Flexible computation of teleseismic synthetics for source and structural studies

The modelling of P and S arrivals at teleseismic distances, for shallow sources, requires the consideration of the interaction of the direct arrivals and their surface reflections (pP, sP and pS, sS). A flexible computational scheme has been developed to allow for distinct crustal and upper mantle structures beneath source and receiver with full allowance for reverberation effects. The seismic response to a point moment tensor source is built up from the reflection and transmission properties of portions of the media in the frequency-slowness domain. This allows control of the propagation processes in the shallower parts of the model. A simple representation is used for the contribution from the lower mantle that can also be adapted to include core reflections and refractions. The response at each receiver is calculated using a restricted slowness integral over a bundle of slownesses surrounding the geometric slowness for each group of phases. Thus, for example, P, pP and sP can be calculated together without the cost of a full reflectivity treatment. This teleseismic calculation scheme provides a good representation of the main P or S arrivals at teleseismic distances over a wide range of frequencies and can be used for both source and structural studies. The method is illustrated by showing the influence of regional upper mantle models on the nature of the seismograms. There can be significant changes in the relative amplitudes of the direct and surface-reflected phases, which would modify inferences about source mechanism. An observational example using broad-band P and S arrivals from an event in the Fiji region, recorded at a network of stations in northwestern Australia, illustrates the way in which surface-reflected phases can provide useful constraints on event depth, and also how deviations from the predicted seismograms for a reference model can be used for structural investigations.