Research
Research Papers
The Earthquake Hazard in Dunedin
I F McCahon, M D Yetton, D R L Cook - Soils and Foundations
Dunedin is located in one of the least seismically active areas in New Zealand. However, while the frequency of severe earthquake shaking is considerably lower in Dunedin than for Christchurch or Wellington, strong shaking will occur in the city. Even the relatively small earthquake of 1974 caused about $2 million (1993 values) of damage, and larger, though infrequent earthquakes could cause many times this amount in direct damage and major disruption to the local economy.
This study attempts to quantify the seismic hazard of Dunedin by adopting current seismic hazard analysis techniques and applying them specifically to Dunedin. Seismic hazard analysis involves three components: a Seismicity Model (a model of earthquake occurrence probability in regions close enough to affect the city), an Attenuation Model (energy loss and wave modification as the seismic waves travel to the basement rock under the city) and a Site Response Model (predicting the changes to the earthquake waves as they propagate up through the gravels, sands and silts underlying the city).
The Seismicity Model developed here makes use of the traditional Gutenberg Richter occurrence relationship (log N = a - b M). In the common case when b is close to one, an approximate tenfold reduction in earthquake occurrence occurs with each step up the Richter magnitude scale. Thus by knowing the number of relatively frequent small earthquakes, the average recurrence interval of more infrequent larger events can be predicted. The basic model has been refined for the central and southern South Island by subdividing the area into a number of seismicity zones. In each zone the maximum credible earthquake has been estimated from previous geological studies or, in the absence of such work, from the length (and, where known, the displacement per event) of the known active faults in the zone.
Probabilistic information is obtained from the number of earthquakes historically recorded in the zone over a given period of time. For magnitudes less than 6 the recorded instrumental data is from 1960-1988; for magnitudes greater than 6 and less than 6.5, 1940-1988; and magnitudes greater than 6.5, 1840-1989. It should be noted that even the period 1840 -1989 is much shorter than the return period for major earthquakes on any of the faults near Dunedin and although this is the maximum record available, this time span is still relatively short.
The earthquake hazard in Dunedin is dominated by relatively infrequent moderate to large earthquakes (magnitude up to 7.5) in eastern Otago, and large to very large earthquakes in the much more seismically active but distant Fiordland region.
The calculated probabilities for various intensities of shaking in Dunedin are comparable with those reported previously by others, and are as follows:
| Modified Mercalli Intensity | Approximate Expected Effect | Average Return Period |
| Intensity VI | Minimal property damage | 30 years |
| Intensity VII | Some property damage Loss of life unlikely | 100 years |
| Intensity VIII | Significant property damage Loss of life possible | 450 years |
| Intensity IX | Extensive property damage Some loss of life | In excess of 2,500 years |
| Intensity X | Catastrophic property damage Major loss of life | Very small probability |
These intensity return periods are for "average" ground conditions and some parts of the city, such as the reclaimed land and the alluvial area of South Dunedin, are likely to experience shaking of up to one intensity unit higher for the city than the average.
This amplification was observed during the 1974 Dunedin earthquake, the deep relatively soft alluvial soils changes the nature of the earthquake shaking by modifying the ground acceleration, velocity and displacement at any frequency. In some areas of the city the earthquake vibrations are amplified and within the city distinct local variation results in particular from gradational changes in the top 30m of sediment. These effects have been considered in the Site Response Model.
Variation in shaking intensity can also be expected (and was observed in the 1974 Dunedin earthquake) from topographic focusing and shielding effects. These effects have not been considered in this study, as current knowledge does not allow analysis.
As well as amplifying earthquake shaking, the alluvial soils of South Dunedin are also potentially susceptible to liquefaction. There is insufficient borelog and soil testing data available to define this hazard accurately. Analysis of typical sites indicates that while liquefaction is unlikely to be widespread, some local areas may be vulnerable to liquefaction damage during major earthquakes.
Much of Dunedin is sited on relatively steep topography, with areas of known instability. Large earthquakes could trigger landslide movement, on both existing landslides, where the movement would probably be limited, and generate new landslides. New landslides would occur only if the slope was already at marginal stability. Areas vulnerable to new earthquake triggered movement are difficult to determine, and the best indication is the distribution of known mass movement features. A significant area of South Dunedin is potentially vulnerable, but probably only during earthquakes of a return period in excess of 150-200 years.
Chapter 8 of the report briefly considers the potential damage to the city in terms of the likely impact of a major earthquake on physical structures. Structures on the alluvial areas of the city should perform adequately provided they have been designed to recent code spectra for flexible soil sites. Damage to engineering lifeline services may be significant. The length of the supply pipelines makes the water supply vulnerable, but this is offset by the number of supply sources and interconnections between them. Reticulation is likely to be damaged in the softer alluvial soil areas. Sewers are similarly vulnerable in the soft soil areas, and a major part of Dunedin sewerage traverses this area.
An in-depth lifelines study for Dunedin has not been attempted. It is recommended that an engineering lifelines study be initiated, and a study of the economic and sociological impact of a major earthquake. Site specific studies should be carried out for key public and service facilities, and a review of planning control on areas of potential mass movement. Further research into the active faults close to Dunedin would be of great benefit in better defining the seismic hazard in Dunedin. Continued compilation of subsurface information with respect to soil amplification effects and liquefaction susceptibility is also necessary.