From the London Mail [caution tabloid]:
Thousands of Californians received earthquake warnings on Thursday after a 4.7-magnitude tremor shook Northern California. The US Geological Survey (USGS) issued a ShakeAlert after the quake struck at 4:49am PT (7:49am ET), about 70 miles offshore of Petrolia.
ShakeAlert messages are triggered when seismic sensors detect a quake of magnitude 4.5 or higher expected to cause significant shaking.