SEISMICITY IN DISTINCT-ELEMENT MODELS OF THRUST AND NORMAL FAULTS

Seismicity is simulated using 2- and 3-D distinct-element, mechanical models of normal faulting and thrusting. A ‘rockmass’ is represented by an assembly of circular, frictional, elastic disks in 2-D, and by frictional, elastic spheres in 3-D. The individual particles are bonded in shear and tension according to a specified strength, resulting in an initially continuous rockmass, which has mechanical properties consistent with natural rock. Faulting of the rockmass occurs in response to loading by progressive bond breakage and coalescence of individual, single-bond ‘micro-cracks’. Seismic events result from individual bond breakage. Strain energy stored at individual contacts is released as kinetic energy during elastic rebound of the two formerly bonded particles and neighbors, and by slip between particles. Because the numerical scheme solves the full dynamic equations of motion, propagation of waves through the bonded material occurs, simulating seismic waves whose velocities are dependent on the stiffness, density and packing of particles. Seismic events are manifest visually as clusters of outward radiating velocity vectors that mark the wave front, and propagate at seismic velocities, marking model ‘earthquakes’.

Quantification of seismicity is done using modified subroutines originally developed to model acoustic emissions in small-scale biaxial tests (Hazzard et al., 1998). Kinetic energy is calculated from the velocities and masses of once bonded particles and their neighbors. Events occuring close together in time and space are clustered and considered to be individual seismic events. Kinetic energy of the source particles is monitored for a time period that is a function of the assembly shear wave velocity. Any new events that occur within this time window are incorporated into the event. If this occurs, the source area is expanded to incorporate the particles involved in the later event. Event magnitude a function of kinetic energy change.

At this stage in these models, seismicity appears to be qualitatively realistic. The next step is to quantitatively determine if the seismicity follows a Gutenberg-Richter type relationship. Application of these modeling techniques to natural structures may lead to predictive models of earthquake behavior, both in terms of location and recurrence.