Q. What does this EQC-funded geological investigation on the coast 30km north of Gisborne reveal?
A. It shows that three or four large tsunamis have struck the East Coast in the past 1200 years. The last large tsunami occurred around 300 years ago. It also shows there were three large earthquakes, probably centred north of Gisborne in the past 1800 years. Each quake is thought to be around magnitude 7.2 and each quake uplifted coastal land by about 3.5m. The earthquakes occurred 1800, 1200, and 400 years ago. The average recurrence interval is 750 years, suggesting the next quake from this source is probably not imminent.
Q. Did this investigation reveal anything new?
A. Scientists have known for a long time that the East Coast is seismically active and can experience earthquakes of magnitude 7.0 and above. They have also known this region has a high risk of tsunamis. This study has provided an insight into the timing and magnitude of some of these events.
Coastal sites that provide good evidence of tsunamis are rare, so hard data such as this helps to ground truth hazard models and adds to overall body of knowledge on earthquakes and tsunamis in this part of New Zealand.
Q. How large were the tsunamis?
A. Scientists estimate the tsunamis were between 9m and 12m high, and would have travelled inland many hundreds of meters in low-lying areas.
Q. Do these findings increase the level of hazard for the East Coast and for Gisborne in particular?
A. No. The findings are consistent with existing modelling. Hard evidence of pre-historic tsunamis is hard to find on the New Zealand coast, and one of the benefits of this study is that it has provided solid evidence of large tsunamis in the past 1200 years. Tsunamis of this size and frequency on the East Coast are accounted for in the National Tsunami Hazard Model.
Q. How was this study site chosen?
A. From previous work on the East Coast, scientists knew that this particular stretch of coast 30km north of Gisborne was a good area to look for evidence of earthquakes and tsunamis. The fieldwork produced better results than they anticipated. The physical evidence of earthquakes and tsunamis was more distinct than they were expecting. This has given scientists confidence that further investigations in this area will be worthwhile.
Q. Are local Council and Civil Defence officials aware of this study and its findings?
A. They are aware that the study has taken place and have an indication of the results. Scientists are scheduled to present detailed findings to both these groups on 16 November.
Q. Would the tsunamis identified in this study have impacted Gisborne City, and if so to what degree?
A. The tsunamis would have affected the entire East Coast of the North Island. It is hard to estimate from just this one study how big a particular tsunami would have been at various places along the coast. However, we know from past fieldwork and from modelling that 5m-plus tsunamis can strike anywhere along the East Coast.
Q.How much did the study cost, how long did it take, and how many people were involved?
A. The study was funded by a $70,000 research grant from EQC. It was undertaken over 2.5 years, and involved seven scientists.
Q. What techniques were used in this study?
A. A 90m-long trench was dug at right angles to Puatai Beach north of Gisborne and geologists looked for evidence of past quakes in the trench walls. They radiocarbon dated shells and wood in the trench walls to establish the dates of the three earthquakes and two tsunamis. The trench also provided information on the height and frequency of past tsunamis.
Q. What is the source of the tsunamis?
A. There are a number of offshore faults in this region and any of them could have produced the tsunamis. It is also possible they came from faults farther afield, such as near South America. Ruptures on the fault which was targeted in this study – Gable End Fault – do not coincide with the dates of the tsunamis. This means that the tsunamis were generated by other faults. Further fieldwork could pinpoint the sources of the tsunamis.
Q. What are the benefits of studies such as this?
A. Studies such as this provide hard knowledge of past earthquakes and tsunamis which can be used to understand and better plan for future events.
Q. Are there people or organisations who have helped with this work and should be acknowledged?
A. Richard Steele, retired from Gisborne District Council, provided field assistance.
Bruce McFadgen of Victoria University of Wellington (School of Māori studies – Te Kawa a Māui) and Nick Beynon provided archaeological surveys.
Matt Watson of ScanTech provided a ground penetrating radar survey.
Peter King, Stan Pardoe, Chris Torrie, Ingrid Collins, and Bernard Card of Whangara Farms Māori Partnership Incorporation, and Wayne Ngata, Maui Tongohau, Anne McGuire, Nori Parata, and Victor Walker of Te Aitanga a Hauiti provided land access consent and support for this work.
Read more here: Gisborne seismic and tsunami hazard:constraints from marine terraces at Puatai Beach (Litchfield et al., 2016)
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