A 16KYR-LONG RECORD OF EARTHQUAKES PRESERVED IN THE DEEP-SEA TURBIDITE RECORD OF THE NORTHERN HIKURANGI MARGIN OF NEW ZEALAND

The northern Hikurangi Subduction Margin, East of North Island of New Zealand contains well-developed series of Quaternary turbidites. This region is characterised by high sediment delivery and intense tectonic activity, which makes it an ideal location for marine turbidite paleoseismology studies. Although offshore active faults may generate large M_w>7 earthquakes with return times as low as 500 yr, the paleoseismic history of the margin is poorly understood because of limited human occupation (1000 yr) and historical record (170 yr).

The three Poverty, Ruatoria and Matakaoa large morphological re-entrants of the margin concentrate recent gravity flow sedimentation. A series of piston cores collected between 2003 and 2008 record continuous successions of turbidites interlaid with hemipelagite and tephras emplaced since the Last Glacial Maximum. Sedimentological evidences indicate three triggering mechanisms for the turbidites: catastrophic floods, volcanic eruptions and slope failures; the latter representing >95% of the overall record. The age of each turbidite is estimated using the Oxcal software from an exceptionally dense set of tephrochronology and radiocarbon ages (0.7 to 2 ages/m). The age together with the facies and the petrophysical properties of the sediment allow the correlation of turbidites in each re-entrant and along the 200 km-long margin. 46 turbidites, named margin events, are synchronous across the margin between 387±170 and 16,446±314 yr BP, among which 41 are deposited by slope failures induced turbidity-currents. The source area of the slope failures at 150-1000 m water depths together with the stable return times between the Late Holocene period and since 16 ka suggest that these margin events are triggered by large earthquakes, which is validated by the correlation of margin events with the sparse onland paleoseismic evidences. The recurrence interval (RI) of paleo-earthquakes over the last 16 kyr deduced from the turbidite record ranges from a few years to 1235 yr, with an average RI of 400 yr.