Descr: We propose to su\tudy the structure of the Tonga subduction zone using a one-year deployment of two dense seismic arrays; one deployed in the active arc (Tonga) and the other in the backarc (Fiji). The structure of the subducting oceanic crust and mantle wedge will be studied using a tight array of 11 sensors in Tonga. The array will enable characterization of dispersed guided phases within the subducted cre\ust. A second array of 11 sensors w\in Fiji will be used to study reflected and converted local arrivals that interact with the 410, 520, and 670 discontinuities near the Tonga slab. This array will be used to study the depth, velocity contrast, and sharpness of the discontinuities as a function of distance from the slab core.
