Imaging Active Fault Zones in the Deep Tonga Slab

Abstract

Two‐thirds of global deep earthquakes occur within the subducting Tonga slab, making it an ideal setting to investigate slab deformation at transition zone depths, and to gain insights into the rupture mechanisms of deep earthquakes. We investigate two concentrated regions of seismicity using relative earthquake relocation and focal mechanism cluster analysis. Earthquakes are relocated using a hypocentroidal decomposition method, incorporating regional arrival times from temporary land and ocean bottom seismometers, as well as teleseismic phases. Global Centroid Moment Tensor (GCMT) solutions are grouped into common focal mechanism types through K‐means clustering of their principal axis orientations. By integrating precise relative earthquake locations with GCMT cluster analysis, we identify distinct seismic patterns. In both study areas, earthquakes align along a dense seismic plane, representing a fault or shear zone. Cross‐correlation relocation of events in the northern zone, recorded by temporary regional seismographs, shows that this fault zone is narrower than ∼3 km. The planes of seismicity are coplanar with the steeply dipping nodal planes of earthquake focal mechanisms and follow the inferred upper boundary of the slab, with seismicity rates decreasing deeper into the slab. We interpret the fault zones as resulting from downward motion of the active Tonga slab relative to stagnant slab material to the west. The fault zones likely represent planes of recurrent thermal shear runaway within the subducted oceanic crust, which has thermal properties suitable for the development of thermal shear instabilities.