Seismic reflection images of active faults on New Zealand's South Island

摘要

Zealand is located along the boundary between the Australian and Pacific plates. Although there are numerous faults associated with this plate boundary setting, few have ruptured during the nearly 200 years of European settlement. Yet, paleoseismology provides clear evidence of relatively recent activity on many of them. Knowledge of the shallow structure and other characteristics of these faults is important for understanding the related seismic hazard and risk. Key properties of faults that produce infrequent large earthquakes are usually determined or inferred from paleoseismological investigations of surface outcrops, geomorphology, trenches, and boreholes. In an attempt to improve our knowledge and understanding of active faults beyond the reach of conventional paleoseismological methods (I.e., deeper than a few meters), we have acquired high-resolution seismic reflection and groundpenetrating radar (GPR) data across the following three fault systems on New Zealand's South Island: (I) a northern section of the transpressive Alpine Fault zone, (ii) numerous reverse faults hidden beneath the very young sediments that cover the northwest Canterbury Plains, and (iii) a critical portion of the reverse Ostler Fault zone in the south-central part of the Island. After subjecting our data to diverse processing procedures, the resultant seismic and GPR sections provide vivid images of the target structures. On the 2D and 3D high-resolution seismic and GPR images of the Alpine Fault zone, we see the principal fault dipping steeply through Quaternary sediments and offsetting the basement. A distinct～25 m vertical offset of basement provides a maximum～1.4 mm/yr dip-slip displacement rate. The more important strike-slip component of displacement has yet to be estimated at this location. Our high-resolution seismic and GPR sections across parts of the northwest Canterbury Plains display a complex pattern of faults and folds beneath a variably thick veneer of flat-lying sediments. Structural restorations of the seismic images suggest 10 - 23% compressive strain, which would correspond to an average strain rate of 20 - 50×10-9/yr if the onset of compression coincided with the accelerated uplift of the Southern Alps approximately 5 Ma. Finally, multiple 2D high-resolution seismic images of the Ostler Fault zone reveal a 45° - 55° west-dipping principal fault and two subsidiary 25 - 30° westdipping faults, one in the hanging wall and one in the footwall of the principal fault. Again, we are able to structurally restore models based on the seismic images. These restorations are compatible with 440 - 800 m of vertical offset and 870 -1080 m of horizontal shortening across the Ostler Fault zone, which translate to a relatively constant deformation rate of 0.3 -1.1 mm/yr since the Late Pliocene - Pleistocene.