SAN DIEGO -- A string of earthquakes along the U.S.- Mexico border over the past few days is rattling nerves. The latest quake hit Monday morning, near Mexicali. Officials said the magnitude 4.9 - downgraded from 5.1 - earthquake struck at 10:29 a.m. The quake was centered 24 miles south-southeast of Calexico and was felt in parts of San Diego, Imperial and Orange counties and as far away as Yuma, Arizona. Late Friday, that same area was hit by a 5.4 earthquake, which shut down factories near the border and left 400,000 people without power. The recent quakes are actually a swarm or collection of quakes that hit roughly around the same time in the same area. Normally, you have a big quake, followed by smaller aftershocks. But a swarm is different. The big quake is not at the beginning of the series and it's not always that much bigger than the other quakes. Monday's quake was followed by a series of aftershocks, including one that measured a 3.6 magnitude. Earthquakes are typical for this area near the border, which is basically the southern extension of the San Andreas Fault. "South of the Salton Sea, the San Andreas tends to develop complex faults - many smaller faults - in several directions. It's most likely related to this system." Swarms tend to range from 2 to 5 magnitudes and tend to repeat. They're not typically a prelude to a major quake.
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