Reflections on 10 years since the Christchurch Earthquake

Last month I had the pleasure of convening the 2021 New Zealand Society for Earthquake Engineering (NZSEE) Annual Technical Conference. Held at the University of Canterbury with over 400 delegates from across New Zealand, the conference reflected on the 10^th anniversary of the Christchurch Earthquake and the positive legacies that have come from the challenges we faced.

We are fortunate to have some of the world’s top earthquake engineers and scientists working in New Zealand, and many were at the Conference. There were enjoyable presentations and lively discussion panels – with the conversations during coffee breaks a reminder of just how valuable meeting face-to-face really is. For me, the three days of the 2021 Conference were a chance for personal reflection on the Christchurch Earthquake. The result is four key areas where I think we have made significant progress, but with challenge still ahead.
 

Improvement of our Earthquake-Prone Building Stock

It is worth remembering that virtually all the Christchurch Earthquake’s fatalities came from older reinforced concrete or masonry buildings not designed to current standards. This was one of the government’s motivations for passing the Building (Earthquake-prone Buildings) Amendment Act 2016, which introduced a nationally consistent approach to identifying, assessing and retrofitting earthquake-prone buildings across New Zealand. The Act also created deadlines for territorial authorities and building owners to undertake relevant seismic risk reduction measures.

Five years since the introduction of the legislation, the numbers from MBIE paint a mixed picture of how we have progressed. Around the time of writing there were over 3000 buildings in the register of earthquake-prone buildings and another 4000 expected to be identified by territorial authorities in high and medium seismic risk areas. Obtaining an estimate of how many earthquake-prone buildings have been strengthened is more challenging, but the number is certainly significant. Furthermore, New Zealanders are increasingly aware that a high earthquake rating (%NBS) is desirable, which is helping drive owners to strengthen their assets.

Our efforts to improve our existing building stock are not without challenges. A number of recent cases have arisen in which owners have not, or are unlikely to, meet their obligations under the Act. This is likely to be the case for many more owners when they find situations in which the economics of retrofitting just don’t stack up. One must ask the question of whether we are ready for the potential implications as more and more buildings will eventually reach their deadlines for strengthening? In 2020 a group of industry representatives penned a letter to the Minister of Finance proposing tax deductions on seismic upgrades to accelerate seismic strengthening work required by the Act and improve the resilience of the built environment. It remains to be seen whether the Government will adopt such an approach.

The waters also still appear plenty muddy when it comes to how the earthquake-prone building portions of the Building Act interact with other relevant legislation, in particular the Health and Safety at Work Act 2015 (HSWA). In 2018, Worksafe issued a policy clarification stating if PCBUs were meeting their obligations under the Building Act then Worksafe would not enforce to a higher standard under the HSWA.
The reality is the HSWA still seems to be a major driver behind organisations’ approaches to seismic risk, and there are countless examples of ‘snap closures’ of buildings identified as earthquake prone – certainly not the intention of the 2017 Building Act Amendment.
 

Low-damage Seismic Design

Arguably, the performance of buildings in the Christchurch Earthquake met the expectations of most engineers – in general, they remained standing and occupants could get out safely, but the structure became significantly damaged in the process. However, this was clearly not society’s expectation, which instead was for buildings to not just protect lives but also be re-occupiable after an event without requiring significant repairs.

The numbers illustrate the story well. The loss of 185 lives is a tragedy no matter how you look at it, but this figure is dwarfed by the 6,000 that died in the Kobe Earthquake in 1995 or the 100,000 plus that lost their lives in Haiti in 2010 – a testament to the quality of the design standards and construction quality of New Zealand’s buildings. On the other hand, the direct economic loss of in excess of $30 billion ranks in the top five costliest earthquakes of all time, and the loss of such a significant portion of the building stock has permanently changed the fabric of Christchurch’s CBD.

