(FORTUNE Magazine) – Two major earthquakes this summer are sending Californians scurrying to stores for emergency supplies in anticipation of the Big One, a catastrophic quake that seismologists predict could shake the state sometime this decade. Since June 28, when tremors rocked Southern California, retailers such as Builders Emporium have reported vibrant sales of emergency items: thermal blankets, crowbars, fire extinguishers, and water purification tablets. Preparedness has its price: an average $500 for a family of four. Even the most blase Angelenos are stockpiling supplies. The hottest earthquake item by far: emergency gas shut-off wrenches. As Jay Collins, vice president of B&B Hardware Sales, explains it: ''People don't want their house to blow up, especially if they're in it.'' Sensing an opportunity, HomeBase Warehouse sells for $79 an item called Quake Saver, a gravity-triggered device that automatically shuts off the gas whenever a temblor measuring 5.5 or greater (like the two big ones this summer) occurs. During the past month, the National Earthquake Information Center in Golden, Colorado, has located more than 500 earthquakes in Southern California measuring 3 or greater on the Richter scale. The center provides via modem a daily quake update, ''Quick Epicenter Determination.'' Computerless citizens who want to know what's shaking can call 303-273-8516 for a recorded announcement.