The effects of normal and shear stress wave phasing on coseismic landslide displacement

Predictive models used to assess the magnitude of coseismic landslide strain accumulation inresponse to earthquake ground shaking typically consider slope-parallel ground accelerations only andignore both the influence of coseismic slope-normal ground accelerations and the phase relationshipbetween dynamic slope-normal and slope-parallel accelerations. We present results of a laboratory studydesigned to assess the significance of the phase offset between slope-normal and slope-parallel cyclicstresses on the generation of coseismic landslide displacements. Using a dynamic back-pressured shearboxthat is capable of simulating variably phased slope-normal and slope-parallel dynamic loads, we subjectedsediment samples to a range of dynamic loading scenarios indicative of earthquake-induced ground shaking.We detail the variations in strain accumulation observed when slope-normal and slope-parallel stresses occurindependently and simultaneously, both in and out of phase, using a range of dynamic stress amplitudes. Ourresults show that the instantaneous phasing of dynamic stresses is critical in determining the amount ofcose ismic landslide displacement, which may vary by up to an order of magnitud e based solely on wave-ph asingeffects. Instantaneous strain rate is an exponential function of the distance normal to the Mohr Coulomb failureenvelope in plots of shear stress against normal effective stress. This distance is strongly controlled by the phaseoffset between dynamic normal and shear stresses. Our results demonstrate that conditions considered byconventional coseismic slope stability models can either overestimate or underestimate earthquake-inducedlandslide displacement by up to an order of magnitude. This has important implications for accurate assessmentof coseismic landslide hazard.