Source retrieval for mantle earthquakes by iterative deconvolution of long-period P-waves

Abstract

Long-period P-waves of eighteen deep and intermediate-depth earthquakes in a moment range 1×10 26−3× 10 28 dyn · cm are analysed by the method of Kikuchi and Fukao (this issue) to iteratively derive point dislocations on a plane. The fault plane is defined as the plane of point dislocations that gives the best fit between the synthetic and observed waveforms. For dip-slip type earthquakes the fault plane is found to dip more steeply than the auxiliary plane. In general, point dislocations tend to cluster into one or a few subevents. We characterize the largest subevent by its maximum moment rate ( M 0) m and its half duration τ from which the associated stress drop Δσ can be roughly estimated. For earthquakes shallower than 400 km, half duration τ is approximately constant, 5 s, so that stress drop Δσ increases from < 100 to > 1000 bar as ( M 0) m increases. The value of τ for the great Banda Sea earthquake of 1963 ( h = 100 km) is exceptionally large (τ~10 s, for which Δσ ~ 700 bar). For earthquakes deeper than 400 km, τ increases from 3 s to 9 s as ( M 0) m increases so that Δσ is approximately constant, 700 bar. Stress drop so obtained approximates to an overall stress drop with a possible exception of the several largest earthquakes. The distribution of point dislocations and the moment rate function indicate that rupture most often expands in a fan-shaped fault area and that a large part of the rupture front ceases propagation in a coherent manner. A sharp contrast in stress drop behavior across a depth of 400 km may be related to the olivine-spinel transition within the downgoing slab of lithosphere.