Sct controller reported B737 descended below assigned altitude; entering a lower MVA.

Attributes
ACN	1557974
Time
Date	201807
Local Time Of Day	1201-1800
Place
Locale Reference	SCT.TRACON
State Reference	CA
Aircraft 1
Make Model Name	B737 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	Final Approach
Route In Use	Visual Approach
Flight Plan	IFR
Aircraft 2
Make Model Name	A320
Operating Under FAR Part	Part 121
Flight Phase	Final Approach
Route In Use	Visual Approach
Flight Plan	IFR
Person 1
Function	Approach
Qualification	Air Traffic Control Fully Certified
Events
Anomaly	Deviation - Altitude Overshoot Deviation - Procedural Clearance Deviation - Procedural Published Material / Policy Inflight Event / Encounter CFTT / CFIT

Narrative:

Aircraft X was turned from the downwind to san and cleared to follow traffic for a visual approach. Aircraft Y was cleared for an approach from straight in behind aircraft X. Aircraft X did not turn towards the airport in an acceptable amount of time causing me to cancel aircraft Y's approach clearance. I stopped aircraft Y at 3800 ft. Which is the MVA in that area and vectored him to a heading of 340 to gain separation with the intent to re-clear him after separation was ensured. Aircraft Y stated that he had continued descent. I issued aircraft Y a low altitude alert and eventually climbed him to 5000 for re-sequencing. Aircraft X should have turned to final sooner. I could have ensured the turn but experience told me that [company] typically turns to final within an acceptable window for this situation to work.

Original NASA ASRS Text

Title: SCT Controller reported B737 descended below assigned altitude; entering a lower MVA.

Narrative: Aircraft X was turned from the downwind to SAN and cleared to follow traffic for a visual approach. Aircraft Y was cleared for an approach from straight in behind Aircraft X. Aircraft X did not turn towards the airport in an acceptable amount of time causing me to cancel Aircraft Y's approach clearance. I stopped Aircraft Y at 3800 ft. which is the MVA in that area and vectored him to a heading of 340 to gain separation with the intent to re-clear him after separation was ensured. Aircraft Y stated that he had continued descent. I issued Aircraft Y a low altitude alert and eventually climbed him to 5000 for re-sequencing. Aircraft X should have turned to final sooner. I could have ensured the turn but experience told me that [company] typically turns to final within an acceptable window for this situation to work.

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.