The North American and Pacific plates meet in California at the San Andreas fault. The plates grind past each other there at as much as an inch-and-a-half a year. Until the plates jam. Then energy builds up, and eventually they lurch - an earthquake. But on some active portions of the fault, the plates tend to just creep along, without many violent jerks. All thanks, it turns out, to a little mineral lube, according to a study in the journal Geology. Geologists got rock cores from two miles down, at the active fault zone. They looked at those fault rocks with an electron microscope. And they found an incredibly thin coating of clay polish on the rocks. They say that clay layer apparently lubricates the plates - like greasing ball bearings - and that it's deposited there by mineral-rich water, sucked into the fault's nooks and crannies. As the fault moves, it opens new cracks for the fluid to get into, which means more lube, and more smooth creeping. A process that's been going on for millions of years. So given enough time, parts of the fault seem to develop their own earthquake prevention mechanism. But for other sections, the clock's still ticking.