E.M. Thomson

Ground motion simulations of great earthquakes on the Alpine Fault: effect of hypocentre location and comparison with empirical modelling

ABSTRACT This paper discusses simulated ground motion intensity, and its underlying modelling assumptions, for great earthquakes on the Alpine Fault. The simulations utilise the latest understanding of wave propagation physics, kinematic earthquake rupture descriptions and the three-dimensional nature of the Earths crust in the South Island of New Zealand. The effect of hypocentre location is explicitly examined, which is found to lead to significant differences in ground motion intensities (quantified in the form of peak ground velocity, PGV) over the northern half and southwest of the South Island. Comparison with previously adopted empirical ground motion models also illustrates that the simulations, which explicitly model rupture directivity and basin-generated surface waves, lead to notably larger PGV amplitudes than the empirical predictions in the northern half of the South Island and Canterbury. The simulations performed in this paper have been adopted, as one possible ground motion prediction, in the ‘Project AF8’ Civil Defence Emergency Management exercise scenario. The similarity of the modelled ground motion features with those observed in recent worldwide earthquakes as well as similar simulations in other regions, and the notably higher simulated amplitudes than those from empirical predictions, may warrant a re-examination of regional impact assessments for major Alpine Fault earthquakes.

Development of Deep Shear Wave Velocity Profiles in the Canterbury Plains, New Zealand

Deep (typically > 1,000 m) shear wave velocity (V S ) profiles were developed across the Canterbury region of New Zealand at nine strong-motion stations using a combination of active and passive surface wave methods. A multimode, multimethod joint inversion process, which included Rayleigh and Love wave dispersion and horizontal-to-vertical spectral ratio data, was used to develop the V S profiles at each site. A priori geologic information was used in defining preliminary constraints on the complex geologic layering of the deep basin underlying the region, including velocity reversals in locations where interbedded terrestrial gravels and marine sediments are present. Shear wave profiles developed as part of this study had characteristics comparable to the profiles from 14 Christchurch sites detailed in a separate study. The profiles developed in the two studies were combined to form region-specific V S profiles for typical deposits, which can be used to improve the accuracy of current three-dimensional (3-D) crustal velocity models of the region.