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Evaluating the Updip Extent of Large Megathrust Ruptures Using Pcoda Levels

When slip at shallow depth occurs during large subduction zone thrust events, P wave energy enters the water layer and establishes pwP, the reverberating waves called “water bounces” that follow pP. For water depths ≥5–6 km (i.e., near the trench) above the shallow slip, pwP manifests in a strong ~10‐s period ringing that can persist for minutes into the teleseismic P wave coda at all azimuths. Deeper slip can generate shorter‐period pwP ringing at trenchward azimuths. At large distances, Pcoda windows have several‐minute‐long intervals free of secondary arrivals. We consider rmsPcoda/rmsP amplitude ratios at distances from 80° to 120° as a potential proxy for occurrence of shallow slip for 39 MW 7.5+ megathrust earthquakes from 1990 to 2016 with estimated slip distributions. Ratios for the 15‐ to 7‐s‐period band have a strong bimodal distribution, with higher average Pcoda/P amplitudes observed for ruptures with slip extending to shallow depth.