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Adrienn Lukacs

Adrienn KovacsPhD title: Geophysical characterisation of the Alpine Fault throughout and beneath Whataroa Valley, Haast River and Turnbull River, Westland, New Zealand

Supervisors: Andrew Gorman, Richard Norris

Email: kovad091@student.otago.ac.nz
 

Geology Background

Adrienn at the Whataroa survey holding a hammer used as a seismic source Adrienn at the Whataroa survey holding a hammer used as a seismic source

The Alpine Fault is a major element of the Australian/Pacific plate boundary in the New Zealand region of the southwest Pacific. It connects the west-dipping Tonga-Kermadec-Hikurangi Subduction Zone in the northeast with the east dipping Puysegur Subduction Zone in the southwest of New Zealand. The Alpine Fault is accommodating two-thirds to three-quarters of the 35-38 mm/yr oblique plate motion (DeMets et al., 1994, Sutherland et al., 2006, Norris & Cooper, 2007). Dextral strike-slip motion along the fault is estimated at 23.1 ± 0.8 mm/yr (Sutherland et al., 2006), while orogen-perpendicular shortening is accommodated by significant ongoing uplift in the Southern Alps on the hanging wall (eastern side) of the fault. Uplift rates vary along the fault (e.g., Norris & Cooper 1995, 1997) with the greatest uplift of 6-9 mm/yr being in the vicinity of Mount Cook (Little et al., 2005). Geological evidence supports the occurrence of three or four significant earthquakes during the last 1000 years with magnitudes as high as Mw = 7.9, with the most recent occurring in AD 1717 (Sutherland et al., 2007), before immigration of Europeans to New Zealand. The most recent risk assessment for the Alpine Fault forecasts a 14 – 29% chance of another significant (magnitude as high as Mw = 7.9) earthquake before AD 2056 (Sutherland el al. 2007). Future Alpine Fault earthquakes of similar magnitude pose a significant risk to population centres and infrastructure throughout the South Island.

The central region of the Alpine Fault exhibits “serial partitioning” into oblique thrust and strike-slip segments, the scale of (<3 km along-strike) appears to be related to the spacing of major rivers (Norris & Cooper 1995, 1997). Geometry suggests these segments merge into a single oblique structure at depths comparable to the topographic relief. Although the Alpine Fault can be directly observed by field geologists, where it is cut by river valleys, its underground structure is often poorly understood. This structure has profound significance on:

  • Evolution of the transpressive orogenic system
  • Ductile and brittle deformation mechanisms, and their interactions
  • Seismogenesis and the habitat of earthquakes

Detailed higher resolution sesmic reflection and gravity research is required to determine the geometries of these structures at the depths.

Summary of research project

Seismic shot holes dug across river terraces in the Whataroa Valley, Westland, New Zealand. Piles of rocks in a row indicate where the holes are. sothern alps in the background Figure 1.: Seismic shot holes dug across river terraces in the Whataroa Valley, Westland, New Zealand

Although it appears as a linear (in small scale rather irregular segments) feature when viewed on satellite and areal images the Alpine Fault does not have outcrops along the Haast and Turnbull Rivers and in the Whataroa Valley. These valleys are filled up with late Quaternary sediment dating from about 18 ka, when the retreat of the LGM ice began, thereby covering much of the geological evidence of the Alpine Fault’s trace. My PhD thesis involves the characterisation of the Alpine Fault in shallow depth (4-7km) in the Whataroa Valley and along the Haast and Turbull Rivers. This includes gaining information about the internal geometry of the sedimentary basin and underlying basement rocks (glacial erosion surface), evolution of deposition (which might connect not only to glaciations but also to changes in see level), successfully imaging of Alpine Fault structure in the near surface and observing evidence for displacement caused by earthquakes on basement rocks and sediments.

