Mapping Depth To Bedrock, Shear Stiffness, And Fundamental Site Period At Centreport, Wellington, Using Surface-Wave Methods: Implications For Local Seismic Site Amplification

Wellington's port (CentrePort) experienced significant damage from the M-w 7.8 Kaikoura earthquake as a result of soil liquefaction, lateral spreading, and shaking-induced damage to structures. To investigate these ill effects and propose mitigation measures to prevent similar damage in future earthquakes, there was a need to quantify the variations in the depth to bedrock, shear stiffness, and fundamental site period (T-0) across the port. To characterize T-0 and develop shear-wave velocity (V-s) profiles for use in seismic site-response analyses, horizontal-to-vertical (H/V) spectral ratio (HVSR) measurements and active-source and passive-wavefield surface-wave testing (i.e., multichannel analysis of surface wave [MASW] and microtremor array measurements [MAM], respectively) were performed across the port. A site-period map developed from 114 HVSR measurements indicates several areas of rapidly changing, complex subsurface structure. Deep (200+ m) V-s profiles developed at six reference locations across the port were used to estimate the depth to soft (V-s > 760 m/s) and hard (V-s > 1500 m/s) rock. T-0 estimates from HVSR measurements (T-0_H/v) at the six reference locations are shown to be related to the depth of hard rock, based on linear viscoelastic transfer functions calculated from V-s profiles truncated at several depths. T-0_H/v measurements at two ground-motion stations near the port are also shown to be in reasonably good agreement with predominant periods of maximum spectral amplification recorded during both the 2016 Kaikoura and 2013 Cook Strait earthquakes, despite that these sites were also being affected by soil non-linearity and potential 3D basin-edge effects.