Sct controller reported an air carrier descended below assigned altitude of 9;000 feet during approa...

Narrative:

I intended to send aircraft X direct to vydda and descend to 9000. I'm not sure if I said something different; missed a read back; or had a pilot deviation. They were still reviewing tapes when I filed this report. I noticed aircraft X descending below 9000 (the altitude I thought I assigned him) and told him to maintain 8500. The MVA in the area was 8600 and I observed him as low as 8000 in the area (he took a while to stop descending when I caught it). I had several law enforcement aircraft circling in a bad spot right where san traffic turns base and it required a lot of attention in that area. It's certainly possible I misspoke on my assigned altitude or missed a read back but like I said; I don't know whether it was my fault or the pilot's at this point.not really sure because I don't even know whose fault it was yet. I was definitely a little mentally fatigued as I was about 7 hours into my shift after a quick turn. I know how hard it is to get adequate rest for a quick turn so I even asked for an extra hour between shifts (which was approved). If I misspoke or missed a read back; I think it was just a combination of having a situation with the two law enforcement aircraft requiring a lot of attention and being a little mentally fatigued and that contributed to me missing the read back or misspeaking on the assigned altitude.

Original NASA ASRS Text

Title: SCT Controller reported an air carrier descended below assigned altitude of 9;000 feet during approach to SAN. The descent was noted at 8;500 feet. The Controller was not sure who made the mistake.

Narrative: I intended to send Aircraft X direct to VYDDA and descend to 9000. I'm not sure if I said something different; missed a read back; or had a pilot deviation. They were still reviewing tapes when I filed this report. I noticed Aircraft X descending below 9000 (the altitude I thought I assigned him) and told him to maintain 8500. The MVA in the area was 8600 and I observed him as low as 8000 in the area (he took a while to stop descending when I caught it). I had several law enforcement aircraft circling in a bad spot right where SAN traffic turns base and it required a lot of attention in that area. It's certainly possible I misspoke on my assigned altitude or missed a read back but like I said; I don't know whether it was my fault or the pilot's at this point.Not really sure because I don't even know whose fault it was yet. I was definitely a little mentally fatigued as I was about 7 hours into my shift after a quick turn. I know how hard it is to get adequate rest for a quick turn so I even asked for an extra hour between shifts (which was approved). If I misspoke or missed a read back; I think it was just a combination of having a situation with the two law enforcement aircraft requiring a lot of attention and being a little mentally fatigued and that contributed to me missing the read back or misspeaking on the assigned altitude.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Attributes
ACN	1382783
Time
Date	201608
Local Time Of Day	1201-1800
Place
Locale Reference	SCT.TRACON
State Reference	CA
Environment
Light	Daylight
Aircraft 1
Make Model Name	Large Transport Low Wing 2 Turbojet Eng
Operating Under FAR Part	Part 121
Flight Phase	Initial Approach
Route In Use	Vectors
Flight Plan	IFR
Person 1
Function	Approach
Qualification	Air Traffic Control Fully Certified
Experience	Air Traffic Control Time Certified In Pos 1 (yrs) 4
Events
Anomaly	ATC Issue All Types Deviation - Altitude Overshoot Deviation - Procedural Clearance Inflight Event / Encounter CFTT / CFIT

Sct controller reported an air carrier descended below assigned altitude of 9;000 feet during approach to san. The descent was noted at 8;500 feet. The controller was not sure who made the mistake.