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

Abstract:

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 M_w 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 M_w8.57 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 M_w8.0 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 M_w 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.