A 2300-Year Paleoearthquake Record of the Southern Alpine Fault and Fiordland Subduction Zone, New Zealand, Based on Stacked Turbidites

Turbidites from three sedimentary basins <10 km offshore of Fiordland, New Zealand, provide a 2300 yr submarine record of large magnitude paleoearth-quakes associated primarily with the Alpine fault and the Fiordland subduction zone. Sedimentary cores comprise proximal sand-silt turbidites, which are stacked with no intervening hemipelagite. Radiocarbon dating of foraminifera constrains the sediment source ages, not direct event ages. Despite reworking, however, the dates are largely in chronological order. Upper slope source regions of the turbidity currents have been largely evacuated during triggering events and replenished by shelf-to-upper-slope sediment transport between events. Age models for the turbidites are constructed via an unconventional application of the OxCal Sequence program. The mean recurrence interval of turbidites decreases southward from similar to 190 yr in George basin, similar to 160 yr in Looking Glass basin, to similar to 150 yr (and possibly as low as 100) in Secretary basin, and the respective coefficients of variability increase from 0.30 to 0.60-0.99. These data imply that turbidites are more frequent and less periodic in the south. Consideration of potential turbidity-current triggering mechanisms, including sediment transport during extreme storm events, indicates that large magnitude earthquakes are the most likely trigger. The paleoearthquake records include turbidites emplaced during two recent interplate thrust earthquakes, as evidenced by excess Pb-210 data, and probably the A.D. 1826 Fiordland earthquake and the well-dated A.D. 1717 Alpine fault earthquake. The recurrence intervals are shorter than recently published recurrence data from the Alpine fault on land, reflecting mixed fault-source earthquake records and potentially increased Alpine fault segmentation offshore.