Dynamic Bilateral Rupture Along a Circular Bimaterial Fault

Abstract:

Faults with large slip-displacement frequently juxtapose blocks of different properties against each other, and by doing so will form bimaterial fault interfaces. Rupturing along bimaterial faults may differ from rupturing along homogeneous faults. We investigated rupture behavior along an experimental bimaterial fault made of cylindrical PMMA blocks (10 cm diameter) sheared in a rotary apparatus. Our cylindrical fault has no free edges and may be considered as a proxy for a quasi-continuous, infinite fault. Both blocks are made of the same PMMA but differ in size; thus, the stiffness difference between them is determined by geometry, not material. The rupture character was monitored by ten sets of rosette strain gauges mounted at ~2.25 mm from the fault surface with data recording at 1 MHz. Our analysis is devoted to spontaneous nucleation and bilateral rupture propagation during stick-slip events. We analyzed the 2D strain in a plane normal to the fault during 46 bilateral ruptures under normal-stresses of 2.3-5.8 MPa. Ruptures propagating parallel and opposite to slip direction are defined as positive and negative, respectively. The rupture fronts were analyzed in terms of rupture velocity, particle velocity and local variations in stress intensity.

Most ruptures propagated at supershear velocity (> 1350 m/s), with 79% of positive ruptures and 60% of the negative ones. The propagation velocity of negative ruptures typically increased with distance from the nucleation site, and in some cases, even doubled the velocity over ~10 cm along the fault. Positive ruptures did not display systematic variations of propagation velocity. The rupture fronts displayed consistent variation of the peak normal-stress: reduction (more tensile) for positive ruptures, and increase (more compressive) for negative ruptures. The absolute magnitude of normal-stress changes was as high as 0.6 MPa, with more intense changes for positive ruptures (5-15% of mean) relative to the negative direction (1-6% of mean). Particle velocity analysis revealed peak velocities of ~2 m/s and distinct slip-pulses of width in the range of 3 cm to 15 cm. We noted strong local coupling between particle velocity and normal-stress variations that is related to the nature of bimaterial interfaces.