W. P. De Lange

New Zealand tsunamis 1840-1982

Abstract An extensive search of newspaper reports, archival material, and the literature has revealed that many more tsunamis have affected the New Zealand coast than hitherto realised. 32 tsunami events are listed, including their probability of occurrence, the maximum runup height, as well as the epicentre and Richter magnitude for those events associated with earthquakes. Most coastal regions of New Zealand are reported as experiencing tsunamis. Generally these events have been associated with earthquakes, although the tsunami source mechanisms have also been attributed to large rotational slumps, submarine slumping along the Chatham Rise, and submarine mud volcanism associated with diapiric intrusions on the continental shelf off Poverty Bay. Tsunami waves and seiching accompanying the Krakatoa eruption of 1883 appear to have been induced by pressure coupling between the atmosphere and oceans. Most tsunamis have affected the east coast. This largely reflects both exposure to pan-Pacific origins and th...

Interactions of natural hazards and society in Austral-Asia: evidence in past and recent records

Abstract Interactions of some of the principal historical natural hazards with human populations in the Austral-Asian region are discussed both from the perspective of the impact of the hazard on humans as well as the effects of human activities and climate change on hazard magnitude and frequency. Basically, the former type of interaction is evident for most hazards, while the latter interaction is primarily confined to terrestrial and coastal flooding, erosion, landslides, sea level rise, drought, and fire. Social vulnerability to natural hazards is related to the resources available to cope with the hazard, level of economic development, the ability to predict the occurrence of a hazard and to adjust and adapt to conditions posed by the hazard, and planning measures embraced by societies. Historical chronologies are presented for a range of hazards. Problems in reconstructing historical records of natural hazards include: interpretations of oral records; lack of supporting artifacts; obliteration of evidence of chronic hazards by higher magnitude events; and the inability to distinguish between the effects of different hazards in sediment records. Nevertheless, useful examples illustrate the effects and awareness of volcanic activity and associated hazards, such as tsunami, by early Maori and subsequent development of avoidance strategies; the effects of widespread land use changes and increases in population on the occurrence of floods, landslides and gullies in China and New Zealand; and the effects of forest conversion and drought on fire hazards in Indonesia.

Modelling of tsunamis generated by pyroclastic flows (Ignimbrites)

Pyroclastic flows entering the sea played a major role in generating the largest tsunamiwaves, arising from the 1883 eruption of Krakatau, Indonesia, which caused a considerabledeath toll, most deaths resulting from the tsunamis. The potential exists for similar eventsto occur in Indonesia and New Zealand.Processes leading to tsunami generation by pyroclastic flows, especially those associatedwith Krakatau-type eruptions, are reviewed. The major processes include:1. Deposition at the shoreline causing a lateral displacement as the zone of depositionmoves offshore.2. Upward and lateral displacement of water caused by the propagation of a watersupported mass-flow.3. Downward and lateral displacement of water caused by the sinking of debris from a segregated flow travelling over the water surface.4. Upward displacement of a large volume of water due to the deposition of acaldera-infill ignimbrite or pyroclastic flow deposit.The pyroclastic flow is modelled as a horizontal piston forcingwater displacement. The flow behaves as a wedge of material displacingseawater horizontally and vertically as it moves outwards from the source.Individual pyroclastic flows are treated as linear features that travel alonga specific direction from the volcano, exhibiting limited lateral spreading.The event duration for the formation of a large pyroclastic flow and thedeposition of the ignimbrite is taken as 200–400 s, with flow velocitiesdependent on the volume of material erupted.For simulations it is assumed that the ignimbrite deposit is elliptical with relativelyuniform thickness and the principal axis orientated along the flow direction. Therefore the tsunami is generated by defining an elliptical source region and defining an effective displacement behaviour at each node within that region. The effective displacement is defined by a start time, a finish time and a vertical velocity. These three parameters determine when the seafloor starts to rise and how far it travels during a model time step. The result is a seafloor disturbance that propagates away from the source.The major difficulty with this approach is determination of the appropriate verticalvelocity. With a real pyroclastic flow the effective vertical velocity at any point isvery high. However the model needs to average the displacement spatially andtemporally. Accordingly we apply the model to pyroclastic flows from Mayor Island, New Zealand to examine the influence of model parameters. To further calibrate the numerical model this study is being undertaken in conjunction with physical modelling of the Krakatau 1883 eruption at the Indonesian Tsunami Research Center, BPPT, Jakarta. Historical data will also be used to refine and calibrate the pyroclastic flow model.

