Evaluating Shallow Slip Extent in Large Megathrust Earthquakes by Analyzing P and Pdiff Coda Arrivals for Water Reverberations

Observations of shallow focus megathrust earthquakes are difficult to obtain due to limited instrumentation in the near-source underwater region, and teleseismic observations have limited resolution that is strongly influenced by poorly resolved shallow seismic velocity structures near the toe of the accretionary prism (Lay et al., 2012). Slip under deep water can establish strong water reverberations that persist into seismic coda that is not usually inverted for the source. In this study, P wave coda is examined for many recent major and great earthquakes with independently determined slip distributions and known tsunami excitation to evaluate the prospect for rapidly constraining the up-dip rupture extent of large megathrust earthquakes. For narrowband filtered (7-15 s) data at an 80°-120° distance range, final event average rms Pcoda/rms P (RMS C/P) ratios above 0.610 are found exclusively for events that are believed to have ruptured to shallow depths, while ratios < 0.610 are found for all but one of ruptures that did not extend to the trench. Average RMS C/P ratios computed at all azimuths for each event are larger at azimuths in the direction of the trench for all cases, and events with deep slip beneath continental margins have pronounced reduction of reverberative coda amplitudes in the landward directions. This procedure is successful in separating ruptures that have shallow slip from those that do not for events larger than MW 7.5. It is likely to be very effective for smaller events as well, as their entire rupture area will span a smaller depth range than for many of these events. Given the overall success of the classification using all azimuths, which is the simplest for fast implementation, an optimal azimuthal windowing measure is not necessary. Our specific procedure is of somewhat limited value for assessing likelihood of enhanced tsunami excitation relative to the seismic moment magnitude of the event for very nearby coastlines; however, it could still be valuable for tsunamis with coastal arrival times that are longer than the average 15-18-minute lag time required for measuring numerous relative coda levels in the 80°-120° distance range.