Neotectonics and assessment of seismic risk


Klos, P. Z., Siddoway, C. S., Langridge, R. (2009). The Ruataniwha Fault: Neotectonic evaluation and seismic hazard. Colorado College Geology Department and GNS Science, New Zealand. 


The previously understudied Ruataniwha Fault lies within the Hawke’s Bay region of New Zealand’s North Island. A diverse spectrum of methods and results aim to classify earthquake hazard along this oblique reverse fault to better inform local bodies of this previously unnoted seismic hazard. Interpretation of LiDAR results, orthophotographs, and ground topography created a detailed map of fault location for ~50 kilometers of its trace. An RTK GPS surveyed vertical and horizontal offsets in Late Quaternary terraces along the fault quantifying amounts of vertical offset up to ~21 meters and possible horizontal offsets up to 2 meters. Correlation of terrace stratigraphy to other previously dated Quaternary surfaces within the region determined age constraints for the offset Q1, Q2, Q3 and Q4 surfaces. Reinterpretation of seismic profiles collected within the Ruataniwha-Dannevirke Basin constrained fault dips in select locations at 56 ±7o to the NW. Data on offset amount, age constraint, and dip angle calculated slip rate along the fault, increasing from zero at the margins to 0.18 ±0.09 mm/yr in the central northern section. Larger scale tectonic interpretations of the 8 km wide anticline (cored by the fault) identified tighter interlimb fold angles in sections of more active surface faulting. Using the regional tectonic similarities, the nearby and geometrically similar Tukituki fault served as a proxy for recurrence estimates of the Ruataniwha Fault. As well, methods identifying “kinks” in the slope of the fault scarp define a recurrence interval of ~7.5 ky. This is similar to recurrence intervals found at the nearby Tukituki Fault. These classifications give a quantifiable assessment of hazard posed by the fault (New Zealand Fault Hazard, Class IV). Using this information, local bodies can know what types of structures are reasonably built near the fault and, most importantly, local bodies can now be informed of this seismic hazard so plans can be made to accurately mitigate this threat.


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