Evidence of subduction quakes under central NZ – Expert reaction

Hikurangi Margins map
The Hikurangi margin. GNS Science.

Scientists have found, for the first time, direct geological evidence of ‘megathrust‘ subduction quakes occurring under central New Zealand.

Subduction earthquakes occur where tectonic plates meet, have the potential to be larger in magnitude than fault ruptures closer to the surface, and are more likely to trigger tsunamis.

Analysing sediment cores extracted from a coastal lake near Blenheim, the researchers found traces of two previously undetected major subduction earthquakes occurring in the area in the last 1,000 years.

“Subduction earthquakes are not a ‘new’ risk for New Zealand, as we have always assumed they can occur, and they are accounted for in our seismic hazard models,” said lead author Kate Clark, of GNS Science, in a media release.

“This study is significant in that it confirms that risk.”

The research is published in the Bulletin of the Seismological Society of America. Further information is available in this GNS Science media release.

the SMC collected the following expert commentary on the new research.

Assoc Prof John Townend, EQC Fellow in Seismic Studies, Victoria University Wellington, comments:

“In order to understand the hazard posed to New Zealand by future large earthquakes, we need to know when such earthquakes have occurred in the past – and how big they were. The challenge with this is that the earthquakes of most interest happen very infrequently.

“This study exemplifies the painstaking, multidisciplinary research needed to recognise earthquakes in the geological record and to tease out the details of when and where they happened and how large an area they affected.

“By analysing sediments and microfossils extracted from core samples collected in Big Bay, near Blenheim, the team has been able to document changes in environmental conditions within the lagoon related to abrupt subsidence. Then, using computer models, the team has shown that the observed subsidence events are consistent with the changes expected during large earthquakes.

“In this case, evidence has been found of two large earthquakes occurring about 350 years apart that ruptured the southern section of the Hikurangi subduction zone, where the Pacific plate is being thrust down beneath the leading edge of the Australian Plate, which forms the southern North Island. These observations provide the first substantiated evidence for large subduction zone earthquakes in this area and will enable New Zealand’s seismic hazard model to be refined.”

Dr Geoffroy Lamarche, Principal Scientist – Marine Geology, NIWA, comments:

“The Hikurangi Margin (east of the North Island) is characterised by the subduction of the Pacific Plate beneath the North Island. The region is complex, with geological faults dissecting the North Island (upper plate) generating the earthquakes of the recent past, and the major interface between the two plates. Although it is well-accepted that such interface can generate great earthquakes (i.e. of magnitude greater than 8), whether it has ever happened, and indeed could happen again, along the SE North Island is still unproven.

“Such great earthquakes have been inferred from conceptual and physical models as e.g., those using measurements of relative plate movements (geodetic), but there are no tangible observations of such events in the historical past, which would ascertain such hypothesis.

“This paper uses geological evidence in the form of sediment changes at Big Lagoon to infer earthquake occurrences. The problem is incredibly complex as many tectonic sources can produce such geological signals. It is by cross checking many data that the authors surmise that the observed sediment changes are related to plate interface great earthquakes.

“Such research is of high relevance to better understand the hazard related to the plate interface below the North Island. It is by developing independent research initiatives with similar findings that we will be able to increase our certainty.

“The present research is of very high interest but still carries a very high level of uncertainty. Other independent studies with similar objectives, e.g. looking at small landslide occurrence along the Hikurangi margin, the rising of coastal terraces, geodetic modelling (the modelling of plate displacement), may eventually corroborate the finding of this research.

“This paper does certainly add an important part in the quest to assess the likelihood of great earthquake occurring along the SE North Island, and participate to advancing our understanding of seismic hazard in NZ, but it is insufficient to prove beyond any doubts that great earthquakes can occur beneath the SE North Island. Indeed, further independent studies, i.e. using different data in different regions and reaching similar conclusions are needed to strengthen the conclusions of this paper.”

Dr Virginia Toy, Senior Lecturer in Geology, University of Otago, comments

“This robust study employs a multi-disciplinary methodology to demonstrate that the fault interface between the Pacific and Australian tectonic plates off the Southern Wairarapa coast is definitely capable of slipping during earthquakes to generate ground displacements and tsunamis. This is similar to the behaviour of the Japan Trench, where slip during the Tohoku-Oki earthquake in March 2011 generated both a large (Mw9.0) earthquake and an unexpectely damaging tsunami. However, the uplift of the ocean floor where the fault comes to the surface required to explain the surface record at the Blenheim study site is only 3.5 m, much less than the 10 m vertical (and 50 m horizontal) displacement that occurred in the Japanese event. There is thus no specific indication from Clark and co-worker’s study that we need to be prepared for such large tsunami as occurred in Japan.

” Nevertheless, tsunami and earthquake hazard clearly exists in New Zealand and the results of the current study will be very informative for appropriate emergency management and civil defense planning.

“One of the points highlighted in the press release associated with Clarke and co-workers’ study is that the two earthquakes identified occurred only ~350 years apart, and this is less than the ‘return period’ that is presently accounted for in our earthquake hazard models. However, it is important to bear in mind that return periods are only statistically robust when calculated for many earthquakes, and that individual events may be separated by far shorter or longer times; for example, the Alpine Fault has a mean return period of around 330 years over the last 8000 years (at least), but the most recent events were in 1717, 1620, 1450 and 1100 AD – clearly individual events aren’t occurring exactly every 330 years.

“Overall, I hope we will see the results of more studies of the sort reported by Clark and co-workers in future. Associated research, comprising careful examination of sedimentary inputs to, and character, of the Pacific-Australian plate boundary interface, and measurement of temperature, stress and fluid pressure at depth are proposed as part of New Zealand’s engagement in the International Ocean Discovery program. Combining these information streams would allow even better predictions of earthquake and tsunami scenarios to be made for the New Zealand region.”