Reconstructing the Landscape Response to Earthquakes on the Alpine Fault, New Zealand

Abstract

Earthquakes are important drivers of erosion and sediment flux from mountain belts because they cause extensive co-seismic and post-seismic landsliding. Despite the importance of earthquakes, few empirical studies have determined how mountain landscapes respond to such events or quantified the contribution of sequential earthquakes to millennial-scale sediment flux. In this study the sediments of two lakes were used to reconstruct the duration and magnitude of sediment flux caused by the post-seismic landscape response of the Southern Alps to sequential large earthquakes on the Alpine Fault, New Zealand. Lake cores were characterised using sedimentological and geochemical techniques, which revealed a repeating sequence of co-seismic, post-seismic and aseismic deposits recording deposition over multiple seismic cycles. Radiocarbon dating together with Bayesian statistical modelling were used to produce a precise chronology for the lake records and confirmed that Alpine Fault earthquakes form megaturbidites, which are overlain by stacks of hyperpycnites that record post-seismic increases in sediment flux. The precisely dated megaturbidites tightly constrain the timing of the last six Alpine Fault earthquakes, which have an average recurrence interval of 271±73 (1σ) years. The lake sediments show that each Alpine Fault earthquake was followed by, on average, six decades of elevated sediment flux that was three to four times higher than the aseismic rate. Post-seismic sediment fluxes accounted for 27±4% of total sediment flux from the lake catchments, supporting the conclusion that large earthquakes are one of the most important drivers of episodic sediment flux from the range front of the Southern Alps.