New research has shed light on why Wellington City experienced so much damage in the 2016 Kaikōura Earthquake.
The quake was by no means the "big one" for Wellington with its epicentre being more than 200km away from the city.
But more than 120 seconds of ground shaking was recorded, with up to 30 seconds of what's considered to be "strong-motion duration" felt across the capital.
Several buildings, with a cluster at Thorndon and the port, were demolished in the aftermath.
Research by Master of Science student Alistair Stronach and Professor Tim Stern from Victoria University's School of Geography, Environment, and Earth Sciences has revealed more about why some buildings in Wellington didn't fare very well.
They have discovered the thickness of soft sediments in the basin beneath Wellington City is up to two times greater than previously thought.
"When earthquake waves pass through layers of sediment, as opposed to basement rock, they increase in intensity and lead to more shaking. This can have a devastating effect on cities, even when earthquakes are located several hundred kilometres away", Stern said.
An infamous example of this effect was the devastation and loss of life in Mexico City in 1985 when seismic waves from an earthquake 400km away were trapped in the basin the city is built on.
The Kaikōura earthquake produced strong waves that got "trapped" in the sediment basin beneath Wellington causing unexpected damage in the Pipitea and CentrePort area of the city, Stern said.
Other earthquake-damaged buildings on CentrePort's land included Statistics House, Customhouse, Shed 39 and the BNZ building. Its container wharf was also severely damaged.
Buildings that had to be demolished up the road in Thorndon included 61 Molesworth St and Defence House.
The research, funded by the Earthquake Commission, used high-precision measurements of the earth's gravity field to make a map of the sedimentary thickness beneath Wellington city.
Measurements were taken throughout the central business district and along the outer hills.
A maximum thickness of about 540m was recorded near Wellington Regional Stadium, which is twice previous estimates.
Stronach said the data from the research will be used in future computer simulations to predict expected shaking in different areas of the city.
"These simulations are vital in planning for building design and identifying parts of the city most vulnerable to intense shaking from both local and distant earthquakes."
The research also found the recently discovered Aotea Fault stretches beyond Clyde Quay Wharf to under Waitangi Park, before heading south, roughly along the line of Kent Terrace.
"Based on our modelling, this fault has several splays—or limbs—across the lower slopes of Mt Victoria and shows up as a steep step in the basement rock beneath the Te Aro part of downtown Wellington," Stronach said.
This fault, along with the Wellington fault running along Tinakori Rd, form the edges of what can be described as a bath tub effect in an earthquake, he said.
"So it's like water sloshing from one side to the other bouncing off coming back and then they reinforce in some places and cancel in others."
Stern said both edges were very steep providing for conditions where a resonance can occur, like in sound waves.
"Where they can get caught backwards and forwards and you get local zones of amplification."