A new study of the soft sediment basin beneath Wellington city has found it is twice as deep and has a steeper side than previously thought, increasing the chances of more severe shaking following an earthquake.
“Together, the greater depth of the basin and its steep edge increase the likelihood that earthquake waves will become trapped and amplified. When this happens, we predict more intense shaking in parts of the city within the central business district,” said lead author Professor Tim Stern from Te Herenga Waka—Victoria University of Wellington.
Close to the Wellington Regional Stadium, the researchers found sediment depths of an estimated 500 metres—about twice the depth suggested by previous studies. Depths of 200 metres were found throughout much of Thorndon, with depths of 150–200 metres beneath Waitangi Park, next to Te Papa.
The study also found the shape of the sedimentary basin beneath the city differs significantly from earlier models. Researchers discovered the western edge of the basin is defined by a steep, fault-like structure extending from Lambton Quay beneath the railway station, continuing west of the stadium and reaching toward the ferry terminal.
Previous work in the 1970s had recognised the surface of this fault structure as the Lambton fault, but only considered it a minor fault. This study is the first to define its subsurface shape and extent.
“Putting our new data into computer models, we found ground shaking following a quake could increase by a factor of about three near the basin’s edge. This more intense shaking means the effects on the city could be more significant than previously thought, even for a moderate or a distant quake. Much of the amplification is due to earthquake waves becoming trapped in the basin and resonating, much like waves caught in a swimming pool or bathtub,” said Professor Stern.
This is what happened after the 2016 Kaikōura earthquake. The quake, 80 kilometres away from Wellington, generated waves that were trapped in the basin, resonated, and caused ground motion in parts of the central business district that exceeded predictions.
Researchers from Te Herenga Waka worked with colleagues from the United States on the study, which used new gravity and seismic reflection methods to determine the basin’s thickness and geometry.
“By combining new digital and satellite technologies with traditional geophysical techniques, the study shows we are now able to better map the shape and depth of basins and thus predict where earthquake shaking in urban environments is likely to be intensified,” he said.
The study is published in the New Zealand Journal of Geology and Geophysics. It was partly funded by the Natural Hazards Commission and Te Herenga Waka.