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St. Patrick's Day began with a rumble in Los Angeles when a 4.4-magnitude earthquake -- now nicknamed the 'Shamrock Shake' -- rocked the region.
The quake, felt by millions, was the biggest to hit the L.A. area since the 1994 Northridge disaster.
NBC News spoke with U.S. Geological Survey seismologist Dr. Lucy Jones who said it's important to be prepared for a larger earthquake, even if there's "no reason to think it's a precursor for the 'Big One.'"
"We really like to use this chance as a time to remind people -- you decided to live in L.A., that means living with earthquakes," Jones said. "Are you ready for it? They are all not gonna be this small."
Today's earthquake demonstrated that California's early warning prototype worked, Jones said. The system gave Pasadena a two-second warning.
Watch "Nightly News with Brian Williams" tonight for more.