Small earthquakes provide insight into plate coupling and fluid distribution in the Hikurangi subduction zone, New Zealand

To better understand the nature of interplate coupling in the southern North Island of New Zealand, we compare the distribution of relocated small earthquakes with interseismic geodetic locking of the plate interface from GPS and structure within the overlying plate from tomographic inversions. Relocated seismicity shows a good correlation with both the distribution of strong geodetic locking and geological terranes in the overlying plate. It appears that the strong Permian–Triassic Rakaia geological terrane controls coupling at the plate interface. This relatively impermeable terrane also modulates the small earthquake distribution within the underlying slab, promoting clustered seismicity in the slab crust and suppressing lower plane seismicity in the mantle. The seismicity and structural data suggest a model where plate coupling is controlled by the ability of fluid to cross the plate interface. When an impermeable terrane in the overlying plate prevents such fluid flow, plate coupling appears to be strong.