numerical modeling of ground deformation and source mechanism related to co-seismic change
in gravitational potential
HUAN-YEN LOO, YANG-GUANG YUAN and HUI-ZHEN SONG Institute of Geology, State Seismological Bureau, P.O. Box 634, Beijing (China)
(Received by publisher May 9,1989)
Huan-yen Loo, Yang-guang Yuan and Hui-zhen Song, 1990. Numerical modeling of ground deformation and source mechanism related to co-seismic change in gravitational potential. In: S. Das and M. Ohnaka (Editors), Earthquake Source. Processes. Tectonophysics, 175: 131-137.
The release of gravitational potential energy is implicit in ground surface deformation, as was seen in the 1954 Fairview Peak, 1964 Alaska and 1976 Tangshan earthquakes where more energy was released by surface subsidence than was radiated as seismic waves.
Based on the numerical results of dip and thrust faulting driven by either gravitational tectonic stress alone or additional external forces, the elastic stress accumulation process in the front of the fault tip is required, as is usual, prior to any faulting slip, but the energy release would be mainly derived from the downward movement of the hanging wall of a normal fault or the footwall of a thrust fault. In other words, there will be a decrease in lactations potential energy in the upper part of the subsiding wall to generate seismic waves, and an increase in elastic strain energy in the lower part either to cause after shocks or to bring about cessation of further fault movement. In addition, by geodetic surveying along a fault prior to an earthquake opportunities exist to judge the nature of the tectonic. force and the damage zonation.
During an earthquake an enormous amount of energy is released in the form of seismic waves and abrupt energy adjustment takes place within the focal body, where the gravitational energy of one subregion is transformed into the elastic strain energy in another, and vice versa. Most re- searchers emphasize the elastic strain energy change during the earthquake process and almost all previous studies of the focal mechanism have taken no account of the gravitational force. This simplification is reasonable for faults with large horizontal movement. However, there are many earthquakes with large vertical displacements such as the 1964 Alaska and 1976 Tangshan earth- quakes. Some theoretical and practical studies have shown that an important energy source for seismic
waves is the released gravitational energy in the earthquake focus body (Dalhen, 1977; Savage and Walsh, 1978; Barrows, 1983). Here, we will study another means of analyzing the characteristics of spatial energy change within the focal body during an earthquake. The fault is treated as a crack in a fracture mechanics approach and the energy change due to gravity alone or additional external forces is calculated using the finite element method.