Characterization of Near‐Surface Velocity Structure at Haast, New Zealand, Using Distributed Acoustic Sensing (DAS) Measurements of Seismicity

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Abstract
Distributed acoustic Sensing (DAS) data collected along a 30 km length of telecommunications fiber crossing the Alpine Fault near Haast enable analysis of interactions between fluvioglacial and seismotectonic processes. Here we use DAS recordings of 25 earthquakes to probe near‐surface structure beneath the Haast river valley. For each earthquake, delayed P‐ and S‐wave arrivals incompatible with a regional velocity model are observed at common locations. We show using a planar slab model that these delayed arrivals are proportional to the thickness of subjacent low‐velocity structures. 3D ray‐tracing reveals basin‐like low‐velocity features of approximately ∼1–4 km width and 200–650 m depth, consistent with independent seismic and gravity measurements collected along partly co‐located lines. We infer these low‐velocity structures to be sediment‐filled erosional basins that may exacerbate ground shaking in large earthquakes. The method developed here is general and highlights the potential of dense DAS measurements to provide high‐resolution subsurface characterization.

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