Earthquakes in the fault zone from Tonga across to Vanuatu is a bit messed up and for good reason. There is the subduction zone to the west and another to the east but in between is crazy
It is a series of ridges or rift faults that are formed by the pacific plate moving westward away from the finger of the
the indo Australian Plate moving north. Since they are moving in different directions they form some rifts and ridges between them.
The four images to the right show some of these faults across the zone.
The top one is Tonga, showing the location of today’s 6.3 magnitude quake inside the curve of the subduction zone on the right. This finger of land is part of the IndoAustralian plate and shows some of the rift faults that define the breakup of this part of the plate and where the pacific plate is pulling away as it moves westward.
The second image is a bit to the west showing the Fiji island of Viti Levu. This shows the continuation of the rift faults into this zone. This fault zone is really a web of pieces that are being dragged apart by the pressure from the tectonic forces moving the pacific plate ever westward.
And below that the third image shows these same faults as they continue across to the islands of Vanuatu. These islands are the result of the collision of the plates causing uplift and volcanic activity.
The area below this line of images is also riddled with faults and is bordered at the bottom by a subduction zone where the Indo-Australian Plate is subducting under the pacific plate.
The forth image below shows this curved subduction zone and the how the rift faults continue from the top of the zone to the bottom.
It’s a kind of Tectonic weaving of plate over plate and knitted by fault zones.
Here they define the edge of the finger of the Indo-Australian Plate that is rising over the pacific plate causing part of it to subduct, and the part of the same plate that is being subducted below the Pacific plate. It’s a sort of tectonic weaving but it produces great stresses and pressures on the faults and is the cause of the huge amount of earthquake activity.