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.

Material contrasts across faults are a common occurrence, and it is important to understand if these material contrasts can influence the path of rupture propagation. Here we examine models, solved numerically, of rupture propagation through one type of geometric complexity, that of a fault branch stemming from a planar main fault on which rupture initiates. This geometry, with a material contrast across the main fault, could be representative of either a mature strike-slip fault or a subduction zone interface. We consider branches in both the compressional and extensional quadrants of the fault, and material configurations in which the branch fault is in either the stier or the more compliant material and configurations with no material contrast. We find that there are regimes in which this elastic contrast can influence the rupture behavior at a branching junction, but there are also stress states for which the branch activation will not depend on the orientation of the mismatch. For the scenarios presented here, both compressional and extensional side branches are more likely to rupture if the branch is on the side of the fault with the more compliant material versus the stiffer material. The stresses induced on the branch fault, by rupture traveling on the main fault, are different for the two orientations of material contrast. We show how the interactions between rupture on the two faults determine which faults are activated.

Fault intersections are a geometric complexity that frequently occurs in nature. Here we focus on earthquake rupture behavior when a continuous planar main fault has a second fault branching off of it. We use the finite element (FE) method to examine which faults are activated and how the surrounding material responds for both elastic and elastic–plastic off-fault descriptions. Compared to an elastic model, a noncohesive elastic–plastic material, intended to account for zones of damaged rock bordering maturely slipped faults, will inhibit rupture on compressional side branches and promote rupture of extensional side branches. Activation of extensional side branches can be delayed and is triggered by continued rupture propagation on the main fault. We examine the deformation near the branching junction and find that fault opening is common for elastic materials, especially for compressional side branches. An elastic–plastic material is more realistic because elevated stresses around the propagating rupture tip and at the branching junction should bring the surrounding material to failure. With an elastic–plastic material model, fault opening is inhibited for a range of realistic material parameters. For large cohesive strengths, opening can occur, but with material softening, a real feature of plastically deforming rocks, open- ing can be prevented. We also discuss algorithmic artifacts that may arise due to the presence of such a triple junction. When opening does not occur, the behavior at the triple junction is simplified and standard contact routines in FE programs are able to properly represent the physical situation.

Fault intersections are a geometric complexity that frequently occurs in nature. Here we focus on earthquake rupture behavior when a continuous, planar main fault has a second fault branching off of it. We use the finite element method to examine which faults are activated and how the surrounding material responds for both elastic and elastic-plastic off-fault descriptions. Compared to an elastic model, a non-cohesive, elastic-plastic material, intended to account for zones of damaged rock bordering maturely slipped faults, will inhibit rupture on compressional side branches and promote rupture of extensional side branches. Activation of extensional side branches can be delayed and is triggered by continued rupture propagation on the main fault. We examine the deformation near the branching junction and find that fault opening is common for elastic materials, especially for compressional side branches. An elastic-plastic material is more realistic since elevated stresses around the propagating rupture tip and at the branching junction should bring the surrounding material to failure. With an elastic-plastic material model, fault opening is inhibited for a range of realistic material parameters. For large cohesive strengths opening can occur, but with material softening, a real feature of plastically deforming rocks, opening can be prevented. We also discuss algorithmic artifacts that may arise due to the presence of such a triple junction. When opening does not occur, the behavior at the triple junction is simplified and standard contact routines in finite element programs are able to properly represent the physical situation.