The Torsional Free Oscillation of the Earth due to a Dislocation

Abstract

The torsional free oscillation due to a dip-slip fault or a strike-slip fault is studied in an earth model with homogeneous mantle and homogeneous liquid core. Because of the simple earth model assumption, only the fundamental mode of order number two, ₀T₂, is investigated.
It is found that the ₀T₂ is effectively excited by a vertical strike-slip fault. The surface amplitude of the free oscillation due to a horizontal or a vertical dip-slip fault is about three percent of that due to a vertical strike-slip fault. Radiation patterns, except for a horizontal or a vertical dip-slip fault, are governed by P₂² (cos θ) (cos 2ψ sin 2ψ).
If a step function time variation fault is assumed, then the maximum surface amplitude of 6×10^-2cm is estimated when the area of the fault is 10⁴km² and the mean offset over the fault-plane is 5m.