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Geometry and faulting history of the Long Spur fault zone, Castle Hill Basin, New Zealand

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Author

  • Dagmar Juul Schou

Summary, in English

Strike-slip faults pose significant earthquake hazards in many populated areas and therefore information about their general behavior is needed in order to attempt to predict future earthquakes. Much of this information is acquired using paleoseismic techniques, which focus on analyzing past earthquakes. These include measurement of displacements of either geomorphic features or beds across faults and searching for evidence for past earthquakes. Paleoseismic techniques are often used on plains but due to logistical limitations, such investigations of mountainous areas are often scarce and limited. The short Long Spur fault zone in Castle Hill Basin, Southern Alps, New Zealand has unusually large and numerous natural vertical exposures as it passes through seven gullies in a colluvial fan. It therefore offers a rare opportunity to examine the geometry and faulting history of a strike-slip fault zone at the surface. The Long Spur fault zone is most likely a sinistral active tear fault between two reverse faults and has a minor normal component. The studied part of the fault zone is unusually wide - up to 210 +/- 10 m - and it is suspected that the width may be due to a releasing bend or stepover zone on the fault. Measurements of displaced beds yield a minimum number of three seismic events on the fault zone and an uncertain minimum recurrence interval of 9700 years. The ratio of strike-slip to normal dip-slip is calculated at 15:1 to 21:1 with a tentative minimum lateral displacement of 150 m to 248 m. The smallest possible slip rate on the fault is 0.5 - 0.7 mm/year, estimated from the minimum slip and the maximum age of the colluvial fan dissected by the fault zone. From this study, it is impossible to say whether displacement on individual fault strands is influenced by the distance between fault strands, but the strike of the fault strands may possibly have a small influence. Displacement on individual fault strands seems to be distributed after a similar pattern in all investigated earthquakes, so that faults with a relatively large offset in one earthquake, will also have large offset in the next, and vice versa for faults with small offsets. Not all fault strands are active in every earthquake, and there are indications that only the northernmost part of the fault zone was active in the latest earthquake event(s). In order to fully describe the geometry and faulting history of the Long Spur fault zone, further studies are suggested to focus on examination of displaced units in combination with dating of event horizons.

Publishing year

2017

Language

English

Publication/Series

Dissertations in Geology at Lund University

Document type

Student publication for Bachelor's degree

Topic

  • Earth and Environmental Sciences

Keywords

  • Long Spur fault zone
  • Castle Hill Basin
  • paleoseismology
  • faulting history

Report number

505

Supervisor

  • Charlotte Möller

Scientific presentation