EVIDENCE FOR TRANSFORMATIONAL FAULTING FROM A DEEP DOUBLE SEISMIC ZONE IN TONGA

DOUBLE seismic zones, planes or earthquakes parallel to the dip of a subducting slab and separated by 2040 km, provide important clues about the earthquake generating mechanisms and strain distribution inside subducting slabs. Double seismic zones have been found at intermediate depths (70-200 km) in many subduction zones1-6 but have not been previously reported in deep slabs. Here, by relocating earthquakes with a hypocentroidal decomposition technique7 and visualizing the earthquake positions and uncertainties in three dimensions, we identify a double seismic zone at depths of 350-460 km in the Tonga subduction zone. Source parameters of the earthquakes determined by waveform analysis suggest different stress orientations for the two zones, with in-plane compression in the lower zone and in-plane tension in the upper zone. The double zone may be due to transformational faulting, as olivine along the edges of a metastable olivine wedge becomes warmer and transforms to spinel8-11.