Adjoint‐State Attenuation Tomography: Method and Application to Northern New Zealand

We develop a novel seismic attenuation tomography method that is formulated under a matrix‐free framework and avoids ray tracing. The key ingredient is the use of the adjoint‐state technique to compute the gradient of the misfit function, defined as the difference between observed and synthetic , to obtain an optimal attenuation model. Two advancements have been achieved over previous ray‐based methods. First, is accurately computed using a grid‐based method to solve the Eikonal equation, followed by solving the governing equation. Second, the inversion mainly requires solving the forward and adjoint equations, with a cost roughly twice that of forward modeling alone. The proposed method is benchmarked by imaging the structure of the Hikurangi subduction zone beneath northern New Zealand. The amplitude spectra of P‐waves from local earthquakes are inverted to estimate at 1 Hz, along with the seismic moment and corner frequency of each event. A total of measurements from 6,478 events at 44 stations are used. The new model shows high in the subducted, cold Pacific plate, adjacent to the low mantle wedge, matching previous results. Our findings reveal contrasting high and low , at depths of km beneath the backarc, may indicating the presence of a lithosphere‐asthenosphere boundary (LAB). The low beneath the backarc originates from depths of 150–200 km to the LAB, ascending along a nonvertical path toward the region beneath the arc, which may result from the dehydration of the deeply subducted slab.