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

Abstract

AbstractDetermining the up‐dip extent of slip during large megathrust earthquakes is important for understanding tsunami generation, potential for subsequent failure of the shallow megathrust, and plate contact depth‐varying frictional properties. Recent measurements of early Pcoda amplitudes relative to direct P signals for the period range 7 to 15 s find high Pcoda/P ratios for many events with finite‐fault rupture models that have slip extending to near deep trenches, resulting in enhanced pwP (water‐reverberation) generation. However, some events with high Pcoda/P measures have finite‐fault model solutions that vary in, or lack, shallow slip. We reexamine six large megathrust earthquakes with unexplained high Pcoda/P ratios (30 July 1995 Chile, MW 8.0; 17 November 2003 Aleutian, MW 7.7; 12 November 1996 Peru, MW 7.7; 7 October 2009 Vanuatu, MW 7.6; 5 May 2015 Papua New Guinea, MW 7.5; and 5 March 2002 Mindanao, MW 7.5), inverting for slip models with depth‐varying dip and varying bathymetry, adjusting kinematic parameters to better allow for the possibility of late shallow slip with associated strong pwP excitation. We confirm that modest patchy late shallow slip (~1 to 3 m) occurred near the trench for the 1995 and 2003 events, accounting for observed high Pcoda levels and minor tsunamis. The 2009 event appears to have minor shallow slip, whereas the 1996, 2002, and 2015 events do not; their coda amplitudes are likely enhanced by early aftershocks. Refined modeling of the later stages of large megathrust ruptures, guided by Pcoda levels, improves resolution of the up‐dip limit of co‐seismic slip.