A Canterbury earthquake expert has created a guide to help engineers produce more resilient safety-critical structures after new research into why earthquake shaking varies across Wellington.  

The study into the behaviour of the land underneath Wellington buildings during the 2016 Kaikōura earthquake has given engineers better understanding on how to make new buildings more seismically resilient in specific areas around the capital, University of Canterbury Professor of Earthquake Engineering Brendon Bradley says. 

“Up until now, engineers typically have used a one-size-fits-all approach following the building code. This new information allows them to decide which locations may need more resilient buildings and which buildings may need less strengthening interventions,” he said. 

“These guidelines provide engineers with methods and tools for a more sophisticated design for buildings like hospitals and emergency facilities, but can also be used for ordinary buildings where clients want more confidence in the expected seismic performance.” 

Funded by Toka Tū Ake EQC, the team analysed different sites around Wellington to help explain why so many buildings had suffered significant damage from the M7.8 Kaikōura earthquake on November 14 2016.  

Sledgehammers were used to create vibrations which were recorded by the sensors to determine the composition of the soils below. Further research is planned in Auckland, Christchurch, Tauranga and Napier using the findings from the Wellington project to help predict how local soils may respond to future events.  

"It was alarming to see the significant damage to buildings in Wellington from relatively lower shaking produced by an earthquake more than 60 kilometres away,” Dr Bradley said. 

Seismic waves are amplified in Wellington soil in a phenomena called the ‘basin effect’. 

The team set out to quantify this precisely and understand why it led to some buildings suffering more damage than others in 2016. It investigated locations with seismic instruments and compared the detailed GeoNet ground-motion data with the damage to buildings.  

“In certain places that the characteristics of the soils amplify the shaking over a narrow range of frequencies that can specifically affect 10-storey buildings, which were badly affected, but 20-storey buildings a lot less; while in other locations the findings could be the opposite,” Dr Bradley said.   

This understanding will help engineers design new buildings and assess existing ones to more accurately predict behaviour in future earthquakes. 

“Because there is a lot of variation in this basin amplification from location to location, it was difficult to recommend simple changes to the building standard, so instead we have created holistic guidance primarily for important safety-critical structures,” Dr Bradley said.