A series of aftershocks hit Southern California and northern Mexico today in the wake of Sunday's 7.2 magnitude earthquake that shook the area, causing widespread damage and at least two deaths and dozens of injuries. The strong quake, with its epicenter in Mexico's Baja Peninsula about 115 miles southeast of San Diego, could be felt as far away as Los Angeles and Las Vegas. As many as 20 million people may have felt the quake. Morgan Page is a geophysicist with the US Geological Service.