The dynamics of laboratory thrust earthquakes near the free surface

Several studies have shown that the asymmetric geometry of thrust faults with respect to the Earth's surface leads to a complex dynamic behavior of updip ruptures, such as asymmetric ground motions and temporal variations in normal stress. Here we use an experimental set-up designed to mimic earthquakes on thrust faults to study how fully developed updip ruptures interact with the free surface. The experimental technique (Rubino et al., 2017) combines ultra-high speed photography and digital image correlation and enables to produce coherent full-field maps of dynamic displacements, velocities, strains, and stresses associated with the ruptures at intervals of one microsecond. The ruptures typically arrive to the free surface at a supershear rupture speed and include a second phase of a trailing Rayleigh disturbance. Both phases involve a temporal increase in velocity magnitude. The full-field experimental measurements visualize how the free surface breaks the symmetry in the velocity field as the rupture arrives, with a larger peak of velocity magnitude in the hanging-wall compared to that in the footwall. Moreover, while the arrival of the rupture is associated with a fault-parallel motion, as slip continues, the motion of the hanging wall becomes sub-vertical and the motion of the footwall includes a large horizontal component. This study provides the first experimental measurements of the stresses associated with the rupture near the free surface. The measurements show small initial increase in normal stress as the rupture breaks the free surface followed by a significant reduction. Additional reduction is observed at the arrival of the trailing Rayleigh rupture, with a temporal complete release in experiments that were conducted under small initial compressive load (indicating possible opening of the interface). All experiments show a significant delay in the response of shear resistance to the variations in normal stress.