So, what has been done to close the gap between engineering design philosophies and societal expectations? There was a massive uptake of low-damage solutions in the Christchurch rebuild, often with the use of structural systems specifically tailored towards low-damage – such as buckling-restrained braces, base isolation and precast rocking systems. The benefits of a low-damage solution would have been immediately obvious to building owners and tenants impacted by the Christchurch Earthquake. The challenge is communicating the benefits to others who have not experienced an earthquake damaging their assets and disrupting day-to-day activities. Some big steps have recently been made on this front with MBIE’s Low-damage Seismic Design project, which aims to get people talking a common language and provide clearer guidance to stakeholders on the benefits of low-damage solutions. If the benefits are clearly demonstrable, the result will over time be better performing buildings and more resilient communities.
 

Updated seismic hazard model

Whilst not directly related to the Christchurch Earthquake, the work that GNS Science is leading to update the National Seismic Hazard Model may have a significant impact on seismic design and assessment. At present, there is heated debate going on around when and how the update will be released. Experience shows that hazard estimates invariably increase in most locations – so how can we as an industry manage this? There will be some delay between release of the hazard model and any update of the seismic loadings standard, so the responsibility is on engineers to inform clients of the risks they face when making decisions on building design criteria or strengthening options: do they choose to stick with current requirements or try to anticipate what an updated seismic loadings standard may look like? This is a particularly challenging conundrum given the requirement to meet legislative deadlines for strengthening buildings.

One of the biggest changes to the hazard model is its treatment of uncertainty. The new model will adopt a ‘logic tree’ approach that incorporates numerous different models, all weighted in accordance with how well they are expected to reflect the ‘truth.’ The result is that rather than getting just a single best estimate of seismic hazard we will get a distribution of hazard, which then requires choices to be made around what level of confidence one would like in their design. Engineers will likely need to dust off their probability and statistics textbooks to ensure they have a thorough understanding.

Sticking with the same theme, a highlight of the NZSEE Conference was the Park & Paulay Lecture, delivered by Beca’s Rob Jury on the topic of ‘Designing for Uncertainty.’ Reflecting on this subject, the lesson I took away was that even if we now have better means of quantifying uncertainty, there are still many additional unknowns that engineers must account for. It is in this realm where our engineering judgement is needed most to manage the inherent coarseness of seismic engineering and ultimately deliver structures that have some margin to remain standing when the unexpected happens.
 

Are we ready to respond to the next big one?

There will undoubtably be many different views on how we as a profession responded to the Christchurch Earthquake and other events in the following years. One thing that is for sure is that we would not have been nearly as prepared without the efforts of those who went on NZSEE post-earthquake reconnaissance missions in the preceding decades. These missions, to places like Mexico (1985), Northridge (1994) and Padang (2009), meant that there was always a group of engineers with relevant post-earthquake experience to help lead the response.

Regardless of how well we might have been prepared (or not depending on your point of view), there were still many lessons to be learnt from the response to Christchurch. These have largely been captured in a number of guidance documents, in particular the ‘Rapid Post Disaster: Building Usability Assessment – Earthquakes’  field guide. What is crucial to complement this guidance though is experienced engineers, and the responsibility for this now lies with MBIE. Hopefully the desire remains to still send Kiwi engineers into post-disaster zones for training purposes as nothing can beat on the ground experience.

One final thought on response to future earthquakes is how technology will support out response – in particular, the use of monitoring and instrumentation systems. Over the last decade there has been a continuing reduction in the cost of accelerometers and increase in mobile connectivity. Systems, such as Beca’s Beacon, mean that owners can have near real-time information on how their assets are likely to have responded during an earthquake. Reflecting on the experience of Christchurch and the approach commonly adopted at the time of ‘if its greater than magnitude 5 we’ll get an engineer in’ shows just how beneficial these systems would have been 10 years ago. The challenge for now though is getting these systems up and running and making sure organisations remain prepared as the memory of what happened in the past begins to fade – our experience has demonstrated that the cost of resilience planning is dwarfed by the small disruptions of the dozens of shakes around magnitude 5 felt across the country each year, with COVID and other non-seismic disruptions further leveraging a proactive approach to preparedness.