Person:

Dmowska, Renata

We seek to characterize the likelihood of multiple fault activation along a branched normal-fault system during earthquake rupture using dynamic finite element analyses. This is motivated by the normal faults in the vicinity of Yucca Mountain, Nevada, a potential site for a high-level radioactive waste repository. The Solitario Canyon fault (SCF), a north–south trending fault located approximately 1 km west of the crest of Yucca Mountain, is the most active of these faults. Based on the results of previous branching work by Kame et al. (2003), branch activation in the hanging wall of a normal fault such as the SCF may be possible for fast ruptures propagating near the Rayleigh-wave speed at the branch junction. Dynamic branch activation along a splay of the SCF during a seismic event could have important effects on the rupture velocity and resulting ground motions at the proposed repository site. We consider elastic as well as a pressure-dependent elastic–plastic response of the off-fault material. We find that based on the regional stress state in the area, the only likely candidates for branch activation in the hanging wall of the SCF are more steeply westward dipping intrablock splay faults. We also find that the rupture velocity for an earthquake propagating updip along the SCF must reach supershear speeds in order for dynamic branch activation to occur. Branch activation can have significant effects on the ground motions at the proposed repository site, 1 km away from the SCF beneath the crest of Yucca Mountain, causing the repository site to experience a second peak in large vertical particle velocities. Elastic–plastic response near the branch junction reduces peak ground velocities and accelerations at the proposed repository site.

We analyze the nucleation and propagation of shear cracks along nonplanar, kinked, and branched fault paths corresponding to the configurations used in recent laboratory fracture studies by Rousseau and Rosakis (2003, 2009). The aim is to reproduce numerically those shear rupture experiments and from that provide an insight into processes which are active when a crack, initially propagating in mode II along a straight path, interacts with a bend in the fault or a branching junction. The experiments involved impact loading of thin Homalite-100 (a photoelastic polymer) plates, which had been cut along bent or branched paths and weakly glued back together everywhere except along a starter notch near the impact site. Strain gage recordings and high-speed photography of isochromatic lines provided characterization of the transient deformation fields associated with the impact and fracture propagation. We found that dynamic explicit 2-D plane-stress finite element analyses with a simple linear slip-weakening description of cohesive and frictional strength of the bonded interfaces can reproduce the qualitative rupture behavior past the bend and branch junctions in most cases and reproduce the principal features revealed by the photographs of dynamic isochromatic line patterns. The presence of a kink or branch can cause an abrupt change in rupture propagation velocity. Additionally, the finite element results allow comparison between total slip accumulated along the main and inclined fault segments. We found that slip along inclined faults can be substantially less than slip along the main fault, and the amount depends on the branch angle and kink or branch configuration.