The physical mechanism of earthquake remains a challenging issue to be clarified. Seismologists used to attribute shallow earthquake to the elastic rebound of crustal rocks. The seismic energy calculated following the elastic rebound theory and on the basis of experimental results of rocks, however, shows a large discrepancy with measurementa fact that has been dubbed the heat flow paradox. For the intermediate-focus and deep-focus earthquakes, both occurring in the region of the mantle, there is not any reasonable explanation yet. The current article will discuss the physical mechanism of earthquake from a new perspective, starting from the fact that both the crust and the mantle are discrete collective systems of matters with slow dynamics, as well as from the basic principles of physics, especially some new concepts of condensed matter physics emerging in recent years. 1. Strss distribution in earth's crust: Without taking the tectonic force into account, according to the rheological principle that everything flows, the vertical and the horizontal strsses must be in balance due to the effect of gravitational pressure over a long period of time, thus no differential strss in the original crustal rocks is to be expected. The tectonic force is successively transferred and accumulated via stick-slip motions of rocky blocks to squeeze the fault gouges, and then applied to other rocky blocks. The superposition of such additional horizontal tectonic force and the original strss gives rise to the real-time strss in crustal rocks. The mechanical characteristics of fault gouge are different from rocks as it consists of granular matters. Thus the elastic modulus of the fault gouge is much lower than that of rocks, and will become larger with increasing pressure. This character of the fault gouge leads to a tectonic force that increases with depth in a nonlinear fashion. The distribution and variation of tectonic strss in the crust are then specified. 2. Strength of crust rocks: The gravitational pressure can initiate the transition from elasticity to plasticity in crust rocks. A method for calculating the depth dependence of elasticity-plasticity transition is formulated, and demonstrated by exemplar systems. According to the actual situation analysis the behaviors of crust rocks fall into three typical zones: elastic, partially plastic and fully plastic. As the proportion of plastic parts in the partially plastic zone reaches about 10%, plastic interconnection may occur and the variation of shear strength of rocks is mainly characterized by plastic behavior. The equivalent coefficient of friction for the plastic slip is smaller by an order of magnitude, or even less, than that for brittle fracture, thus the shear strength of the rocks for plastic sliding is much less than that for brittle breaking. Moreover, with increasing depth a number of other factors can further reduce the shear yield strength of rocks. On the other hand, since earthquake is a large-scale damage, the rock breaking must occur along a weakest path. Therefore, the actual fracture strength of rocks in a shallow earthquake is assuredly lower than the normally observed average shear strength of rocks. The typical distributions of averaged strength and actual fracture strength in crustal rocks varying with depth are schematically illustrated in the paper. 3. Conditions and mechanisms of earthquake: An earthquake will lead to large volume expansion, and the expansion must break through the obstacles. The condition for an earthquake to occur may be as follows: the tectonic force should exceed the sum of (a) the fracture strength of rocks, (b) the friction force of fault boundary, and (c) the resistance from obstacles. Therefore, the shallow earthquake is characterized by plastic sliding of rocks that break through the obstacles. Accordingly, four possible patterns for shallow earthquakes are put forward. Deep-focus earthquakes are believed to result from a wide-range rock flow that breaks the jam. Both shallow earthquakes and deep-focus earthquakes are the slip or flow of rocks following a jamming-unjamming transition. 4. Energetics and precursors of earthquake: The energy of earthquake is the kinetic energy released from the jamming-unjamming transition. Calculation shows that the kinetic energy of seismic rock sliding is comparable to the total work for rocks' shear failure and for overcoming the frictional resistance. There will be no heat flow paradox. More importantly, some valuable seismic precursors are likely to be identified by observing the accumulation of additional tectonic forces, local geological changes, as well as the effect of rock state changes, etc.