Evaluating the Updip Extent of Large Megathrust Ruptures Using P_coda 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.

Shallow Megathrust Slip During Large Earthquakes That Have High P Coda Levels

“…Journal of Geophysical Research: Solid Earth enough to produce persistent pwP reverberations extending long into the P coda . However, the clear evidence of early discrete impulsive ground motions, which are likely due to early aftershocks in the distance profiles (Figure 3d), accounts for the high P coad /P measure (Figure 1), as speculated by Lay and Rhode (2019).…”

“…Two of the events clearly fall into the scenario involving rupture that includes slip at shallow depth on the megathrust with associated strong pwP ringing that enhances the high coda amplitudes (Figure 17a). This is the basic scenario invoked by Lay and Rhode (2019) and . For the 30 July 1995 Chile and 17 November 2003 Aleutian events, late shallow slip of~1 to 3 m efficiently generates associated pwP reverberations in the deep trench with 12-to 15-s period, accounting for observed very high relative P coda levels for these two events (extending well beyond the time interval we invert).…”

Enhancing Tsunami Warning UsingPWave Coda

“…Lay & Rhode, focus on teleseismic P waves recorded at epicentral distances from 80+° to 120° where there is a several minute‐long time interval between the first P arrival and the arrival of secondary seismic phases such as PP and PKIKP . For a large earthquake rupture that radiates direct P waves for a duration of less than 2 min there is about 2 min of immediately ensuing P coda duration in which no other arrival from radial Earth structure is expected.…”

“…Summing measures from many stations suppresses effects of individual station/path complexity and enhances sensitivity to energy emerging from the source region. Band‐pass filtering the data to emphasize the period range of pwP expected for rupture under deep water (Lay & Rhode, ; use a passband of 7 to 15 s) near the trench increases the sensitivity of the measures to the updip limit of slip during the earthquake as suggested by Figure .…”

“…The approach presented here builds upon an initial study by Lay and Rhode (), which introduced the concept of using the strength of teleseismic P coda relative to P as a proxy for the occurrence of some slip or no slip at shallow depth under deep water for large megathrust ruptures. That work follows on early studies of P coda generation for submarine faulting (e.g., Ihmlé & Madariaga, ; Ward, ; Wiens, , ).…”