Rupture Process of the Tonga Earthquakes of 3 May 2006 (Mw 8.0) and 19 March 2009 (Mw 7.6): Intraslab Thrust Deformation Below the Megathrust
Monday, 15 December 2014: 8:00 AM
The Tonga subduction zone is one of the most seismically active areas in the world, with a high rate of plate convergence between the subducting Pacific plate and the Tonga arc from 15°S to 25°S.However, seismically recorded large interplate thrusting events along the arc have not been larger than about Mw 8.0, and the degree of interplate seismic coupling has been inferred to vary from moderate to very low going from northern Tonga to southern Tonga [e.g., Scholz and Campos, 2012]. The possibility of this region experiencing a low probability maximum magnitude event of Mw8.57 up to 9.14 has been raised, but a paucity of geodetic observations and their distance from the Tonga trench have precluded assessments of slip deficit accumulation on the megathrust [Wallace et al., 2005]. Faulting mechanisms of historical large shallow earthquakes are uncertain [e.g., Okal et al., 2011], and some recent large events have been normal faulting or thrust-faulting intraslab events, so catalog-based assessments of seismic coupling are uncertain. Two recent large thrust-faulting earthquakes in 2006 and 2009 further complicate such assessments. The 3 May 2006 Mw8.0 event was located 100km west of the trench and caused a substantial regional tsunami, so it is sometimes assumed to be an interplate rupture. However, finite-fault modeling of teleseismic body waves supports a slip distribution centered about 60 km depth, consistent with centroid depths from W-phase (60.5 km) and global CMT (65 km) inversions. This was an intraslab thrust event with estimated stress drop of ~10 MPa. The fault plane is difficult to resolve unambiguously using teleseismic body waves, but joint inversion with static offsets from local GPS stations favors a shallow-dipping plane subparallel to the megathrust, or possibly a steeply dipping plane with triggered megathrust faulting. The 19 March 2009 Mw 7.6 thrust event occurred below the trench, and finite-fault and W-phase inversions indicate a 45km deep centroid. The fault plane is also difficult to establish. There continues to be a paucity of large megathrust earthquakes in Tonga.