  Based on the geodynamics, an earthquake does not take place until the
momentum-energy excess a faulting threshold value of rock due to the movement
of the fluid layer under the rock layer and the transport and accumulation of
the momentum. From the nonlinear equations of fluid mechanics, a simplified
nonlinear solution of momentum corresponding the accumulation of the energy
could be derived. Otherwise, a chaos equation could be obtained, in which chaos
corresponds to the earthquake, which shows complexity on seismology, and
impossibility of exact prediction of earthquakes. But, combining the
Carlson-Langer model and the Gutenberg-Richter relation, the magnitude-period
formula of the earthquake may be derived approximately, and some results can be
calculated quantitatively. For example, we forecast a series of earthquakes of
2004, 2009 and 2014, especially in 2019 in California. Combining the Lorenz
model, we discuss the earthquake migration to and fro. Moreover, many external
causes for earthquake are merely the initial conditions of this nonlinear
system.
