The abstract on this [1] makes me think something similar could be happening here: low attenuation implies the resonance would take quite a number of cycles to die out like the one discussed in the article.

Here’s another abstract [2] describing resonance with periods near 10 seconds lasting a long time.

[1]: https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/1999...

[2]: http://science.sciencemag.org/content/299/5615/2058