Seismic velocities beneath creeping gas hydrates slides: analysis of ocean bottom seismometer data in the Tuaheni Landslide Complex on the Hikurangi Margin, New Zealand.

Abstract

In April-May 2014 an international collaboration of scientists (SCHLIP-3D) investigated the relationship between gas hydrates and slow moving sub-marine landslides with the Tuaheni Landslide Complex (TLC) located on the Hikurangi Margin, New Zealand. The research voyage TAN1404, on the vessel R/V Tangaroa, used both P-Cable and Ocean Bottom Seismometers (OBS) supplied by GEOMAR. The main goal was to assess the hypothesised mechanisms that could explain the creep-like deformation observed within the TLC. These mechanisms include; A) Hydrate dissociation, in which bottom-water warming would cause excess pore pressure due to the dissociation of gas hydrate which could lead to seafloor destabilisation in the upper region of the landslide. B) Hydrofracturing, where the cyclic buildup and release of free gas under low-permeability sediments in the upper region of the landslide may lead to mobilisation of the sediments. C) Hydrate-glacier, where the gas hydrate allows for slow plastic deformation, much like glacial creep. This thesis focuses on OBS data collected during the TAN1404 voyage, to obtain P and S wave velocity profiles. Analysis of the resulting velocity profiles Vp and Vs, allows for the distinction between the hypothesised mechanisms; a decreased Vp and unchanged Vs would indicate the presence of free gas from dissociation of the gas hydrate, a decreased Vp and Vs could indicate fracturing and support the hydrofracturing hypothesis, elevated Vp and Vs indicates the presence of gas hydrates and supports the hydrate-glacier hypothesis. Evidence for the hydrate-glacier hypothesis was observed in the analysis of this thesis, with gas hydrate thought to be forming with localised distribution, close to the Bottom-Simulating Reflector (BSR) pinchout. Also investigated in this thesis was the velocity structure in a region of a double-BSR in the immediate vicinity of the TLC, with the goal to investigate indications for gas hydrate between the two BSRs; the results were inconclusive.