Newly discovered hidden faults in Hamilton, an area once thought devoid of any active faults, pose a potential seismic risk.
However, as the faults haven’t presented themselves on the surface, it is difficult to directly assess how frequent and intense activity on these faults have been.
To explore this activity more accurately, a University of Waikato team, led by Professor David Lowe and Dr Vicki Moon, will examine the liquefied volcanic-ash layers found in many small lakes scattered amidst the faults to assess the frequency and activity of the faults in the last 20,000 years.
“This project will be groundbreaking, as no one has ever identified volcanic-ash layers in lakes as being liquefied, no one has used CT scanning to identify the phenomenon, and the term ‘tephra seismites’ (‘tephra’ is a Greek word for volcanic ash) that we are using has also never been used before,” says Professor Lowe.
The idea to use the liquefied ash layers stems from sediment cores Professor Lowe extracted from lakes in the Hamilton lowlands in his early PhD days in the 1980s, which showed unexplained holes in the volcanic-ash layers preserved in the cores and ash-filled ‘cracks’ in the underlying lake sediment.
In 2015, Dr Moon and Dr Willem de Lange (Waikato University) were alerted to a possible fault in the northern suburbs of Hamilton (exposed during subdivision excavations). With a grant from EQC, Moon and de Lange investigated further and, with the aid of LiDAR and graduate-student research projects, subsequently discovered a total of around 25 faults in and beyond Hamilton.
It was then that Professor Lowe put two and two together and realised that the unexplained holes and ‘cracks’ associated with the ash layers in the lakes were the result of liquefaction due to seismic activity.
Professor Lowe was funded by EQC in another project (on ash-fall hazard) to investigate very thin (< 1 mm) volcanic-ash deposits in the lakes. He took this opportunity to re-core three lakes to obtain new cores, to successfully test the idea of using CT scanning to identify the liquefied ash layers.
The liquefaction in the ash layers is similar to the liquefaction experienced in the 2010-2011 Canterbury earthquakes, the only difference being the fine sands and silts (of the ash layers) have been injected downwards rather than upwards as they were in Canterbury. These ash injection features are now termed ‘tephra seismites’.
The first part of the project, which officially started on 1 October, is to map the occurrence of tephra seismites in the Hamilton lowlands and identify which layers have been liquefied in the lake sediments by extracting cores from up to 30 lakes around the Hamilton area. Some of the lakes are close to faults and others are more distant.
“Each core will then be CT scanned at Hamilton Radiology (by radiographer Nic Ross) to see if we can determine a pattern that suggests which faults have been active.
“Even before starting the project, we know from the previously obtained lake core records that there’s been some activity, possibly up to three or four earthquakes in the last 20,000 years. This means some of the faults could be considered active,” says Professor Lowe.
A cyclic resistance model will be developed as part of the project, which will establish the minimum amount of shaking that could have caused each instance of liquefaction.
A piece of equipment will also be brought out later in the project from Bremen University (Germany) called a Vibrocrawler, which can float on top of lakes as a mobile cone penetration testing system. It will be used to validate the cyclic resistance model.
Throughout the research the team will be liaising with interested parties such as the Waikato Regional Council, who are very supportive of the work which should result in the research team providing earthquake hazard maps for Hamilton.
“These maps will greatly improve hazard planning and mitigation strategies by policy makers, and provide data for the National Seismic Hazard Model which informs the New Zealand Building Code,” says Professor Lowe.
“Hamilton’s building codes relate to the region being classified (in the National Seismic Hazard Model) as having a ‘relatively low to moderate seismic risk’. But that classification is based on few recorded earthquakes in the area’s history, so what we find could have a bearing on Hamilton’s building standards, which could also have cost implications.”
The research methods could also be applied to other regions in New Zealand that are otherwise difficult to characterise seismically, such as Hawke’s Bay or the Manawatu.
The team will also be working with local iwi and to look at how the project affects them.
“These groups can also provide useful information to us, as Ngāti Wairere’s main pā site was by the Waikato River and has a hot spring. There is in fact a whole line of hot springs in Hamilton that hasn’t been highlighted, running out to Temple View, which aligns with one of the main newly-discovered faults. In return, we hope our findings can help iwi by identifying important landscape sites that might be at risk of potential earthquake activity.”
Other researchers involved in the project include Dr Max Kluger (Bremen University), Associate Professor Rolly Orense (Auckland University), Dr Pilar Villamor (GNS Science), and Professor Richard Johnston (Swansea University, UK).
The study is being supported by funding from Te Pūtea Rangahau a Marsden (Marsden Fund) and the Endeavor Fund of the Ministry of Business, Innovation and Enterprise (MBIE).
The project will run for three years until 2023.