Dynamic Rupture through a Branched Fault Configuration at Yucca Mountain, and Resulting Ground Motions

E L Templeton,J R Rice,R Dmowska,H S Bhat

doi:10.1785/0120090121

Abstract

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.

Highlights

We seek to characterize the likelihood of multiple fault activation along a branched normal-fault system during earthquake rupture using dynamic finite element analyses
We investigate the likelihood of branch fault activation for splay faults on the extensional side of the Solitario Canyon fault (SCF), with a range of splay fault angles from small angles for more steeply westward dipping splays to larger angles for conjugate eastward dipping faults
We repeat simulations with constant prestress state like those of Kame et al (2003), using the model shown in Figure 6a to characterize branch fault activation, for Ψ 30°, for sub-Rayleigh and supershear rupture for range of branch angles ranging from 10° to 60°

Summary

Introduction

We seek to characterize the likelihood of multiple fault activation along a branched normal-fault system during earthquake rupture using dynamic finite element analyses. Poliakov et al (2002) identified locations of possible branch activation by investigating the dynamic stress field surrounding a rapidly propagating semi-infinite nonsingular mode II distance-weakening shear rupture in an elastic material. They suggested that the prestress state and rupture velocity control the location of zones where the crack-tip stress field could violate a Mohr–Coulomb (MC) failure criterion and potentially nucleate rupture along a preexisting branch. They suggested that the prestress state and rupture velocity control the location of zones where the crack-tip stress field could violate a Mohr–Coulomb (MC) failure criterion and potentially nucleate rupture along a preexisting branch. Kame et al (2003), Bhat et al (2004), and Fliss et al (2005) extended the work of Poliakov et al (2002) by including preexisting branch faults to investigate, in the framework of slip-weakening modeling of spontaneous rupture, how rupture velocity, prestress state, and

KILOMETERS

Line 2

Results and Discussion

Conclusions