Date: 202012
Local time of day: 1801-2400
Location: ZZZ.ARTCC, US
Flight conditions: VMC
Aircraft: Small Transport
Operator: Corporate
Operating under: Part 91
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Reporter organization: Government
Reporter function: Enroute
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Time Pressure; Situational Awareness; Distraction; Confusion; Communication Breakdown; Workload
Anomaly: ATC Issue All Types; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter CFTT / CFIT
Detector: Automation Air Traffic Control; Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance
Contributing factors: Airspace Structure; Human Factors; Procedure
Primary problem: Human Factors

Narrative: Volume during this session was not particularly high; but complexity and workload changed quickly; and a sequence of events took my attention away from aircraft that didn't need much of it; including the aircraft in question. Aircraft X descending into ZZZ. My initial descent clearance was to 15;000 ft. An eastbound departure was potentially in conflict with 3 other aircraft; including Aircraft X. Deconflicting this aircraft took much of my attention during this time. Aircraft X ended up being no factor. Once clear of all other aircraft; I gave Aircraft X descent clearance to 11;000 ft. Read back was not as clear as I thought; but I had other things to worry about; including two aircraft simultaneously calling for VFR flight following and a conflict alert between VFR aircraft I was providing flight following to and a Mode-C intruder which took much of my attention because they were tracking to pass about 1 mile apart and the Mode-C intruder was climbing into the other VFR aircraft. Once that situation cleared up and as Aircraft X was descending through 12;000 ft. I received a call from FSS (Flight Service Station) which I decided to answer on the assumption that it would be a faster phone call than it turned out to be.Their request was for a Special VFR departure (SVFR). I had to take time to verify that SVFR was possible from this airport and since SVFR is a very rare request in my area of specialization I then had to look up the phraseology to give the clearance. By the time the clearance was delivered and read back; at least 2 minutes had passed. As I disconnected the FSS call; Low Altitude Alert showed Aircraft X at 7;900 ft.; descending below the MIA (Minimum IFR Altitude) of 8;000 ft. I immediately issued a low altitude alert with a clearance to climb to 10;000 ft. immediately. After the pilot responded I advised the pilot that his descent clearance had been to 11;000 ft. and asked if he had ZZZ in sight. Pilot responded that he had the field in sight. I cleared Aircraft X for a visual approach and gave instructions to contact Tower. It did not appear that he observed the climb clearance that accompanied the Low Altitude Alert; and the aircraft was at 7;300 ft. when I cleared Aircraft X for a visual approach.It is difficult to recommend any actions or changes to prevent re occurrence. This particular sector is quite complex; with high terrain; military airspace; two approach control facilities; arrivals to and departures from ZZZ1 in all directions. Convergence between ZZZ arrivals/departures to the south of ZZZ and ZZZ1 arrivals/departures to the southeast of ZZZ1. Also ZZZ2 arrivals and departures to the northwest of ZZZ2 all come together in the vicinity of ZZZ3. Also several satellite airports where pilots cannot reach us on the ground resulting in FSS relays. 11 adjacent sectors adjusted for typical sector configuration with multiple combined but the actual number of individual adjacent sectors is much higher and a relatively heavy VFR workload. It can unexpectedly become quite busy; even with a small number of aircraft in the sector. When this happens; it usually entails coordination calls from one or both of the approach control facilities; the military controlling agency; FSS; and one of the 11 adjacent sectors; and often includes aircraft that require a higher-than-normal workload. The timing of these factors is not controllable; so when several things happen at once; it usually can't be predicted; it can become difficult to manage; and it's often over before a D-side (Radar Assist) can make it to the sector to help.




Synopsis: Center Controller reported they were distracted by a high workload and missed an erroneous read back of a descent clearance and the aircraft descended below the Minimum IFR Altitude.