Study of the impact of seamount subduction on the outer wedge of the Hikurangi margin from combined lab analyses of rock properties and marine seismic data

University Of Texas At Austin, Austin TX

Investigators

Abstract

A subduction zone runs along the Hikurangi margin of New Zealand, and the faults along the margin move at different rates. Along this subduction zone, sediments on the incoming Pacific tectonic plate are either stripped off and transferred to the upper tectonic plate or carried down deep into the mantle on the lower, subducting tectonic plate. Some of this may be due to the distribution of seamounts on the subducting oceanic plate. Seamounts can enhance sediment subduction by taking the sediments with them. Fluids from the sediments may in turn affect movement on the faults. To improve understanding of how faults slip, this study looks at the effects of seamounts on subduction along the Hikurangi margin. This project uses existing marine seismic data to calculate seismic wave speeds and properties of rocks and sediments beneath the continental shelf of the Hikurangi margin to depths of 7-10 km. The study compares those properties with laboratory experiments on rock samples from the Hikurangi margin to get a better understanding of subduction zone processes. The study supports the training of graduate students. An educational model will be developed and used for undergraduate education and at local high schools. Through hands-on experiments, students will discover connections between rock properties and seismic hazards. The megathrust fault along the Hikurangi subduction zone of New Zealand changes its slip behavior dramatically from south to north along the margin. The megathrust along the southern section of the margin is geodetically locked and will likely slip coseismically. However, along the central and northern margin it appears to creep and generate regular slow-slip events. Recent studies have also shown that the ground motion of regional seismic waves is amplified much more at the northern Hikurangi margin than to the south. To gain a better understanding of these geological processes, likely factors that control slip behavior will be studied. These include accretionary wedge and underthrust sediment properties and the impact of seamounts on the accretionary prism of the Hikurangi margin. In this synthesis project existing marine active-source seismic data will be used to image seismic velocity structure and reflectivity of sediments and the underlying basement of the Hikurangi margin to depths of 7-10 km. The imaged properties of the shallow subduction zone will be compared with ultrasonic wavespeeds measured in rock samples from representative segments along the Hikurangi margin. Confining and pore pressures will be applied to the rock samples within the range expected for the subduction zone setting to determine their consolidation characteristics and stress history. In a two-year project the following hypotheses will be addressed: 1) Seismic velocities of the accretionary prism in southern Hikurangi are higher than northern Hikurangi in proportion to the differences in the degree of consolidation of Neogene rocks accreted along the margin. 2) The differences in compaction and dewatering inferred from seismic velocities along the Hikurangi margin is driven locally by the lateral stress imparted by subducting seamounts. 3) More severe shaking at the northern Hikurangi margin after the 2016 Kaikoura earthquake was caused by lower shear-wave velocities broadly distributed in the prism. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.