Seismic moment tensors and source depths determined by the simultaneous inversion of body and surface waves

Abstract

We present a linear moment tensor inversion method that incorporates both body and surface wave data. The source coordinates are fixed during the inversion, and the source depth is determined by applying the inversion at several depths and taking the depth that produces the minimum residual. The inversion is carried out in the time domain for body wave data and in the frequency domain for surface wave data. The fundamental Rayleigh waves at the period of 256 s from IDA stations and the long-period P-waves from WWSSN and GDSN stations, whose angular distances are from 30 to 90°, are used as the data. The simultaneous inversion of both body and surface wave data sets has several merits. Because of the body wave constraints, the present method gives reasonable solutions at shallow depths (< 30 km) where long-period surface wave inversions become unstable. Although the inversion using only body wave data is unstable in the sense that it allows for a variety of solutions with a slight change in the assumed depth, probably because of the noise present and poor coverage of body wave data to determine the source mechanisms, the inclusion of surface wave data makes the solution stable. This is because the surface wave data constrain nodal-plane strikes. We may also expect better resolution in source depth by including the body wave data because the local crustal structure above the source can be taken into account to generate the Green's function. We applied our method to two large earthquakes, one occurring at the triple junction off Boso Peninsula, Central Honshu, on 18 September 1984 and another at the outer rise area off the Chile trench on 16 October 1981. The scalar moments of the Boso-Oki and Chile earthquakes are estimated to be 3.2 × 10 26 dyn cm and 6.3 × 10 26 dyn cm, respectively. The ratio of the minimum absolute eigenvalue of the moment tensor to the maximum, which may measure the non-double-couple component, is less than 10% for both earthquakes. This suggests that their sources are essentially a double couple. The source depths of both events are determined as ∼ 30 km. The Boso-Oki event is indicative of hinge faulting within the Pacific slab at the triple junction. The Chile event is consistent with compressional bending stress in the deeper part of the lithosphere at the trench-outer rise area off Chile.