Apparently some morons are making a bad April Fools joke. See: What’s Shakin’? L.A. Earthquakes Don’t Foretell the "Big One” This Time
Quote:
Last night, the USGS put out an alert that someone was sending out a hoax email warning of an impending magnitude 7.4 earthquake in the Southern California area. The email is set up to look like it's coming from the USGS itself and is clearly some idiot's idea of a funny April Fools' prank. By coincidence, I wrote the article below last night; I had originally planned for it to post later today, but given the hoax letter I figured it was apropos to put it up more expediently. So, just to be clear: There is no way to foretell a mag 7.4 earthquake with any certainty, the letter is a hoax, and the real science, as usual, is far more interesting than anything some bonehead can make up.

The linked article goes on for a nice (not April Fools) discussion of aftershocks and foreshocks.

Shocking! shocked
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"Toto, I've a feeling we're not in Kansas any more."
-Dorothy, in The Wizard of Oz