Receiver Function Studies

A.M. Reading, B.L.N. Kennett, M. Sambridge

Estimating seismic structure from teleseismic receiver functions is a popular method of using earthquake energy to explore the crust and upper mantle. Assuming an isotropic, horizontal-layered crust, most of the converted energy which provides information on receiver structure is found on the radial component. In the normal scheme the receiver function is formed by deconvolving the radial component with the vertical component to remove the influence of the source and source-side structure but the record is still dominated by the presence of the high-amplitude initial P-pulse.

A geometric rotation to the LQT frame (Figure 1) with the L component along the apparent direction of propagation of P reduces the effects on the orthogonal Q component and has been used in a number of studies. An alternative is to make a transformation to remove the free-surface effects from three-component seismograms [Kennett, 1991], reconstructing the incoming P and S-wavevectors together with the (horizontal) H-wavevector (Figure 1). The resulting S-wavevector contains all the converted (i.e., SV) motion while the H-wavevector describes the transverse, horizontal (i.e., SH) motion. A new style of receiver function can then be created by deconvolving the S-wavevector with the P-wavevector.

Figure 1: Seismic energy arriving at a receiver. a) station/event reference frame, ZRT, b) ray coordinate reference frame, LQT, as used by Vinnik [1977] and c) the PSH wavevector reference frame as used in this study. Incidence angle = i.

We illustrate the effect of the transformation on the observed receiver function and on the inversion for seismic structure using three contrasting stations in Western Australia (figure 2). MBWA is a new permanent station on the Pilbara Craton where there is a sharp Moho at a depth of 30 km. Station WT08 in the central Yilgarn Craton shows exceptional conversions from the Moho. Whereas WS03 is a poor example of a receiver waveform, showing a low signal-to-noise ratio and a low-amplitude Moho conversion. This station is located in the Yerrida Basin at the northern extremity of the Yilgarn Craton, which was strongly influenced by the Capricorn Orogen.

The effective surface velocities need for the transformation to PSH vectors,were determined empirically by finding the values which most completely remove the initial P-pulse. For stations MBWA and WT08, located on old cratonic crust, moderately fast P and S velocities (5.8 and 3.4 km/s) were appropriate, whereas slower velocities (5.5 and 3.1 kms/s) were used for station WS03, in the Yerrida Basin. The receiver functions were stacked to improve the signal-to-noise ratio.

Figure 2: Observed receiver functions, radial(upper) and S-wavevector (lower) for stations in western Australia. Note the absence of the initial P-pulse on the receiver functions' calculated for the S-wavevector. MBWA is a 6-event stack, WT08 a 9-event stack and WS03 a 2-event stack.

The S-wavevector receiver functions are inverted for 1-D shear wavespeed structure using the Neighbourhood Algorithm (NA) approach. The quality of the inversion is improved because we are no longer expending effort to match the large initial P-pulse. Tests on synthetic S-wavevector receiver functions (compared with radial receiver functions for the same model, both with added noise) also show a faster, improved misfit reduction in the same number of iterations.

The results of the inversions for the these three stations are shown in Figure 3 which compares the The best-fit structure obtained from the radial receiver function and the S-wavevector receiver function.

Figure 3: Observed (black) and synthetic (blue) receiver functions i) radial and ii) S-wavevector) and associated seismic velocity models iii) radial and iv) S-wavevector). The P:S velocity ratio and S-velocity structure corresponding to the best-fit seismic structure are shown by red lines. The best fit is determined by minimizing the least-squares difference between observe and modeled waveforms. The yellow-green model density plots indicate the proportion of better-fitting models in that region of parameter space. All models were calculated with the same number of iterations so the wider swath of green in the left-hand velocity plot for WS03 (velocity inverted from radial receiver function) shows the distribution of profiles is wide and not constrained well by the data. P:S ratios are shown since they are part of the velocity profile although their determination is not affected by the S-wavevector approach.

For station MBWA, the fit between observed and synthetic receiver functions is improved, and although the overall structure (e.g., depth to Moho) does not change substantially, some low-velocity zones are removed and it is now possible to fit the converted waveform corresponding to a sharp Moho.

WT08 shows such a high-amplitude Moho phase conversion that it is limited by the search bounds imposed on the model.

WS03 shows a very significant improvement. The low-amplitude Moho conversion was not fitted (Figure 3c, iii) by the inversion algorithm using the standard, radial receiver function but has been successfully fitted and a structure determined (Figure 3c, iv) using the S-wavevector receiver function. Although the record for WS03 has a low signal-to-noise ratio, the pulse at just less than 5 seconds is a known feature of records from this region and the main features of the structure at for WS03 arises from signal rather than noise.

The determination of seismic structure from S-wavevector receiver functions is likely to be most useful: