Seismotectonic hazard evaluation for the Kawarau River power development

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Hancox, G.T.; Beanland, S.; Brown, I.R. 1985 Seismotectonic hazard evaluation for the Kawarau River power development. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 384 63 p.

Abstract: Planning for the proposed Kawarau Power Development has involved the consideration of six schemes, which include alternative sites for five dams, four powerhouses, and several canal and tunnel routes located on the Kawarau River between 12 km and 35 km upstream from the Kawarau/Clutha confluence. Engineering investigations aimed at assessing the feasibility of the Kawarau development have included studies of regional geology and active faulting, and detailed engineering geological investigations at specific sites in the Kawarau valley. This report is concerned with the evaluation of seismotectonic hazards at these sites. Except for some sections of the canal routes which will be partly in alluvium, all of the proposed structures will be founded on schist bedrock. The sites are situated in an area where there are a number of major active and potentially active, mainly reverse faults, among which the Nevis-Cardrona Fault System faults and the Pisa, Dunstan, Moonlight, Ostler, and Alpine faults are likely to contribute to seismotectonic hazards that may affect the proposed structures. Such hazards include: a. Tectonic ground deformation (ground surface rupture and tilting) and strong ground motions associated with maximum credible earthquakes (MCE) on nearby and distant active faults; b. Ground motions from regional seismicity; c. Reservoir induced seismicity. The historic seismic record, which is relatively short (about 150 years), shows Central Otago currently to be an area of low seismicity and is of uncertain value in assessing the probability of future large magnitude earthquakes in the region. Probabilities of future fault movements and large magnitude earthquakes have therefore been estimated from geological data (active fault traces), which indicate that major earthquakes (> M 7.0) have occurred repeatedly in Central Otago in the last 500000 years. The Nevis-Cardrona Fault System faults (NW Cardrona, Nevis, Gentle Annie and Muddy Creek) and the Pisa Fault are closest (0-4 km) to the proposed sites. Except for the Gentle Annie and Muddy Creek faults, these faults are classed as active, although all are considered capable of future primary fault movement. Studies of local geology and late Quaternary (last 500000 years) movements on these faults has enabled faulting recurrence intervals to be determined. The NW Cardrona Fault is the most active fault in the Kawarau valley, having undergone several movements in the last 23000 years, at an average recurrence interval of about 6500 years. The mean recurrence interval for the Nevis Fault is about 4000 years in the Nevis valley, but the fault is not known to have moved in the Kawarau valley in at least the last 2300 years. The Pisa Fault is not known to have moved in the last 35000 years. Although Late Quaternary activity cannot be demonstrated on potentially active faults (Gentle Annie, Muddy Creek), they are regarded as capable o future movement. From subjective assessment of fault data (recur rence intervals, elapse time, tectonic relationships) the estimated probabilities of fault movements in the Kawarau valley in the next 150 years are: < 0.1% far the Pisa Fault, < 2% for the Gentle Annie and Muddy Creek faults, about 4% for the Nevis Fault, and 2-12% for the NW Cardrona Fault. In the event of these low probability events occurring, about 1-2 m of primary fault movement is considered the range of likely future movement on the NW Cardrona, Gentle Annie and Muddy Creek faults. Such faulting may affect some of the proposed reservoir areas or canal routes. Movement on the Pisa Fault is not expected to affect directly any of the proposed sites as it is some 2.5 km downstream from the nearest site. In response to primary fault movements in the Kawarau valley, secondary displacements, tentatively estimated at 300-600 mm (30% of the primary movement), may occur on subsidiary faults within the Nevis-Cardrona Fault System (Gibbston Flats and Mt Difficulty faults principally) affecting both canal and tunnel routes. Small secondary displacements (possibly 10-100 mm, or less) are also possible on major rock defects (crushed and sheared zones) which are present in the schist bedrock at most sites. Overall, the hazard from tectonic deformation at the Kawarau power development sites in the next 150 years is considered to be extremely low to low. From geological data, the Maximum Credible Earthquake (MCE) in the Kawarau valley is considered to be about Ms 7.4, and may be generated by movement on either the NW Cardrona, Gentle Annie, or the Pisa Fault at estimated 150 years probabilities of < 2-8%, < 2%, and < 0.1% respectively. In the event of the MCE the estimated maximum average peak ground accelerations at the proposed sites are 0.44 to 0.53 g horizontal, and 0.59 to 1.03 g vertical. These values are likely to be subject to uncertainty of about plus or minus 0.2 g. From historic seismicity combined with inferences from geology (active faults in the region) the estimated return periods for earthquakes felt at Queenstown are 12 years for MM VI, 54 years for MM VII, and 250 years for MM VIII. Based on these estimates the 150 year ground acceleration in the Kawarau valley is estimated to be in the range 0.18 to 0.25 g. Similar accelerations are estimated for MCEs on distant active faults. For most of these faults the 150 year probabilities of the MCE occurring are low < 20%, although for the Alpine Fault the probability is moderate to high (36-80%). The MCE on the Alpine Fault (Ms 8.5) would produce a ground acceleration of about 0.14 g in the Kawarau area. Strong ground motions are not expected to affect significantly any of the proposed power development sites or any of the existing large landslides in the Kawarau valley. However, an earthquake with felt intensities of about MM V, possibly producing ground motions of up to 0.1 g, may initiate small local failures on schist debris slopes steeper than about 34°. In view of the very small size of the proposed reservoirs the probability of reservoir induced seismicity is regarded as very low. (auth)