When the levee breaks: the largest channel-wall collapse ever described

Abstract 3D seismic reflection data from the Hikurangi trench-axial channel-levee system, offshore New Zealand, reveal the largest channel-wall failure yet described. Wholesale collapse of both channel banks along a 68 km stretch created a mass transport deposit (MTD) containing blocks up to 4 km long, with a preserved volume of 19 km 3 . Subsequent deposits have erosional contacts into the MTD, confirming that its original volume was even larger. Channel-walls typically collapse locally, yet synchronous failure of both channel flanks is documented here, implying a catastrophic external trigger. For most of its length, the MTD overlies deposits imaged as high-amplitude reflectors, representing the pre-failure channel floor. Locally these older channel deposits were excavated, transported across and down channel, and then deposited in imbricated stacks, such that the volume of displaced materials exceeded that of the initial failure. Metadata studies of other channel-wall derived MTDs confirm this to be the largest channel-wall collapse documented. MTDs initiated by large-scale levee-collapse like that documented here pose an under-appreciated risk to seafloor infrastructure both within channels and over regions extending at least twice the channel width into their overbank areas.