 A tectonic earthquake begins by an initial rupture at a point on the fault surface, a process known as nucleation.The scale of the nucleation zone is uncertain, with some evidence, such as the rupture dimensions of the smallest earthquakes, suggesting that it is smaller than while other evidence, such as a slow component revealed by low-frequency spectra of some earthquakes, suggest that it is larger.The possibility that the nucleation involves some sort of preparation process is supported by the observation that about 40% of earthquakes are preceded by foreshocks.