Evaluating the Updip Extent of Large Megathrust Ruptures Using Pcoda Levels

Abstract

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.