This study detects temporal changes in the wave speeds of long-period Rayleigh and Love waves after the 2004 Great Sumatra Earthquake, which were measured from repeating earthquakes. Seismic observations reveal that the Rayleigh-wave speed reduction is more than that of Love waves by a factor of six. Love waves are much more sensitive to the S-wave speed of the shallow crust than Rayleigh waves in isotropic media (i.e., the S-wave speed is the same in all directions). One would therefore anticipate considerable Love-wave speed reduction if the S-wave speed reduction results from the near surface after great earthquakes. However, the observations indicate the opposite. The result of waveform modeling suggests that these unexpected observations can be suitably explained by an increase in radial anisotropy as a result of increasing fluid-filled horizontal cracks after great earthquakes.
Yu, W., Song, T.-R. A., Su, J., & Lin, J.-T. (2021). Rayleigh-Love discrepancy highlights temporal changes in near-surface radial anisotropy after the 2004 Great Sumatra Earthquake. Journal of Geophysical Research: Solid Earth, 126, e2021JB022896. https://doi.org/10.1029/2021JB022896