California will hold large-scale emergency drills preparing more than 9.3 million people for a magnitude 7 or greater earthquake tomorrow. Seismologists believe the US state is decades overdue a “Big One” tremor measuring magnitude 7 or greater on the Richter scale. A massive fault line zone in the Bay Area of Southern California is thought to be most at risk with experts giving a 72% chance of a large tremor within the next 30 years.