The relative speed with which the two sides of a geological fault move past each other. Typically, slip rates are measured in millimeters per year. This figure applies to the motion of a fault over an extended period of time, since most faults slip only during earthquakes; in at other times, the two sides are "locked." Thus, a slip rate of 6 mm/yr does not mean that two structures built directly across from each other on either side of a fault will move past each other at 0.5 millimeters per month, 6 millimeters per year, or 60 millimeters per decade. They may, for example, remain relatively "fixed" for many years until they are suddenly offset several meters in a large earthquake.