For decades, geophysicists have used models of friction along faults, tested in the lab, to assess earthquake risk. The amount of friction along a fault, the thinking went, would determine whether and how powerfully an earthquake would strike. When friction is high, rocks along the fault snag against each other, locking into place, and tension builds over time. With enough tension, these slabs of rock eventually give way all at once and lurch forward violently, causing an earthquake. In contrast, if the rocks can consistently slide past one another gently, in slow motion, about a few millimeters a year, the tension is released. No snagging and breaking, no earthquake. This is an ongoing process known as "creep." But it turns out complexity is much easier to assess than a fault's frictional properties. To determine complexity, Tsai and his colleagues measured fault density as well as the average "alignment" of faults?"how close they are to parallel?"taken from California