INEXPENSIVE PHYSICAL MODEL OF ELASTIC DEFORMATION AROUND A STRIKE-SLIP FAULT

The use of physical models in structural geology courses is a widely employed strategy for active learning. Education research has demonstrated that effective active-learning strategies employed in science courses significantly increase student learning and decrease the incidence of failure (Freeman et al., 2014, PNAS).

The deforming medium in the physical models used to illustrate Earth structures is often dry/wet sand, wet clay, dry powder, or some layered combination of these. Physical models of strike-slip faults are commonly built upon rigid lower plates that are slid horizontally past one another along straight edges that parallel the direction of relative motion. As the lower plates are moved relative to each other, the deforming medium is sheared and produces various deformation effects on its upper surface that are of interest to students: en echelon cracks or folds, sags and pop-ups, and small faults that coalesce into a through-going fault.

Elastic deformation away from the principle fault trace has been described many times since it was noticed in the great San Francisco earthquake of 1906 (Reid, in Lawson, 1908). Elastic effects were reported in the M6 South Napa earthquake of 24 August 2014, documented by relative displacement of GPS stations before, during and after the main shock (Hammond et al., 2014, c/o http://geodesy.unr.edu), by progressive displacement of markers along the ground surface, and by various observations of deformation by residents of the Browns Valley community through which the West Napa fault passes.

We developed a small (~30 cm x 30 cm x 6 cm), inexpensive (<~$10) physical model to help students visualize elastic deformation adjacent to a strike-slip fault. The model is made of foamboard, open-cell foam rubber in 2.5 cm-thick sheets, and adhesives, and is usually topped by a deforming medium (sand, clay, powder). Frictional drag along a vertical fault through the foam causes elastic strain in the foam, decreasing over ~10 cm from the fault surface to the line along which the foam is adhered to the foamboard below. Given enough displacement of the foamboard at the base of the model, slip releases elastic strain in the foam and produces surface rupture and audible sound. Details of the model design and use are available via http://CroninProjects.org/Abbuhl/.