Rupture propagation and focusing of energy in a foam rubber model of a stick slip earthquake

Abstract

Dynamic displacements and velocities measured at the surface of a block of foam rubber are analyzed for stick slip events on faults with three different geometries; elliptical, rectangular (aspect ratio of 3.33), and long thin (aspect ratio of 12.66). Over 100 separate events were recorded, each with up to eight simultaneous point measurements of particle displacement. The positions of the measurements were along the fault trace and to distances of a few fault lengths. Acceleration of the rupture front is observed as the rupture propagates out from its point of nucleation. Deceleration of the rupture is also observed when it enters a region of lower effective stress. In the direction of rupture propagation the static displacements along the fault trace increase, and the rise times decrease. The combined effect causes a rapid increase in particle velocities. For a predominantly unilateral rupture, increases in particle velocities by a factor of 2 or 3 over those recorded near the center of the fault trace are common. At higher rupture velocities, there is a larger increase in the particle velocity (or decrease in the rise time) for a given increase in the static displacement. The amplitude of the far‐field displacement is similarly strongly dependent on the nature of the rupture. Differences in far‐field displacements of an order of magnitude are seen for measurements at equal distances from a unilateral rupture. When the rupture initiates at depth, a much more uniform distribution of particle displacements and velocities is obtained. Afterslip, i.e., motion occurring significantly slower than the time required for a shear wave to travel the length of the fault, is also observed. Comparison of our model results with surface static displacements following major earthquakes suggests that observed variation in static displacements may be due in part to variations in the bedrock stress field. Such stress heterogeneity can lead to a division of a major earthquake into a series of several subruptures, each of which rupture coherently, but the event as a whole behaves incoherently.