Direct Observations of Surface-Wave Eigenfunctions at the Homestake 3D Array

Date: 4/26/2019

Time: 11:45 AM

Room: Vashon

Ever since 1885, when Lord Rayleigh first predicted the existence of waves that travel along a free surface, observations interpreted to be surface waves have been remarkably useful for helping determine Earth structure and earthquake source properties. Yet despite the theory for both Rayleigh and Love waves being well accepted, and the theoretical predictions accurately matching observations, the observation of their quantifiable decay with depth has never been measured in the Earth's crust. The primary difficulty of confirming this decay of motion with depth is that nearly all seismometers are placed at or near the Earth's surface, or in isolated borehole installations. In this work, we address this gap in observations by making direct observations of both Rayleigh-wave and Love-wave eigenfunction amplitudes over a range of depths, using data collected at the three-dimensional Homestake Array for a suite of nearby mine blasts. Observations of amplitudes over a range of frequencies from 0.4-1.2 Hz are consistent with theoretical eigenfunction predictions, with clear exponential decay of amplitudes with depth, and a reversal in sign of the radial-component Rayleigh-wave eigenfunction at large depths, as predicted for fundamental-mode Rayleigh waves. Minor discrepancies between the observed eigenfunctions and those predicted using estimates of the local velocity structure suggest that the observed eigenfunctions could be used to improve the velocity model. Our results confirm that both Rayleigh and Love waves have the depth dependence that they have long been assumed to have. This is an important direct validation of a classic theoretical result in geophysics, and provides new observational evidence that classical seismological surface-wave theory can be used to accurately infer properties of Earth structure and earthquake sources.

Presenting Author: Victor C. Tsai

Authors