Initial testing to detect and characterise hidden faults in urban areas of New Zealand using shear-wave seismic surveys
Christchurch was rocked by a 6.3 MW scale earthquake at 12:51 local time on 22 February 2011. The epicentre of the earthquake was located only ten kilometres to the southwest of the city centre, at a depth of only five kilometres. The damage in the historic city centre, which was attributable to the unfavourable foundations (soil liquefaction), was so severe that large parts of the city were declared uninhabitable, and approx. 12,000 houses had to be demolished, and another approx. 100,000 houses were damaged.
The earthquake was caused by the reactivation of a fault hidden beneath the city that had previously been undetected. This led to a change in thinking by the authorities and the inhabitants, because of the awareness that hidden, but recently-active faults of this kind could also be located beneath other populated areas in New Zealand. A means was therefore sought to identify and characterise faults of this kind. In 2014, the New Zealand Earthquake Commission therefore funded a two-year bilateral research project between the Institute for Geological and Nuclear Science (GNS Science) and LIAG, with the aim of using shear-wave reflection seismic technology, in an exemplary way, to explore for faults of this kind in urban areas.
The coastal town of Whakatane on North Island was selected as the pilot area because a major fault was thought to lie beneath the town. The very well explored Edgecumbe Fault lies only approx. 30 km away and when a strong earthquake reactivated it in 1987, it gave rise to a highly visible trace on the surface. This fault was therefore used in a go-no go experiment to methodologically calibrate the technique before carrying out further investigations within the built-up area of Whakatane. The calibration experiment delivered such successful results that surveying in Whakatane began immediately. Approx. 4.5 km of seismic lines were acquired in the urban area during surveys carried out over 10 nights, and were then interpreted onsite. The underground structures revealed by the seismic survey included the trace of a hidden fault zone with a throw of over 10 m in young sediments, which cut through a previously unexpected zone that crosses the whole city and also affects the central hospital.