Shallow Megathrust Slip During Large Earthquakes That Have High P Coda Levels
Abstract
Determining the up-dip extent of slip during large megathrust earthquakes is important for understanding tsunami generation. A recently introduced approach shows that high Pcoda/P amplitude ratios for period range 7-15 s are observed for events for which finite-fault rupture models indicate slip extending to near trenches, resulting in enhanced tsunami. However, for some events with high Pcoda/P measures, published finite-fault models differ with regard to occurrence and amount of shallow slip. We re-examine six large subduction zone thrust earthquakes with high Pcoda/P ratios, using fault models with depth-varying dip and varying bathymetry and adjusting kinematic parameters to allow for the possibility of late shallow slip and associated strong pwP excitation. For events of 30 July 1995 Chile (MW 7.9) and 17 November 2003 Aleutian (MW 7.8), we confirm that late shallow slip (~1-3 m) and associated pwP excitation occurred near the trench, accounting for observed very high P coda levels for these events. The slip model of 12 November 1996 (MW 7.7) Peru earthquake occurred where the Nazca ridge subducts has the primary slip under the coastline with minor shallow slip in the southern region that can account for the modest observed tsunami. The 7 October 2009 Vanuatu (MW 7.6) event has minor slip at shallow depth that contributes to coda level, but an early aftershock contributes to high coda amplitudes. The 5 May 2015 Papua New Guinea (MW 7.5) event does not have shallow slip, but an along-strike triggered aftershock produces the high Pcoda level. The 5 March 2002 Mindanao (MW 7.5) event has relatively compact slip without shallow rupture, with complex early coda that likely results from scattering or an early aftershock. For the events with shallow rupture, the shallow slip is patchy and separated from the concentrated slip at larger depth. Reduced along-dip rupture expansion velocity allows the late slip to be captured. This suggests that the shallow megathrust slip may be triggered by the deeper slip, either dynamically driving conditionally stable regions to fail or by triggering unstable patches within a predominantly stable sliding regime. Improved modeling of the later stages of large megathrust ruptures, guided by Pcoda levels, will improve resolution of the up-dip limit of slip.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.T51H0394W
- Keywords:
-
- 4315 Monitoring;
- forecasting;
- prediction;
- NATURAL HAZARDS;
- 7230 Seismicity and tectonics;
- SEISMOLOGY;
- 8170 Subduction zone processes;
- TECTONOPHYSICS;
- 8488 Volcanic hazards and risks;
- VOLCANOLOGY