Earthquake is a natural disaster that causes enormous loss to human society.Earthquake prediction is a major challenge to scientists from seismology and other relevant fields.However, the mechanism of earthquake is far from clear, and the mainstream view in the international seismology community is that earthquake is unpredictable. Based on some new concepts and knowledge from physics, the current article scrutinizes the gestation and occurrence of earthquake from a novel perspective, formulates a rational mechanism for earthquake. It is dissected that the basis of the erroneous cognition of earthquake unpredictability originates in misconceptions about the seismic principles and in a misunderstanding to the self-organized criticality. Earthquake is indeed consistent with the laws of self-organized criticality, that it is in principle impossible to make a medium-or long term prediction, yet the short-term predictability for earthquake should not be excluded. Prerequisites for a successful short-term earthquake prediction are as follows: a correct understanding to the principle of earthquake, availability of sufficient precursors pertinent to earthquake gestation and occurrence, geological data at large concerning the regions of interest. Traditional seismology is based on the continuous solid medium assumption, by which earthquake is attributed to the brittle fracture of rocks, as summarized in the concept of elastic rebound. This viewpoint is not at all in accordance with actual observations, unable to explain a lot of earthquake phenomena, and furthermore unable to establish truly pertinent earthquake precursors,consequently it concludes the unpredictability of earthquake. As the crust is composed of rock blocks and fault gouges there between, it must be treated as a discrete system, and knowledge of granular matters can be applied to understand the gestation and triggering of earthquakes. The fundamental picture for a seismogenic process is that the tectonic forces are transmitted in a form of force chain and the rock blocks move in the stick-slip fashion. By carefully analyzing the distributions of the strength and the tectonic force upon the crustal rocks with depth, we formulated a mechanism of earthquake in that rock blocks flow via the plastic sliding, and the earthquake is essentially a jamming-unjamming transition confronted by rock blocks. Thus the elastic rebound as mechanism for earthquake can be abandoned. Our novel earthquake principle and the relevant understanding to the earthquake processes can explain many seismologic phenomena that usually could not be understood in terms of traditional seismology, for instance, the heat-flow paradox is resolved,and the cause of deep-focus earthquake also becomes clear. A feasible pathway to obtain the earthquake precursors correctly and to realize the short-term prediction of earthquake is also presented.