Every week at noon, we hear SF emergency sirens fill the air with their hypothetical shrieking. This isn't unusual.

What is unusual, and what you need to know, is that they will go off tomorrow, Thursday, at around 10:15 a..m. Jim Herd at SFCitizen has word that it's all part of an earthquake drill. Behold:

CITYWIDE SIRENS TO SOUND AS PART OF EARTHQUAKE DRILL What: The City’s Outdoor Public Warning System Sirens will be activated as part of the statewide Drop, Cover and Hold earthquake drill on October 15, 2009 at 10:15 am. The sirens will be sounded, followed by a voice message indicating that it is a test. This is the first time the City is utilizing the system citywide at a time other than the regular Tuesday noon weekly test.

San Francisco residents and visitors should not be alarmed by the fact that the sirens are sounding at a time other than the regular Tuesday test.

When: Thursday, October 15, 2009 at 10:15 am

Where: 92 locations citywide

Why: The City is participating in the Great California ShakeOut, the statewide earthquake drill. To date, more than 280,000 people in San Francisco have registered to participate in the drill, and more than 6.4 million have registered statewide. For more information, please go to www.shakeout.org/sanfrancisco.

So, please don't run out into the street, arms flailing about in hysteria. The sirens are a test, only a test.