Catherine Chagué-Goff

Sedimentary differences between the 2002 Easter storm and the 15th-century Okoropunga tsunami, southeastern North Island, New Zealand

Internationally, a key problem in reconstructing chronologies of coastal hazards is the ability to distinguish between storm and tsunami deposits. This situation has been exacerbated by the low number of known locations where both types of deposit occur along the same stretch of coastline. The sedimentological characteristics of a tsunami and a storm deposit laid down on the same stretch of coastline on the southeast coast of the North Island, New Zealand, are distinctly different. The 15th-century tsunami was probably caused by fault rupture in the Cook Strait region, whereas the Easter 2002 storm was generated by a meteorological depression centred some 900 km to the southeast. The differences include areal extent, thickness, and grain-size characteristics. The tsunami deposit thins abruptly at the margins, fines inland, is more poorly sorted, has entrained rip-up clasts, and has an erosional lower contact, often with a buried soil. The storm deposit has a highly variable grain-size distribution with a marked coarsening at its landward extent, is better sorted, coarser, and has a sharp, non-erosional lower contact associated with buried vegetation and soil. The coarser grain size is probably the result of differences in sampling regime as opposed to wave energy. The storm deposit extends about 40 m inland as opposed to about 200 m for the tsunami. Variations in the preservation of evidence are a reflection of the age of deposition. Records of tsunamis and storms in New Zealand indicate that there are probably several coastal sites where both types of deposit can be compared.

Palaeotsunami deposits: a New Zealand perspective

Abstract Over the past few years, geological investigations of coastal sediments in New Zealand have provided evidence of past tsunami. The identification of palaeotsunami deposits is helping to increase our knowledge about the sources, magnitude and frequency of these events. This paper briefly outlines both the key diagnostic characteristics used in New Zealand to identify palaeotsunami deposits and the emerging findings related to recent research.

A tsunami (ca. 6300 years BP) and other Holocene environmental changes, northern Hawke's Bay, New Zealand

Abstract Sediment cores collected in a coastal lagoon a few kilometres east of Wairoa, northern Hawkes Bay, New Zealand, were examined using sedimentological, geochemical, palynological and micropaleontological analyses. A distinct short-lived catastrophic saltwater inundation (CSI) about 6300 years BP and possibly other minor marine incursions are preserved in the coastal estuarine to lagoonal freshwater sedimentary sequences, which have been deposited in the last 6500 years. The CSI is characterised by a gravel unit that thins landward and decreases in particle size to sand, within a sequence consisting mainly of brackish estuarine muds. Diatom assemblages indicate a marked change from the shallow brackish estuarine muds to marine gravels and sands to brackish estuarine muds. The marine influence in the gravel and sand is also shown by the presence of marine dinoflagellates and a peak in Na/Rb. Sedimentological, chemical and paleontological (in particular diatoms) evidence indicates it is a CSI. We conclude that this was a tsunami and propose the most likely propagating mechanisms. Marine influence decreases upcore and totally freshwater conditions are evident in the upper section of the cores. The geochemistry of the sediments mainly reflects the change in stratigraphy, with distinct signatures for tephra (Na, Fe, Cr), organic-rich and peat units (As, Br) and the coarse gravel-sand CSI unit (Na/Rb, Cr, Fe), but it is also indicative of changes in depositional environment. The change in chemistry (Na/Rb) in the CSI event is indicative of a saltwater influence, whereas a marked change in S content suggests a sudden change from brackish to freshwater conditions shortly after 4800 years BP. Another peak in S and Br content about 3200 years BP may indicate another temporary change to brackish conditions.

