For the purpose of investigating the influence of metastable olivine(MO) phase transformations on both deep seismicity and stagnation of slabs,we constructed a 2-dimensional finite element thermal model for a 120 Ma-old 50°dipping oceanic lithosphere descending at 10 cm/yr with velocity boundary layers,which would mitigate the interference of constant velocity field for the slab. The resulting temperatures show that most of intermediate and deep earthquakes occurring within the Tonga slab are occurring inside the 800℃and 1200℃isotherm,respectively.The elevation of olivine transformation near～410 km and respective persistence of metastable olivine and spinel within the transition zone and beneath 660 km would thus result in bimodal positive,zonal,negative density anomalies,respectively.These results together with the resulting pressure anomalies may reflect the stress pattern of the Tonga slab:(i) slab pull force exerts above a depth of～230 km;(ii) MO existence changes the buoyancy force within the transition zone and facilitates slab stagnation at a depth of 660 km;(iii) as the subducting materials accumulated over 660 km,deepest earthquakes occur due to MO transformation;(iv) a flattened‘slab’ may penetrate into the lower mantle due to the density increment of Sp transformation. For the purpose of investigating the influence of metastable olivine (MO) phase transformations on both deep seismicity and stagnation of slabs, we constructed a 2-dimensional finite element thermal model for a 120 Ma-old 50 ° dipping oceanic lithosphere descending at 10 cm / yr with velocity boundary layers, which would mitigate the interference of constant velocity field for the slab. The resulting temperatures show that most of intermediate and deep earthquakes occur within the Tonga slab are occurring inside the 800 ℃ and 1200 ℃ isotherm, respectively. elevation of olivine transformation near ~ 410 km and each persistence of metastable olivine and spinel within the transition zone and beneath 660 km would therefore result in bimodal positive, zonal, negative density anomalies, respectively.These results together with the resulting pressure anomalies may reflect the stress pattern of the Tonga slab: (i) slab pull force exerts above a depth of ~ 230 km; (ii) MO existence changes the buoy ancy force within the transition zone and facilitates slab stagnation at a depth of 660 km; (iii) as the subducting materials accumulated over 660 km, deepest earthquakes occurrence due to MO transformation; (iv) a flattened’slab ’may penetrate into the lower mantle due to the density increment of Sp transformation.