Stochastic waveguide in the lithosphere: Indonesian subduction zone to Australian craton

S U M M A R Y Earthquakes occurring in the Indonesian subduction zone recorded in the cratonic regions of northern Australia have long, high frequency coda associated with both P and Swaves. Similar behaviour is seen for earthquakes within Australia on paths within the cratonic regions. The complex character of the coda and its rapid spatial variation indicate that the high frequency waves have travelled within a zone of strong, multiple scattering with very high intrinsic Q . There are subtle variations in the nature of the coda that can be described through a coda Q . The variations in coda are associated with the position along the arc and the nature of the transition from the subduction zone to the continental lithosphere. The character of the coda observations can be captured with numerical simulations in models derived from seismic tomography for northern Australia and the neighbouring subduction zone, incorporating lower crustal and mantle heterogeneity with much longer horizontal than vertical correlation length. High frequency calculations to high frequency (16 Hz) for long propagation distances (up to 1500 km) have been carried out using finite difference modelling on the Earth Simulator. The variations in coda properties can be associated with the way in which the seismic energy gets ducted into the stochastic waveguide associated with the lithosphere. The most efficient transfer occurs where continental lithospheric material is adjacent to the earthquake zone.