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The 3 May 2006 (Mw 8.0) and 19 March 2009 (Mw 7.6) Tonga Earthquakes: Intraslab Compressional Faulting Below the Megathrust

Created 17/10/2025

Updated 17/10/2025

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The Tonga subduction zone is among the most seismically active regions and has the highest plate convergence rate in the world. However, thrust events confidently located on the plate boundary have not exceeded Mw 8.0. The possibility of a low probability maximum magnitude event of Mw 8.6 to 9.1 has been raised, but a paucity of geodetic observations and their distance from the Tonga trench have precluded direct assessment of megathrust slip deficit accumulation. We analyze two major thrust fault earthquakes that occurred in central Tonga in 2006 and 2009. The 3 May 2006 Mw 8.0 event has a focal mechanism consistent with interplate thrusting, was located west of the trench, and caused a moderate regional tsunami. However, long-period seismic wave inversions and finite-fault modeling by joint inversion of teleseismic body waves and local GPS static offsets indicate a slip distribution centered ~65 km deep, about 30 km deeper than the plate boundary revealed by locations of aftershocks, demonstrating that this was an intraslab event. The aftershock locations were obtained using data from 7 temporary seismic stations deployed shortly after the mainshock, and most lie on the plate boundary, not on either nodal plane of the deeper mainshock. The fault plane is ambiguous and investigation of compound rupture involving co-seismic slip along the megathrust does not provide a better fit, although activation of megathrust faulting is responsible for the aftershocks. The 19 March 2009 Mw 7.6 compressional faulting event occurred below the trench; finite-fault and W-phase inversions indicate an intraslab, ~50-km deep centroid, with ambiguous fault plane. There continues to be a paucity of large megathrust earthquakes in Tonga.

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Field Value
Title The 3 May 2006 (Mw 8.0) and 19 March 2009 (Mw 7.6) Tonga Earthquakes: Intraslab Compressional Faulting Below the Megathrust
Language eng
Licence Not Specified
Landing Page https://data.gov.au/data/dataset/b5ebc781-ed53-4b01-9caf-fe61ad923e89
Contact Point
Geoscience Australia Data
clientservices@ga.gov.au
Reference Period 22/04/2018
Geospatial Coverage Australia
Data Portal Geoscience Australia

Data Source

This dataset was originally found on Geoscience Australia "The 3 May 2006 (Mw 8.0) and 19 March 2009 (Mw 7.6) Tonga Earthquakes: Intraslab Compressional Faulting Below the Megathrust". Please visit the source to access the original metadata of the dataset:
https://ecat.ga.gov.au/geonetwork/srv/eng/csw/dataset/the-3-may-2006-mw-8-0-and-19-march-2009-mw-7-6-tonga-earthquakes-intraslab-compressional-faulti