Gisborne seismic and tsunami hazard : constraints from marine terraces at Puatai Beach

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Litchfield, N.J.; Cochran, U.A.; Berryman, K.R.; Clark, K.J.; McFadgen, B.G.; Steel, R. 2016 Gisborne seismic and tsunami hazard : constraints from marine terraces at Puatai Beach. Lower Hutt, N.Z.: GNS Science. GNS Science report 2016/21 vii, 99 p.

Abstract: The East Coast, Gisborne District, is at risk from large earthquakes on active faults lying offshore to the east. These earthquakes are also a likely trigger for tsunamis. Earthquakes on offshore faults can raise the coastline, creating uplifted beaches, and so we can use these landforms to provide a record of past earthquakes and tsunamis. In this study we obtain much greater insight into raised beach records than previous studies by excavating a 90 m long trench across three raised beaches at Puatai Beach. The beach is approximately 30 km north of Gisborne. The trench provided information on the timing, magnitude and frequency of past earthquakes on the Gable End Fault. This fault lies about 5 km offshore and runs from just north of Gisborne to Tolaga Bay. The trench also provided information on the height and frequency of past tsunamis that have struck the East Coast. Investigation of the trench confirmed that there are three raised beaches at Puatai Beach. The beaches were raised in three earthquakes, and were then covered by slopewash, most likely from slips and washed down onto the beaches during storms. The earthquakes occurred approximately 1800, 1200, and 400 years ago and each raised the Puatai Beach coast by around 3.5 metres. These results confirm previous calculations that the Gable End Fault ruptures roughly every 750 years, producing earthquakes of around magnitude 7.2. However, the average amount (3–4 m) and rate of uplift is higher than the long-term average, and may suggest that the involvement of other faults such as the Waihau Fault. The time since the last uplift event (400 years) is shorter than the average repeat times (750 years), suggesting the next earthquake is likely not imminent. Within the slopewash sediments on each raised beach are thin sandy layers, many containing shells. We interpret these to be storm or tsunami deposits, with most probably of tsunami origin. Two of the tsunamis occurred approximately 1000 and 300 years ago. These ages are slightly younger than the earthquakes which raised two of the beaches and within slopewash, so the tsunamis were probably not triggered by the Gable End Fault. There is evidence for tsunamis at other sites along the North Island East Coast, as well as raised beaches and submarine landslides which occurred at approximately 1000 and 300 years ago. Further work could pinpoint the sources of these tsunamis. The highest tsunami sand layers were deposited 9–12 m above mean sea level. These heights that are within, but at the upper end of, the expected tsunami heights for this coastline as modelled in the National Tsunami Hazard Model. In addition to the data on earthquakes and tsunami, the trenches also included archaeological remains. These were mainly fireplaces, probably from prehistoric transient Māori occupation, not established settlements. It is possible that Māori witnessed the last earthquake around 400 years ago and tsunami around 300 years ago. (auth)