SR22
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 3
Situational Awareness; Communication Breakdown; Workload
ATC Issue All Types; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Issued New Clearance
Chart Or Publication; Human Factors; Airspace Structure; Procedure
Came back from break to relieve controller working [two combined sectors]. Traffic was moderate with work to be done so I plugged in as coordination with Vancouver Controller was taking finishing. Controller finished work needed to be taken care of so time could be allowed for and uninterrupted briefing could take place. Upon completion of the briefing I accepted the sector and began working and by this point traffic was building from a moderate pace to a heavy and busy workload. As the Controller was leaving the sector a D side (Radar Assist) was placed on the sector due to the rise in traffic and business. I can't recall but I don't think I heard the coordination with Vancouver regarding Aircraft X nor did I hear if the relieved controller giving the D side a briefing said anything about Aircraft X. Once I was working I heard the D side trying to get communications on Aircraft X realizing that the altitude was below the MIA of 11500 ft. Due to the mountainous terrain I was unable to reach the Aircraft X on the main or BUEC (Backup Emergency Communications) frequency. I decided to relay using a commercial jet off of Vancouver. The relay was successful and Aircraft X was able to climb out of the MIA to 13000 ft. Communication and Radar was re-established. During initial coordination with Vancouver no altitude was given to the Controller prior to me assuming the sector.
ZSE Controller reported an aircraft flying below the Minimum IFR Altitude after relieving another controller.