Waveform response to the morphology of 2-D subducted slabs

Abstract

Summary To evaluate waveform responses to morphological variations of the subducted lithospheric slab, scalar waveform solutions are computed for 2-D slab models perpendicular to the arc. The dependence of the waveforms on the background 1-D velocity model and on the wavelet frequency is analysed. The waveform variation due to the background velocity model is characterized by gradual changes in amplitude and traveltime over the take-off angle; such variation can be distinguished from the slab-generated waveform anomaly, which is restricted to a narrow take-off angle range and often severely alters the wavelet frequency. Therefore, much information can be gained through the waveform modelling of slabs in a homogeneous background velocity model. The characteristics of the waveform anomaly, such as slab-diffracted waveform broadening, depend on the ratio between the wavelength of the main source wavelet and the thickness of the slab. The slab-diffracted broadening, which has low-frequency characteristics, may be described more accurately as a wavelet broadening rather than a pulse-width broadening. When the wavelength is shorter than about one-half of the slab's thickness, a restoration of normal amplitude that is accompanied by waveform distortion is observed for waves propagating parallel and down-dip to the slab. Considerable waveform variations in arrival time, amplitude and wavelet-width exist among different morphological models of the slab, and may provide some useful guidance in inferring the real slab morphology from waveform data.