“Los Angeles is caught in a vise,” says John Shaw, an associate professor of structural and economic geology at Harvard who was half of a research team that discovered a large, active crack in the earth, capable of causing destructive earthquakes, under Los Angeles. The researchers announced their discovery in March 1999. The crack, or fault, does not break the Earth’s surface, so it remained hidden until Shaw and a colleague from the University of California, San Diego, found it in 1998. Los Angeles “is locked between converging sections, or plates, of Earth’s crust, carrying North America and part of the Pacific Ocean floor. As the plates collide, rocks beneath the city are shattered and cut by faults of many shapes and sizes.” Stress building up along parts of the newly discovered fault, then rupturing, could cause a series of large shakers every 250 to 1,000 years.