THE ENERGETICS OF GRAVITY DRIVEN FAULTING

BARROWS, L.J. Andrews Environmental Engineering, 3535 Mayflower Blvd. Springfield, IL 62707, lbarrows@andrews-eng.com.

Faulting can be modeled as either of two different boundary-value problems. These involve fundamentally different energetics. In displacement-bounded faulting, locked-in elastic strain energy is transformed into seismic waves. In force-driven faulting, the forces that create the stress on the fault supply work or energy to the faulting process. Half of this energy is transformed into seismic waves plus work done in the fault zone and half goes into an increase in locked-in elastic strain. In displacement-bounded faulting the change in locked-in elastic strain drives slip on the fault. In force-driven faulting it stops slip on the fault. Tectonic stress is reasonably attributed to gravity acting on topography and the Earth's internal density variations. The intensity of this stress is comparable to the stress drops commonly associated with tectonic earthquakes. The gravity collapse seismic mechanism assumes the fault fails and slips in direct response to the gravitational tectonic stress. Gravity collapse is an example of force-driven faulting. In the simplest case, energy that is released from the gravitational potential of the topography and stress-causing density variations is equally split between the seismic waves plus work done in the fault zone and the increase in locked-in elastic strain. The release of gravitational potential energy requires a change in the Earth's density distribution. Gravitational body forces are solely dependent on density so a change in the density distribution requires a change in the body forces. This implies the existence of volumetric body-force displacements. The volumetric body-force displacements are in addition to displacements generated by slip on the fault. They must exist if gravity participates in the energetics of the faulting process.