The Makassar Strait tsunamigenic region, Indonesia

The Makassar Strait region has had the highest frequency of historical tsunamievents for Indonesia. The strait has a seismic activity due to the convergenceof four tectonic plates that produces a complex mixture of structures. The maintsunamigenic features in the Makassar Strait are the Palu-Koro and Pasternostertransform fault zones, which form the boundaries of the Makassar trough.Analysis of the seismicity, tectonics and historic tsunami events indicatesthat the two fault zones have different tsunami generating characteristics.The Palu-Koro fault zone involves shallow thrust earthquakes that generatetsunami that have magnitudes that are consistent with the earthquakemagnitudes. The Pasternoster fault zone involves shallower strike-slipearthquakes that produce tsunami magnitudes larger than would normallybe expected for the earthquake magnitude. The most likely cause for theincreased tsunami energy is considered to be submarine landslidesassociated with the earthquakes. Earthquakes from both fault zonesappear to cause subsidence of the west coast of Sulawesi Island.The available data were used to construct a tsunami hazard map whichidentifies the highest risk along the west coast of Sulawesi Island.The opposite side of the Makassar Strait has a lower risk because it isfurther from the historic tsunami source regions along the Sulawesicoast, and because the continental shelf dissipates tsunami wave energy.The greatest tsunami risk for the Makassar Strait is attributed tolocally generated tsunami due to the very short travel times.

Seasonal, interannual, and decadal variability of storm surges at Tauranga, New Zealand

Abstract A database of storm surge events was constructed for two sites in south‐eastern Tauranga Harbour, New Zealand, for the period 1960‐mid 1998. Storm surge events were defined as occasions when the residual level between the predicted high tide level and recorded water level exceeded 10 cm. The residual was determined at high tide only (every 12.4 h), with 954 storm surge events found over the 38.4‐year period analysed. The magnitude and frequency of storm surge events varied considerably between 1960 and 1997, withamarked shift evident c. 1976. The period from 1976 to 1997 corresponded to a reduced storm surge frequency and magnitude, compared to the period 1960–76. Wavelet analysis of 125 years of wind storm annual frequencies showed strong fluctuations at inter‐decadal periods. Therefore, it is suggested that the frequency of storm surges varies in response to a coherent inter‐decadal oscillation in surface temperature over the Pacific Ocean, known as the Inter‐decadal Pacific Oscillation (IPO), that reversed phase c. 1976. The El Nino Southern Oscillation (ENSO) also affected the number of days exceeding the storm surge threshold per year, with La Nina events being associated with more storm surge days. The presence of significant decadal variations indicates that annual exceedence probability distributions may misrepresent the storm surge hazard. The available data indicates that there are extended periods when the IPO increases the hazard, and others when the hazard is decreased. Existing analyses of storm surge hazard for the Bay of Plenty have largely been based on data obtained during a period of reduced hazard. Conditions that were associated with larger and more frequent storm surges during 1960–76 may be expected to prevail again over the next few decades.

The hydrodynamics of the southern basin of Tauranga Harbour

Abstract The circulation of the southern basin of Tauranga Harbour was simulated using a 3-D hydrodynamic model ELCOM. A 9-day field campaign in 1999 provided data on current velocity, temperature and salinity profiles at three stations within the main basin. The tidal wave changed most in amplitude and speed in the constricted entrances to channels, for example the M2 tide attenuated by 10% over 500 m at the main entrance, and only an additional 17% over the 15 km to the top of the southern basin. The modelled temperature was sensitive to wind mixing, particularly in tidal flat regions. Residence times ranged from 3 to 8 days, with higher residence times occurring in sub-estuaries with constricted mouths. The typical annual storm events were predicted to reduce the residence times by 24%–39% depending on season. Model scenarios of storm discharge events in the Wairoa River varying from 41.69 m3/s to 175.9 m3/s show that these events can cause salinity gradients across the harbour of up to 4 PSU.

