Uppermost mantle structure of the Australian continent from Pn traveltime tomography

Abstract

Past studies of the seismic structure of the Australian continent have dominantly exploited surface wave tomography for the mantle, with seismic refraction, receiver functions, and ambient noise used for crustal structure. The 3-D structure has been summarized in the Australian Seismological Reference Model (AuSREM), for which the zone immediately below the crust is the least well characterized. Pn traveltime tomography provides a way of improving structural information on the uppermost mantle across the continent. We have exploited waveforms from larger events across Australia recorded at both permanent and portable stations since 1993, supplemented by bulletin arrival times. To compensate for the large velocity contrasts, with much faster mantle wave speeds in the center and west, all events were relocated using the AuSREM model. After relocation, consistent patterns of traveltime residuals are obtained. We extract Pn as the first arrival in the distance range 1.8° to 15°. We use the FMTOMO approach to invert the travel-time residuals to generate a P wave speed structure with a resolution of 3°×3°. There is strong heterogeneity in Pn wave speed in the uppermost mantle across the continent. The fastest Pn wave speed of 8.36 km/s beneath the Precambrian cratons of western and central Australia is 3.99% faster than the global ak135 model, and the slowest Pn wave speed 7.66 km/s on the eastern margin is 4.74% slower. The slowest velocities in the uppermost mantle are found along the eastern margin of the Australian continent beneath the Phanerozoic orogenic belts, with links to Neogene volcanism.