Abstract: A tentative analysis of the mechanism of mantle circulation flow, continent drifting and earthquake origin is first clearly made, giving a simple mechanics-based explanation of the nature. Then the effects of earthquake mechanism on ground motion and wave propagation are delineated in a simple manner by accounting for the different types of earthquake source, break propagation, as well as the variations of rock property. It is shown on the basis of principles of mechanics that at least two physical parameters characterizing the seismic intensity must be specified because at the earthquake source two parameters, the stress jump and the velocity jump of particle motion, should act simultaneously whenever a sudden break occurs according to the discontinuous (jump) wave theory. While it is shown that the break propagation speed C b together with the break plane area may be the third key parameter influencing definitely the time form of unloading function at the source. Since the fault break is an unloading process in the sense of mechanics and the break lead to stress jump and velocity jump so some concepts so far defined seem not exact or even of misunderstanding. Thus some comments on a number of basic concept in engineering seismology and earthquake engineering are made tentatively, aiming at arising an academic discussion.