Nct controller experienced operational error when descending aircraft to altitude below MVA.

Attributes
ACN	688435
Time
Date	200602
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : nct.tracon
State Reference	CA
Altitude	msl bound lower : 4000 msl bound upper : 5000
Environment
Flight Conditions	IMC
Weather Elements	Rain Turbulence
Light	Night
Aircraft 1
Controlling Facilities	tracon : nct.tracon
Operator	common carrier : air carrier
Make Model Name	B737 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	descent : approach
Route In Use	arrival : on vectors
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : approach
Qualification	controller : radar
ASRS Report	688435
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Events
Anomaly	non adherence : required legal separation
Independent Detector	aircraft equipment : altitude alert other controllera
Resolutory Action	controller : issued new clearance flight crew : took evasive action
Consequence	faa : investigated
Miss Distance	horizontal : 12000 vertical : 300
Supplementary
Problem Areas	ATC Human Performance
Primary Problem	ATC Human Performance
Air Traffic Incident	Operational Error

Narrative:

During very busy traffic while sequencing aircraft X to sfo and oak airport simultaneously; I descended aircraft X to 4000 ft instead of 5000 ft over the vicinity of mount diablo. Low altitude alert activated as aircraft X descended through 4400 ft 2 mi north of mount diablo peak depicted on radar scope. I advised aircraft X to climb back to 5000 ft.

Original NASA ASRS Text

Title: NCT CTLR EXPERIENCED OPERROR WHEN DSNDING ACFT TO ALT BELOW MVA.

Narrative: DURING VERY BUSY TFC WHILE SEQUENCING ACFT X TO SFO AND OAK ARPT SIMULTANEOUSLY; I DSNDED ACFT X TO 4000 FT INSTEAD OF 5000 FT OVER THE VICINITY OF MOUNT DIABLO. LOW ALT ALERT ACTIVATED AS ACFT X DSNDED THROUGH 4400 FT 2 MI N OF MOUNT DIABLO PEAK DEPICTED ON RADAR SCOPE. I ADVISED ACFT X TO CLB BACK TO 5000 FT.

Data retrieved from NASA's ASRS site as of January 2009 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.