Signatures of natural catastrophic events and anthropogenic impact in an estuarine environment, New Zealand

The sedimentary record of known natural catastrophic events and anthropogenic activity in an estuarine environment is assessed using sedimentological, chemical and geochronological techniques. Shallow cores collected from intertidal and salt marsh sediments in Ahuriri Estuary, Hawkes Bay, reveal a variety of signatures of natural and human disturbance. Evidence for the 1931 Hawkes Bay earthquake, which resulted in an uplift of one to two metres in the Napier area, is given by a change from silt- to sand-dominated sediment in the lower estuary, which is consistent with a shift toward higher energy depositional conditions following uplift. However, based on physical properties of sediments, the 1931 uplift event does not appear to have caused major changes in depositional conditions in the upper estuary. Similarly, no changes were recorded at one site in the lower estuary, which seems to represent an area of low energy depositional environment. Although the 1960 Chilean tsunami resulted in structural damage in Napier, it did not produce any recognisable sedimentological and geochemical signature in the sedimentary record, suggesting that the study sites were possibly beyond the limit of sedimentation of the tsunami. Post-European settlement impact is mainly restricted to the lower estuary, where increased concentrations of Zn, Cr, Pb and Cu are attributed to industrial discharges. Evidence of agricultural runoff is shown by an increase in Cu concentrations within a fine-grained depositional environment that is distal from industrial sources in the town of Napier. Chemical data (Cl and S) suggest a change in the depositional environment in the upper estuary due to increased freshwater influx and/or decrease in seawater influence. Dating by 210Pb suggests that this occurred around the middle part of the 19th century, and might be attributed to river flooding in the region at that time.

Palaeotsunamis and their influence on Polynesian settlement

The 11 March 2011 Tōhoku-oki tsunami caused widespread devastation to coastal communities in Japan. This event however was merely the latest, yet largest, of several similar occurrences in the Pacific that include the 2007 Solomon Islands, 2009 South Pacific and 2010 Chilean tsunamis. All have had their predecessors, and a growing data base of palaeotsunamis in the Pacific suggests recurrent events comparable with, and of larger magnitude than their recent historical counterparts. Here we show that evidence for regional palaeotsunamis provides an opportunity to re-evaluate hypotheses used to explain the punctuated history of human settlement patterns across the Pacific. In particular, the almost two millennia ‘long pause’ in eastward migration, and the abandonment of long distance sea-voyaging in the 15th century, may be related to palaeotsunamis, with potential sources including the tectonically active Tonga-Kermadec trench, the Kuwae caldera collapse, and the more distant Pacific-wide Ring of Fire.

The Australian tsunami database A review

There has been a significant increase in the number of peer-reviewed publications, critical reviews and searchable web-based databases, since the first substantial tsunami database for Australia was published in 2007. This review represents a complete reorganization and restructuring of previous work coupled with the addition of new data that takes the number of events from 57 (including 2 erroneous events) to 145. Several significant errors have been corrected including mistaken run-up heights for the event of 19 August 1977, Sumba Island, Indonesia, that suggested it was the largest tsunami in Australia’s history. The largest historical event in the database is now the 17 July 2006, Java, Indonesia, tsunami that had a run-up height of 7.90 m at Steep Point, Western Australia. Although estimated wave heights of 40 ft (∼13 m) were noted for the 8 April 1911 event at Warrnambool, Victoria, no run-up data were provided. One of the more interesting findings has been the occurrence of at least 11 deaths, albeit for events that are generally poorly defined. Data gathered during the construction of this database were rigorously reviewed and as such several previous palaeotsunami entries have been removed and other potentially new ones discarded. The reasons for inclusion or exclusion of data are discussed, and it is acknowledged that while there has been an almost three-fold increase in the number of entries the database is still incomplete. With this in mind the database architecture has been brought in line with others in the region with the ultimate goal of merging them all in order to provide a larger, interrogatable and updatable data set. In essence, the goal is to enhance our understanding of the national and regional tsunami hazard (and risk) and to move towards an open-source database.

Impact of Tsunami Inundation on Soil Salinisation: Up to One Year After the 2011 Tohoku-Oki Tsunami

