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

Abstract

An analysis of boulders displaced during the September 2010 M W 7.1 Darfield (Canterbury) earthquake provides noninstrumental 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 8Á970 cm laterally from hosting soil sockets of B1 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 (000Á0158) ridges above c. 400 m elevation but not on NE-, NW-and SE-striking ridges. The prevailing boulder horizontal displacement azimuth of 2509208 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 (B108) 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 (B1 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 bedrockÁsoil interface. Remapping of boulders following the February 2011 M W 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.

Strong proximal earthquakes revealed by cosmogenic 3He dating of prehistoric rockfalls, Christchurch, New Zealand

“…DR3). PGV estimates from known sources are <13 cm/s even when considering possible PGV amplification of as much as 60% at the study site due to topographic effects and site conditions (Khajavi et al, 2012). Strong earthquake shaking above the estimated PGV threshold generated by earthquakes on proximal faults thus provides the most likely explanation for the origin of the Rapaki paleorockfalls.…”

Chronology and processes of late Quaternary hillslope sedimentation in the eastern South Island, New Zealand

“…We note that OSL dating of infill sediments behind prehistoric boulders could provide an independent method for constraining major prehistoric shaking events, as it is difficult to envisage a cause other than earthquakes (Khajavi et al, 2012) for episodic displacement (albeit small) of such large boulders. Assuming each of the infills is related to a seismically induced displacement, PB2 (Fig.…”

The 2010–2011 Canterbury Earthquake Sequence: Environmental effects, seismic triggering thresholds and geologic legacy

“…The majority of boulder displacement vectors were oriented subparallel to maximum instrumentally recorded peak ground displacements from proximal seismometers (Fig. 17D) (Khajavi et al, 2012). Some large displaced boulders showed no evidence for rotation and the ground between the boulder and source socket was undamaged, suggesting that the boulders were temporarily airborne during transient strong ground shaking with vertical PGAs exceeding 1 g due to topographic amplification along ridge crests.…”

“…Boulders ranging from 10 to 1000 kg in mass were displaced distances of N 0.1 to 2 m from shallow (b 50 cm) soil sockets on low angle slopes (b10-15°) on summit surfaces during the M w 7.1 Darfield earthquake (Khajavi et al, 2012) (Fig. 15H).…”

“…Measured PGAs were not used to conduct site-specific dynamic analysis of the rockfall triggering because no PGA measurements were obtained directly at rockfall source sites. Topographic and geometric effects may have amplified horizontal PGAs by N 30-80% (Khajavi et al, 2012;c.f. Beuch et al, 2010), implying that the instrumentally recorded PGAs may under-estimate rockfall triggering PGAs at the source cliffs.…”