Virtual Deep Seismic Sounding for a Uniform Measure of Crustal Thickness

Abstract:

After over one hundred years of research since the Moho was discovered by seismic refraction, the seemingly straightforward, essential goal of building a global model of the crust is still an on-going endeavor. In addition to seismic refraction/reflection and dispersion of surface waves, scattered wavefield on the receiver-side generated by distant earthquakes (crustal receiver functions) made significant contributions to this effort. However, the most commonly used phase, Ps, is quite small and requires stacking of data from many earthquake sources. To this end, virtual deep seismic sounding (VDSS), which uses SV-to-P conversion off the free surface as a virtual source to generate P-wave reflection off the Moho ( SsPmp phase), has been shown to be effective.

At distances less than about 55°, the reflection off the Moho is post-critical, resulting in large amplitudes. Thus, high-quality data from a single earthquake, with no stacking and little filtering, yield a clear signal from the Moho. To minimize source-side scattering, the sources were originally limited to deep earthquakes. Lately, using particle motions, this limitation was overcome by separating P- and S-wave portions of the signal and remove source-side scattering by deconvolution. This advance paves the way for using the large move-out of SsPmp to minimize the trade-off between crustal thickness and the average speed of P-waves in the crust. VDSS has been successfully applied to array data over apertures of more than 1,000 km and in places where the crustal thickness reaches 75 km. Simplicity of the VDSS, deep-penetrating power of earthquakes, low cost and minimal environmental impact attest to the potential of this robust method for providing a uniform measure of crustal thickness when details of the Moho are not the primary concern.