Crustal Thickness Behind the Tonga Arc Measured with pP Precursors

Abstract:

Underside reflections from internal discontinuities within the lithosphere, particularly the Moho, can sometimes be observed as precursors to the depth phase pP. If these precursors are recorded by dense arrays of seismometers, the signal processing techniques developed for reflection surveys using controlled sources at the surface can be applied to create an “upside-down” seismic reflection profile. Previous studies with this technique have largely been used to measure the depth of upper-mantle discontinuities or the thickness of continental crust. Here we use recordings from Japan’s Hi-net array of intermediate- and deep-focus earthquakes in the Tonga-Kermadec subduction zone to measure crustal thickness within the Lau and South Fiji Basins and along the Lau Ridge. Our measurements sample areas that have been inaccessible to traditional surface seismic measurements. We have used modeling to address complications of bathymetric variability at the bounce points and low velocities in the upper mantle in Tonga. We have found the crust to vary in thickness from about 7 km to 20 km. Our measurements agree with existing measurements of crustal thickness from surface seismic refraction surveys where they overlap and demonstrate the feasibility of using this technique to measure crustal thickness in oceanic regions.