Moving from earthquake prone to earthquake proof
Imagine a time when a magnitude 7.5 rupture on the Wellington Fault is as insignificant as a thunderstorm. Where the buildings we live and work in are built to withstand the ground shaking and ground deformation caused by earthquakes.
This isn’t just a dream. Everyday scientists and engineers are working side by side to improve building, infrastructure, and urban design to be safer and more resilient to the effects of earthquakes. Sadly, to date, this progress is often made from lessons hard won.
On 3 February 1931, New Zealand's deadliest earthquake shattered the cities of Napier and Hastings. At least 256 people died in the magnitude 7.8 earthquake. The silver lining is that this devastating event led to the introduction of New Zealand’s initial earthquake building standards in 1935. For the first time, new buildings were required to be built to withstand horizontal forces, constructed to act as one unit and use materials resistant to shaking.
Ninety years on, the building standards have gone through many iterations to accommodate changes in building materials, design principles and huge advancements in our understanding of earthquake mechanics.
MODELLING THE NEXT ONE
In 2022 the National Seismic Hazard Model (NSHM) was updated to incorporate the wealth of new knowledge gleaned from a global record of earthquake information and learnings over several decades.
The NSHM calculates the likelihood and strength of earthquake shaking that may occur in different parts of the country over specified time periods, for example, the next 10, 50 or 100 years. Among other things, it is used to inform building codes.
However, ground shaking is only part of the picture of earthquake risk. During an earthquake the ground surface can alter significantly by rising, subsiding, tilting, forming bulges or spreading along a fault line. Naturally this deformation carries risks for the buildings and infrastructure in the rupture’s war path.
WHAT ARE ACTIVE FAULTS AND WHY ARE THEY A HAZARD
Faults known to have caused this surface deformation in the last 125,000 years are known as active faults. We are interested in them because they indicate areas prone to surface rupture during large earthquakes.
Since 1848 there have been 12 earthquakes – an average of one every 15 years – in which active fault ruptures have displaced the natural and built environment, causing extensive down time and repair costs.
Notably among these includes the 1855 M8.2 Wairarapa Fault rupture that markedly transformed the Wellington region landscape and affected its subsequent urban development.
In 1987 the M6.5 Edgecumbe earthquake caused substantial subsidence that compromised the Rangitaiki plains flood protection scheme and contributed to cascading flood damage 30 years later in 2017.
However, the direction of land movement isn’t the same in each earthquake. In many cases the old adage of ‘what goes up must come down’ rings true. The 1931 Napier Earthquake caused uplift of most of Ahuriri Lagoon, on which the airport and more recently, houses, have been built, but the longer-term earthquake record is for subsidence on the Hikurangi Subduction Zone, potentially causing a flood risk to these now developed areas.
Knowing where and how the surface will rupture along faults and respond to an earthquake is critical information for arming ourselves against the next seismic shockwave, whether through more informed urban planning or mitigations such as retrofitting and building stronger and safer buildings and infrastructure.
MORE RESILIENT WITH ADVANCING TECHNOLOGY
And we have never been in a better position to being one step ahead of what nature will throw at us. In the past we have waited for disasters to happen to know what the aftermath looks like. But today, new technology can uncover more of Mother Nature’s secrets that can help us pre-empt the next big event.
Technology such as LiDAR (laser tech) can be used to detect past land movement and active fault locations with incredible resolution – down to individual backyards! In the past 15 years alone, LiDAR has turned up around 100 new active faults. We can use this dynamic landscape data to figure out how much the ground moves in earthquakes and forecast the places most at risk of displacement.
This rich data can inform where and how we build, and with the right mitigations and building design, potentially redefine the areas currently deemed unsuitable for development.
BUILDING BETTER
This is good news at a time when land availability is at a premium. Presently, development avoidance zones are set in places likely to experience a surface rupture during large earthquakes. But with more than 500 faults in New Zealand we can’t avoid them all, especially on city edges where urban sprawl is already distancing communities from town centres. Rather, we need smarter cities fortified with better cost-effective engineering options, including ways to retrofit established infrastructure that would be costly to rebuild. For example, pipelines that can bend but not break where they cross active faults.
Hazards only become disasters when they aren’t mitigated. Through ongoing commitment to data collection, research and innovative urban and engineering design we can weather the storm and live more safely and prosperously in our shaky isles. By investing in mitigations now, we can create more social and economic certainty and redirect the huge sums saved from disaster response, rebuilding and near untenable insurance premiums into lifting economic growth.