The 2022 MW 7.3 Southern Sumatra Tsunami Earthquake: Rupture Up-Dip of the 2007 MW 8.4 Bengkulu Event

On 18 November 2022, a large earthquake struck offshore southern Sumatra, generating a tsunami with 25 cm peak amplitude recorded at tide gauge station SBLT. Our W-phase solution indicates a shallow dip of 6.2°, compatible with long-period surface wave radiation patterns. Inversion of teleseismic body waves indicates a shallow slip distribution extending from about 10 km deep to near the trench with maximum slip of ∼4.1 m and seismic moment of $1.05\times {10}^{20}$ Nm (M_W 7.3). Joint modeling of seismic and tsunami data indicates a shallow rigidity of ∼23 GPa. We find a low moment-scaled radiated energy of $4.15\times {10}^{-6}$, similar to that of the 2010 M_W 7.8 Mentawai event ($3.1\times {10}^{-6}$) and other tsunami earthquakes. These characteristics indicate that the 2022 event should be designated as a smaller moment magnitude tsunami earthquake compared to the other 12 well-documented global occurrences since 1896. The 2022 event ruptured up-dip of the 2007 M_W 8.4 Bengkulu earthquake, demonstrating shallow seismogenic capability of a megathrust that had experienced both a deeper seismic event and adjacent shallow aseismic afterslip. We consider seismogenic behavior of shallow megathrusts and concern for future tsunami earthquakes in subduction zones globally, noting a correlation between tsunami earthquake occurrence and subducting seafloor covered with siliceous pelagic sediments. We suggest that the combination of pelagic clay and siliceous sediments and rough seafloor topography near the trench play important roles in controlling the genesis of tsunami earthquakes along Sumatra and other regions, rather than the subduction tectonic framework of accretionary or erosive margin.