Source-Receiver Geometry of Teleseismic Imaging

By definition, teleseismic earthquake sources originate at a great distance from the receiver array and have ray turning points in the middle-to-lower mantle (>400 km depth). As a result, source wavefields are nearly vertically incident, planar wavefronts arriving beneath the receiver array. Thus, the recorded wavefield is a combination of the source along with the P and S scattered modes generated by the incoming plane wave. Figure 1 shows a schematic example of the teleseismic wavefield and the modes we use for imaging. The synthetic teleseismic wavefield shown in Figure 2 illustrates the combined source and receiver wavefield recorded by the surface array. The first arrival is the source wavelet followed by the forward-scattered P-to-S conversion seen near 13 seconds on the radial and the backscattered P-to-P phase on the vertical near 20 seconds.

 

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Figure 2 Synthetic seismograms calculated for a crustal model with an 8 km Moho offset.

As demonstrated by the synthetic seismograms, the plane-wave source creates secondary sources in the form of free surface reflections. These reflections retain a significant amount of energy and produce subsurface reflections often several times larger in amplitude than their forward-scattered counterparts. In theory,free-surface multiples provide additional information that may be used in a more traditional upgoing/downgoing wavefield framework. However, without a means to discriminate between forward and backscattered phases propagating as both P and S waves we may expect a significant amount of contamination between modes. Component rotation from [V,N,E] into the [P,Sv,Sh] system using the free-surface transfer matrix Kennett (1991) allows us to identify phases based on polarization direction. Cross-contamination by modes not traveling nearly parallel to the predicted source wave propagation direction still occurs.