Without a model, it is impossible for a geophysicist to study the possibility of forecasting earth quakes. We will define a quantity, the event-degree, in the paper. This quantity plays an important role in the model of quakes forecasting. In order to make a simple model, we make a hypothesis of earth quakes. The hypothesis is: "(i) There are two kinds of earth quakes, one is the triggered breaking (earth quake), the other is spontaneous breaking (earth quake). (ii) Most major quakes in continental plates such as Eurasian Plate, North America Plate, South America Plate, Africa Plate and Australia Plate are triggered breaking. (iii) These triggered quakes are triggered by the evolution of high pressure centers and low pressure centers of the atmosphere on the plates. (iv) How can the evolution of the high pressure centers trigger a quake in quantitative sense? It depends on the event-degree, the extent of the high pressure center, the rate of evolution of event-degree and the the degree of stored energy for reaching breaking point."