Dash 8-400
Air Traffic Control Developmental
Distraction; Situational Awareness
ATC Issue All Types; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Issued New Clearance
Human Factors; Procedure
There were aircraft sequenced near Glacier Peak and the R-side told the front one; the one that descended below the MIA (Minimum IFR Altitude); to fly a heading and descend and maintain 13000 ft. The read back was covered by our paired freq and the R-side said something along the lines of 'read back blocked; try again'. I don't recall what the pilot read back but he must have missed the altitude because he made the turn and descended below the Glacier Peak MIA. The R-side immediately issued a low altitude alert once we saw the MSAW (Minimum Safe Altitude Warning) flashing and the aircraft climbed to 13000 ft.I should have suggested to split the sector earlier because we had a few instances of the paired frequencies stepping on each other.
I was training on 01/31 combined during a period of metering. The overall complexity wasn't particularly challenging; but the frequency congestion was very; very high. I was metering several aircraft on the GLASR Arrival; with a pair of aircraft at the back of the sequence. In the midst of a huge amount of frequency congestion; several aircraft were keying up at once on different transmitters (up to four airplanes talking at once in different parts of the sector and not hearing each other). I had Aircraft X turned to a 250 heading in the vicinity of Glacier Peak (Minimum IFR Altitude is 126). I instructed the pilot to descend to 130 and heard him read back 130. I then moved on to other things. Moments later; the MSAW (Minimum Safe Altitude Warning) alert activated; showing the aircraft below the MIA. I issued a safety alert to pilot and he climbed back up to 13000 ft.If I had asked for the sectors to be split; that would've probably prevented the issue from happening because the frequency congestion would've been reduced.
Seattle Center Controllers reported an aircraft descended below the cleared altitude as well as the Minimum IFR Altitude.