COMPUTATION OF THREE COMPONENT RECEIVER FUNCTIONS (R3) BY BEAM FORMING OF REGIONAL VERTICAL COMPONENT SEISMOGRAMS BEFORE DECONVOLUTION

In receiver function (RF) analysis, we interpret Ps conversions from broadband seismograms of teleseismic events (from 30 to 90 degrees away). Because of the near vertical ray paths of these events, vertical components are assumed to contain mostly P phases (including P reverberations) and the horizontal components record mostly Ps converted phases (including reverberations ending in a S phase). The horizontal component of the seismogram is deconvolved by the vertical (assumed to be the P-source function) to recover a RF (assumed to be a time series of Ps conversion coefficients and P-to-S reflection coefficients). Noise in the vertical component, especially scattering from localized 3-D structure, decreases the signal to noise ratio. A cleaner estimate of the incoming P-waveform can be made by beam forming the vertical component of the seismogram from a given station with those recorded at neighboring stations (from upwards of three hundred km away). This beamed vertical component is averaged from stations across different geological terrains. As a result, local variation in delay times of P reverberations will not be coherent across stations and will be removed. This results in an estimate of the P-waveform that is relatively free of P-reverberations. We then deconvolve all three components of the local seismogram by the beamed vertical component and produce a three component receiver function (RF3). The vertical component of the RF3 will contain local P-reverberations, thereby contributing P-arrival times that are independent of the Vp/Vs ratio. When combined with radial (Ps) components of the RF3, this can reduce the ambiguity in trade-off between estimates of depth and Vp/Vs ratio to a given horizon. Comparison of RF3s with traditional receiver functions may yield better interpretations than using either method independently.