OE þeorf, OIc. þjarfr 'unleavened' < PGm. þerba- 'stiff' < *trep-ó-.
As seen, this root is attested with o-grade, e-grade and Ø-grade.
*(s)dʰerbʰ- is found in OHG sterban 'to die', with an s-mobile that devoices the initial consonant. OE has deorfan 'to perish'.
CS strƅbi 'to make ripe', And OCS u-strabljǫ, -strabiti 'to make healthy', which is a *-eie/o-causative derivation with o-grade in te root.
Also in this root o-grade, e-grade and Ø-grade is attested.