Tsunami Hazard for the Auckland Region and Hauraki Gulf, New Zealand

The Hauraki Gulf is a semi-enclosed sea next to the largest population centre in New Zealand, the Auckland metropolitan region. The potential tsunami hazard is of concern to regional and local planners around the Hauraki Gulf. The Hauraki Gulf has recorded 11 tsunamis and one meteorological tsunami (rissaga) since 1840.The historical tsunami data are relatively sparse, particularly for the largest events in 1868 and 1883. Moreover, local sources may produce damaging tsunamis but none has occurred during recorded history. Therefore numerical modelling of potential tsunami events provides a powerful tool to obtain data for planning purposes. Three main scenarios have been identified for numerical modelling:1. A teletsunami event from an earthquake off the West Coast of South America. Historically this region has produced the largest teletsunamis in the Hauraki Gulf.2. A tsunami generated by a local earthquake along the Kerepehi Fault. This fault bisects the Gulf, has been active during the last century at the southern inland end, and is overlain by a considerable thickness of soft sediment that may amplify the seismic waves.3. A tsunami generated by a volcanic eruption within the Auckland Volcanic Field. This field has involved a series of mainly monogenetic basaltic eruptions over the last 140,000 years. Many of these eruptions have involved phreatomagmatic eruptions around the coastal margins, or within the shallow waters close to Auckland.

Potential Tsunami Hazard Associated with the Kerepehi Fault, Firth of Thames, New Zealand

Geophysical data have identified four submarine segments of the Kerepehi Fault, roughly bisecting a back-arc rift (Hauraki Rift). These segments have been traced through the shallow waters of the Firth of Thames, which lies at the southern end of the Hauraki Gulf, New Zealand. No historical or paleotsunami data are available to assess the tsunami hazard of these fault segments.Analysis of the fault geometry, combined with paleoseismic data for three further terrestrial segments of the Fault, suggest Most Credible Earthquake (MCE) moment magnitudes of 6.5–7.1. Due to the presence of thick deposits of soft sediment, and thesemi-confined nature of the Firth, the MCE events are considered capable of generating tsunami or tsunami-like waves. Two numerical models (finite element and finite difference), and an empirical method proposed by Abe (1995), were used to predict maximum tsunami wave heights. The numerical models also modelled the tsunami propagation.The MCE events were found not to represent a major threat to the large metropolitan centre of Auckland City (New Zealands largest population centre). However, the waves were a threat to small coastal communities around the Firth, including the township of Thames, and 35,000 ha of low-lying land along the southern shores of the Firth of Thames.The Abe method was found to provide a quick and useful method of assessing the regional tsunami height. However, for sources in water depths < 25 m the Abe method predicted heights 2–4 times larger than the numerical models. Since the numerical models were not intended for simulating tsunami generation in such shallow water, the Abe results are probably a good guide to the maximum wave heights.

Utilizing cone penetration tests for landslide evaluation

Pore pressure and shear strength are two important parameters that control the stability of slopes. These parameters can be derived in-situ by cone penetration testing (CPT) with pore pressure measurements. This paper presents the results from three static, vibratory and dissipation CPT profiles deployed into a landslide headwall at Pyes Pa, Bay of Plenty, New Zealand. The landslide strata consist of volcanic ashes and ignimbrites. Studying the stability of slopes in this area using in-situ geotechnical testing is of societal-economic importance since several other landslides within comparable strata caused considerable property damage. Three CPT profiles were collected across the headwall of the slide scar with 2 m spacing in undisturbed sediments using static, vibratory and dissipation test modes. Static CPT results are used to evaluate soil grain size variations, geotechnical parameters of sediments such as shear resistance, probable slip surface and sensitivity of sediments. Liquefaction potential of sediments is assessed using vibratory CPT results. For dissipation tests, the cone remained stationary in the sediment for ∼60 min to monitor pore pressure dissipation at the depths of 6, 9 and 11 m. With the use of pore pressure dissipation data, values of soil horizontal permeability are calculated. The liquefaction probability from static CPT results is compared to liquefaction potential evaluation from vibratory CPT. Last but not least, an unstable soil layer is defined based on static CPT, vibratory CPT and dissipation results.

Oil dispersal modelling: reanalysis of the Rena oil spill using open-source modelling tools

ABSTRACT Oil spill forecast modelling is typically used immediately after a spill to predict oil dispersal and promote mobilisation of more effective response operations. The aim of this work was to map oil dispersal after the grounding of the MV Rena on Astrolabe Reef and to verify the results against observations. Model predictions were broadly consistent with observed distribution of oil contamination. However, some hot spots of oil accumulation, likely due to surf-zone and rip current circulation, were not well represented. Additionally, the model was run with 81 differing wind conditions to show that the events occurring during the grounding represented the typical likely behaviour of an oil spill on Astrolabe Reef. Oil dispersal was highly dependent on prevailing wind patterns; more accurate prediction would require better observations of local wind patterns. However, comparison of predictions with observations indicated that the GNOME model was an effective low-cost approach.