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Abstract Most large tsunamis are generated by earthquakes on offshore plate boundary megathrusts. The primary factors influencing tsunami excitation are the seismic moment, faulting geometry, and depth of the faulting. Efforts to provide rapid tsunami warning have emphasized seismic and geodetic methods for quickly determining the event size and faulting geometry. It remains difficult to evaluate the updip extent of rupture, which has significant impact on tsunami excitation. TeleseismicPwaves can constrain this issue; slip under deep water generates strongpwPwater reverberations that persist as ringingPcodaafter the directPphases from the faulting have arrived. Event‐averagedPcoda/Pamplitude measures at large epicentral distances (>80°), tuned to the dominant periods of deep waterpwP(~12–15 s), correlate well with independent models of whether slip extends to near the trench or not. Data at closer ranges (30° to 80°) reduce the time lag needed for inferring the updip extent of rupture to <15 min. Arrival ofPPandPPPphases contaminates closer distancePcodameasures, but this can be suppressed by azimuthal or distance binning of the measures. Narrowband spectral ratio measures and differential magnitude measures ofPcodaand directP(mB) perform comparably to broader band root‐mean‐square (RMS) measures.Pcoda/Plevels for large nonmegathrust events are also documented. Rapid measurement ofPcoda/Pmetrics after a large earthquake can supplement quick moment tensor determinations to enhance tsunami warnings; observation of largePcodalevels indicates that shallow submarine rupture occurred and larger than typical tsunami (for givenMW) can be expected.
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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,Pwave energy enters the water layer and establishespwP, the reverberating waves called “water bounces” that followpP. For water depths ≥5–6 km (i.e., near the trench) above the shallow slip,pwPmanifests in a strong ~10‐s period ringing that can persist for minutes into the teleseismicPwave coda at all azimuths. Deeper slip can generate shorter‐periodpwPringing at trenchward azimuths. At large distances,Pcodawindows have several‐minute‐long intervals free of secondary arrivals. We consider rmsPcoda/rmsPamplitude ratios at distances from 80° to 120° as a potential proxy for occurrence of shallow slip for 39MW7.5+ 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 averagePcoda/Pamplitudes observed for ruptures with slip extending to shallow depth.
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- Award ID(s):
- 1802364
- PAR ID:
- 10459386
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 46
- Issue:
- 10
- ISSN:
- 0094-8276
- Page Range / eLocation ID:
- p. 5198-5206
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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