Small Transport; Low Wing; 2 Turbojet Eng
Air Traffic Control Fully Certified
Confusion; Distraction
ATC Issue All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Issued New Clearance
ATC Equipment / Nav Facility / Buildings; Procedure
Aircraft X was on a radar vector on the downwind level at 4000 ft. in a 3700 ft. MVA. Two VFR targets were in the area just flying around. Aircraft X had one of them in sight; I descended him to 3000 ft. while still in the 4000 ft. MVA. The TCIC (Tower Controller in Charge) noticed it and told me about it. I corrected the altitude while Aircraft X was descending through 3700 ft. He descended to 3100 ft. before correcting to 3700 ft. It was a significant event. I was too busy dealing with the two VFR targets to catch what I had descended Aircraft X to. The low altitude alarm never went off. The 3700 ft. MVA is based off of a 2700 ft. prominent obstacle about 7.5 miles northeast of where the event happened. The terrain elevation where the event happened was about 390 ft. The aircraft was never less than 2600 ft. away from the ground. The 3700 ft. MVA east of Eugene airport has one identifier way up at the north end of the MVA area; where barely any IFR traffic goes. I would suggest putting another '37' identifier at the south end where aircraft fly most often.
EUG TRACON Controller reported vectoring an aircraft below an unlabeled MVA. Controller stated that display MVA labeling should be improved.