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

DOUBLE seismic zones, planes or earthquakes parallel to the dip of a s ubducting slab and separated by 2040 km, provide important clues about the earthquake generating mechanisms and strain distribution inside s ubducting slabs. Double seismic zones have been found at intermediate depths (70-200 km) in many subduction zones1-6 but have not been previ ously reported in deep slabs. Here, by relocating earthquakes with a h ypocentroidal 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. Sou rce parameters of the earthquakes determined by waveform analysis sugg est different stress orientations for the two zones, with in-plane com pression 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.