But we believe stress is pretty much uniform over an area like the Mojave. Old faults in the Mojave may be 5 million to 6 million years old, whereas new faults may be only 10,000 years old. Because of the mechanics of faulting, new faults require greater force to slip than old ones. But both fault sets can slip simultaneously when activity shifts from old to new faults, Ron says. As faults rotate and age, they become unfavorably oriented relative to stress and therefore less able to accommodate slippage.
Scientists use two key methods to prove a fault has rotated.
First, they can locate sedimentary rock, which is assumed to have formed when sediment deposited horizontally, and observe tilts in enormous sections. Second, they can take paleomagnetic measurements that is, measure the direction of the magnetic field in a rock. Our absolute frame of reference is the Earth's magnetic field because it always runs north to south. When rock is being formed from lava or even sediments, it contains magnetic grains. When lava cools or sediment deposits, the grains acquire the magnetic properties of the field and align parallel to the Earth's magnetic field, which is always north-south.