Seismically induced boulder displacement in the Port Hills, New Zealand during the 2010 Darfield (Canterbury) earthquake.

An analysis of boulders displaced during the September 2010 MW 7.1 Darfield (Canterbury) earthquake provides non-instrumental constraints on the variability, distribution and origin of strong ground motion during major earthquakes. Boulders ranging in mass from 10 to 5000 kg were displaced 8–970 cm laterally from hosting soil sockets of <1 cm to 50 cm depth at several sites in the Port Hills, roughly 35 km southeast of the earthquake epicentre. Boulder displacement was observed on N-striking (000–015°) ridges above c. 400 m elevation but not on NE-, NW- and SE-striking ridges. The prevailing boulder horizontal displacement azimuth of 250±20° is subparallel with the direction of instrumentally recorded transient peak ground horizontal displacements. Boulder displacement distance has no correlation with displacement azimuth, boulder mass or soil socket depth and has a partial correlation with slope angle. The lateral displacement of many boulders from low slope (<10°) ground surfaces on ridge crests exceeds nearby instrumentally recorded peak ground displacements at lower elevations by up to an order of magnitude, implying that seismic waves were amplified at the study sites. Preliminary 2-D FLAC modelling suggests that topographic amplification may explain this observation. The co-existence of displaced and non-displaced boulders at proximal (<1 m spacing) sites also suggests small-scale ground motion variability and/or varying boulder-ground dynamic interactions relating to shallow phenomena such as variability in soil depth, bedrock fracture density and/or microtopography on the bedrock–soil interface. Remapping of boulders following the February 2011 MW 6.2 Christchurch earthquake reveals no subsequent relocation despite locally recorded horizontal and vertical ground accelerations well in excess of the Darfield earthquake and pervasive rockfalls and landslides elsewhere. This study successfully identifies some of the major controls on spatial ground motion variability at non-instrumented locations and highlights the complexity of ground response at different spatial scales and for different earthquake characteristics. [ABSTRACT FROM AUTHOR]