The long-term effect of tsunami inundation on soil salinisation was assessed following the 2011 Tohoku-oki tsunami in two areas on the Sendai Plain, near Sendai airport in the Miyagi Prefecture and Matsukawa-ura near Soma in the Fukushima Prefecture. Data gathered over four sampling seasons 2, 5, 9 and 11 months after the tsunami near Sendai airport show that the salt content generally decreased with time. Concentrations were nevertheless higher in February 2012 than in October 2011, probably due to capillary action and evaporation following long periods with little precipitation in the winter, while the lower concentrations in October were attributed to dilution due to intense rainfall prior to the sampling period. In February 2012, the area with chloride concentrations over the guidelines for the establishment of rice seedlings still extended for nearly 1 km between 2.45 and 3.33 km inland. Chloride concentrations also reached the guideline values at the land surface 1.71 km inland. This corresponded to the limit of the area deemed not suitable for rice production by local rice farmers. However, recent observations revealed that rice crops were not only halted in 2011 but also in 2012, probably due to high salinisation of soil and/or surface and groundwater. Our study shows that soil salinisation was still recorded to nearly 15 cm depth in areas with fine-grained organic-rich soil ~2.5 km from the shoreline 11 months after the tsunami, and that water-leachable ions were preferentially retained in organic-rich muddy sediment and soil, reflecting the long-term impact of tsunami inundation. In Matsukawa-ura, salt crusts still covered the area flooded by the tsunami in February 2012 and both the soil and muddy tsunami deposit were characterised by high chloride and sulphate concentrations. The latter might also lead to sulphide toxicity. Remediation measures have been implemented in certain areas, but further research needs to be carried out to test the effectiveness of the measures being used to allow rice production to resume.

Unearthing earthquakes and their tsunamis using multiple proxies: the 22 June 1932 event and a probable fourteenth-century predecessor on the Pacific coast of Mexico

Tsunami deposits have been widely studied in temperate latitudes, but the intrinsic difficulties associated with tropical coastal environments, and the intensity of bioturbation in these habitats, limit the possibilities of analysing these formations. Here, we investigate the deposits on the Colima coast of Mexico, which overlies the subducting Rivera and Cocos Plates, in order to reconstruct the tsunami inundation history and related hazard. We developed a multi-proxy study aimed to recognize and date historical and palaeotsunami deposits, including historical data on the effects of a known tsunami, geomorphological mapping, stratigraphic, grain size, organic matter content, diatoms, geochemical composition, magnetic susceptibility, and anisotropy of magnetic susceptibility, together with radiometric dating (210Pb and 14C). We identified two probable tsunami deposits at Palo Verde estuary including a historical event associated with the Mw 6.9 earthquake on 22 June 1932 and a palaeotsunami most likely generated by a similar event in the fourteenth century. This work shows that it is possible to identify both historical and palaeotsunamis in the tropical environment of Mexico’s Pacific coast. These data will serve to enhance our understanding of tsunami deposits in tropical environments and of the regional tsunami hazard.

The value of a Pacific-wide tsunami database to risk reduction: putting theory into practice

Abstract The recent 2011 Tōhoku Tsunami showed yet again the devastating impact that these events can have on coastal communities. Even prior to the 2004 Indian Ocean Tsunami there had been a growing awareness of the need to document a record of past tsunamis for risk reduction purposes. The bulk of such early databases were based on historical data. Only in recent years have palaeotsunami databases started to be collated. When one considers that the Pacific region accounts for 85% of known historical tsunamis worldwide, it is unsettling that we have only documented 11 palaeotsunamis throughout all Pacific Island countries (PICs). The way forward to enhance our understanding of palaeotsunamis, and to better understand the magnitude and frequency of events from local, regional and distant Pacific sources, is to gather data from each PIC. By collating data from each island it should be possible to map the spatial and temporal distribution of past events over the last several thousand years throughout the entire Pacific region. These data will provide the essential baseline information needed for achieving more effective disaster risk reduction for PICs.

Tsunami runup and tide-gauge observations from the 14 November 2016 M7.8 Kaikōura earthquake, New Zealand

The 2016 Mw 7.8 Kaikōura earthquake was one of the largest earthquakes in New Zealand’s historical record, and it generated the most significant local source tsunami to affect New Zealand since 1947. There are many unusual features of this earthquake from a tsunami perspective: the epicentre was well inland of the coast, multiple faults were involved in the rupture, and the greatest tsunami damage to residential property was far from the source. In this paper, we summarise the tectonic setting and the historical and geological evidence for past tsunamis on this coast, then present tsunami tide gauge and runup field observations of the tsunami that followed the Kaikōura earthquake. For the size of the tsunami, as inferred from the measured heights, the impact of this event was relatively modest, and we discuss the reasons for this which include: the state of the tide at the time of the earthquake, the degree of co-seismic uplift, and the nature of the coastal environment in the tsunami source region.