PERTURBATIONS TO DETRITAL 10BE FOLLOWING THE 2016 KAIKŌURA EARTHQUAKE, NEW ZEALAND

Extreme events, such as large-magnitude earthquakes or storms, disrupt landscapes and may trigger transient periods of increased erosion rate. Understanding the magnitude, duration, and lasting impact of such perturbations to Earth’s surface requires detailed study of a landscape before and after catastrophic events. Here, we analyze cosmogenic beryllium-10 (¹⁰Be) in river sands to examine the landscape response to the M_w 7.8 Kaikōura earthquake, which triggered ~30,000 landslides in New Zealand’s South Island in 2016. While the assumption of uniform erosion used to estimate long-term erosion rates from ¹⁰Be is violated by abrupt excavation of deep (>1m) material, we aim to quantify the magnitude that ¹⁰Be is disrupted by these changes to sediment production and transport. Our dataset consists of a four-year timeseries (pre-earthquake; 2017 – 2019) of detrital sand samples from five catchments spanning the high-relief Seaward Kaikōura Range, which experienced strong ground motion during the earthquake. Preliminary results reveal a broad spatial trend in pre-earthquake ¹⁰Be concentrations, which range an order of magnitude among catchments and correlate well with basin-averaged topographic metrics. Post-earthquake measurements show an approximately two-fold ¹⁰Be dilution between pre- and post-earthquake samples in two catchments that experienced the most intense landsliding, in agreement with prior observations of deep-seated erosional events in other locations. Temporal and spatial variability in this dataset reveal complexity in the post-earthquake ¹⁰Be response that may be related to landslide distribution, sediment routing, or remaining impacts from prior landslides. We examine these factors and evaluate the ability of instantaneous ¹⁰Be sampling to estimate time-averaged erosion rates in regions that experience frequent, extreme surface process events. By constraining the ¹⁰Be response to the Kaikōura earthquake, we aim to understand the role that such events play in the long-term evolution of tectonically active mountain ranges.