Seismic and gravity surveying locations in West Land, New Zealand Figure 2.: Seismic and gravity surveying locations in West Land, New Zealand

I am primarily making use of new high-resolution seismic reflection and gravity data sets that were collected along the Haast River and Turbull River in January 2009 and in the Whataroa Valley in January-February 2011. The experiments’ small sources combined with more closely-spaced shot point and receiver locations has greatly increase resolution over earlier geophysical experiments. The geophysical transects cover 4 - 5 km are orientated perpendicular to the Alpine Fault. The lines extend inland as far as possible (i.e., until limited by the extreme topography of the Southern Alps or river channels) in order to maximise the coverage of the hanging wall. Due to the rugged topography and temperate rainforest vegetation of the Southern Alps and West Coast of the South Island, suitable locations for collecting seismic reflection and detailed gravity data across the Alpine Fault are rare. The combination of gravity and seismic data enables the development of a subsurface model that supports a steeply-dipping Alpine Fault cutting through a thick section of coastal sediments overlying a strongly-glaciated basement. However, the interpretation supports a thick sedimentary sequence within the coastal plain – possibly consisting of interbedded outhwash gravels and marine sediments. The results provide information that will help characterise the type of fault motion that will occur on the Alpine Fault during a future earthquake. The gravity and seismic models correlate to each other, filter the uncertainty and present better images of the geometry.

Haast gravity and seismic models

Seismic and gravity surveying location along the Haast River, Westland, New Zealand
Figure 3.: Seismic and gravity surveying location along the Haast River, Westland, New Zealand

Seismic cross section with geological interpretation along the Haast River.
Figure 4.: Seismic cross section with geological interpretation along the Haast River.

Basement rocks and surface models based on gravity models along the Haast River
Figure 5.: Basement rocks and surface models based on gravity models along the Haast River


Turnbull gravity results

Gravity stations and shot points locations along the Turnbull River, Westland, New Zealand
Figure 6.: Gravity stations and shot points locations along the Turnbull River, Westland, New Zealand

Cross section along the Turnbull River. Basement rocks and surface models based on gravity models.
Figure 7.: Cross section along the Turnbull River. Basement rocks and surface models based on gravity models.


Whataroa seismic and gravity models

Shot points and gravity stations location in the Whataroa Valley, Westland, New Zealand
Figure 8.:Shot points and gravity stations location in the Whataroa Valley, Westland, New Zealand

Preliminary seismic model across the Whataroa Valley
Figure 9: Preliminary seismic model across the Whataroa Valley. Model was created in the program RAYINVR (Zelt and Smith, 1992). Gravity models were applied to the geometry, thus the basement rocks surface correspond to real elevations (figure on the top). On the bottom the reflected waves are demonstrative: arrivals coloured blue represent the actual arrival times, black represents the model arrivals counted by RAYINVR. The model and actual arrivals fit well, but other shots must be added to the model to reduce uncertainty

Three merged shots and reflections from the Alpine Fault (red dashed line)
Figure 10:Three merged shots and reflections from the Alpine Fault (red dashed line)

Cross section along the Whataroa Valley with the basement rocks and surface models based on gravity surveying.
Figure 11.: Cross section along the Whataroa Valley with the basement rocks and surface models based on gravity surveying.

Conference Presentations

  • Adrienn Kovács and Andrew R. Gorman: An integrated seismic and gravity investigation of the Alpine Fault at Haast, 2010 GeoNZ Conference, Auckland, New Zealand. (poster presentation)
  • Adrienn Kovács and György Less: Age determination on Numulites Perforatus with biometric method in Magyarvalkó, Romania, 2007 National Student Research Conference, Szeged, Hungary (oral presentation)
  • Adrienn Kovács and György Less: Age determination on Numulites Perforatus with biometric method in Magyarvalkó, Romania, 2007 Young Specialist Conference, Bakonybél, Hungary (oral presentation)

Professional Membership

  • American Geophysical Union, student member
  • Geoscience Society of New Zealand, student member