Tectonic Geomorphology and Seismic Hazard of the Mt Fyffe Section of the Hope Fault

Abstract

The northeast-trending transpressive Hope fault is a major tectonic element of the active Pacific-Australian plate boundary zone through New Zealand. This study presents geomorphic and paleoseismic field data from the Mt Fyffe section of the Hope fault, which in turn is used to develop a seismic hazard map for the adjacent area. The Mt Fyffe section is a 12 km long, 1 km wide zone of deformation that changes in strike and slip rate from 275° and 16 ± 5 mm/yr in the southwest, to 235° and 2 to 4.8 mm/yr in the northeast. Slip is transferred from the Mt Fyffe section to the Jordan thrust and related structures. Deformation along the Mt Fyffe section has been divided into four structural domains, from southeast to northwest: an extensional step-over, a series of four en-echelon wedges, a contractional step-over, and a contractional domain. Near surface fault zone kinematics recorded by tectonic geomorphic landforms are interpreted to reflect the change in strike of the fault zone, topographic loading and the related fault zone break-out along the range front. The south-western Mt Fyffe section has ruptured at least once between 660 AD and 1800 AD, and the north-eastern end ruptured at least once between 1410 and 1640 AD, and possible since 1640 AD. A rupture of the Mt Fyffe section with the Conway section is the foundation fault for Kaikoura. It is estimated to have a Mmax of greater than 7. Probabilistic seismic hazard models (Stirling et al., 2002; in press) estimate a rupture of the Hope fault will result in peak ground accelerations (PGA) for the 150 and 475 year events at Kaikoura of 0.45 to 0.6 g and 0.85 to 2.0 g (midpoints) respectively. Results of a deterministic seismic hazard assessment using the foundation fault, indicate PGA at the Kaikoura township will be between 0.64 g (after Stirling et al, 2000) and 0.31 g (after McVerry et al 2006), lower than that calculated by probabilistic methods. Detailed geomorphic mapping has defined two levels of seismic hazard avoidance zones along the Mt Fyffe rangefront. Zone A contains major structures that accommodate most offset and Zone B contains secondary, smaller scale deformation.