To recognize the seismogenic process, mechanism and behavior of seismic precursors, as well as look for the ways of earthquake prediction the granular physics principle is applied based on the basic structure of the crust and mantle and existing experimental observations, where we treat the earth crust and mantle as large scale discrete matters. Main outcomes are: A granular model of the structure and movement of the earth crust and mantle is established. A formation mechanism of the tectonic forces, which causes the earthquakes, and a model of propagation for precursive signals are proposed. Properties of the seismic precursive signal and its relevance with the earthquake occurrence are illustrated, and principle of ways to detect the effective seismic precursor is elaborated. In the paper the mechanism of palintectic earthquake is also explained by the jamming-unjamming transition of the granular flow. Some earthquake phenomena which were previously difficult to understand are explained, and the predictability of the earthquake is discussed. Due to the discrete nature of the earth crust and mantle, the continuum theory no longer applies during the quasi-static seismogenic process. In all in this paper, based on granular physics, we study the causes of earthquakes, earthquake precursors and earthquake prediction, and a new understanding, different from the traditional seismological point of view, is obtained.